Fan noise

[MarkusBass]

I think that it is important to understand how this part of the amp works. The voltage across R435 (available on T1 and T2 test points - there is a possibility to set up bias using these test points) is amplified by A302.A. Amplified signal is available on TP3 test point. Later it is compared (by A301.A working as a comparator) with the voltage on TP2 test point and if it is greater, positive voltage is provided through D311 diode to Q301 transistor'a base. Only in this case the fan gets 24V supply. I think that loud playing is not enough, you should provide a signal from a signal generator to see whether it works. Of course in this case you shouldn't use a speaker but rather some resisitor (>200W). Alternatively you could simulate this situation by providing to pin 3 of A301 a voltage that is greater than the voltage on TP2 but I would do it very carefully - possibly with a resitor like 47-100k. This should force Q301 to work. If it does, you don't need to check the transistor.

Marek

[lowell]

Marek,
I got 35vac on the speaker jack through a resistor and left it going for 10minuts. I monitored the TP2 and TP3. TP3 measured -12vdc and TP2 was -.5vdc. The fan voltage did not change from the -12vdc throughout this test... then I saw some heat or smoke... couldn't tell if it was smoke or not, hopefully just some compound or dust burning a bit... oops. Does this mean Q301 is bad? Or could A301 have a problem?

[MarkusBass]

Lowell,

I wouldn't stress the amp for such a long time. The voltage change on the test point should be immediate after you increase the input signal to the amp.
I still suggest that you try to understand how this part of the amp works. I must admit that I was wrong when I tried to explain this in my previous post; the first OpAmp is not just a simple amp but it is configured as integrator. Here: http://www.wisc-online.com/objects/i...?objID=SSE5303 it is explained quite well how it works (press the Next button to go to the next screens).
I'm not sure whether Q301 is suspected. I would say rather that it does not get proper voltage from A302.A. The voltage on TP2 should be -5V (-15V divided by 20k and 10k resistors) and it should be always the same. But the voltage on TP3 should change when you increase input signal. Would it be possible that A302.A op amp does not get the signal from R435? This is not correctly shown on the schematic but I would say that the signal (AC) on R435 should be available on H9.1 and H9.2 and later on T1 and T2 (don't confuse it with TP1 and TP2). Is it possible that between H9 and T1/T2 there is some kind of a plug and it is disconnected (or there is no contact)?

Marek

[lowell]

Marek thanks for the link... will read on. However, the amp finally kicked into 24v mode on the fan and the noise went away! I then tried running the amp with VR3 disconnected and the amp works and there is no noise. I am going to replace VR3 and see if that's the problem. If I'm on the wrong track let me know. I will read that link info you sent me, thanks again.

[MarkusBass]

Without VR3 there is no -12V on the fan so it's no wonder that there is no noise. VR3 provides -12V to the fan and you can easily check it by measuring its output voltage. If it's -12V, it works correctly.

Marek

[lowell]

Marek... when I disconnected one lead on VR3 the fan still had -12v and there was no noise...??? don't know why or how. I'll replace it anyway and let you know what is happening after that. Thanks for the integrator link that helped me understand the circuit alot. I don't understand how these circuits (comparator and integator) would cause this noise though and only at the -12v and not the 24v.

[lowell]

so the replacement of the fan v-regulator took care of the fan noise... however when biasing the amp I'm not getting a reading for the bottom power amp (across H8). Carvin said to bias to 3-5ma across the ballast resistor. The upper power amp I was able to bias but the meter reads 0amps across the lower power amp ballast resistor. I tried adjusting the VBE multiplier pot with no difference. I did not want to fry anything so I shut it down. I tested the 2 150ohm resistors on the VBE mutliplier and they're fine. ??

[MarkusBass]

I suggest to measure voltages on Q206 (base, collector and emitter). Are you measuring the voltage directly on emitter resistors of the output transistors? And are you doing this using milivolt range of your DVM?

Marek

[lowell]

Marek,
I misread your post and replaced Q206. There is now bias current flowing, however it is rising rapidly after turning on the amp. It starts around 1ma across the emitter resistor and rises. I shut the amp off after it hits 5ma.

[lowell]

Ok so I was sitting over the amp on my bench stewing over why this thing was "running away" on me. I then thought about how the heatsink plays a role in the circuit - I did not have the heatsink attached to the power module. Once attached, the amp worked great. I slowly raised the bias until it both amps were running around 3ma. I walked a way for a minute and heard the mains fuse blow. When I ran back the entire power amp that I had just serviced was shot. All output xsistors measured shorted. At this point I had not placed any "bird poop" on that side of the board. Could running the amp for 5-10 minutes w/o this white goop have caused the power amp to overheat and blow? This is the only conclusion I can draw being that it was working ok until I turned up the bias after about 5 minutes of having the amp on. It seems extreme to me though considering that it was such a short period of time and I was not running any signal into it. I will replace everything again, luckily I ordered double everything in case this happened. Then I will retry w/ thermal doodie and heatsink attached.

[lowell]

Ok the heatsink compound made a big difference as the amp is running for a while now w/ no problems.

I replaced everything in the top power amp (see schematic) for a second time around, added thermal compound and mounted everything to heatsink. Solid state amps have so many interracting circuits it's hard for me to keep track of what's going on. I guess I'll get better at it the more I get my hands dirty. Tube amps are much easier and simple for me... but that won't stop me from learning more about ss as I'm very interested in that as well. The whole idea of thermal coupling and solid state biasing circuits i.e. servos/vbe multipliers has been a new thing for me. Do you ss experts find that most amps you see for guitar, bass etc that are solid state operate in similar class? I'm finding AB to be the most common that I've seen, however the GK100RB I worked on was in Class G I believe... called so I think because of the lower and higher supply rails.