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Author Topic: Help with Gallien-Krueger 200GT guitar amp  (Read 1786 times)

smackoj

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Help with Gallien-Krueger 200GT guitar amp
« on: February 28, 2019, 09:27:22 AM »
I have worked on this GK 200GT for a little too long and believe the 1976 "boat anchor" output transformer is bad. Rather than searching for a replacement, I would  like to replace the whole power amp board with a chip amp, either class AB or D. The problem I have, if it is actually a problem?, is that the preamp section which I like a lot, except for the "Current Limiter" pot, has a negative -8.7 vdc signal wire to the power amp board. Is that too much voltage and needing to be changed to a positive voltage before using a chip amp power board? I believe the preamp is working correctly but I am not sure because the schemo is hard for me to read and not sure if the signal from R85, the reverse 10k pot called "Current Limiter" is showing a negative voltage feed to the power amp? Review of the Owner's Manual shows that the Current Limiter function is to change the power output of the amp (I would just as soon remove it). Would anyone care to look at this and offer suggestions?  thanks


joecool85

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Re: Help with Gallien-Krueger 200GT guitar amp
« Reply #1 on: February 28, 2019, 11:22:07 AM »
I have worked on this GK 200GT for a little too long and believe the 1976 "boat anchor" output transformer is bad. Rather than searching for a replacement, I would  like to replace the whole power amp board with a chip amp, either class AB or D. The problem I have, if it is actually a problem?, is that the preamp section which I like a lot, except for the "Current Limiter" pot, has a negative -8.7 vdc signal wire to the power amp board. Is that too much voltage and needing to be changed to a positive voltage before using a chip amp power board? I believe the preamp is working correctly but I am not sure because the schemo is hard for me to read and not sure if the signal from R85, the reverse 10k pot called "Current Limiter" is showing a negative voltage feed to the power amp? Review of the Owner's Manual shows that the Current Limiter function is to change the power output of the amp (I would just as soon remove it). Would anyone care to look at this and offer suggestions?  thanks

There shouldn't be any DC going to the power amp on the signal line.  If there is, something is wrong upstream.
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smackoj

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Re: Help with Gallien-Krueger 200GT guitar amp
« Reply #2 on: February 28, 2019, 09:06:39 PM »
I will double check the breed of voltage. I thought I read DC but it's been several days so I will test it again and report back. Thx mucho

phatt

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Re: Help with Gallien-Krueger 200GT guitar amp
« Reply #3 on: March 01, 2019, 07:44:26 AM »

This is very confusing,,
You say the OT is shot, and you hate the Current limiter but if the OT is dead then the Current limit would not be working as it only limits the power amp section.

How about you explain the symptoms and let the more qualified folks here help you find out what might be a simple fix.
 I'd guess the limit circuit might be faulty.

Looks like the Limit voltage controls the Emitters of Q1&Q2 at the power amp input. Probably works like a master gain control for the power amp.

I maybe wrong but likely the Power Tx has 30/30VDC split supply the negative side likely has a much lower current ability. So It would not be able to power a 50 Watt Chip Amp.

Phil.

smackoj

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Re: Help with Gallien-Krueger 200GT guitar amp
« Reply #4 on: March 01, 2019, 06:23:49 PM »
Thanks Phil; Yes, this amp has confused me quite a lot. Here is the original problem. I would turn the amp on everything would lite up (it has 5 red leds on the face. Within a minute or less 4 of the leds would turn off, all but the one indicating power on, and no signal would go thru the amp. If I shut it off and try it on again within a few minutes, nothing, power led on but no signal moving. If I would wait a long time, say half a day, and turn on, it would light up all the leds and started to amplify. Note; once when I first had it on and I turned the Master Vol. about a quarter inch, the leds went dead and so did the amp.

Also I made an error on my original post. I was measuring the voltage on the brown wire which is the Current Limiter send from the pre to the power amp (I thought it was being used as a pre-gain and thought it was carrying the guitar signal to the amp section). I did measure that pot at the time and did not get any change in the voltage as I rotated the 10k pot.

Thanks for the expert advice,     Jack D

tonyharker

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Re: Help with Gallien-Krueger 200GT guitar amp
« Reply #5 on: March 02, 2019, 07:01:46 AM »
Probably something simple like a dry joint or a duff pot

phatt

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Re: Help with Gallien-Krueger 200GT guitar amp
« Reply #6 on: March 02, 2019, 07:56:52 AM »
Well obviously there is an issue with the limit control system and as *Tonyharker* has noted it's likely simple.

clues;
I would think that the Limit leds should only light up when the signal reaches the set threshold. maybe a short flash at power on until the voltages settle but as they all come on then something is wrong around Q26.
So check the voltages around Q26,, all the test voltages are on the schematic.
Check you have -9VDC at ALL points marked (H). See if that voltage changes over time.

Namely points (H) at
Q3 & Q4
Q10 & Q11
Q23 & Q24
And also Q15 & Q16, the trem circuit.

