When I first purchased this Little Bear T10 Pro 6N2 valve Phono Turntable RIAA Preamp preamplifier Ver2.5 I did no electrical mods. Just bought Fisher Telefunkens and a Russian Rectifier tube off Ebay. Bought tube damper rings and feet. New audiophile power cable from Audio Advisor and enjoyed it for a few months. I never got the dreaded hum issue that some other buys experienced. Then I decided to recap and replace all the caps and resistors on the signal DC side. Attached is the schematics.

My first step from stock I soldered in the following Caps and the Little Bear worked great. I did not measure these caps when I first put them in.

MUNDORF - MEAIO-0.47, 0.47uF 450V, Mcap EVO Oil Series
Two AMTRANS- 100pF, 100VDC, AMCH Polypropylene Film Cap
Two AMTRANS - 0.1uF 630V, Amtrans AMCO Metalized Polyester (PET) Caps. Success@!
Sounded great with no problems. My soldering skills are good enough. I successfully completed several learning to solder kits before attempting this. Including my Enron? Power transformer kit that is well regarded for students in EE school.

But then I attempted to do the entire signal side by replacing all the resistors. I soldered everything at 430 degree F. With Wonder Solder. That's when disaster stuck! No audio coming out from the preamp.

In answer to your questions I commented below:

1. I was performing a continuity test with my DMM. black probe on the AC ground input and using the red probe to trace the circuits. I could make the schematic with where I probed and report if I got a beep or not. It would be helpful in my understanding how circuits work. I wish I took an actual Electronics course and brought this in to have the teacher got over it with me.

2. I examined all the solder joints with my magnifying professional workbench light and examine each solder joint. They all seems fine to me. I did a few touch ups just in case.

3. Used my multimeter to test the RCA jacks and there was a signal.

4. All my tubes glow the typical strength. I swapped the tubes around several times. They all glow.

5. "The fact that the valves glow means that the heater circuit is OK. What about the HT circuit ? Check across the leads of the same color from the transformer with the DMM on AC."
- I measured all the power leads coming off of the E-Transformer. Within specs so no problem there. my DMM and report the transformer ac output and further readings along the circuit.

The little caps do not measure right. I used my new Craftsmen Multimeter with one log unsoldered from the circuit board to measure the capacitance.

C15, C16 - measured 80 pF - AMTRAMS-73776 100pF 100VDC 630V, Amtrans AMCO Metalized Polyester (PET) Caps
C9 , C10 - measured .093 uF - AMTRAMS-75772 0.1uF 100VDC 630V, Amtrans AMCO Metalized Polyester (PET) Caps
MUNDORF-76426 MEAIO 0.47uF 450v
C19, C20 - measured 70 pF - Capacitor - Silver Mica, 500 V, 10pF - 1000pF - Capacitance: 10 pF
C13, C14 - measured 370 pf
C7, C8, C11 - measured .47 uF
C12 - measured way off at .007 uF (so I guess I need to buy a new cap at this position. Im wonder if I somehow fried it in my soldering and or testing)

Resistors:
Location: Near R23 and R9 - MUNDORF - MEAIO, 0.47uF 450V, Mcap EVO Oil Series
Location: Near R9: - AMTRANS- 100pF, 100VDC, AMCH Polypropylene Film Cap, gold-plated OFHC Leads
Location: R24 - KIWAME Resistor 220R / 2 watt, 1.0%, Axial, Tinned Copper Leads, Flameproof Silicone Coated

Location: R8, R6, R3, R10, R12, R14 - PRP Resistor 3M3 / 0.5 (1/2) Watt, Metal Film, 1%, 100ppm, Non-Magnetic – All failed

Location: R9 - KIWAME Resistor 910R / 2 watt, 1.0%, Axial, Tinned Copper Leads, Flameproof Silicone Coated

Location: R18, R11 - KIWAME Resistor 2K2 / 2 watt, 1.0%, Axial, Tinned Copper Leads, Flameproof Silicone

Location: R16, R5, R2 - KIWAME Resistor 62K / 2 watt, 1.0%, Axial, Tinned Copper Leads, Flameproof Silicone

Location: R7 - PRP Resistor_62499 100R / 1 Watt , Metal Film, 1%, 100ppm

Location: R15, R23 - KIWAME Resistor 330K / 2 watt, 1.0%, Axial, Tinned Copper Leads, Flameproof Silicone – All failed

Then I decided to replace all these KIWAME (carbon) resistors with Takman REY (Poly) resistors. Still no audio coming out of the preamp.

