Attached is a table showing what the voltages should be and what I measured as well as a schematic. Some voltages are high, some are low, some by a lot, some only a little. They are highlighted. There were high and low voltages before the repair but now some are worse and there are a few more. There are no burnt components.

The radio had been running fairly well lately despite the odd voltages. A few days ago a small encapsulated PC board had a cap blow out. I rebuilt the circuit on a separate PC board. This circuit, PC-2, is highlighted in orange and is in the upper right of the schematic. Most voltages are similar to what they were before this happened. Pin voltages that did not come back to normal are highlighted in light blue. All components in rebuilt circuit check out good yet some voltages in blue on the attached table are far from what they were before the repair.

I noticed noise about the time I noticed the damaged PC-2. The noise is still present after replacing the PC-2 circuit but I can't find a faulty component nor can I even narrow it down to a specific part of the circuit. The noise is definitely coming from the circuit and is not coming from the power supply or through the antenna. It is present as soon as sound can be heard from the speaker. It does not change as the unit warms up. The noise is a crackling, rumbling sound occurring at random, usually a few times a minute. It is not constant, it comes in short bursts. It reminds me of the sound lightning makes on an AM radio. I'm guessing it may be another bad cap but everything I can see looks good. I hope it's not in one of the other PC boards such as PC-4 in the audio circuit.

The noise is not as loud if a dummy antenna is attached. I don't know if there is noise being transmitted.

I can pick up circuit noise with an inductive probe but I can't tell where it comes from. Cap C-7 coming from pin 8 of tube V-1 has always had more circuit hiss than any other component yet it checks out good and replacements are no different. C-8, C-9, C-14 have slightly less hiss.

I've tapped and wiggled each tube and each pin and everything else but there are no bad connections and the tubes appear to be good.

I only have a DMM to test with and I'm not familiar with how the operation of one tube could affect what's measured on another tube. If anyone out there has any ideas I'd appreciate it if you can give me some idea where to look.
Thanks

 

Here are some observations and suggestions from examining the C-540 schematic and voltages table.
You say that you are not familiar with tube-to-tube interactions, so I have gone into some detail. These old tube circuits are complex and intricate, so this note has grown long in the writing. Don't panic!

Schematic. Some points that are bothering me.
1] In PC-1 the Rx oscillator AC signal is shorted to ground at pin 3 via an unmarked capacitor. I think that the unmarked capacitor should connect pins 1 and 3. Also, I think that the .005 capacitor on PC-1 should connect pins 2 and 4.
2] At connector TS-1 (mic lead) I cannot see how pin 1 (RED) gets 0V/ground on transmit from the right hand of the two mic circuits. For the Tx oscillator and the mic pre-amp to work plus Rx oscillator disable, 3 cathodes and PC-1 100k must all be pulled down to 0V. On the left hand mic circuit (Rx) the lead to/from pin 1 is o/c, allowing the red and green wires to float up to around 50V, determined by the 100k on PC-1 and V-2A pin 2, triode plate.

Sometimes circuit designers put in deliberate "mistakes" in order to prevent copying and/or to show patent infringements.
Perhaps that is occuring here...

Voltages. Nice, comprehensive table!
1] All the filament voltages and some of the plate voltages rise about 10% between "should be" and "measured". This suggests a common cause in the power supply or thereabouts. The PSU is unregulated so the cause could potentially go all the way back to the mains supply. All these voltage changes won't be related to change of PC-2.
2] Tx oscillator cathodes (V1 pins 1 and 6) are low at the same time as Rx oscillator plate (V2 pin2) is high. A significant discrepancy, not explicable if both Tx and Rx are working ok.
3] Note that your "should be" filament voltages are less than the values at the bottom left of the schematic. I can't explain why the schematic voltages are at the upper limit of the tube specs, 6.3 +/- 10%.
4] Ideallly you need two sets of voltage readings: one for Rx and one for Tx (see Schematic [2]). Changing between Tx/Rx will really jiggle the operating conditions of most of the tubes.
5] When measuring in and around the Squelch and ANL circuits, make sure that you have consistent context: all these controls in consistent positions between readings - squelch, ANL switch, volume. For reading Rx quiescent levels its also important to have consistent RF input, preferably none from a dummy impedance in the antenna connector.

