Completed Project What is the use of Level Set and 12AU7 circuit in Pilot FA-540 AM/FM/AUX radio?

Thread Starter

Rahulk70

Joined Dec 16, 2016
522
Hi folks,

I recently scored a Pilot FA-540 on eBay. The one I received was from a console. But the models sold usually is the non-console type. So, while going through the circuit from the service manual I noticed that the original FA-540 had a "LEVEL SET" pot tuner and a 12AU7 circuit. But in the console unit I received the 12AU7 circuit is missing. What is the specific use of the 12AU7 circuit? Is it necessary to add it?

I've attached a picture of the unit I've received. I've also attached the schematics for a better understanding along with picture of the non console unit from an eBay ad (https://www.ebay.com/itm/Audiophile...h=item4b5947ee14:g:1LwAAOSwLr1cIYIT:rk:6:pf:0) to show the tube and the level set.

Thanks in advance.
 

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Dodgydave

Joined Jun 22, 2012
10,242
It looks like an output volume control with a Cathode follower amplifier, so you can limit the sound output to feed into headphones or tape recorder.
 
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Thread Starter

Rahulk70

Joined Dec 16, 2016
522
It looks like an output volume control with a Cathode follower amplifier, so you can limit the sound output to feed into headphones or tape recorder.
One question, sorry if it sounds stupid. So if the 12AU7 and level control was present will I be able to get sound output to a speaker directly from AUDIO OUTPUT? Because currently I need to run the output a speaker with amplifier like a simple computer speaker to get audio, directly connected to a speaker there isn’t any output.

Also what is the exact need for a preamp and power amplifier? Because when I purchased this radio the seller has said it works best with SA-232 and SP-210 preamps.

Another thing I find a little confusing is that there is already an antenna for the AM at the back but there are some connections for an external 75ohm and 300ohm antenna it seems. Are these just regular wires if these particular resistance?
 

Dodgydave

Joined Jun 22, 2012
10,242
Preamp stages contain the Bass Treble controls, and usually Aux /Mic inputs which have different input resistances.

75 ohm and 300 ohm antennas are for FM.

75 ohms is a coaxial type for a Dipole,

300 ohms is a flat twin wire type you can pin up on a wall or between trees etc.

Sound will be available at the audio output but low in volume for a connection to an amplifier, if you're using a computer or other Non Valve amp, you need to use a Transformer to isolate the high voltage.
 

Sam Spastic

Joined Dec 20, 2018
4
My guess is that the shielded cable that went to the SET LEVEL control now goes directly to the OUTPUT jack and maybe thru a .1 capacitor. Cost savings!
 

Thread Starter

Rahulk70

Joined Dec 16, 2016
522
Preamp stages contain the Bass Treble controls, and usually Aux /Mic inputs which have different input resistances.

75 ohm and 300 ohm antennas are for FM.

75 ohms is a coaxial type for a Dipole,

300 ohms is a flat twin wire type you can pin up on a wall or between trees etc.

Sound will be available at the audio output but low in volume for a connection to an amplifier, if you're using a computer or other Non Valve amp, you need to use a Transformer to isolate the high voltage.
Thanks for the detailed reply. I'll try to make an antenna and see ow the FM works. The seller told me that he was able to get good reception with the Tuner. So, maybe its the missing antennae the is causing me issues I guess.

Also can I skip the preamp and directly use a power amp?

I used a computer speaker (which has an amp already in it) for amplifying the AUDIO OUT. Sorry if this sounds stupid but this tuner has a transformer on it. So, it isn't the hot chassis type tuner. I was under the impression that only the hot chassis type tuner needed transformer isolation.
 

Thread Starter

Rahulk70

Joined Dec 16, 2016
522
My guess is that the shielded cable that went to the SET LEVEL control now goes directly to the OUTPUT jack and maybe thru a .1 capacitor. Cost savings!
Upon thinking it kinda makes sense compared to the desk model which has the LEVEL SET and 12AU7. The console model since it goes inside it doesn't need the level set at the back for volume control. I think its controlled from the amp itself.
 

