GK 400RB Distorted Output

[Jake H]

Hi There,

I'm working on a GK 400RB (the schematic is attached). Originally the amp had blown output transistors and Q7 had blown up (and Q10 looked suspect) as well as some of the surrounding resistors (22ohm and 1k, I also replaced the 33ohm 5 watters as the values were way off). I've replaced everything that was damaged and the amp makes noise, but it's not working properly. There is a distorted output when you start to get into any usable volume range. I suspect that there is a problem with the bias network, however I can't track down what the cause is. I basically was reduced to a shot gun approach, where I was just replacing parts that I suspected could be the next culprit, but the problem persists.

Here are my voltages:

Q7
B 1.6V
C 57V
E 1V

Q10
B -.4
C -56.7
E .171 (yup positive)

bias network
R15 .025V (with the trim pot cranked I can move it to -.2V, but then R18 is still wonky)
R18 .29V

The junction between R17 and R18 is -28.62

I'm getting 6.7mV across R26-27

I'm also getting my + and - 15V at points A and B
Let me know if there is anything else that I can report. Thanks!

[52 Bill]

Whenever an amp takes out a bunch of parts, sometimes it's easy to miss a bad resistor or even a transistor that needs replacement. I will assume that you do not have a scope and cannot look at the waveform. So it sounds clean until it gets louder?

Continue checking voltages in the bias circuit and also check the limiter circuits. Meter test Q3-Q6.

I also replaced the 33ohm 5 watters as the values were way off

I assume that this is a typo and you meant 0.33ohm.

[drewl]

Sounds like the bias is way off, or a stage still not amplifying correctly.

As Bill said, you have to check everything in the output stages.

I've been seeing massive intermittent preamp pc board issues with the RB series.

[Jake H]

Hey Bill,

Thanks so much. I had read on the 5 watters that they were 33 ohms when they in fact said "R 33 Ohm" - and never looked back... FIXED. I appreciate the help immensely.

Good eye

For clarification though, and this doesn't have to be answered, and maybe it's a can of worms, but why was the bias of Q10 so off compared to Q7? Considering the 5 watters create a voltage divider scenario, shouldn't they have been both pretty far out? Does D2 have something to do with this?

Thanks again