Noise.
1] I suggest that you turn the volume down to zero when attempting to chase noise. R-16, if it hasn't been changed, will be very old and likely crackly. Make sure that the wiper has really good connection at the ground end. This configuration might help you to ascertain whether the noise is being generated in the audio part (V-5B, V6). Or, whether the noise is sneaking around, somehow, through the IF stages (V-3), which it shouldn't normally be able to.
2] RF-borne noise should be stopped by C-7 and L-2, but note that there is a potential sneak path via C-8, C-9 past the dummy antenna. Particularly if the Tx oscillator is still on in Rx, see Voltages [2].

Hoping that at least some of this helps. Many thanks for the opportunity to revisit a tube circuit in earnest!

 

Here are some observations and suggestions from examining the C-540 schematic and voltages table.
You say that you are not familiar with tube-to-tube interactions, so I have gone into some detail. These old tube circuits are complex and intricate, so this note has grown long in the writing. Don't panic!

Schematic. Some points that are bothering me.
1] In PC-1 the Rx oscillator AC signal is shorted to ground at pin 3 via an unmarked capacitor. I think that the unmarked capacitor should connect pins 1 and 3. Also, I think that the .005 capacitor on PC-1 should connect pins 2 and 4.
2] At connector TS-1 (mic lead) I cannot see how pin 1 (RED) gets 0V/ground on transmit from the right hand of the two mic circuits. For the Tx oscillator and the mic pre-amp to work plus Rx oscillator disable, 3 cathodes and PC-1 100k must all be pulled down to 0V. On the left hand mic circuit (Rx) the lead to/from pin 1 is o/c, allowing the red and green wires to float up to around 50V, determined by the 100k on PC-1 and V-2A pin 2, triode plate.

Sometimes circuit designers put in deliberate "mistakes" in order to prevent copying and/or to show patent infringements.
Perhaps that is occuring here...

Voltages. Nice, comprehensive table!
1] All the filament voltages and some of the plate voltages rise about 10% between "should be" and "measured". This suggests a common cause in the power supply or thereabouts. The PSU is unregulated so the cause could potentially go all the way back to the mains supply. All these voltage changes won't be related to change of PC-2.
2] Tx oscillator cathodes (V1 pins 1 and 6) are low at the same time as Rx oscillator plate (V2 pin2) is high. A significant discrepancy, not explicable if both Tx and Rx are working ok.
3] Note that your "should be" filament voltages are less than the values at the bottom left of the schematic. I can't explain why the schematic voltages are at the upper limit of the tube specs, 6.3 +/- 10%.
4] Ideallly you need two sets of voltage readings: one for Rx and one for Tx (see Schematic [2]). Changing between Tx/Rx will really jiggle the operating conditions of most of the tubes.
5] When measuring in and around the Squelch and ANL circuits, make sure that you have consistent context: all these controls in consistent positions between readings - squelch, ANL switch, volume. For reading Rx quiescent levels its also important to have consistent RF input, preferably none from a dummy impedance in the antenna connector.

Noise.
1] I suggest that you turn the volume down to zero when attempting to chase noise. R-16, if it hasn't been changed, will be very old and likely crackly. Make sure that the wiper has really good connection at the ground end. This configuration might help you to ascertain whether the noise is being generated in the audio part (V-5B, V6). Or, whether the noise is sneaking around, somehow, through the IF stages (V-3), which it shouldn't normally be able to.
2] RF-borne noise should be stopped by C-7 and L-2, but note that there is a potential sneak path via C-8, C-9 past the dummy antenna. Particularly if the Tx oscillator is still on in Rx, see Voltages [2].

Hoping that at least some of this helps. Many thanks for the opportunity to revisit a tube circuit in earnest!

Thanks for the detailed reply! I will look things over and get back to you again soon with some answers to several of your questions and other details which I left out (not knowing they'd be important.)

In the meantime, about my guess that the noise might be from a leaky cap. I have never actually heard that kind of noise so it really is a guess. Could a leaky cap cause high pitched crackling and low pitched rumbling simultaneously (but not in unison)? There is no popping like a brief arc through an insulator might sound. These go n for several seconds then go quiet till the next burst.