My first suspect would be that Q26 has a problem,, maybe the Zener diode (D3) has died.

At Q26 also check you have -8.4VDC at points (G), as Q26 also supplies the neg rail for the whole preamp section.
HTH, Phil.

teemuk

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Re: Help with Gallien-Krueger 200GT guitar amp
« Reply #7 on: March 02, 2019, 11:41:36 AM »
You will loose a lot of the 'tubey' characteristics inherent to that amp if you replace the output section with a generic chip amp.

Current limiter is a control for power amp's phase inverter overdrive. Yes, I know it's a solid-state amp. They designed the output secton to follow similar architecture as tube amps (not generic SS) so substitution with chip amp will erase the designed-in tube emulation and PA soft clipping.

Just my opinion.

smackoj

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Re: Help with Gallien-Krueger 200GT guitar amp
« Reply #8 on: March 02, 2019, 01:10:32 PM »
I will get the voltages Phil, thanks.  I agree the sound would change using a chip amp Teemuk but I was working from the idea that fixing this would be to difficult and/or expensive. However it looks like I may be able to avoid going with a chip amp.

smackoj

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Re: Help with Gallien-Krueger 200GT guitar amp
« Reply #9 on: March 02, 2019, 07:48:31 PM »
Hello gents; I am having a little trouble checking voltages because the PCBs have no markings to designate the exact position of any of the parts on the board. However, I feel confident that I know where Q26 is and I read:  C= -30 VDC
B= -9 VDC and E= +0.7 VDC   I am not exactly sure where to test for the (H) voltage by the Cur Limit pot but all three legs of the pot read +0.65 VDC  This pot is reverse taper and I had it set at 10 which equals max current limiting. I rotated the pot to six diff settings and it seemed to stay very close to +0.7 VDC at all settings. One time when I had the probe on leg 1 of this pot I got a -8.5 volts for just a second then it went back to +0.7.  Thanks again for the knowledgeable advice.

Jack D

DrGonz78

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Re: Help with Gallien-Krueger 200GT guitar amp
« Reply #10 on: March 03, 2019, 03:19:52 PM »
Typically a solid state amp does not have an output transformer but this amp has one. Are you using a load on the output? The norm with solid state amps is to not have a load connected while servicing the amp and once it is stable you connect one. So should smackoj have a dummy load or speaker connected to the amp?
“A person who never made a mistake never tried anything new.” -Albert Einstein

g1

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Re: Help with Gallien-Krueger 200GT guitar amp
« Reply #11 on: March 03, 2019, 08:56:23 PM »
Yes, in this case with the OT a load should always be connected.  Better safe than sorry.

Have you checked Q26 with your meter?

smackoj

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Re: Help with Gallien-Krueger 200GT guitar amp
« Reply #12 on: March 03, 2019, 09:49:28 PM »
Having such a big hunk-o-iron OT was my first suspicion but thanks to mi amigos aqui (my friends here) I started checking more carefully and thoroughly the Current Limiter pot and, yes, the voltages on Q26. Note; I wasn't sure about having a load on the OT so in early tests I had it connected to load. Now however I have been working on the amp with the OT disconnected. I realize that could make changes to the voltages throughout although I think I have this amp stable again. The Current Limit pot went crazy when I gave it a small turn under power. It went from -0.5 vdc to -30 vdc with about a quarter inch turn on the pot and it immediately released a foul odor. I killed the power (I had it on the light bulb limiter) and replaced the pot and my first tests seem to indicate that may have been the only problem? I am pooped from this amp today so I will start again tomorrow with putting the OT back in and see if this old girl wants to dance or play bingo?

I will report back asap with hopefully some good news!  Muchas Gracias Amigos

g1

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Re: Help with Gallien-Krueger 200GT guitar amp
« Reply #13 on: March 03, 2019, 10:15:53 PM »
If you are getting -30V on the pot, Q26 has likely shorted.
Q26 is being used as a voltage regulator.  It should have a constant -9V on the emitter (H) no matter what the pot setting.  The zener D3 on Q26 base is what determines the emitter voltage.
All the current limit pot does is put some resistance between the supply node H and the R4/R5 junction in the power amp.
The only way to get -30V on the current limit pot is if Q26 is shorted from C to E.

smackoj

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Re: Help with Gallien-Krueger 200GT guitar amp
« Reply #14 on: March 04, 2019, 10:44:15 AM »
Thanks g1, I will check Q26 again now that I have the pot replaced and know it is functioning properly. And I am seeing on the shematic that zener D3 limits voltage to 12v? Also, I need to try and be sure I am actually testing Q26 because the PCB does not supply any parts numbers.

Later addition to this post: Q26 has -9.0 on the E, -22.0 on the C and -9.5 on the B



muchas gracias,   Jack D
« Last Edit: March 04, 2019, 02:35:26 PM by smackoj »

 

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