Thread Starter

Rahulk70

Joined Dec 16, 2016
522
My guess is that the shielded cable that went to the SET LEVEL control now goes directly to the OUTPUT jack and maybe thru a .1 capacitor. Cost savings!
Exactly! I opened up and did an inspection today and like you said through the 0.1 cap in parallel with a 220k resistor. I’ve decided to put a 12AU7 back. I’ve wired it and now just waiting for the 1Meg pot to arrive on Wednesday which I ordered. Also the FM reception is working now after adding the antenna. I’ve put a dipole type connected between D1 & Ground. Can I use a 12AU7A instead of 12AU7? I’ve brand new if the 12AU7A only. Also what’s the exact use of this cathode follower?
 

MisterBill2

Joined Jan 23, 2018
11,596
I looked at the schematic circuit drawing and the "level set " and 12 AU7 feed what is called an output connector to feed either an external amplifier or a pre-amplifier with tone controls. If it was part of a console setup then it was built to feed a specific amplifier and so the variable control would not be required. Thus making the effort to add that part of the circuit may not be any benefit to you, depending on what you choose to connect this tuner to.
 

Thread Starter

Rahulk70

Joined Dec 16, 2016
522
I looked at the schematic circuit drawing and the "level set " and 12 AU7 feed what is called an output connector to feed either an external amplifier or a pre-amplifier with tone controls. If it was part of a console setup then it was built to feed a specific amplifier and so the variable control would not be required. Thus making the effort to add that part of the circuit may not be any benefit to you, depending on what you choose to connect this tuner to.
Hi,
Yes my unit was part of a console once. That’s why it’s missing the cathode follower. But I’m planning to is use it non console unit now. Hence the addition of cathode follower.
 

MisterBill2

Joined Jan 23, 2018
11,596
Hi,
Yes my unit was part of a console once. That’s why it’s missing the cathode follower. But I’m planning to is use it non console unit now. Hence the addition of cathode follower.
It depends entirely on what you are trying to do. That is a good monophonic tuner, if you are assembling a vintage system then yes, that is the way to go. But if it is strictly fro listening enjoyment then adding an internal IC stereo decoder would solve both challenges by providing a decent drive level as well as stereo sound.
 

Thread Starter

Rahulk70

Joined Dec 16, 2016
522
It depends entirely on what you are trying to do. That is a good monophonic tuner, if you are assembling a vintage system then yes, that is the way to go. But if it is strictly fro listening enjoyment then adding an internal IC stereo decoder would solve both challenges by providing a decent drive level as well as stereo sound.
Finally I managed to get the 1.5Meg pot. It’s was really difficult to find in regular shops. Even the one I got is really old from the 60s, made by Clarostat. Checked with multimeter and the total is only 1.28Megs. But since the original cathode follower needed only 1 meg I think this should be okay. Now the volume is good and it works perfectly fine.

I tried adding a 5751 and it sounded quite hollow to me though it's again is better than a 12AU7. Is it okay to replace the 12AU7 with a 12AX7?

Also I've a ECC83 mini single tube preamp from an old German Emud 713ST radio. I'm trying to mode it to add it as a preamp stage to the audio output from the tuner.
 

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MisterBill2

Joined Jan 23, 2018
11,596
Power transformers do run warm on most commercially built audio equipment, 116F is warm but not hot. It would be wise to go through the whole tuner after it has been on for an hour or more and see if anything is HOT. I was once asked to repair a Monarch brand receiver with a burned out power transformer. That was because of a 15 amp line fuse instead of the 2 amp one. The second IF amplifier tube had an internal short circuit and when the 220 ohm cathode resistor burned up they replaced it with a ten watt resistor which just ran very hot. The power trans ran much hotter and burned out completely. Warm is OK for tube stuff, hot is not. At least not usually, except for tubes handling power.
 

Thread Starter

Rahulk70

Joined Dec 16, 2016
522
Power transformers do run warm on most commercially built audio equipment, 116F is warm but not hot. It would be wise to go through the whole tuner after it has been on for an hour or more and see if anything is HOT. I was once asked to repair a Monarch brand receiver with a burned out power transformer. That was because of a 15 amp line fuse instead of the 2 amp one. The second IF amplifier tube had an internal short circuit and when the 220 ohm cathode resistor burned up they replaced it with a ten watt resistor which just ran very hot. The power trans ran much hotter and burned out completely. Warm is OK for tube stuff, hot is not. At least not usually, except for tubes handling power.
So, that's good news then :). I'm currently recapping the unit. I must say this unit has pretty good reception especially the AM even at night when regular radios can't capture.

