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Old 29th February 2012, 11:50 AM   #1
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Default Amplifier getting hot on one channel only

Hi folks, I have just finished working on my JVC VN-700 amp, or so I thought. So far in the power amp, I have replaced all output transistors, caps, driver transistors, bias diodes, and the differential pair on one of the amps.
Now, the amp runs and sounds fine, with no audible distortion.

BUT, after carefully setting the dc offset to 1-2 mV on each channel and adjusting the bias as per the service manual, the heatsink on one channel is getting quite/very warm while the other channels heatsink is barely above ambient temperature, after about half an hours use at low power output (2-5 Watts).

I set the bias by measuring the current through the emitter resistors and set it to 10 mA (as per instructions). Both channels sound the same etc, and don't sound distorted. Could one channel be having oscillation issues, or could it be something like the output transistors not matched well to each other? Something else I noted when I set the dc offest, the meter wouldn't give a steady figure and would fluctuate up and down a bit. I have included a screenshot of the amplifier schematic.
Thank you all in advance for your help, it really is appreciated
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Old 29th February 2012, 12:10 PM   #2
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Osvaldo de Banfield's Avatar
Join Date: Dec 2011
Location: Barrio Garay,Almirante Brown, Buenos Aires, Argentina
Yes, I believe you can have oscillating the amp, or may be a fake cap, although it is new (It happen several times to me) or reversed wired. Sometimes the painting in the PCB are wrong.
Osvaldo F. Zappacosta. Electronic Engineer UTN FRA from 2001.
Argentine Ham Radio LW1DSE since 1987.
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Old 29th February 2012, 12:19 PM   #3
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I haven't reverse wired any caps, the two power amp driver circuits are on a pcb 'card' that fits into a slot, much like a computer card, and they are interchangeable so I can compare them side by side etc.

I have just been doing some google searching and reading, and discovered that if replacing transistors of a different type with a different bandwidth due to differences in the internal capacitance can induce oscillation.

On the amplifier that is good, I replaced the differential pair on the input with two BC547. The originals were 2SC1345, and were not well matched and gave me 250 mV dc offset. That channel is okay, but the channel that is getting hot is still running the old 2SC1345. Looking at the schematic, it seems transistor X403 is part of the negative feedback loop and the original 2SC1345 may not be compatible with the new driver transistors. I will try replacing them with another pair of BC547 and see how that goes. Of course it may have no effect but its worth a go. At least then both channels will be running the same transistors etc.
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Old 1st March 2012, 12:55 PM   #4
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Okay, well today I went through a bunch of bc547 transistors, and found the best matched pair and used them to replace the original 2SC1345, as per my idea in the previous post. And sure enough, after I set the dc offset and the idle current bias, the amplifier channel was getting far less warm than it was before, barely noticeably different from the other channels heatsink, but that could be down to the other channels heatsink having one side exposed to fresh air on the side of the amp. Having had the amp running for an hour under the same conditions etc, it only gets very slightly warmer than the other channel.

So, I do not know if my theory in my previous post was correct or not, or why exactly the channel was getting hot, but its all good now and sounds great for an amplifier of its age. I didn't go through the circuit with a scope, that was my next plan should the transistors fail to fix it. If anyone has any ideas on what could have been the problem then I'd like to hear them, so I can try to better understand the systems at work
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