The radio does pick up signals from the antenna as it should. They're fainter than usual even when the noise is quiet. It also transmits but I haven't measured RF power yet. I did find out the noise is not being transmitted.

I figured the noise was quieter when a dummy ant was attached because the noise getting into the signal line would be partially grounded out through the load and L-3. My inductive pick up does not 'hear' the noise on any of the power wires.

I've wondered why C-7 has so much hiss. Where is it coming from? Replacing it with others does not change this. Maybe it is from R-7? I know some resistors can cause different kinds of noise. When running right, there is only a little bkg hiss from the speaker without any signals. If this is from C-7, then removing this could really quiet the bkg noise floor. I've tried other uF values for C-7 but the hiss was always there.

About the bkg noise, I recently learned of 2 ways to reduce this. One is to put a 0.001 uF cap between the volume wiper line and ground to short out the highs. At the speaker end, it takes about 20 uF to 50 uF across the speaker terminals to do the same thing. I use electrolytics connected back to back to act like a non-polarized electrolytic. They seem to remove a slightly broader range of hiss frequencies than the small cap on the volume wiper.

I also had the idea for a simple soft start feature to reduce stress on these old, original tube filaments during the high current cold start. I just happened to have a 3 position switch as the on-off switch. If I wire a small 120 V, 50 W, bulb to the center position, I can stop at this position for a few seconds and the filaments warm up slowly, then I go over to full on. This is on the schematic. The whole radio gets partial voltage momentarily, but I see no problems. If I didn't have a 3 position switch I could plug the radio into an outlet, switch and plug in light assembly to achieve the same thing. It's a nice simple feature, and the only one I could make since I don't have a stash of old transistors like I do these other old parts. All soft start circuits I've seen online are far more complicated. I prefer individual components to transistors and ICs so I can tinker and see what I'm doing rather than working with 'black boxes' containing unknown circuitry. All wiring on this radio is point to point aside from the small PC boards. They are encapsulated but fortunately the schematic gives the internal circuits.

More soon. Thanks.

 

Here are some observations and suggestions from examining the C-540 schematic and voltages table.
You say that you are not familiar with tube-to-tube interactions, so I have gone into some detail. These old tube circuits are complex and intricate, so this note has grown long in the writing. Don't panic!

Schematic. Some points that are bothering me.
1] In PC-1 the Rx oscillator AC signal is shorted to ground at pin 3 via an unmarked capacitor. I think that the unmarked capacitor should connect pins 1 and 3. Also, I think that the .005 capacitor on PC-1 should connect pins 2 and 4.
2] At connector TS-1 (mic lead) I cannot see how pin 1 (RED) gets 0V/ground on transmit from the right hand of the two mic circuits. For the Tx oscillator and the mic pre-amp to work plus Rx oscillator disable, 3 cathodes and PC-1 100k must all be pulled down to 0V. On the left hand mic circuit (Rx) the lead to/from pin 1 is o/c, allowing the red and green wires to float up to around 50V, determined by the 100k on PC-1 and V-2A pin 2, triode plate.

Sometimes circuit designers put in deliberate "mistakes" in order to prevent copying and/or to show patent infringements.
Perhaps that is occuring here...

Voltages. Nice, comprehensive table!
1] All the filament voltages and some of the plate voltages rise about 10% between "should be" and "measured". This suggests a common cause in the power supply or thereabouts. The PSU is unregulated so the cause could potentially go all the way back to the mains supply. All these voltage changes won't be related to change of PC-2.
2] Tx oscillator cathodes (V1 pins 1 and 6) are low at the same time as Rx oscillator plate (V2 pin2) is high. A significant discrepancy, not explicable if both Tx and Rx are working ok.
3] Note that your "should be" filament voltages are less than the values at the bottom left of the schematic. I can't explain why the schematic voltages are at the upper limit of the tube specs, 6.3 +/- 10%.
4] Ideallly you need two sets of voltage readings: one for Rx and one for Tx (see Schematic [2]). Changing between Tx/Rx will really jiggle the operating conditions of most of the tubes.
5] When measuring in and around the Squelch and ANL circuits, make sure that you have consistent context: all these controls in consistent positions between readings - squelch, ANL switch, volume. For reading Rx quiescent levels its also important to have consistent RF input, preferably none from a dummy impedance in the antenna connector.

