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No left channel output

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Hello all,

I am having an issue with a voice of music integrated amp that I am trying to restore. I have re-capped the amp section with the help of some generous folks here in this forum. After the recap everything worked, but there was quite a bit of hum from the speakers. So after reading through some other posts I decided to rewire the tube heaters and twist the wire and keep it as close to the chassis as possible to hopefully eliminate some hum. It did seem to help a little, but now I cannot get any sound out of the left channel. I do hear a small amount of hum coming out of it, but no music. I have ruled out the inputs, speaker wires, etc... All tubes seem to be operational, and switching tubes from left to right channel makes no difference. I have attached the layout and schematic from the manual as well as a pic of the current internals (yes, I know it is messy, the channel with issues is on the right in the pic).

I have retraced all my steps, everything is where it should be. So now I need some assistance. Any thoughts? All connections seem to be fine, but could it be a soldering issue?
 

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Try by passing all inputs from selector and connect directly to preamp input stage to see if both channels work then? you have in the bottom righthand side of pic link bare wires are any of those touching each other. if it was working fine but with hum ok then you rewired the heaters to see if that stopped the hum then I say somewhere there is the problem you may have disturbed a wire or a soldering joint has been disturbed? see if that helps you
 
thanks for the suggestions. So, I am really new at all of this. to compare voltages between the channels I am assuming I need to have the amp plugged in and powered on? Then flip it over to access the bottom of the tube socket? Trying to think of the logistics of that without damaging anything. Especially me :)..
 
thanks for the suggestions. So, I am really new at all of this. to compare voltages between the channels I am assuming I need to have the amp plugged in and powered on? Then flip it over to access the bottom of the tube socket? Trying to think of the logistics of that without damaging anything. Especially me :)..

I've often used a couple of wood 2x4/2x6 pieces or a simple wood frame to support an inverted amp, and keep the tubes safe.
A half-hour to make something can prevent some expensive and dangerous accidents that usually happen IME when the amp is 'propped up' on something temporary.

I agree with McCurwen about not moving gear that's powered up.
I use grabber test leads or alligator leads for voltage measuring - connect leads, power up, measure, record, power down, connect + lead to next test point, etc.. Slow, I know..... but I can stand back without my hand(s) in the amp. For the steady-handed, another method would be a clip-on lead to ground, and one hand with the + probe. Keep your other hand away from the amp.

Don't forget that the HV caps may retain a lethal charge after you've powered down the AC supply- be careful moving those test leads.
 
I have retraced all my steps, everything is where it should be. So now I need some assistance. Any thoughts? All connections seem to be fine, but could it be a soldering issue?
As mdamp suggests, injecting signal into the preamp section directly is one tactic, but you need to figure out exactly what you are connecting to, to make that work.

If the heater wiring was the only thing you worked on between the time the amp was operating (with hum) and not operating, I'd suspect the heater circuits, or some connections nearby. All the tubes are heating up? Be sure to check your (AC) heater voltages when you troubleshoot, along with recording the DC voltages on the plates, grids and cathodes.

Do you have a spare preamp tube?

BTW, I'd double-check the schematic against the actual amp, since the tube designations are different on the schematic and pics/part list, as you've no doubt noticed. (Output tubes are V2/3 on the schematic, V 1/4 on the pic and parts list.)
 
All the tubes are heating up. I just rewired them again with different wire and same result. I do not have a spare preamp tube, it is a 6eu7. I can get one off of ebay though. I have 2 new el-84's to replace the 6bq5's, but switching those out does not make any difference with the output problem. There is something getting out of the left channel, I hear a little bit of hum and some, what sounds like static, coming out. It's almost as if the channel balance pot is busted or something.

As for checking the voltages, can you assist me with which pins to put the volt meter on for the output tubes? Do i need to check voltages across any of the caps? As I noted in the beginning of this post I am new to all of this and trying to learn as I go. The heater pins are 4 and 5 on the 6bq5/el84's. Not sure on the 6eu7 as it is soldered to a circuit board, but I do know where the heater wires are soldered into the board at.

Thanks again for your assistance!
 
As for checking the voltages, can you assist me with which pins to put the volt meter on for the output tubes? Do i need to check voltages across any of the caps? As I noted in the beginning of this post I am new to all of this and trying to learn as I go. The heater pins are 4 and 5 on the 6bq5/el84's. Not sure on the 6eu7 as it is soldered to a circuit board, but I do know where the heater wires are soldered into the board at.