These are my readings for the following Resistors on my v2.5 little bear (test bed phono preamp)
R3 3.3m reads 1.66m
R6 3.3m reads 1.66m
R10 3.3m reads 1.66m
R8 3.3m reads 1.66m
R12 3.3m
R14 3.3m
R24 2.2k reads 1.93k
R25 2.2k reads 1.93k

On my new Little Bear T10 v2.6 I couldn't get any readings at all on the following resistors. But I could on my test bed v2.5 phono stage. My v2.6 works perfectly fine. I cannot figure out why there are not Multimeter Vdc readings. This is impossible to figure out. sigh.

R22, R23, R15 and there are a few more on that side of the board. I gave up trying to measure them.

My next step is to repopulated the original 10 and 100 pF disc capacitors. tonight I plan to solder those in at C15, C16, C19, C20.

I checked the three tube type switches several times. Right now I have 3 Fisher telefunken 12ax7a tubes and a new rectifier from Russia. I need to try the stock Chinese 6N2 tubes next and flip those switches. I replaced every resistor except 4 of them. I also used a table top lighted work bench magnifying glass and all the soldier joints seem fine. Also didn't see any splash over solder making a short. I'm pretty good at that after learning on the 3 kits I built already for practice. I built that Enron power transformer and it worked the first time.
I purchased a new Craftsmen Multimeter that measures capacitance at Sears. I'm a little irked that my EXTECH EX320 doesn't. I bought it to learn and had no idea it couldn't.

Got a new Craftsmen Multimeter that does measure capacitance. Made a bunch of measurements with the caps unsoldered from the board. Many of the new parts are way off!

 

 

Alright, I just went down to Fry's Electronics and bought two new 100 pF and 10 pF caps to replace

C15, C16 - measured 80 pF - AMTRAMS-73776 100pF 100VDC 630V, Amtrans AMCO Metalized Polyester (PET) Caps
C19, C20 - measured 70 pF - Capacitor - Silver Mica, 500 V, 10pF - 1000pF - Capacitance: 10 pF

Also put back the original .47 cap at location C12
Still no audio...

 

I found this little bear V2 schematic on the internet. which schematic matches your little bear?

 

I found this little bear V2 schematic on the internet. which schematic matches your little bear?

Yes, it's the same basic circuits. I have that one printed up as well. I was thinking of marking up a printout with my readings if that would help others figure out what was wrong with mine.

 

You do not seem to be taking a logical approach to fault finding. Do both left and right channels have the same fault ? Try injecting a signal into the grid of each valve starting at the last one in the chain and note if you can hear it on you main amplifier. The main amplifier needs to be connected to the output of this pre amplifier. (The right hand one in your schematic.) If you don't have a signal generator use the headphone out signal from a battery powered radio. Connect the common (The band on the jack plug closest to the cable.) to the common of your amplifier. (The negative rail or the outside metal of one of the phono sockets.) Connect the tip of the jack plug via a capacitor (10 to 100 nF) and a resistor (10K to 100K) to a test probe. Turn on the radio and plug in the jack plug. Touch the test probe on the grids of each valve starting at the right hand one. Note the resuts and post them in your next post. (In the days before transistors I would just touch the grid pin of the valve with my finger. there would normally be enough mains hum picked up to hear it on the output.) Also note the voltages with respect to the negative rail onthe grid, anode and cathode pf each valve and post the results.

Les.

 

Les, thank you for the response and advice. I will try to perform this tonight. I read the Dummies Guide to Electronics, completed 3 beginners learning to solder kits including that Elenco AC/DC Triple Output Power Supply Kit
https://www.amazon.com/gp/product/B0002DT0GU/ref=oh_aui_detailpage_o03_s00?ie=UTF8&psc=1

I may have built it successfully the first try but don't know much what to do with it. Instructions didn't cover how to use it much for troubleshooting.


1.Connect the common (The band on the jack plug closest to the cable.) To the common of your amplifier. (The negative rail or the outside metal of one of the phono sockets.)
* this would be the RCA jack output on the phono preamp? I'm not sure what you mean by "closest to the cable". I assume your talking about connecting the black RCA interconnect cable from the Phono stage RCA output to the RCA input of my Melody iA3 tube amp.

2. Connect the tip of the jack plug via a capacitor (10 to 100 nF) and a resistor (10K to 100K) to a test probe.
*I'm lost here. Does "jack plug" mean my RCA interconnect cable? Which one?

