G-K Backline 600 Pre amp low power and distorting

[TwinBeech]

P/P measured on the scope. The 1 V P/P is a nice sine wave. The .5V P/P is distorted as in the picture. Lowering the signal did not change the shape of the signal at pin 7.

[J M Fahey]

Short answer: you do not have a problem there, move on.

What is your actual problem?

How much does that amp put out on a speaker load?

Show the output waveform through clean channel, no attenuation, with all tone controls flat, no treble/bass/mids enhancers pushed in , boost on 0, master volume (if any) on 10, rising volume slowly until output starts to clip, with 100 to 200mV RMS 400 to 1000 Hz sinewave input.
Set contour pot fully towards U7-1.

Post Vpp and Vrms on the load.

[nickb]

P/P measured on the scope. The 1 V P/P is a nice sine wave. The .5V P/P is distorted as in the picture. Lowering the signal did not change the shape of the signal at pin 7.

The top half of U7 is just a scooped filter. It should not be distorting like that at that small signal level. But I find it very odd that it it still perfectly linear as that is not what I'd expect from a busted opamp. That said, the other side is perfectly clean so you can turn the contour control to the clean side and check the speaker output.

Also you can plug into the return jack to bypass the preamp and drive just the power amp- try that and see what happens.

[TwinBeech]

After replacing the power module with a new one from G-K the clip light came on a lot when it used not to; the output was weak on both channels and distorted on both.

With a 200mv sine wave at 833Hz input the clean A channel with A Gain max, Master gain max, output at spkr was 3V P/P. Clean sine wave. Going to the overdrive channel with gain at max slightly distorted sine wave of about .5V P/P.

I am going to look at the pre amp output next. With the new power board I would expect it to be ok. I'll see.

Cheers

[Jazz P Bass]

I would test it at a lower input amplitude.

100mv.

[TwinBeech]

I will have to make a new pad to use 100mv. Here are voltages from the O'scope at the op amps. U5-7=17.5v; U7-1=17.5v; U7-7=9v; U8-7= 5v; U8-1 2v some distortion; U9-7 =1.6v clean; U9-1 .1v; Out to Power amp = .1v.
Using same input signal at the RETURN jack I get 1.5v P/P out to the power amp. Also the spkr terminal shows 60v P/P there with the input at the RETURN. I only stay on for a few seconds as the load gets hot fast. Without the voltages from G-K I don't know if these are ok. Something is attenuating the signal in the clean channel.

[nickb]

I would test it at a lower input amplitude.

100mv.

The signal was 200mVpp not rms (see above). 100mV rms is 288mVpp.

TwinBeech you need to specify DC, RMS Vp or VPP with each measurement - don't make us guess.

[nickb]

I will have to make a new pad to use 100mv. Here are voltages from the O'scope at the op amps. U5-7=17.5v; U7-1=17.5v; U7-7=9v; U8-7= 5v; U8-1 2v some distortion; U9-7 =1.6v clean; U9-1 .1v; Out to Power amp = .1v.
Using same input signal at the RETURN jack I get 1.5v P/P out to the power amp. Also the spkr terminal shows 60v P/P there with the input at the RETURN. I only stay on for a few seconds as the load gets hot fast. Without the voltages from G-K I don't know if these are ok. Something is attenuating the signal in the clean channel.

Have you checked the send and return jack switches? It sounds like one or both contacts may be bad. Either clean or patch send to return with a (short) lead.

[TwinBeech]

Sorry about the confusion. I will list the type of measurements. I get the 200mv P/P to be 141mv rms if my formula is right. Following the signal with a scope I note the voltages I see on the screen which are peak to peak. I have not heard back from G-K for the proper voltages. I will query again.

Cheers

[nickb]

Sorry about the confusion. I will list the type of measurements. I get the 200mv P/P to be 141mv rms if my formula is right. Following the signal with a scope I note the voltages I see on the screen which are peak to peak. I have not heard back from G-K for the proper voltages. I will query again.

Cheers

Not quite: 200mVpp = 100mVpk =70.7mV rms

[TwinBeech]

Right. This theory takes me back to 1961 and electronics in the AF.

