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In Reply to: RE: power xformer for a vintage Kenwood int amp. posted by BS64 on August 17, 2014 at 08:12:13
Usually the four fuses are for B+/- rails, which are after the rectifier. Trace the lead of the B+ transformer leads and check for AC output. Could be merely your rectifiers is out
good luck
Follow Ups:
Indeed do some further testing. Transformers rarely fail, and when they do there is usually obvious signs of over heating. Most likely failure in vintage amps is the big electrolytic capacitors get dried out and cause cause the rectifiers to blow due to the increased peak currents.
No fusible link inside the bell ends of the xformer.
There are no obviously damaged parts. Here's the story: I use this KA-3700 int amp to drive a pair of outdoor speakers by the pool. The music suddenly stopped. I went inside and found the red power LED out with the switch still on. The vents over the xformer were very (relatively speaking) hot. Turned the switch off and let it sit for a while but still no power. Took the lid off and poked around inside and see nothing obvious. The tops of the two biggest caps, 6300 mf, are both intact. I had a tube amp that was blowing the B+ fuse several years ago. Took it apart and found the top of one of the caps about to come off. Replaced all 4 and it was good to go.
I am not an electronics whiz, but I will do what I can with a dvm. No worries, safety first.
Here's a couple quick checks.
After the unit has been unplugged for a while...
DVM set to ohms measure across the big caps, they should start out low ohms and charge up to near infinity ohms. There are probably four total but two are in parallel on one side (B+) and two on the other (B-) so you don't really need to check all four.
DVM set to diode check, measure across your rectifiers they should measure a diode drop on direction and open circuit the other
Check obvious things like fuses and switches
If everything above checks out time to do some under power tests. Without a schematic or looking at the board. I would say check the B+ and B- across the big caps set to DC volts. I don't know what is should be exactly, but it should NOT be zero or close to. I would guess around +/- 45Vdc. Then set the meter to AC volts and measure the ripple on the caps. Should be near zero volts but one or two percent of the DC voltage is probably okay five or ten percent or if one side is noticeably higher than the other then your caps are drying out and should be replaced.
If the B+/- is good then you probably have low voltage supply problem and I would need to see a schematic to tell you were to look.
The 4 fuses that are visible were removed and ohmed out - all good. I verified that the power switch (rotary) is closing. I will perform the checks you recommend later this week. Many thanks!
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