3. Turn on the radio and plug in the jack plug. Touch the test probe on the grids of each valve starting at the right hand one. Note the resuts and post them in your next post.
*Plug in "jack plug" to where?

4. Also note the voltages with respect to the negative rail onthe grid, anode and cathode pf each valve and post the results.
*touch my red positive probe with the 100nF cap and 10k resistor to rails B+1, B+2 and B+3?

 

1 The jack plug (Probably a 3.5mm type) This is to plug into the headphone out socket on your battery powered radio. Assuming it is a socket for stereo headphones then the plug that goes into it has three connections. One is the tip of the plug. (one channel) The next is a narrow band of metal (Which is the other channel. I cant remember which is lef and right but it does not matter.) then there is a longer section of metal (The common.) which probably has the thread that the cover screws onto. (If you are using a plug salvaged from some old headphones then the cover where the wires are connected will be moulded on.) RCA connector and phono connector are just different names for the same thing.

2 & 3 The 3.5mm jack plug plugs into the headphone socket of the battery radio. (Which needs to be turned on and tuned to a station so that we get a signal to inject into your pre amplifier.)

4 You measure the voltages with your multimeter. The black lead connected the negative supply rail. This is the same potential as the outer connection on the input and output connectors of your pre amplifier. You then touch the red probe on each ot the nine points and note the reading for each.

Injecting a signal into the grid of each valve in turn is a way of seeing wher the signal is getting lost. So if ypu inject the signal from the radio into the grid of the right hand valve and you hear it coming out of the speaker on you main amplifier we then know that the last stage (the right hand valve is working.) If you then inject the signal from the radio into the grid of the middle valve and you don't hear it from the speaker then we know the fault is the middle stage of the pre amplifier. We can then do more tests to localise the fault to a component.

The DC voltage tests may also give us a clue to where the fault is.

Les.

 

Alright, I attempted to do what you told me but I'm sure I still did not understand. I made a YouTube video trying to do these tests.

C9 123 vDC
C8 124 vDC
R8 124 vDC
C11 3.2 vDC
R24 3.2 vDC
RCA red jack out 2.9 to 5.5 vDC

I think I know how I failed to perform the signal test. I didn't inject the radio signal to each tube separately. But I probably didn't do any of this right to begin. I need clarification on where to place my multimeter probes and my Radio leads before I try again.

Little Bear T10 v2.5 troubleshooting youtube video

 

The voltage tests are done without injecting any signal and with the power to the pre amplifier switched on.
The tests using the radio to inject signals at various points are done with your main amplifier connected to the output of your pre amplifier so you can hear the output of the pre amplifier.

Is the fault on the left hand, the right hand or both channels ? (As I asked in post #6)
I will have another attempt to get you to measure the grid cathode and anode of each valve (Tube in US terminology)
We will use the schematic posted by JoJester in post #4 We will start with the first stage of the left hand channel (V1b)
Connect the negative (Black) probe of your meter to any point marked GND. (I have previously called this the common or negative rail.)
Connect the positive (Red) probe of your meter to the grid (Pin 7) of V1b What does the meter read ?
Connect the positive (Red) probe of your meter to the cathode (Pin 8) of V1b What does the meter read ?
Connect the positive (Red) probe of your meter to the anode (Pin 6) of V1b What does the meter read ?

Answer those 4 questions and I will give instructions to test the next stage.

Les.

 

Concur with Les. During this time you are testing in the quiescent mode of operations. Just follow the instructions.

 

The voltage tests are done without injecting any signal and with the power to the pre amplifier switched on.
The tests using the radio to inject signals at various points are done with your main amplifier connected to the output of your pre amplifier so you can hear the output of the pre amplifier.

Is the fault on the left hand, the right hand or both channels ? (As I asked in post #6)
I will have another attempt to get you to measure the grid cathode and anode of each valve (Tube in US terminology)
We will use the schematic posted by JoJester in post #4 We will start with the first stage of the left hand channel (V1b)
Connect the negative (Black) probe of your meter to any point marked GND. (I have previously called this the common or negative rail.)
Connect the positive (Red) probe of your meter to the grid (Pin 7) of V1b What does the meter read ?
Connect the positive (Red) probe of your meter to the cathode (Pin 8) of V1b What does the meter read ?
Connect the positive (Red) probe of your meter to the anode (Pin 6) of V1b What does the meter read ?

Answer those 4 questions and I will give instructions to test the next stage.
Les.

Well, I was sure if you wanted me to turn on the phono preamp and inject a signal while taking these readings. So I tried a few things and post the results here.