Noise.
1] I suggest that you turn the volume down to zero when attempting to chase noise. R-16, if it hasn't been changed, will be very old and likely crackly. Make sure that the wiper has really good connection at the ground end. This configuration might help you to ascertain whether the noise is being generated in the audio part (V-5B, V6). Or, whether the noise is sneaking around, somehow, through the IF stages (V-3), which it shouldn't normally be able to.
2] RF-borne noise should be stopped by C-7 and L-2, but note that there is a potential sneak path via C-8, C-9 past the dummy antenna. Particularly if the Tx oscillator is still on in Rx, see Voltages [2].

Hoping that at least some of this helps. Many thanks for the opportunity to revisit a tube circuit in earnest!

Okay, here's what I have:
Schematic
1] I hadn't noticed the unmarked cap in PC-1. The 'PCs' are encapsulated circuits so I can't change anything internally without building a completely separate circuit. I could possibly estimate the value of the unknown cap by measuring the capacitance between the various other pins and solving for the unknown. I can't change those parts of the circuit though. The PCs came encapsulated from the factory. They are the only PC boards in the whole radio.

2] The first attachment is the original mic wiring diagram. It shows the connections in receive mode. However, it looked wrong for transmission because when the bottom switch is up for transmitting, the red power wire is shorted to the mic shield which is grounded at terminal 3 of TS-1. I re-drew the connections for the top mic switch to eliminate the shorting of the red wire to gnd so power would go through the mic instead. I just assumed that was the direction current would go. I didn't look beyond that. It seemed like the modulated power from the red wire on terminal 2 would go to the grid of V-5A and that's as far as it needed to go. I see now that I was looking at it backwards. Current can flow from the white wire, terminal 5, grid on V-5A, through the mic to the shield gnd. I was looking at the red wire as being a power source but since it goes to the Tx circuit it is actually taking the signal from the mic. The original mic wiring was right, I just understood it wrong.

As for the rest of your comments about what's needed for the Tx to work and the Rx to be disabled, I can't follow at all so I can't say anything more.

Voltages (Attachment 2 are some voltage measurements from a month ago for comparison.)
1] The power requirements range from 110-130 VAC. My mains is about 123 V. I've recently replaced some old electrolytic caps in the power supply. I have a few notes from after I built the radio and the numbers I have don't match exactly either. I haven't looked at them again until a few weeks ago. As expected from age and some replaced caps (not all new), the voltages were still not perfect. What I can't figure out now is why some are very far off now after the recent cap blow out and repair compared to a month ago (see attached voltages from 11-12-20.) Specifically tube 1, pin 2 should be -11 V. It was -3.4 V when new, a month ago was -0.4 V and now climbed to +1.6 V. Also tube 4, pins 6 & 7 were fine before but are now half what they were. Tube 6, pins 5 & 6, the plate & grid were 255 and 260 V when new. All measurements this year have been about 315 V, an increase of 21%. It always was the hottest tube. It also had a distinctive blue glow on the glass in one area which shows up very well in the photo taken in July but that is almost invisible now. Glass has a blue glow when hit by electrons so I'd say electrons aren't being accelerated as hard as before. Plate voltage, pin 5, is more positive and so are the grid voltages, pins 6 & 1, and cathode, pin 2, is less positive (more negative). So why wouldn't there be more electron acceleration?

2] No comment.
3] The voltages written on the schematic were measured at a different time from the tube pin voltages, that's why they differ a bit. One recent change was I noticed the original resistor, R-21. 5K, 7W, was running very hot and was only 4.5K so I replaced it with two 10K in parallel. They are each 2W, totaling 4W, about half the original 7W yet they don't seem as hot. I thought an internal burn path or loose wire may have something to do with the noise but it had no effect. I also can't explain the high filament voltages.

4] The instructions only give voltage readings for V-1 in Tx mode. I did notice they can be very different from in Rx mode. I can get the Rx voltages if they'll help.

5] The instructions did say to take measurements with volume at max, squelch at min, and ANL off. I did that except for the volume which I had lowered because of the extreme volume when some pins were touched. There is a note that pin 7 voltage on V-4 will vary with squelch setting but I only measured with it at min. The squelch and volume have always worked fine. I did have some trouble with ANL recently but found it was due to an unseen intermittent short between pins 8 & 9 which are crowded. All the short did was mimic flipping the ANL on-off switch.

