Phono Stage Problem

custom10

custom10

Active Member
Can someone please provide some help towards solving an issue after my first recap. There are attached schematics of the circuitry and I will try and explain my senario. The left channel volume level is very low as compared to the right when using the phono input. All other stages work fine including the tape playback (using cd player) and the radio AM/FM. Only the phono stage is faulty. If I plug the CD player into the phono input same problem so it is not my table. When the unit is in mono (via switch) both channel levels are the same and the balance adjustment dial functions as correctly. In stereo mode when I turn balance fully left there is still sound coming out of right speakers and the left channel level does not change much. I have checked and double checked my solder joints and wiring etc. I used the one by one process when changing the caps using matching value orange drops along the way. In addition the following changes were made. R300A on the left input stage measured 3.6meg so I changed it to 3.3 as per the drawing. C400 (bottom left)cap was a 40uf 400V I made this a 47uf 450V cause thats the closest I had. This does feed the phono stage. C314 another electrolitic is 8uf as per original but is now 450V instead of 350. Again only the phono stage has the problem and the balance dial works fine when using the other inputs, there is no hum, crackle, pop the sound is really good...I am stumped please advise. Thanks very much.
 

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Can you get a good close up photo of the phono section and post it, it might help us find the problem.
 
This is a shot of the section...hard to tell exactly what is going were but it follows the schematic, the tube is socket for V9 12ax7
 

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I'd check out those orange drops to make sure they are the right values and soldered in properly. I only see 2 0.22s on the schematic but it looks like you have 4 in the actual picture? Then again, we don't see all the circuit either.

See what happens when you swap them around; if the quiet channel follows a cap, that could be the problem.
 
Interesting. I have never seen a single stage magnetic phono section before. I'm not seeing an RIAA equalization either.

Regardless... it seems that one tube does the amplification here so you have a good circuit for the heater. I would first check the accuracy of the resistors in the plate and the cathode circuits. Those control the operating points of the triodes.

Just measure them in circuit, powered down, Rumble filter off, with the tubes in. There might be something happening there. If not you will want to trace the audio path from input to the tube grid and from the downstream side of the plate capacitor through the tape switch.
 
Although I posted on finding the huge imbalance on the phono stage I thought I should post on these implicit questions.

custom10 said:
. . . When the unit is in mono (via switch) both channel levels are the same and the balance adjustment dial functions as correctly.
Click to expand...

That is to be expected. Your explanation of your troubleshooting is spot on. You have located the problem in the phono section pretty well.

custom10 said:
In stereo mode when I turn balance fully left there is still sound coming out of right speakers and the left channel level does not change much.
Click to expand...

If you mean when you fade to the left with the balance there is still a little bit still coming from the right, this can be bleed through. If it is substantial, there is a problem, but not related to the phono imbalance.

Wiring a balance control has caused me great hair loss. It is totally counter intuitive. What it does is in the center position it does nothing to either channel. When you "fade" into the left channel it has no effect on the left channel but lowers the right. Screwy sounding but works well in the real world.

custom10 said:
I have checked and double checked my solder joints and wiring etc. I used the one by one process when changing the caps using matching value orange drops along the way. In addition the following changes were made. R300A on the left input stage measured 3.6meg so I changed it to 3.3 as per the drawing. C400 (bottom left)cap was a 40uf 400V I made this a 47uf 450V cause that's the closest I had. This does feed the phono stage. C314 another electrolytic is 8uf as per original but is now 450V instead of 350. Again only the phono stage has the problem and the balance dial works fine when using the other inputs, there is no hum, crackle, pop the sound is really good...I am stumped please advise. Thanks very much.
Click to expand...

All of this is fine, I always suggest bumping the voltage ratings a little anyway. I don't see your component selections as being a problem.
 
Thanks for the input here, I have not had a chance to dig into this more since I posted on the weekend however there are some differences between the schematic and the actual config. I did not use the schematic when changing the caps etc. I just did them one for one. As andyman pointed out there are some descrepencies(two extra caps). I did swap out the 12ax7 for another and no help...same issue. I need to add these two other caps into the circuit drawing and try and repost this. Maybe I should just take them out and see what happens? After work today...Got to go for now thanks again folks.
 
I assume the circuit worked OK before the recap. Regardless, might look closely at the tube socket--(power off and plug pulled)--use a good light and a magnifying glass--look down from the top at the metal pin sockets, make sure none are broken, bent, pushed out of position, etc. Clean the socket with De-Oxit, spray a liitle into the socket, work a tube in and out say 5 times to spread it around give it about 10 minutes to dry. Chances of this being the problem is probably less that 10%, but worth doing.
Tom
 
Clean the socket with De-Oxit, spray a liitle into the socket, work a tube in and out say 5 times to spread it around give it about 10 minutes to dry. Chances of this being the problem is probably less that 10%, but worth doing.
Tom[/QUOTE]

Thanks I will check into that.