Thanks again for your assistance!

You have a schematic which is a good start.
For any project, I usually Google "xxxx tube data' to get the info for each tube in the circuit. (Print out the first page of the data with the pinouts.)
Then I pencil the pin #s for each tube on the schematic.

Remember that the pins are numbered clockwise looking at the tube bottom, i.e. looking at the socket from under the chassis.

Once you do your research, you can come back here to get confirmation on the pin#s. You can do a lot of 'troubleshooting' thinking with a pencil and/or highlighter and printed copies of the schematic.
EDIT: Looking again, I notice that the pin numbers are marked on the schematic - notice that the heater pin#s are marked in the power transformer area of the schematic.
If the 6EU7 socket is soldered to the PCB, you can probably mark some info right on the underside of the PCB, and probe the solder pads with a sharp probe.

You have a multimeter?
Do you have test leads with alligator clips or grabber ends? They are useful, and cheap from China. Some meters come with a selection of probe tips, so you may have what you need already.

If one channel is working very well, I'd assume that the power supply is probably OK - I'm assuming those are the caps you refer to?

For safety, I'd measure the voltage on the PS caps with probes to make sure they are discharged before sticking my hands into the chassis.
 
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All the tubes are heating up. I just rewired them again with different wire and same result. I do not have a spare preamp tube, it is a 6eu7. I can get one off of ebay though. I have 2 new el-84's to replace the 6bq5's, but switching those out does not make any difference with the output problem. There is something getting out of the left channel, I hear a little bit of hum and some, what sounds like static, coming out. It's almost as if the channel balance pot is busted or something.

Can you figure out where to connect a alligator jumper lead (or tack-solder a (thin) wire) to connect the two signal inputs together, and 'feed signal' to both halves of the preamp tube?
 
It's almost as if the channel balance pot is busted or something.

The balance pot is labelled VM12481 on the schematic, I think.
Can you figure out how to check it with your ohmmeter (with the amp powered OFF) ?

BTW, that balance pot would be another spot where you could tie the two channels together for testing, if you wanted to check the output section.
 
You have a schematic which is a good start.
For any project, I usually Google "xxxx tube data' to get the info for each tube in the circuit. (Print out the first page of the data with the pinouts.)
Then I pencil the pin #s for each tube on the schematic.

Remember that the pins are numbered clockwise looking at the tube bottom, i.e. looking at the socket from under the chassis.

Once you do your research, you can come back here to get confirmation on the pin#s. You can do a lot of 'troubleshooting' thinking with a pencil and/or highlighter and printed copies of the schematic.
EDIT: Looking again, I notice that the pin numbers are marked on the schematic - notice that the heater pin#s are marked in the power transformer area of the schematic.
If the 6EU7 socket is soldered to the PCB, you can probably mark some info right on the underside of the PCB, and probe the solder pads with a sharp probe.

You have a multimeter?
Do you have test leads with alligator clips or grabber ends? They are useful, and cheap from China. Some meters come with a selection of probe tips, so you may have what you need already.

If one channel is working very well, I'd assume that the power supply is probably OK - I'm assuming those are the caps you refer to?

For safety, I'd measure the voltage on the PS caps with probes to make sure they are discharged before sticking my hands into the chassis.

I do have a multi-meter. It does not have leads with alligator clips or grabber ends, just pointed probes.

As for the tubes themselves, for the EL-84's, the heater pins are 4 & 5, grid 1 is pin 2, grid 2 is pin 9, grid 3 and the cathode look to be in series on pin 3 and the anode is pin 7.

For the 6eu7, the heaters are pins 1 and 2 (I know where these are on the PCM because I soldered new leads to them when rewiring the heaters). pin 4 is the cathode of channel 1, pin 5 is the grid and pin 6 is the anode of channel 1. pins 7, 8, and 9 are the anode, grid, and cathode respectively of channel 2.
 
Can you figure out where to connect a alligator jumper lead (or tack-solder a (thin) wire) to connect the two signal inputs together, and 'feed signal' to both halves of the preamp tube?

I am not sure I follow you here. The singnals come from the input selector so I think I can find those, but not sure how I would feed it into both halves of the preamp tube, since it is soldered to the PCB.
 
The balance pot is labelled VM12481 on the schematic, I think.
Can you figure out how to check it with your ohmmeter (with the amp powered OFF) ?