I usually troubleshoot with no antenna attached. I only had the dummy attached because I have to transmit to take voltage readings on V-1. That's when I noticed the reduced noise level.

Noise
1] Volume, R-16, is original but only has one tiny noisy spot. It and the squelch are otherwise quiet. Grounds are as good as any other, they connect to a terminal strip bolted to the chassis. The only grounds soldered to the chassis are in the power supply.

2] I've checked, and substituted, caps C-7 & C-8 and they appear good.
How would I know if the Tx oscillator is on when in Rx mode? How would this affect the noise?
---
New developments
Besides replacing R-21, the 5K resistor mentioned, I increased insulation around the bundles of caps I needed to use in the power supply. At one point I heard a faint very high pitched whine sound which I traced to wire 1 on PC-2. The .01 uF caps seemed good but when I moved my probe to the EDGE of one of them it detected the high pitched steady whine. I replaced it and solved that new noise. I also double checked all connections in that circuit. I felt better but the original crackling and rumbling noises were still present.
HOWEVER, during this work, I noticed that if I let the unit run for several hours, the noise slowly diminished, even with the dummy antenna off. Over quite a few hours the noise slowly decreased and is almost totally gone now. I would have expected a noisy component to be faulty and continued use might make it go bad enough to be identified more easily, not get better with time. Rx & Tx seem normal enough. I rarely hear anyone. That could be due to age of the tubes and also because my antenna is indoors. My condo doesn't allow outside antennas such as this, especially not on the roof.
I haven't had a chance to take any new voltage readings to see if they've changed in any way since the noise mysteriously disappeared. I intend to take measurements after about 10 min (the warm up time suggested) and again hours later to see if anything changes with time and temperature.

Thanks for your great help. I wouldn't be surprised if something changes again. Till then, if you have any answers, comments, questions, or are just curious, just ask.

P.S. I just thought of a question. As I mentioned, I've had to jerry-rig antennas to use indoors. This analog, discrete component, circuit has been very tough. I probably would have burnt out a transistor radio by now. If problems do begin to develop in the Tx from trying to match antennas, where would I need to look to fix it? Would it be an invisible problem within a tube or would some other component burn out, say from overheating?
Thanks.

 

This radio is possessed. While taking voltage measurements yesterday it became sensitive to my hand being close to the circuit. There was a constant approx. 60 Hz noise raising my noise floor that covered faint signals. Frequency drift was happening as much as several times a second, fast, slow, with snaps. I monitored the main power supply voltage and it varied between 318 and 314VDC. The voltage level had no effect on noise or frequency shift.

It had been stable until I turned it off to take measurements, then it shifted frequency. This made me think of a heat problem. 4 stores later I had a can of freeze spray. I sprayed each component. The only thing sensitive to the temperature change was L-4 which is used to shift the channel positions on the dial. It was very sensitive at first, but became less so after the first spray. I retuned it but channel positions shifted by the time it was plugged back in. L-4 shifts the channels toward lower frequencies when it is cooled, then they seem to come back after it warms up enough. They can shift completely off the dial making it sound quiet, like there's nothing wrong. The mica caps on the coil are fine, even the ceramic caps don't react. The coil isn't loose but it is sensitive if the radio is bumped. The coil slug does not move freely. Why it should be so sensitive to vibration or temperature when nothing is bothering it is beyond me.

Today the noise is so low I can't believe it. I run it with the cover off so it doesn't get so hot that frequencies shift again. The noise I first reported, which had lasted for weeks, is gone. Today the staticy 60 Hz buzz I had yesterday is gone too. Once I turn it off tonight, I expect it will be out of whack until it warms up for an hour or so tomorrow. I have no idea what to expect.

 

Today, Thurs. 12-10-20, the frequencies were indeed shifted before the circuit had warmed up. After about 3 hours of running with the case open the channel positions were very close to where they should be and seemed to have stabilized. It didn't need much tuning to touch it up. I still have the almost uniformly staticy broadband hiss with sharp peaks raising the noise floor just above the normal soft bkg hiss. Call it crabgrass on my lawn.