With the Phono stage powered on and used an RCA cable from the Output of the phono stage connected to an amplifier input.
Ohm meter
Pin 8: 43 ohm
Pin 7: 44.9 ohm
Pin 6: 0 ohm

Vdc meter
Pin 8: 6Vdc
Pin 7: -.01 Vdc
Pin 6: 123 Vdc

Then I tried using my radio as a source and touched the positive (right side) signal to each of those pins but heard no sound at all coming from the amp.

 

Les wanted you to NOT inject a signal. Just power it up, wait 30 seconds, then measure and record the pin voltages with the tube in the tube socket.

Quiescent means "at rest."

On edit ...

I'm using the 2.2 version schematic for the part identifications.

I noticed no one asked you to measure the power supply voltages.
Using the same quiescent condition, with no input signal, do the following:

Negative lead (black) on ground.
Red lead on the following capacitors:
negative lead of C2 (marked VDD on schematic) :
positive lead of C3 :
positive lead of C4 (marked VAA on schematic):
positive lead of C5 (marked VBB on schematic) :
positive lead of C6 (marked VCC on schematic):

 

Les wanted you to NOT inject a signal. Just power it up, wait 30 seconds, then measure and record the pin voltages with the tube in the tube socket.

Quiescent means "at rest."

Ok great. Those Vdc readings are with no signal. I'm ready for more.

 

yeah ... I edited my post to include the power supply voltages.

 

Hi StylinLP38,
First an apology. I noticed that you connected the resistor and capacitor for injecting a signal in parallel. I looked back at the instructions I gave you in post #6 and realise that I did not make it clear enough that they should be connected in series. WILL YOU READ POSTS, ANSWER THE QUESTIONS AND DO WHAT IS REQUESTED. (And only what is requested as you seem to zero electrical knowledge.)
FOR THE THIRD TIME OF ASKING. Is the fault on the left hand, the right hand or both channels ?
You should NEVER try to measure resistances with power applied to a circuit. (It is likely to destroy your DVM) When I first saw the voltage readings I thought that they looked about right. When I decided to do a calculation to get an idea od the voltage Vcc I realise that something was not right. The 6 volts on the cathode across the 2.2K resistor meant that the current through the valve was 6/2200 = 2.7 mA The same current would be flowing though the 330K anode resistor. This would mean that the voltage across this resistor was 330000 x 2.7/1000 = 891 volts. This is not possible in you circuit so something is not right.

Can you measure the five supply voltages as Joe requested in post #13
Negative lead (black) on ground.
Red lead on the following capacitors:
negative lead of C2 (marked VDD on schematic) :
positive lead of C3 :
positive lead of C4 (marked VAA on schematic):
positive lead of C5 (marked VBB on schematic) :
positive lead of C6 (marked VCC on schematic):

Les.

 

I too thought the voltages were off. I also think there is a problem with R26 in the power supply. It is listed as 10 ohms but that does not provide sufficient filament voltage, I think it should be 1 ohm. The resistor could be a documentation error.

 

Hi Joe,
I think R26 should be about 10 ohms. I have traced the filiment wiring. Each valve has the two halves of it's filiments in parallel (6.3 V 300 mA) the three valve filiments are connected in series 6.3 x 3 = 18.9 volts the voltage drop across the 10 ohm resistor will be 3 volts (10 x 0.3 = 3) So the total voltage required is 18.9 + 3 = 21.9 The peak voltage from the 15 volt winding will be 15 x 1.414 = 21.21 and as the capacitors are quite large the average voltage will be close to the peak voltage.

Les.

 

Hi Les,

Your right. I obviously erred back in the early stages prior to finding the correct model. I didn't revisit each and every test I did. Yes, I knew the filament wiring was in series, and using the 12AX7s it's still in series, thanks to the center-tapped filaments allowing a 12 volt tube to use 6.3V transformers. I certainly will correct that and check a few of the voltages out ....

All the research I've done on the 6N6s tells me they are, other than filaments, equvalent to the 12AX7.

 

Ok I did what you asked. I measured the list but the last 3 caps measured all over the place as long as I kept the probes on.

Negative lead (black) on ground.
Red lead on the following capacitors:

negative lead of C2 (marked VDD on schematic) : 0.5 mV
positive lead of C3 : 5 mV to 1.5 mV
positive lead of C4 (marked VAA on schematic): 17 mV
positive lead of C5 (marked VBB on schematic) : 7.0 mV to 2.0 mV
positive lead of C6 (marked VCC on schematic): 7.7 mV to 4.0 mV