Help with GK MB150-III

[drewl]

Problem with the power amp, Q14 and R16 were open, must have pulled too much current.

Checked all the remaining transistors and diodes, emitter resistors, no obvious faults, but I'm getting around 4v across R15 and R27 on the other rail as soon as the red fault/power up light turns blue.

powering up with a light bulb limiter, the neg. side of sine wave is clipped until it warms up then will put out about 7v RMS and as the 90W limiter bulb comes on amp settles down with correct voltage across R16 & R27, but probably because the rails have dropped a bit being limited.

my base voltage on Q19 and 20 is fluctuating with no signal around 1.2v on Q19 and .2v on Q20.

Amp is pretty close to A1 Rev. and above in this doc.

So basically bias voltage jumping around/unstable and negative portion of wave clipped until warmed up in a few seconds.

Power supplies are all good and clean.

[drewl]

Yeah, with no load there's the +V rail on the output until the fault light turns blue then it drops down to about zero volts.

So something is flaky with the neg. side.

[nevetslab]

Problem with the power amp, Q14 and R16 were open, must have pulled too much current.

Checked all the remaining transistors and diodes, emitter resistors, no obvious faults, but I'm getting around 4v across R15 and R27 on the other rail as soon as the red fault/power up light turns blue.

powering up with a light bulb limiter, the neg. side of sine wave is clipped until it warms up then will put out about 7v RMS and as the 90W limiter bulb comes on amp settles down with correct voltage across R16 & R27, but probably because the rails have dropped a bit being limited.

my base voltage on Q19 and 20 is fluctuating with no signal around 1.2v on Q19 and .2v on Q20.

Amp is pretty close to A1 Rev. and above in this doc.

So basically bias voltage jumping around/unstable and negative portion of wave clipped until warmed up in a few seconds.

Power supplies are all good and clean.

You'll need to replace Q14 & R16 before everything else makes sense....that's your Voltage Gain stage with the bias circuit in the middle of the chain Q14-Q7-Q16 & Q15. And, using GK's fuseable resistors in those stages.

[drewl]

Yeah, replaced Q14 and R16 andQ15, Q16 and Q7 are the next step

[drewl]

Damn, replaced those and it's still screwy.

[nevetslab]

Damn, replaced those and it's still screwy.

If I'm seeing wandering voltage levels across the output stage emitter resistors, I'm then looking at the base voltages of the two drivers (Q21 & Q22), as well as the bases of Q19 & Q20. If they're wandering, back up to watch the current thru the Voltage Gain stage, across R16 & R27. I'll back up to where I find stable readings, then move forward again to see if I can find where that fluctuation is. I've never opened the loop up on these GK amps, but with the loop closed, a quiescent instability can show in more than one stage. I normally sense the voltage at the emitter of the driver stage, setting them for around 400mV above/below the output buss, as setting at that small mV level on the output stage emitters seems too touchy. I also have a power analyzer to monitor the power draw of the amp, if I'm setting closer to the threshold of turning on the outputs. Current draw will always begin to rise near that threshold, as I've heard Enzo mention often.

There may be one or more resistors that got stressed when Q14 and R16 failed. And.....of course, GK uses lead-free solder (and the tiniest solder pads!), plus cut the excess leads way too short to make it easy to repair questionable solder joints. Usually I get burned by that on the power supply end of their amps, with the xfmr secondary lead header, the three bridge rectifiers and the filter caps.

[drewl]

It's just tricky trying to take readings before it starts going nuts.

The outside rail transistors Q14, 15, 21, 22, had stable voltage on the base.

It's the inside amps Q7, 16, 17, and 18 that were all over the place.

Working back, so were Q4, 5 & 6, so we'll see if they are causing the problem.

[drewl]

And that did it!
Q4 and 5 breaking down.

Now to clean up the board and put it through its paces.

Gotta' figure out how to mount some of the transistors that some previous "repairer" stripped the chassis holes.

Maybe larger plastic screws.

Thanks Steven

[nevetslab]

And that did it!
Q4 and 5 breaking down.

Now to clean up the board and put it through its paces.

Gotta' figure out how to mount some of the transistors that some previous "repairer" stripped the chassis holes.

Maybe larger plastic screws.

Thanks Steven

My pleasure! Plastic screws? I haven't had the MB-150 on the bench.....uh, scratch that.....this is the all-metal small bass combo amp. Yes, I have had this on the bench, at least one of the versions built. I'm not a big fan of plastic screws....only use them when no other choice is forcing me. Being a machinist, I have metric and domestic thread taps to deal with those, and sometimes have to enlarge the stripped-out hole with the next size, assuming the semi mounting hole will accomdate.

Anyway, nice to hear success achieved!

[drewl]

Well I was wary of larger screws making collector contact to ground, but I found some longer metric screws that did the trick.