Ideas for intermittent on Gallien Krueger 1001RB

[solarbass]

My spouse bought a used Gallen Krueger 1001RB at a low price; it turns out to have intermittent issues with the power amp.
I apply a 200 Hz sine at the recommended amplitude and measure the power amp input on my scope.
When the problem occurs the positive excursion of the sine wave at the output is clipped, negative sine is OK.
Looking further Q2A base positive is clipped.
Problem does not happen all the time - intermittent - seems to happen when cold.
Oddly when I put the scope probe on D2 Anode or cathode the problem stops and did not reoccur.
Scope probe is a good Tek 10:1 probe about 18 pF
Got the problem to happen again and probed R15 both sides then R16 both sides.
Probed R16 then the problem went away.
Thought it might be mechanical so I replaced D2 with no change, also probed very lightly on R16 and did not flex the PCB.
Not too good at Transistor topologies - can someone help out with what the Q6, Q4,Q5 circuit is doing here ?
What are the diodes D6 and D7 for ?
What would be a good set of next steps to get this amp fixed up ?
Thanks in advance,
Solarbass

[Jazz P Bass]

Q2A base is clipped because that is the feedback input from the power amp output.
Can you probe the preamp side of the input cap also (C13) to make certain that it is not a preamp problem.
Q4, 5 & 6 are a constant current source for the long tailed pair input stage. (Q2 & Q2A).
The diodes are for steering the mute signal.
It sounds to me like a solder problem.

[drewl]

I've usually found intermittant pc board connections on the preamp board, pots, jacks etc.

Try running into the FX return jack to find the source of the problem.

[R.G.]

Solder joint or cracked trace. Well, or cracked component/lead. Those are listed in order of probability.

I would resolder all the joints, especially if this amp was manufactured after lead-free solder became mandatory. I would use some 60-40 rosin-core solder doing that, too. The closer this amp was manufactured to the time when lead free solder was started, the more likely it is to have solder problems.

Power amp PCBs take a lot of mechanical and thermal stress, too. They're more likely to get cracked traces than preamp PCBs.

And mechanical failures of components you have with you always.

[solarbass]

Investigated further with scope probe while failing and working.
Q2 base side looks great always.
Thought R16 might be open when failing. There is a big difference when working & failing on R16 low side / Q10 base but is actually larger signal there when failing than when working, would drop out if R16 opened up.
I think I will replace Q2 I ordered some overpriced replacements. After that Q10 gets the swap.
Checked R11 and R14 both signals look fine working or failing.
Pressed on stuff with a plastic probe for mechanicals, does not change things.
Touching R16 or D2 with scope probe (maybe discharging the cap in the probe to those nodes) makes it start working.
Thanks for the help I will post again if I get it working.

[Jazz P Bass]

Have you attempted any reflowing of the solder?
Also, if you replace Q2, you really should replace Q2A.
Ideally they should be a matched pair.

[soundguruman]

P Bass man is right on.
Resolder the entire board. Clean the controls and jacks. Clean trim pots on circuit board! That may just do the trick.

[J M Fahey]

I wouldn't replace Q2/Q2a , it's very unlikely they have intermittent problems.
They are either good or bad, and any "replacement" (NTE and such) will be iffy at best.
Fully agree with resoldering as suggested above.
Bad connections *do* suffer from intermittent problems.
Just don't overdo it, very little 60/40 solder applied with a good iron usually does the trick.
Good luck.

[solarbass]

Just to follow up, I ordered some replacement transistors and it turns out replacing Q4 & Q5 solved the intermittent issue. These see a higher VCE so probably got tired and stressed into a failure mode.
In addition some time later I replaced Q11 as it had some small leakage as mentioned in another post on this most excellent site. This reduced power-on transient noise.
Power amp works well now. A minor turn-off issue with the preamp which I will try to fix today with the diode/capacitor fix also posted here.
Thanks again all for the help.

[ohjustbyteme]

question for you knowledgeable people....i have a used GK 1001RB - the amp was working ok, but had an episode last sunday at church, it went to protect for a minute, output dropped, and it burned 2 tweeters, went back to ready, and when signal was applied went back into protection.
i did a brief look and sniff for burned resistors in the amp , none to be found... I'm afraid to put this amp back online to test for fear of burning more speakers etc. ideas where to begin to look for trouble? also one of the pots contour seems to act oddly, the amp doesn't put out sound until its turned completely up.
Thank you for your help....

[52 Bill]

Hey byteme!
You are running the amp with the tweeters in biamp mode? It is unusual for the power amp chip that drives the tweeters to put dc voltage on the output without complete self-destruction, unless maybe a power supply issue.

If you give the amp a good hard bump with your fist does it cause any reaction from the amp? You can try this with no speakers connected and just watch to see if it comes out of protect mode or not.

This amp is fairly simple as far as circuits go, but they are packed very tightly into a small space, which makes servicing a bit difficult. What sorts of skills do you have and what sorts of test equipment do you have available?

[drewl]

I wouldn't hook speakers to it if is in protect mode.

It's possible only the tweeter amp blew, I forget if it's visible or under a heatsink.




And I just worked on one two days ago and can't remember- yikes!

[ohjustbyteme]

gk 1001rb protect issue

Hey byteme!
You are running the amp with the tweeters in biamp mode? It is unusual for the power amp chip that drives the tweeters to put dc voltage on the output without complete self-destruction, unless maybe a power supply issue.

If you give the amp a good hard bump with your fist does it cause any reaction from the amp? You can try this with no speakers connected and just watch to see if it comes out of protect mode or not.

This amp is fairly simple as far as circuits go, but they are packed very tightly into a small space, which makes servicing a bit difficult. What sorts of skills do you have and what sorts of test equipment do you have available?

OK guys.. no i was not running in bi amp mode. and the tweeter ic was not burned. thank you for the quick reply....

i can solder well. i have a VOM, a DVM, and an old Eico O scope.. i also have an old tube eico signal generator.

[52 Bill]

Check for dc voltage on the speaker output. Test all of the power supplies to see if they match or come close to the specs on the schematic. Inspect the pc board for any cold solder joints, etc. Pushing on different parts and different sections of the board may help here too.

[drewl]

Usually these have intermittent preamp board problems.

Reflowing and cleaning the pots and jacks usually takes care of the problem.