I can add some caps to chop a little off the crabgrass but it'll still be there. Cooling L-4 only shifts the frequencies to longer wavelength, up to 10 channels, it doesn't affect the noise level.

 

Hello again!

Many thanks for further posts of information and particularly the photo of lit tubes - very nostalgic for me!
A blue glow in the audio power tube is normal and indicates absence of leaks. Check out some of the websites on tube-based guitar power amplifiers for more pictures of different colour glows.

For the past few days I have been looking for specification level info for the C-540, but to no avail. Some wonderful mail order catalogues from the 1960's but no hard data. This means that I'm near the end of what I can suggest to do without RF test equipment.

Some consumer feedback from 1966!
https://worldradiohistory.com/hd2/I...tronics-Illustrated-1966-05-OCR-Page-0052.pdf


From post #3

I like the heaters' soft start feature, missed that on my first run through.

Noise in C7, etc. Shouldn't be coming from R7 as the attached tube is off in Rx mode. I'm now more certain that the noise will be coming from the audio output via the modulator winding (blue wire in schematic). If it gets into C9 and therefore back into RF pre-amp then audio frequencies should be stopped at the IF transformer T-1. You can isolate the audio stage with zero volume control as discussed before.

Damping capacitors will cut out good signal as well as unwanted.


From post #4

Thanks for new diagram of mic switching, much clearer. I'm sorry for any confusion caused about possible mixed Rx/Tx modes, I was unsure what to interpret from the post #1 schematic.

I would encourage you now to try and understand more about the Tx/Rx switch over, activated by the wire running back into the circuit from connector TS-1, pin1. Some questions to ponder:

What element voltages (cathode, grids 1 & 2, plate) are needed for normal tube operation?​

Which tubes are off when the wire is high?​

Which tubes are off when the wire is low/grounded?​

Good to hear that you have some (perhaps original) instructions, with typical voltages. See if you can, for example, work out the DC operating point for one or more tubes.
This might help https://www.vtadiy.com/book/chapter-3-vacuum-tubes-as-amplifiers/

I don't know much about antenna matching/tuning, but instinct says to keep a dummy resistive load in place while experimenting.

From post #5

Rx LC tuning circuits are extremely prone to outside influence, any disturbance: mechanical, temperature or stray capacitance eg a hand. Bear in mind that at least one of the Rx oscillator tuning capacitors will have a negative temperature coefficient (perhaps C-12, N-750). If you get differential temperatures in that area then frequency drift might well be in the opposite direction to what you expect.

Seems like you're well into fault finding mode. Many previous fault-finding exercises have taught me: logical/systematic approach, patience and ... expect the unexpected!!

Fascinating project. Cheers for now.

 

Hello again!

Many thanks for further posts of information and particularly the photo of lit tubes - very nostalgic for me!
A blue glow in the audio power tube is normal and indicates absence of leaks. Check out some of the websites on tube-based guitar power amplifiers for more pictures of different colour glows.

For the past few days I have been looking for specification level info for the C-540, but to no avail. Some wonderful mail order catalogues from the 1960's but no hard data. This means that I'm near the end of what I can suggest to do without RF test equipment.

Some consumer feedback from 1966!
https://worldradiohistory.com/hd2/I...tronics-Illustrated-1966-05-OCR-Page-0052.pdf


From post #3

I like the heaters' soft start feature, missed that on my first run through.

Noise in C7, etc. Shouldn't be coming from R7 as the attached tube is off in Rx mode. I'm now more certain that the noise will be coming from the audio output via the modulator winding (blue wire in schematic). If it gets into C9 and therefore back into RF pre-amp then audio frequencies should be stopped at the IF transformer T-1. You can isolate the audio stage with zero volume control as discussed before.

Damping capacitors will cut out good signal as well as unwanted.


From post #4

Thanks for new diagram of mic switching, much clearer. I'm sorry for any confusion caused about possible mixed Rx/Tx modes, I was unsure what to interpret from the post #1 schematic.

