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G-5500 No Bias On Right Channel

audio_exp

audio_exp

Member
I have no Bias on Right channel when I measure across R71/R77?
I get some audio but it's distorted. Left channel is fine.
Audio is clear in headphones until I put an 8ohm load the right channel.
I have checked all the resistors around the driver section and the 2 driver transistors look good.
All of the small signal transistors test OK.
Kind of lost, any ideas?
 
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Hello. No bias, + distorted sound on a load, could point to some transistor unable to hold the current needed, but performing fine with headphones. It could be some driver transistor or even one output transistor. I'd test both. Drivers and output transistors. check the emitter resistors are ok and not open.
 
elnaldo said:
Hello. No bias, + distorted sound on a load, could point to some transistor unable to hold the current needed, but performing fine with headphones. It could be some driver transistor or even one output transistor. I'd test both. Drivers and output transistors. check the emitter resistors are ok and not open.
Click to expand...

I’m going crazy I have checked every resistor including the emitter resistors (R71/77 0.33ohm), replaced the 2 driver transistors (TR15/17) No Change?
I replaced a few caps just to see if and improvement can be made and no luck (C06/C08/C26/C30/C32/C613/C629).
All outputs test good in circuit and match the working left channel.
I compared the working left channel components against the non-working right channel and can’t find any difference other than almost no bias in the right CH?

So strange.
 
Maybe the bias pot is bad. I just found one on a stereo I'm repairing. blown transistors took it out Take it out and check it with a ohm meter.
 
tonysrep said:
Maybe the bias pot is bad. I just found one on a stereo I'm repairing. blown transistors took it out Take it out and check it with a ohm meter.
Click to expand...
Yes did that already put it on a meter and rotated it and it has continuity throughout it range.
 
Most likely cold solder joint somewhere in the bias circuit, or OPT/emitters/drivers connections.
 
CrSens said:
Most likely cold solder joint somewhere in the bias circuit, or OPT/emitters/drivers connections.
Click to expand...

Check that and even reflowed almost all points on left ch, Also checked D04/601 Varistor (STV-3H) and it good also.
Could it be something in the protect section, are there separate bias supplies for Left and Right?
My schematic is a bit fuzzy I just can seem to find the gremlin!
 
Protection circuit trigger is separate for each channel, but power supply is common for both channels.
Probably nothing to do with protection circuit.
 
Driver or bias transistors backwards? did you confirm the pinout of the replacements? If so, perhaps one output transistor is bad. Not shorted, but not working.

Measuring some voltages at the drive transistors and bias circuit can show something.

I'd stop replacing components, and start with a schematic, methodically, to check voltages and compare both channels WITHOUT RISKING THE AMP. Please attach your DMM to the test point you need to check, and THEN power-on the unit.
 
Drivers are in correctly.
I agree I’m starting to think the outputs are bad even though they measure ok.
Replacing the 44 year old caps with “Conductive Glue” all over them with new low ESR high quality caps IS a good idea. I fully understand that just changing out a lot of parts is not always a good idea because it can make finding a problem even harder but not in this case.
Besides I am planning to re-cap the entire unit anyway but just wanted to rule out and potential issues caused by bad caps around the driver section.

So I just wanted to ask since you can’t get the original outputs anymore can the 2SC2578 be replaced with a 2SC4467? And can the 2SA1103 be replaced with a 2SA1694?
 
just had the same issue with a receiver I was working on the bias transistor was installed backwards, the screen printing on the board was incorrect.
 
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