Yes, adding a fuse is definitely a must in these old HV equipments. I tried touching a neon tester on the body of the tuner while in ON position and it didn't light up. But once I turned the tuner off and touched it lit the neon tester but when I touched I didn't get electrocuted or anything. I thing I must try to get this earthed if possible later on.

Also one thing I noticed that was when I put the level set to max and try to control the volume from the speaker while playing a song the song volume fades out for a while and then it comes back again. but if the level set is mid or kept low then this issue doesn't occur. DO you have any idea what might be the cause?
 

MisterBill2

Joined Jan 23, 2018
11,596
So, that's good news then :). I'm currently recapping the unit. I must say this unit has pretty good reception especially the AM even at night when regular radios can't capture.

Yes, adding a fuse is definitely a must in these old HV equipments. I tried touching a neon tester on the body of the tuner while in ON position and it didn't light up. But once I turned the tuner off and touched it lit the neon tester but when I touched I didn't get electrocuted or anything. I thing I must try to get this earthed if possible later on.

Also one thing I noticed that was when I put the level set to max and try to control the volume from the speaker while playing a song the song volume fades out for a while and then it comes back again. but if the level set is mid or kept low then this issue doesn't occur. DO you have any idea what might be the cause?
The short term fade-out is probably due to a capacitor charging or discharging after you adjust the control. Is there any distortion during that time? That would be a clue. It may be that this is the reason that the control is not normally used as a volume control, as well. I can't open the drawing PDF on this computer, but my guess is that it is a combination of big capacitor and high resistance causing the drop. So I will need to use a different comuter that can open a PDF later and see if I can provide a better answer.
 

MisterBill2

Joined Jan 23, 2018
11,596
OK, now having examined the PDF from a much earlier post, my guess is that turning the "level set" pot all the way up applies the AFC voltage on C46 directly to the grid which cuts the volume until things settle. Without modifying the tuner the simple fix is to add a volume control someplace down stream of the tuner. It does not look like there is any option of adding one to the tuner without some serious changes, although you could replace the control with a concentric shaft one and use the second pot as a conventional volume control on the output. BUT if you are a serious collector of old equipment that is not an option since it is not original.
 

Thread Starter

Rahulk70

Joined Dec 16, 2016
522
The short term fade-out is probably due to a capacitor charging or discharging after you adjust the control. Is there any distortion during that time? That would be a clue. It may be that this is the reason that the control is not normally used as a volume control, as well. I can't open the drawing PDF on this computer, but my guess is that it is a combination of big capacitor and high resistance causing the drop. So I will need to use a different comuter that can open a PDF later and see if I can provide a better answer.
There isn't any sort of distortion. Just the sound fading in and out. But after some experimenting, I found that this issue occurs more when I turn the level set above mid.
 

Thread Starter

Rahulk70

Joined Dec 16, 2016
522
OK, now having examined the PDF from a much earlier post, my guess is that turning the "level set" pot all the way up applies the AFC voltage on C46 directly to the grid which cuts the volume until things settle. Without modifying the tuner the simple fix is to add a volume control someplace down stream of the tuner. It does not look like there is any option of adding one to the tuner without some serious changes, although you could replace the control with a concentric shaft one and use the second pot as a conventional volume control on the output. BUT if you are a serious collector of old equipment that is not an option since it is not original.
I guess I'll keep the controls as such for maintaining its originality. Also, the fade out the issue is more prominent if the input signal fed in has its bass boosted. Turning down the bass solves the fade out issue too.
 

MisterBill2

Joined Jan 23, 2018
11,596
So my suggestion of adding an extra volume control, downstream of the tuner, would probably solve the problem. Really, it might even be solved just by having a series resistor that could be bypassed for "loud" and inserted for "not loud" modes. It would be simple enough to consider experimenting with the idea.
 
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