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Old 9th December 2011, 03:04 PM   #1
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Default Bose 1800: DC Offset Issues

Hello everyone,

I've been working on a Bose 1800 from back in the day. I've been working on repairing the one broken channel. I've almost finished it and I was foolish enough to think I did when I tested it.

Heres what happening:

The amp gets turned on, for about 10-15 seconds DC offset for this side shows nominal values <100mV. After that time frame the voltage spikes to positive rail voltage and falls down and goes back up. The LED meter on the front panel lights up full signal/clipping when the offset spikes up to rail voltage.

I didnt observe the DC offset long enough to see the voltage spike. I hooked up signal and connected a speaker. Listed to about 15 seconds of Frank Sinatra then poof.

I've done some work to the amp here's a summary:
1) all new output transistors
2) repaired op-amp supply voltages
3) replaced part of differential amp, VAS, and Bias transistor.
4) replaced filter caps and zener diodes for op-amp supply

The other channel on this amp is working and I've been using it to source parts out of for testing as I'd rather not make multiple orders to Mouser with 10+ shipping on each.

Heres some pics, there were some damaged trace when I got the amp some years ago:
Initial:
Click the image to open in full size.
Click the image to open in full size.

Removed Power Transistors / Heatsink:
Click the image to open in full size.
Click the image to open in full size.


Damaged Traces that need to be repair..somehow. :
Click the image to open in full size.
Click the image to open in full size.

Heres the schematic:
Click the image to open in full size.
http://i.imgur.com/ETpss.png

Last edited by jamesdean; 9th December 2011 at 03:09 PM.
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Old 9th December 2011, 03:10 PM   #2
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Join Date: Nov 2008
Location: Oakmont PA
Use a hot air gun to see if you can trigger the fault. I suspect you have either a trace that is a hairline failure or a component that will test good until it heats up a bit.
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Old 9th December 2011, 03:11 PM   #3
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I seem to have broken my thread somehow hmmm
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Old 10th December 2011, 04:14 AM   #4
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That was an excellent suggestion. Heat was part of the problem. Q3, part of the VAS, was over heating and unable to fully perform. I repaired its connection to the main heatsink. I tested the amp out-of-chassis for about a half hour without incident. Soon after though, it had a few spikes of DC and toasted my cheap test driver.

I think Q3 has been weaken by all the cycles of usage the during my past few weeks of working on this unit. It had been detached from the heatsink the entire time. I would like to replace it but I'm having trouble source an adequate transistor.

It was originally a Motorola MPS-U10. Replacements I've found either didn't dissapate enough heat or were too slow in Ft. Bose used the MPS-U10 when RCA stopped making the 2N6175.

When Bose made this change out they said to install a 10k thermistor across a resistor that goes from base to collector of the bias transistor Q4. I accidentally broke the thermistor and have no idea what type to replace it with...
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