HELP diagnosing guitar amp Bass pot please

[sonolink]

Hi guys,
A friend asked me if I couild fix his Hughes & Kettner Staesman Quad EL84. The amp would loose sound after a few minutes of playing. I replaced the tubes and the amp is now working ok, I've had it on for more than an hour and it sounds fine BUT the Bass knob doesn't seem to work or do much. So I wrote to H&K and they sent me the attached service manual with the preamp schem but TBH I'm a bit lost here...

Can someone please help me pointing me in the right direction towards what/where to look for?
Thanks in advance
Cheers
Sono

 

[beatnik]

Bass potentiometer seems to be dual section 500K log on schematic.

Not uncommon to see 16mm pots fail on guitar amps, particularly when the pots are PCB mounted, the carbon tracks can easily crack with the kind of abuse that combo amps have to endure during transport.

You might be able to check the pot is varying resistance while in circuit, if there is some resistance change then you probably should look elsewhere.

 

[beatnik]

It could be it's loosing bass before the tonestack, so the bass control doesn't have much to begin with. Check frequency response on the output of the preamp tubes

 

[sonolink]

You might be able to check the pot is varying resistance while in circuit, if there is some resistance change then you probably should look elsewhere.

Thanks for your help Beatnik. I'll check the pot resistance.

It could be it's loosing bass before the tonestack, so the bass control doesn't have much to begin with. Check frequency response on the output of the preamp tubes

Excuse my ignorance, how can I do that?
Cheers
Sono

 

[beatnik]

Thanks for your help Beatnik. I'll check the pot resistance.



Excuse my ignorance, how can I do that?
Cheers
Sono

Signal generator and oscilloscope

 

[JohnRoberts]

Perhaps easier to check the pot with VOM using ohms scale. Should read roughly marked resistance between the potentiometer's two outer leads. The wiper (the middle pot lead) will read varying resistance between the two outer leads as it is turned.

Do this with power turned off... be careful.

JR

 

[CJ]

check outside pins to the center pin, if infinite ohms from 1 leg to the center then the pot is bad.

thanks for the schemo!

sometimes bass controls are subtle, like on some Marshall amps, maybe tune the guitar E string down and see if the bass control works down there.

 

[sonolink]

Thanks for all the input guys!
I'll try all those suggestions and report back

Signal generator and oscilloscope

Ok, so that would be sending a 1kHz sine wave into the amp input and connect the scope across GND and the preamp output tube terminal BUT I can't find that anywhere (I'm probably blind on this hehe), actually I have the impression that this amp's preamp doesn't have any tube at all!! I'm probably mistaken but the two ECC83s seem to serve a different purpose (reverb, FX loop maybe?). Of course I might be completely wrong on this one, this circuit looks way more complex than a Plexi or an AB763!
If I'm right on this then maybe I should connect the scope somewhere at the output of the preamp (TP13)?

Cheers
Sono

 

[ccaudle]

the preamp output tube terminal BUT I can't find that anywhere (I'm probably blind on this hehe), actually I have the impression that this amp's preamp doesn't have any tube at all!!

The schematics are labelled pretty clearly: page 26 of the document you linked has "Preamp" in the title box and is all op-amp, page 27 of the document has "Power Amp" in the title box and has the tubes. Unfortunately, the page labeled "Power Amp" also has the ECC83 pre-amp tubes, which makes it unnecessarily confusing.
It appears that the "Preamp" page has all the solid state circuitry, then the "Send PRE-TUBE" signal gets sent through a connector to the tube section where it goes through the preamp tubes, then comes back to the solid state circuitry through the "Ret PRE-TUBE" signal to the tone controls.

So you can check Send PRE-TUBE to verify that the solid state circuitry has all the low frequencies you would expect, then check Ret PRE-TUBE to make sure the tube preamp section did not lose too much of the bass along the way to the tone stack.
Op-amp U8B buffers the output of the tone stack, so you can compare the input to the tone stack and the output of U8B to see if the tone controls operate like you would expect.

 

[Ike Zimbel]

Thanks for all the input guys!
I'll try all those suggestions and report back



Ok, so that would be sending a 1kHz sine wave into the amp input and connect the scope across GND and the preamp output tube terminal BUT I can't find that anywhere (I'm probably blind on this hehe), actually I have the impression that this amp's preamp doesn't have any tube at all!! I'm probably mistaken but the two ECC83s seem to serve a different purpose (reverb, FX loop maybe?). Of course I might be completely wrong on this one, this circuit looks way more complex than a Plexi or an AB763!
If I'm right on this then maybe I should connect the scope somewhere at the output of the preamp (TP13)?

Cheers
Sono

I would suggest a lower frequency test tone, like 100Hz if you are looking for issues with the bass EQ.
I would also suggest that you do an amplitude sweep through the whole device and see if it's lacking in low-end overall. If so, that could point to an off-value electrolytic capacitor in the signal path. If the amp gets warm inside and is a few years old, it could be that the e-caps are failing.
IF, you see rolled off response from the whole amp, you can then break up the circuit using the Send and return jacks to try and isolate where the bad cap(s) may be. Do a sweep from the Input to the Send, and one from the Return to the Output.

 

[sonolink]

The schematics are labelled pretty clearly: page 26 of the document you linked has "Preamp" in the title box and is all op-amp, page 27 of the document has "Power Amp" in the title box and has the tubes. Unfortunately, the page labeled "Power Amp" also has the ECC83 pre-amp tubes, which makes it unnecessarily confusing.
It appears that the "Preamp" page has all the solid state circuitry, then the "Send PRE-TUBE" signal gets sent through a connector to the tube section where it goes through the preamp tubes, then comes back to the solid state circuitry through the "Ret PRE-TUBE" signal to the tone controls.

So you can check Send PRE-TUBE to verify that the solid state circuitry has all the low frequencies you would expect, then check Ret PRE-TUBE to make sure the tube preamp section did not lose too much of the bass along the way to the tone stack.
Op-amp U8B buffers the output of the tone stack, so you can compare the input to the tone stack and the output of U8B to see if the tone controls operate like you would expect.

Hehe I wish the schematic was so neat and clear as your indications mate!
Thanks a lot for the clarification and suggestions.

I would suggest a lower frequency test tone, like 100Hz if you are looking for issues with the bass EQ.
I would also suggest that you do an amplitude sweep through the whole device and see if it's lacking in low-end overall. If so, that could point to an off-value electrolytic capacitor in the signal path. If the amp gets warm inside and is a few years old, it could be that the e-caps are failing.
IF, you see rolled off response from the whole amp, you can then break up the circuit using the Send and return jacks to try and isolate where the bad cap(s) may be. Do a sweep from the Input to the Send, and one from the Return to the Output.

Of course! I wasn't thinking when I said 1kHz...

Thanks again for all the useful tips dear friends. I will follow your indicatrions and report back
Cheers
Sono

 

[Alva Goldbook]

So the sound would drop out intermittently? And replacing the power tubes just fixed that? Are we talking it would drop out for a split second, or the amp would just go silent after a while?