BTW, that balance pot would be another spot where you could tie the two channels together for testing, if you wanted to check the output section.

There are 2 pins from the balance pot into the PCB, so I could put the multimeter across those pins I guess??

Not sure how I would tie the 2 channels together though. Could I just remove the pot then put a piece of wire in the holes that were vacated but the pot bridging the 2 sides?
 
I do have a multi-meter. It does not have leads with alligator clips or grabber ends, just pointed probes.
$10 on ebay should get a handful of those delivered...not super quality, but adequate, IMO.
As for the tubes themselves, for the EL-84's, the heater pins are 4 & 5, grid 1 is pin 2, grid 2 is pin 9, grid 3 and the cathode look to be in series on pin 3 and the anode is pin 7.

For the 6eu7, the heaters are pins 1 and 2 (I know where these are on the PCM because I soldered new leads to them when rewiring the heaters). pin 4 is the cathode of channel 1, pin 5 is the grid and pin 6 is the anode of channel 1. pins 7, 8, and 9 are the anode, grid, and cathode respectively of channel 2.
It sounds like you have those correct.
FYI, you'll hear folks talking about plate(anode) screen (G2) grid (G1) as well.
 
There are 2 pins from the balance pot into the PCB, so I could put the multimeter across those pins I guess??

Not sure how I would tie the 2 channels together though. Could I just remove the pot then put a piece of wire in the holes that were vacated but the pot bridging the 2 sides?
You don't need to unsolder or remove anything for this 'quick and dirty' sort of troubleshooting. Just think things through before you put a jumper between two points!:eek:

If you look at the schematic, you can see that the balance control will 'short to ground' either input when the wiper is at the 'ends' (either full CW, or full CCW). When the balance control is in the middle, each signal line is separated from ground by 500k resistance (half the pot 1M).

So one 'end' of the pot has the R channel signal, the other 'end' the L channel signal. If you clip a lead from one end of the pot to the other, the two signals will be 'added' together.

If something in the 'bad channel' is shorting its signal to ground, then both channels will 'go quiet' if they are joined together. If the L & R outputs play (mono) full volume, then the 'bad channel' should be investigated back toward the input.

This is where some test leads with alligators or grabbers on the ends can really help.
 
Ok. The channel balance pot has 3 pins on the pcb. One appears to be common ground then one for left and one for right. I set the multimeter to 200 ohm ( other choices are 2k 20k 200k and 2000k). I put the ground lead on the common pin then with the pot turned to send everything to the left channel i got a reading of 25.5. With the pot all to the right channel i get 25.4. So they appear to be even. With the pot in the middle i do not get any reading. Which seems to make sense. So looks like the pot is ok i think.
 
I am not sure I follow you here. The signals come from the input selector so I think I can find those, but not sure how I would feed it into both halves of the preamp tube, since it is soldered to the PCB.

Temporarily connect the two grids together? That's where the signal 'enters' the preamp tube.

You've already tried feeding a known good signal into each input (L,R) separately?
BTW, the AUX, PHONO and TUNER inputs are identical, so you can use any of them, I think.
 
Ok. The channel balance pot has 3 pins on the pcb. One appears to be common ground then one for left and one for right. I set the multimeter to 200 ohm ( other choices are 2k 20k 200k and 2000k). I put the ground lead on the common pin then with the pot turned to send everything to the left channel i got a reading of 25.5. With the pot all to the right channel i get 25.4. So they appear to be even. With the pot in the middle i do not get any reading. Which seems to make sense. So looks like the pot is ok i think.

I'm not clear how you are connecting your ohmmeter probes.
One probe (for resistance it doesn't matter which one) should be on the middle terminal of the pot, the other probe at one end of the pot. Turning the knob should give resistances from low up to close to 1 Meg. This is the sort of reading that's a lot clearer with an 'old' meter (with a needle...).;)
 
Temporarily connect the two grids together? That's where the signal 'enters' the preamp tube.

You've already tried feeding a known good signal into each input (L,R) separately?
BTW, the AUX, PHONO and TUNER inputs are identical, so you can use any of them, I think.

Yes. I have taken the rca's from my source (phone with adapter) and fed both sides of The input. Just to simplify i took the red rca from the source and put it in the a input and it was fine. When i out the same rca lead from the source in the b input, nothing.

I am starting to wonder if i fried the preamp tube??
 
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