I found my problem. As you suggested to check the send and return pins I found an open. The problem was corrosion on the pins of the return connectors. Thanks to all who took the time to reply. Seems like I always end up chasing ghosts because of the years since I have been actively working on electronics. Just do it now as a hobby to keep my and my son's amps going.

Cheers

[J M Fahey]

Right. This theory takes me back to 1961 and electronics in the AF.

I found my problem. As you suggested to check the send and return pins I found an open. The problem was corrosion on the pins of the return connectors. Thanks to all who took the time to reply. Seems like I always end up chasing ghosts because of the years since I have been actively working on electronics. Just do it now as a hobby to keep my and my son's amps going.

Cheers

Sorry but I can neither like your post nor congratulate you.

The problem you had is so common it became a *stickie** here : http://music-electronics-forum.com/t28549/ and I (wrongly) assumed you had already checked that, specially after scoping all of the amp, finding proper signal all over the place and also checking that amp could be properly driven if straight fed signal.
So you had us all chasing ghosts for 25 posts before checking that?

So much so that I even posted:

Short answer: you do not have a problem there, move on.

You even replaced the power module, for Godīs sake.

Ok , let this thread stay to show that shotgunning does not work .

[TwinBeech]

Your arrogance may be appreciated in Argentina but here in the US it is rude. The power module was replaced for another fix that was not posted. You may be an expert but if you are not willing to help people who are not as competent as you think you are why do you bore us with your pompass answers. Just don't reply. Thanks to most of the other posters I have solved some really complex problems with several amps this year. They were patient and helpful to us who are not as smart as you think you are. Thanks to the other posters who took the time to help someone who has forgotten most of the electronic theory but is still interested in playing with it I have been able to repair some amps that were not economically worth repairing. Again, THANK YOU to all who tried to help me with this problem. Every little tip helps to open a door that may offer a fix.

[Jazz P Bass]

Troubleshooting is an acquired skill.

I think you took a big step forward on this one.

Rock On.

[J M Fahey]

The power module was replaced for another fix that was not posted.

Hiding that only confuses the situation.

After replacing the power module with a new one from G-K the clip light came on a lot when it used not to; the output was weak on both channels and distorted on both.
I am going to look at the pre amp output next. With the new power board I would expect it to be ok. I'll see.

Quite misleading info if you hide that important and expensive move was made for another, still undisclosed reason
I am very patient as you would easily find if you only checked the thousands of helping posts here , and have no problem in guiding somebody "by hand", step by step , even along weeks (just check it) in a complex repair.
And of course I get nothing out of it, just the pleasure of helping.

But by the same token, I expect the helped one to do his homework, specially the most basic one, itīs the least he can do, specilly considering the free and often complex advice heīs given.

Contrary to what you think, noobs, specially absolute noobs, are given the most detailed help.

You seemed to be well above that , proven by your apparently Pro troubleshooting, including injecting signal, following and measuring it along the path, scoping it, etc. , even considering waveform changes in parallel paths served by the same Op Amp, etc. , so you threw us all (not only me) in a wild goose chase for weird problems, until nickB asked what by then (post #23 ) would have been long ago tested.

You also were annoyed at the distortion channel actually distorting (post #1) , clipping Fets clipping, etc.

I was wasting time considering possible active switching problems, bad compressor/limiter, bad muting, possible thermal problems, even poorly plugged ribbon connectors, and of course others were also analyzing possible problems, when we were all bitten by the cheesiest problem of them all.

Which of course was screaming at us in the face, since there was perfect signal before the switching jacks, amp could be properly driven by injecting signal after them (in fact, into the second one and to be more precise the damaged one), you were measuring that, we donīt have the amp on the bench like you and yet we were left polishing our crystal balls to guess what you were not saying.

In any case the meat in my last answer was that shotgunning is the worst repair technique possible, and I stick by that concept ........ and you looked like the king of shotgunners by fully replacing the power module ... only to find it didnīt solve the problem

Ok, ok, you now say you didnīt replace it to solve this problem ..... thanks for hiding that important piece of information