Electronics > Repair
Vintage Kenwood Receiver Volume Issues
Arty30:
Thanks, like I mentioned, I have already replaced the capacitors except for the main filters and output couplers due to their size. I think I will go ahead and replace the transistors on the tine amp first and go from there.
squadchannel:
oops. missed the mention of it.
If the problem occurs on all inputs and radio, then the MAIN AMP block is suspect, but first it is going through the NF TONE block.
First, bypass the NF TONE block and make sure the NF TONE block is not the problem.
You need to isolate the problem area.
If you have an oscilloscope, you can also check to see if the signal is passing properly through each transistor. compare the left and right channels from the first stage to the last stage to find what is wrong.
Arty30:
This unit is one that has a helpful pre-out/main-in jumpers, which I have already tested separately and found to be functioning properly.
floobydust:
Another approach I have used is to inject a 1kHz test tone and measure/compare ACV between channels and stages, to locate a signal problem.
But you have to know your multimeter, if it is AC or DC-coupled on ACV. Cheaper DMM's get it wrong if you have say 30VDC with 0.5VAC signal riding on it, the DC saturates the ADC front-end and it gives bullshit readings. Not a problem for a scope, any DC bias.
Arty30:
Its been a while, between being busy, getting covid etc, I have finally solved the problem. I went and replaced the four 2sc458s on the tone amp, however this turned out to be premature. doing voltage measurements, I found that two of the four were not biased on the base. surprisingly the culprit was two 220K resistors that were open circuit, I have never seen resistors go open on their own. weirdly they were small grey dipped resistors, the style that sort of look like a dog bone, and were unique to this board only. the last small annoying fault I'm chasing is the stereo lamp is not functioning. if i tune into a string station is does not illuminate. it is the MPX board that is responsible for this, I will post a screenshot of the part of schematic. There are three transistors that appear to be cascaded, Q7 is off, so Q8 is on, thus Q9 is off and the lamp is out. This doesn't change if I tune in or out of a station, and Im not great with these circuits to tell where the "control" signal is coming from. sorry for the essay, but if anyone has any input I would be grateful, Thanks.
Arthur
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