Hi guys,
Just wanted to clarify some doubts I had with biasing issues on SS amplifiers. I am really interested on working on amplifiers and anything related to audio. I have been reading on amplifiers.
So here is the series of events on the project I am working on:
- Output transistor Q6050 was shorted and one half of the emitter resistor R6100 was open (left channel) and the B+ fuses were open.
- So I borrowed the two output transistors and the emitter resistor from a working channel which was not used. Replaced the fuses however I didn’t replace the driver transistors(I should have!).
- Powered on the unit and measured the biasing voltage at the test points and it was within specs given on the SM. However I should add that I didn’t monitor it long enough.
- Played some signal through it and it was working perfectly fine.
- So assembled the unit and returned it to the owner to be used until the replacement parts arrived.
- Checked it at his place, was working fine and after I got back he called me back saying that the amp had failed and that there was no output.
- I checked the amplifier again the same left channel had shorted. The two output transistors were shorted and the B+ voltage fuses were blown. The emitter resistor was intact this time though.
As per what I have read online regarding what could blow the output stage (some reasons):
- The speaker being shorted - I checked the speakers and I didn’t measure a short across the terminals. However it had a crossover circuit and only what I could read was a capacitor charging and both speakers showed similar behaviour. The L and R speaker sounded same to me when it was working.
- Cold/broken solder joints - I have checked for these and didn’t come across any obvious points.
- Other components failed - I have checked almost all the components on the left channel and it measures ok.
- Biasing issues.
I would like to clarify about the biasing circuit used on this amp. I have done some research on Vbe multiplier circuits but most of the circuits I came across the variable resistor is connected to the base of the Vbe multiplier. However in this unit the VR is connected to another transistor’s base Q6010. I am finding it a bit hard to grasp how this works.
Could someone kindly explain.Q6010 is placed next to one of the drive transistor and it has got what looks like thermal compound applied to it.
What is the reason for this?If it was a biasing issue and somehow it caused the output transistors to short out,
note B-C-E were shorted - why didn’t it take out the drive transistors? The drive transistors still checks out ok.
All of the components related to the biasing part of the channel checks out ok but hypothetically speaking what component failure for e.g.: the potentiometer’s contacts loosing connection, some kind of thermal effect messing up with the biasing circuitry’s transistors?.
If its some kind of an issue which only causes the fault when power is applied what further troubleshooting should I be doing on this circuit? At the moment I am waiting for the replacement parts to arrive but meanwhile I would like to figure out what exactly caused the fault - could I power up the amp without the output transistors and see if the proper voltages are preset which is shown on the SM and lets say if the drive transistors are faulty, ‘leaky’ for instance would I be able to measure that?
Based on what I have detailed above, what’s your opinion for the reason for the fault to occur?
Thanks in advance for the help.
P.S - I couldn’t find replacement for some of the transistors and resistor so here are the equivalents that I have ordered
2SC1740S-S ——— 2SC2240
2SA1930 ——— A1837
0.22 Ohm 2W emitter resistors —— 0.22 Ohm 5W
Also note that there are some film capacitors which looks a bit black, I am not sure it was like it to start with. It measures ok especially when compared to a working circuit. Hope you can see it in the photo