TL Audio PA-1 : I get only medium and high frequences on channel B

[remiroo]

Hi there,

at first, i beg your pardon about my english..

Then I just got a second hand PA-1 and one of the channel (B) give only medium and high frequences using Mic, +48 or instrument input
like old phone souding..
The channel A works fine.

I changed tubes, clean the scratchy pots and PCB , had a look inside at every capacitors (just a look, i don't have the ability of testing them now..) and they look ok.
The input and output gains seem to be ok.

I need advice about this kind of problem, I guess it could come from a bad connection or broken solder on the tubes sockets on this channel..or dead capacitors..

Does anybody identify there a common thing that I'm blind about as a newbie ?

best regards from France

 

[pucho812]

If you have a working channel and a non working channel you can compare the working channel to the non working channel and see what is what.

looking at a photo of the inside of the unit, I see some ic chips. It's possible you have a bad chip. I would also think you may have a bad capacitor or two.
But these are just stabs in the dark.
As always check PSU is in working order and go from there.

 

[scott2000]

Are the rotary switches for the filters working right?

 

[remiroo]

If you have a working channel and a non working channel you can compare the working channel to the non working channel and see what is what.

looking at a photo of the inside of the unit, I see some ic chips. It's possible you have a bad chip. I would also think you may have a bad capacitor or two.
But these are just stabs in the dark.
As always check PSU is in working order and go from there.

Thank you for the advice of comparing both, I will follow this way

 

[remiroo]

Are the rotary switches for the filters working right?

Thank you for the schematics,
the HF filter rotary seems to be ok but the LF filter rotary has no effect, all the low range is missing

 

[remiroo]

just to hear the difference..

 

[scott2000]

Hopefully someone can chime in with some advice based on the clues. Not sure if the cathode caps would create any issues you're experiencing. Where's PRR

 

[remiroo]

So for the moment I found two transistors a bit weird, but i'm not a pro and i'm no be sure I tested them the good way.. some transistors can't be tested with multimeter isn't it ?

On the IC7 I can read values 1100 one side, 100 the other.. does it means that its bad one ?

On the Q8 I read 1100 one side , and nothing the other..

what is weird is about the Q8, I can't find it on the differents PA-1 schematics.. it's the same for another litlle transistor, Q3..

sorry for my lack of kwnoledge about electronic..

 

[JohnRoberts]

dried out electrolytic capacitors will lack low frequency response...

JR

 

[remiroo]

dried out electrolytic capacitors will lack low frequency response...

JR

THank you, I may change all the electrolytic capacitors to be sure they are ok..

 

[JohnRoberts]

THank you, I may change all the electrolytic capacitors to be sure they are ok..

some hand held meters can measure capacitance.

JR

 

[remiroo]

Hello there,
So all the electrolytic capacitors have been changed, and a resistor who looks bad and was weak.
The sound is fine on channel A but le problem is unchanged on channel B...
Do you think it could come from a component on the Low pass Filter or High pass filter on channel B, which cuts all frequencies instead of filtering them ? like a bad chip ?
I noticed too that from the beginning, the signal is weaker on channel B than on channel A..
I don't know how the unit works sorry about my low knowledge about electronics..
I had a look very close to the rotary switches for the filters and they seem to be ok..
any idea would be welcome
best regards

 

[scott2000]

You mentioned earlier that the schematics weren't matching with what you were seeing?. It's possible there are different versions. Maybe you have the older one without the meters and the schematic is for the one with the meters.? Or the other way around?
Not sure what would change but, it will make things a bit more challenging to figure out where things are at. Like pucho mentioned, you have a working channel that you could try to measure and compare at different spots. But it looks like some lethal voltages in there so there's that.

I would suspect the actual switches and not the components related to them but, it's hard to know if that's your problem. Maybe someone can help with a descriptive way to test with your limited tools . Could always swap stuff from one channel to the other but it can get very destructive if not used to it and it's possible to make things worse.
You'll figure it out ,but be careful.

 

[Ike Zimbel]

Hi there,

at first, i beg your pardon about my english..

Then I just got a second hand PA-1 and one of the channel (B) give only medium and high frequences using Mic, +48 or instrument input
like old phone souding..
The channel A works fine.

I changed tubes, clean the scratchy pots and PCB , had a look inside at every capacitors (just a look, i don't have the ability of testing them now..) and they look ok.
The input and output gains seem to be ok.

I need advice about this kind of problem, I guess it could come from a bad connection or broken solder on the tubes sockets on this channel..or dead capacitors..

Does anybody identify there a common thing that I'm blind about as a newbie ?

best regards from France

I suggest you try the un-balanced output on both channels and see if they match. If you get full spectrum sound out of the un-balanced o/p, it points to an issue with output balancing module.

 

[remiroo]

I suggest you try the un-balanced output on both channesl and see if they match. If you get full spectrum sound out of the un-balanced o/p, it points to an issue with output balancing module.

Thank you for the advice
I already tried the un-balanced outputs and the problem is unchanged on channel B, so it does'nt come from the output balancing module.

 

[pucho812]

Are you certain all the opamps are working? Are they getting the correct voltages they need to work?

 

[remiroo]

You mentioned earlier that the schematics weren't matching with what you were seeing?. It's possible there are different versions. Maybe you have the older one without the meters and the schematic is for the one with the meters.? Or the other way around?
Not sure what would change but, it will make things a bit more challenging to figure out where things are at. Like pucho mentioned, you have a working channel that you could try to measure and compare at different spots. But it looks like some lethal voltages in there so there's that.

I would suspect the actual switches and not the components related to them but, it's hard to know if that's your problem. Maybe someone can help with a descriptive way to test with your limited tools . Could always swap stuff from one channel to the other but it can get very destructive if not used to it and it's possible to make things worse.
You'll figure it out ,but be careful.

Thank you for the advice,
I just had a look closer to the potentiometers and just found on the B out gain potentiometer that the central terminal is broken, I don't know how it could affect the frequencies, but I guess from there could come the loss of level on this channel ?

 

[scott2000]

the B out gain potentiometer that the central terminal is broken, I don't know how it could affect the frequencies, but I guess from there could come the loss of level on this channel ?

Neat!. Looks blobby there. Wonder if it was previously broken. But hard to tell in pic.
If it is in fact not making contact, that is the pot's wiper so an important area. Could affect either or both of your scenarios depending on where it is.