AU-9900 No Bias Change - Left Channel

Zekeman

Zekeman

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I'm restoring an AU-9900 and am getting a L-ch bias reading of 9mV which will not adjust with the turn of the pot. The entire driver board is recapped, has new transistors (except for the TR19-22), new zeners, new diodes, and new Bourns multi-turn trim pots.

The voltages read to spec (below).

Any advice on what to check next would be greatly appreciated.

Zeke

AU-9900 F-2580 Driver -1 No bias L-CH.png
 
When you turn the bias trimpot to increase bias does the voltage between TR09 B-E change (i.e. decrease, thus turning TR09 gradually off) and if it does, does the voltage at C-E change? (increase), what voltage do you get here?

If the above is ok - check the voltage across R51 and/or R53 which should also increase. ;)
 
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TR09 B-E voltage does not change with the actuation of VR03 however, the C-E does adjust.

Across R51 I get a 1.18 V drop and across R53 I get a 1.2 V drop.

The Lch bias reading is currently 9.38mV.
 
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Further checking reveals:

Both R61 and R63 have a 230 mV drop.

TR 19 B-E 0.57v, C-E 0.57v
TR 21 B-E -0.59v, C-E -0.42v

On the R channel (One that adjusts properly) the voltage drop across the comparable resistors is 0.57v

So clearly the lack of voltage drop on the L channel resistors is an issue.
 
Well despite seeming like they are turned on - those 2 transistors are only passing a small current, check the resistors, if OK try changing the transistors?
 
TR-19-22 are the only transistors that are original on this pcb. I'll R and R them and measure the resistors.
 
Zekeman said:
I took more measurements today...some red flags...but have more to check.
Click to expand...

Pin 15 on the driver board is missing its supply (no 42V) - you may have a blown fuse (F01 on F-2566 PCB), OR a broken connection between Pin 15 (F-2580) and Pin 04 on the F-2581 PCB.
 
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That was my scres-up...I had the red wire #06 on 2566 disconnected per the bias testing procedure in the manual. Corrected measurements are here...

BTW the driver board in my unit is F-2583 and NOT 2580 which is what is in my service manual...
AU-9900 F-2570 non-adjustable Lch bias.png
 
I can't see much wrong with that now except voltage at TR09 collector indicated twice 1.3V and 1.87V ? - also collector voltage of TR13 1.7V (not -1.7V?) - and the readings for C of TR03 looks like it needs a minus sign. If all of those are just typo's - have a go at adjusting the bias again, as I think that should work.
 
Yes those were typos. So I attempted to measure the bias according to tie service manual again (disconnect number 6 wire on F2566 circuit board and insert the multi meter's leads between the tab in the wire) and the damn thing sits at 9 mA and will not budge when I turn the potentiometer
 
What you should see is a definite change in the voltage across TR09 C-E when you adjust that trimmer, we know you have around 3.6V there, (-1.7V <----> +1.87V) does this voltage change? When this voltage increases so should the bias, as it should turn on TR15 & TR17, I guess you are sure your meter is ok?
 
I clipped meter leads from the TR09 C to E (positive to negative) and I go no change while actuating the pot. I also pulled TR09 to check it and its OK.

Re my meter - It reads the R-channel OK.

I'm going to reverify...
 
Well, that suggests the pot itself faulty/wrong value, or something wrong in the bias chain comprising D05 / TR07 / TR05 / TR09 or any of the components that support them - resistors included.
 
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