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Old 24th September 2011, 09:02 AM   #1
diyAudio Member
Join Date: Sep 2011
Location: Texas
Default No Power on High Side Bias Com-Tech 400

I hate to keep asking questions, however I recently ordered in a Crown Com-tech 400 with the intention of using it for parts (since sound is garbled in both channels). Unfortunately though, after a series of poor decisions and instances of bad luck, I ended up destroying the input board on my original Com-tech 400. Once again, thanks to everyone who helped me with this problem.

Anyway, I am desperately trying to piece these two amps together so that I have one working amplifier in the end. I have been in touch with Crown technical support and have narrowed down the problem with this new amplifier:

1. Someone probably plugged another amplifier into the inputs on this amplifier
2. Most of the electrolytic capacitors were blown up when the previous owner did this
3. I replaced most of the electrolytic capacitors with new ones (at least the ones that were blown)
4. I have determined that there is NO power flowing through R32 on both output boards. According to Kevin at Crown, this indicates that the high side bias is not working on this amplifier. Also, the problem is most likely not on the input board.

The chief problem that I'm experiencing is that I am getting noise in place of the high side portion of the amplifier through my speakers. I can hear music from my computer, however it is very scratchy and unpleasant to listen to.

I have checked and replaced Q18 (a transistor that Kevin said is likely to fail if I have an issue on an output board) on channel 1 without any change.

Also please note that I am experiencing this problem on both channels. Both IOC lights flash in rhythm with the music, indicating that THD is well above .5%

My ultimate question is... what could be causing this problem? Should I check the power delivery system? I can transfer the entire power delivery system (transformer, etc.) from my old Com-tech 400 to the new one, however I currently don't know how to tell if this is even broken.

Where should I start looking?

Here is a link to the schematic:

Thanks again for the help. Once I learn more, I promise to contribute to other members threads more!
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Old 24th September 2011, 10:32 PM   #2
diyAudio Member
Join Date: Sep 2011
Location: Texas
Hey, here is an update:

I spent a while working on my amplifier this morning and have deduced that the output boards are probably good (I have checked all of the diodes on the boards and they are fine). Also, I checked a few more transistors and they are fine too.

I also replaced C110 and U101/U102 to be sure that these components weren't causing any problems. Finally, I replaced the entire power control and delivery system (power output board, transformer, etc.) to no avail.

By this process of deduction, I am reasonably concluding that the problem I'm experiencing lies on the input board or the connection between the PIP card and the input board. I may have missed something on the output boards, but the chances of this happening are small.

What could cause my amp's high-side bias to be completely absent if there is no problem on the output boards, power control board, or display board?

I also checked the cables between the display board and the input board for problems by the way

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Old 25th September 2011, 03:35 AM   #3
Tajzmaj is offline Tajzmaj  Slovenia
diyAudio Member
Join Date: Mar 2011
Location: Ljubljana
What about input op amp? At MA1200 I once find a dead one......
It is cheap and easy to try solution....
Regards, Taj
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Old 25th September 2011, 06:32 AM   #4
diyAudio Member
Join Date: Sep 2011
Location: Texas

Thanks for the reply, however I replaced both of my input OPAMPS with ones from my parts Com-tech 400 and I didn't see any difference.
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Old 3rd October 2011, 02:59 PM   #5
diyAudio Member
Join Date: Sep 2011
Location: Texas

After much troubleshooting, I finally fixed the problem with my amplifier!!

It turns out that R147, R149, R247, and R249 had been blown open when the previous owner had connected another amplifier to the inputs of this amp and had cranked up the volume way too much.

If you have a similar problem with your Crown Amp (garbled audio in both channels) and you are not sure what caused this issue, check these resistors!

I also noticed that the two input POTS (the red ones on the input board) were out of spec and were very dirty. I ended up replacing these with two that were showing 200 ohms of resistance (the originals were reading 230 ohms).

Also, please note that blown resistors may not show any external signs of problems! The ones that I replaced looked almost brand new!

I hope this helps!
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