OK so I have traced down the two extra .022 caps in this circuit that are not on the schematic. One connects to pin 2 then ground, the other connects to pin 8 then ground. These caps were the old paper/wax type and I changed them not realizing that they were not on the schematic. I could go ahead and remove them so that everything is following the schematic? Due to my limited knowledge I tought I would run this by you guys first, is this "normal" to see caps connected like this? and yes I did not have this problem before the recap. Thanks
Terry
 
custom10 said:
Can someone please provide some help towards solving an issue after my first recap. There are attached schematics of the circuitry and I will try and explain my senario. The left channel volume level is very low as compared to the right when using the phono input. All other stages work fine including the tape playback (using cd player) and the radio AM/FM. Only the phono stage is faulty. If I plug the CD player into the phono input same problem so it is not my table. When the unit is in mono (via switch) both channel levels are the same and the balance adjustment dial functions as correctly. In stereo mode when I turn balance fully left there is still sound coming out of right speakers and the left channel level does not change much. I have checked and double checked my solder joints and wiring etc. I used the one by one process when changing the caps using matching value orange drops along the way. In addition the following changes were made. R300A on the left input stage measured 3.6meg so I changed it to 3.3 as per the drawing. C400 (bottom left)cap was a 40uf 400V I made this a 47uf 450V cause thats the closest I had. This does feed the phono stage. C314 another electrolitic is 8uf as per original but is now 450V instead of 350. Again only the phono stage has the problem and the balance dial works fine when using the other inputs, there is no hum, crackle, pop the sound is really good...I am stumped please advise. Thanks very much.
Click to expand...


How was the phono output before the recap?
 
kaplang said:
How was the phono output before the recap?
Click to expand...

The left side was always slightly of less volume but very slightly. The balance function was correct in that there was 0 sound from the right side when dialed over to the left. There was no hum, crack etc. It sounded fine. Now I can only get it right in mono.
 
I'm just wondering if the Rumble filter switch or the FUNCTION Switch couldn't be partially or totally the fault here.

1.) Parts were replaced one for one, and no modifications made.
2.) Unit was working satisfactorily before the re-cap.
3.) O/P states he double checked the solder joints (check one more time just to make sure.)
4.) Tube was replaced and no change.

The next component in line is the rumble Filter switch. Has the O/P checked the unit with switch in both positions.

I'd clean all of the switches and pots AGAIN with De-Oxit, and Toothbrush where applicable.

Larry
 
Well maybe I should have called this post did he fix it or just confuse himself or both?..lol. Before I began (see pic yellow caps 1 and 3 from left to right)there were 2 old caps so I changed them with the new orange caps. These caps do not show on my drawing but I was not looking at that as I went. So I ended up with four on the stage. I just cut them out and bravo all good. Tell me why did this work OK before with the 2 extra caps to ground from pins 2 and 8?
The other pic is just for posterity and now is as the schematic....learn something new every day i tell ya:scratch2:
 

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The pins you had them on might be the answer. Pin 2 is a grid, or input. Pin 8 is a cathode.

Hanging extra capacitors to ground from the input just adds to the load capacitance, but you have a series cap that blocks DC as well as keeps the additional capacitance from being seen by the input. You are saying it was in parallel with the 1M ohm resistor?

Paralleling a capacitor across the cathode resistor brings the cathode to AC ground. It also adds some gain. You are saying it was across, in parallel with the 2.2k ohm and 5.6 ohm resistors? (BTW, between those resistors are where feedback RIAA EQ is often brought back to the first stage.)

The net results of using a grid on one side and a cathode on the other must have caused an imbalance.

So you are saying that without these two caps you have balanced sound now?
 
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I can't really tell from the pictures but is there any chance that the extra capacitor was originally soldered to pin 3 rather than pin 2 and you just got them mixed up? That would have made them both extra cathode caps which would have made more sense. I have often come close to making that kind of mistake in close work with my almost 50 year old eyes.

In any case it seems like removing has solved the problem.

mike
 
Grainger49 said:
The pins you had them on might be the answer. Pin 2 is a grid, or input. Pin 8 is a cathode.

Hanging extra capacitors to ground from the input just adds to the load capacitance, but you have a series cap that blocks DC as well as keeps the additional capacitance from being seen by the input. You are saying it was in parallel with the 1M ohm resistor? Yes on PIN 2 to chassis ground

Paralleling a capacitor across the cathode resistor brings the cathode to AC ground. It also adds some gain. You are saying it was across, in parallel with the 2.2k ohm and 5.6 ohm resistors? (BTW, between those resistors are where feedback RIAA EQ is often brought back to the first stage.) Yes on PIN 8 to chassis gound so in parallel with these
The net results of using a grid on one side and a cathode on the other must have caused an imbalance.

So you are saying that without these two caps you have balanced sound now?
Click to expand...

Yes the balance is fine now with equal levels from each side. Maybe mashaffer
is right I might have made the mistake on the left channel by connecting to PIN 2 rather than the PIN 3 cathode. What are the advantages of having these cathode caps in place? I can add them back to 8 & 3 if it is considered advantageous to have them in the mix. Thanks guys
 
I would leave them out if it is sounding good and you have enough gain. The bypass caps would increase gain above their cut off frequency but I would worry that they would interfere with that loop from the plate of the bottom triode to the cathode voltage divider on the other tube. I am not sure what kind of strange interaction you might get.

mike
 
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