Gallien Kruger GT200

[gbono]

I'm working on a GT 200 where the preamp board is working great but no level from the output board. I found Q1/2 were damaged but also noted that R1 and C1 are swapped on the output board. I get a strong signal from the preamp board to the PA but almost no signal at base of Q1. Assuming C1 provides DC block for preamp out??? What am I missing?

[Enzo]

SO is there signal between the cap and resistor?

[gbono]

Yes there is a signal between the cap and resistor but almost nothing at the base of Q1,,

[g1]

It looks like R1 and C1 are in reverse order from what is shown on schematic.
This doesn't matter but makes me suspect C1, or Q1.

[gbono]

Well C1 and Q1/2 have been replaced but the bias on Q1 is not right.
VB = +9.5V
VE=+10.4V
VC=+10.6V

node "I" is +12.6V

Q1 is turned off - where does Q1 get base bias from?

[Enzo]

Bias is the difference between emitter and base, not just the base voltage.  SO with the base esentially at ground for DC, look at the schematic.  Your emitter then must be negative to turn on the Q1.

Note the top and bottom halves of this amp are the SAME, not opposite polarities as usual.  The upper side acts like a plain single ended amp, then the OUTPUT from the speaker winding becomes the signal for the bottom side.  That signal is inverted from the input signal, so the NFB acts more or less as a phase inverter.

ANyway, over left, R4, R5 are the emitter resistors for Q1,2.  The join, but not at ground, they are wired up and over to the right with the current control and H supply of -8.9V.   H comes from Q26 and ultimately from the -30v rail.

So... do you have voltage H at that control?  At the joint of R4,5?   My first suspect is an open R4, but surely a missing supply there would do it too.

Note by the voltages on the schematic the top and bottom halves mirror one another, so the voltages are all the same.  Use Q2 side to compare.

[gbono]

Duh R2 is grounded - the issue was the current lim knob was turned to 0 so voltage at R4 was positive. So there is an amplified signal at the collectors of Q1/2 but still no output level into 8 ohms??? Q7/8 have 1.7 to 2 v on their bases. Wondering if I should test the OT?

[Enzo]

I see no reason to suspect the OT yet, but I do have some shop wisdom:

never think up reasons not to check something.

SO go ahead.

But remember, the signal for Q2 comes directly FROM the speaker side of the OT.  Seems like it works then.

Before you test the OT, apply signal to the amp, and see if signal is on the hot pin of the extension speaker jack.

Look at the speaker jacks.  Note the main jack requires the signal to come THROUGH the extension jack contacts, so if they are bent or dirty... no sound.  And if you plug the speaker into the extension jack, there is nothing in the main jack to complete the circuit to ground, and... no sound.

A quick test?  Two speakers, one into each jack.  If that works then we can be fairly certain the extension jack contacts are bad.

[gbono]

Plugged in two resistive loads to the main and extension jacks - no output.
Okay simple didn't work so it's time to pull the OT and test it - unless there are any other suggestions?

[g1]

Check voltages at Q26 first.  That is where the DC for the current limit pot comes from.
Seem to recall a similar issue recently and the problem was in the Q26 area.

[gbono]

Thanks g1 - I have noticed that the voltage between R4 and 5 comes from the current limit pot and node H. With the pot turned to "10" I get about -8V between the resistors and Q1 conducts. As I turn the pot towards "0" I end with +8 or so volts?! This isn't how I envision the current limit should work.  I'll look at Q26 again.

[mandu]

When you replaced Q1 and Q2, probably the transistor pins are not same as the replaced ones, even if the same number. The pin connections some times vary between manufacturers. Regards.

[willpirkle]

O/T: hey could you please tell me the P/N for the Accutronics reverb tank in that 200GT? I'm trying to find a replacement for a 1978 GK210G and the schematics appear to be nearly identical, so I am thinking I can use the same tank. The part number should start with "4" TIA.

[gbono]

Amp is finally repaired - I had to replace the output transistors, .3 ohm emitter resistors, driver xistors, etc. There is some crossover distortion in the output sine-wave - any ideas on how to minimize this?   

[Enzo]

That is what the bias adjustment is for.  May I assume there is one?