I would encourage you now to try and understand more about the Tx/Rx switch over, activated by the wire running back into the circuit from connector TS-1, pin1. Some questions to ponder:

What element voltages (cathode, grids 1 & 2, plate) are needed for normal tube operation?​

Which tubes are off when the wire is high?​

Which tubes are off when the wire is low/grounded?​

Good to hear that you have some (perhaps original) instructions, with typical voltages. See if you can, for example, work out the DC operating point for one or more tubes.
This might help https://www.vtadiy.com/book/chapter-3-vacuum-tubes-as-amplifiers/

I don't know much about antenna matching/tuning, but instinct says to keep a dummy resistive load in place while experimenting.

From post #5

Rx LC tuning circuits are extremely prone to outside influence, any disturbance: mechanical, temperature or stray capacitance eg a hand. Bear in mind that at least one of the Rx oscillator tuning capacitors will have a negative temperature coefficient (perhaps C-12, N-750). If you get differential temperatures in that area then frequency drift might well be in the opposite direction to what you expect.

Seems like you're well into fault finding mode. Many previous fault-finding exercises have taught me: logical/systematic approach, patience and ... expect the unexpected!!

Fascinating project. Cheers for now.

Good to hear from you again.

Blue glow:
That tube has had that glow since the radio was new. The thing that bothers me now is that despite the higher voltages in that tube, the glow has nearly disappeared sometime in the last month. If I hadn't seen that I wouldn't suspect anything. Might it have something to do with the higher noise floor I'm hearing the past couple weeks? The plate voltage is high, but the grid voltage is just as high so I'm guessing it may be intercepting most electrons?

C-540 specs.
Attached are the specs as listed in the instruction manual.
The tube voltages on my charts are from the same manual. It's original. I built the radio using these instructions.

I just read the Electronics Illustrated review. It is very accurate. Especially about the sensitivity to movement. To tune it I have to have the cover off. To take voltage measurements the cover has to be on so it can be upside down. It's bound to bump a little with all that rotation. If I thought the slug in L-4 was loose I would put a dab of something to fill a gap or increase the friction so it wouldn't move. Everything is tightly connected and nothing wobbles. I think I mentioned how today it is not sensitive to my hands being near the circuit. This is normal. The strange sensitivity the other day was unusual. On startup channels were about 7-10 positions too low on the dial. This jives with the direction of shift when I cooled L-4. After about a 3 hour warm up today (in the past 5 min was enough) the channel positions on the dial went back to where I'd set them yesterday. It's never had drift like this before but it's been stable 2 days in a row now. Fingers crossed.

Post 3.
This is now getting beyond any troubleshooting I've ever done on this before. What would cause noise from V-6 to go up the blue wire to the Tx oscil V-1B? I mentioned temporarily attaching large and small caps across, or in place of some, especially C-7, looking for hiss reduction but there was no noticeable effect. I did remove C-7 & 8 for this. Most components are on very short leads and their wires wrapped around terminals so that it is impossible to remove them intact.

Post 4.
I'll try to trace power and signal flow in Tx and Rx mode & see if I can gain any insights. This is beyond me.
About your questions:
1. the voltages needed for proper operation are given on the charts I uploaded. They are the top rows of voltages, the "should be" values. I don't know enough to know if a non matching voltage is acceptable, too far out, or good or bad.
2 & 3. I don't know when a tube is on vs off vs on but output is low enough not to make any difference. I know a grid voltage varies the electron flow to the plate, but some are high, some are low, same with cathode and plate voltages. How can I know how much is too little or too much. As my backwards reading of the mic circuit shows, I don't even know which direction the signals are supposed to be going.
Not all plate wires have resistors in series so I can't measure a voltage drop to calc the plate current as a means of determining if the tube is on or off. The question is not something I've ever had to consider, ever.

I'll look at the amplifier link, but I'm not even sure what you mean by "DC operating point". Cathode voltages range from 0 to +47 V. Plate voltages range from -0.5 to +315 V. I have NO experience to tell what is out of whack and what isn't or why. If I knew what to do to alter the pin voltages I would have done that. I did try a bit with no success.

Electronics isn't my field of expertise, only a hobby. I've never had more than a multimeter so I was always limited. I understand you can only do so much without the right equipment but you have been a very big help. Even with the radio sometimes changing from day to day at least I was able to get a start of figuring out what to look for. If you get any ideas, please leave me a message. I can use all the help I can get.
Thanks again.