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Dim Bulb Tester Burns Bright-Dim-Bright-Dim

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You Need..

a Fester Tester..:yes:
 

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HAHA...nice!

Avionic...any ideas on where to turn next? I've tested all the resistors around the emitters and they are good. Does that pulsing sound like a resistance problem to you?
 
Is the protect relay clicking in sync with the toggling dim bulb?
 
Bias adjustable now? Replace the bias trim pot? It likely took a beating in that mess..... You can check it also, but do replace them in both channels.
 
go to a higher wattage lamp (or parallel another lamp in) and dial down the idle current to miniumum.
 
Whatcha usin? double it?

watch the base(pnp) to base(npn) voltage on the outputs ~1.2v

This uses a Vbe multiplier (with resistor adjust at the base) instead of bias diodes and a resistor. Manual does have instructions on going to minimum idle current. (full ccw?)

dbt's can do strange things when their wattage is too close to normal wattage drawn - they are intended for extreme cases.
 
1. The factory installed TR5 was installed with the flat part resting in the heat sink. I installed the KSC 945 CLTA the same way...problem is the leads on the original are reversed relative to the flat...so my problem is partially solved.

2. The unit is fully functional and sounds great.

3. The bias adjust pot tests good.

4. The issue now is that the lowest I can get the bias setting - even with the pot fully CCW is ~24 or so...it can be in increased but not adjusted lower than 24...could this be a function of the all-new outputs?
 
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does it say in the manual to test it like that ? have you tried the mv across each pair of resistors ? it may also tell you something if you measure across them individually
 
Yes that is the exact process outlined in the SM...
 

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Zekeman said:
Yes that is the exact process outlined in the SM...
Click to expand...

ok it just looked a bit odd that's all .
just looked again and the sketch looks wrong ..i would go off the schematic bit at the top
i reckon your red lead should be the other side of the resistor so you measure both at the same time .your measurements should half or more less half ..i would expect it to drop to around 12mv
my brains not working tonight not sure if it will measure double across both as opposed to one ..
you should however measure across both resistors though
 
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Something is wrong with the bias setting on this amplifier - is it the same for both channels ?

The procedure is unclear - as Pete says, the text says measure across 2 emitter resistors - but the diagram shows probes across 1 resistor.

You will measure twice the voltage you are now seeing if you move your probes to measure the volt drop across 2 resistors. Check the emitter resistors (all of them) to make sure they are the correct value(s).

It could be leaky output transistors/drivers, or you may find you have wrong value resistor(s) (drifted ?) or possibly out of spec gain for one or more transistors. OR, worst case, you may have to manually adjust the values of some resistors to accomodate the new semiconductors.
 
The procedure shows measurement across the two emitter resistors and has worked properly on the other four AU-7900's I have worked on. I looked at the sm again last night and notice that the trimmer pot is spec-ed at 470 ohm...when I pulled the pot to test it yesterday I seem to recall it reading in the 300's so even though it tested ok it may not be delivering the correct resistance value?... I am going to order some new pots today.

The other channel adjusted ok ... dead-nuts-on.

Attached are the test points on the board and schematic...for the channel in question
 

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I installed new pots. Bias dialed in perfectly on both channels.

Thanks again to all for your valuable advice!
 
Glad to hear of your success. Just as a point of reference about Sansui and their VR pots. On my G-8000, I found that the trim pot for the bias was opposite of what they suggested in the adjustment section of the manual. Instead of starting the adjustment with the pot all the way counterclockwise, I had to start with it all the way clockwise. I suspect that they ran out at some point in the production line and replaced it with the reverse pots. For this reason, I now check all pots on my Sansuis with an ohm meter prior to adjustments. Have fun with your "new" amp.
 
Bubba,

Thanks....I am learning a lot. The new pots I installed operated in reverse as you mentioned...but they dialed right in...dead-nuts-on! The test points in the SM on this model are valid.
Thanks.
 
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