I recently purchased a Lafayette LR-5555. I initially cleaned it and checked for DCV on the outputs. Everything seemed ok so I used it for a couple of days while I waited on new power supply caps. It worked but seemed a bit anemic. So I replaced the caps without taking any voltage readings first. After installing the new caps I had 42vdc and -53vdc. By the way both caps had dropped to 6000 mfd from 10000 mfd. I have gone as far as disconnecting everything from the power supply and installing new rectifiers. I have 32vac on both taps of the transformer and after passing through the rectifiers it’s unbalanced even before going to the filter caps. If I remember it was 24vdc and 28vdc or there about. After hooking up the filter caps it’s 42vdc and -53vdc. There’s nothing else in the circuit but the bleeder caps and they are both 1.5K ohms. Any ideas. I’m stumped. The rectifiers are all good. The only thing I can think of is a bad transformer but it reads balanced at the taps.
I need help with a power supply problem.
- Thread starter Jon Fox
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Keep in mind that the rectifiers simply convert the sine waveform into full wave rectified, not DC (google for pix)
Connectors or wire wrap posts at psu pins 7,8,9???
Don't unwrap the wire wrap.
Did you see R3 for tuner power. Possible tuner fault, is Tuner working somewhat or not at all.
Try lifting R3 is not already done
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Prefer to unsolder the pins rather than unwrap if you need to go down this path. The wire wrap makes better electrical connection that resoldering the wire.
Check dc offset of both power amp channels
I did disconnect the tuner but still had the problem. The voltage after rectifying is pulsating DC. Once filtered it’s steady DC. I got the power supply sorted and have balanced output at 43vdc +and-. Now I have to figure out why my relay isn’t latching.
Do have a schematic. But I got the power supply issue resolved. Now I have to figure out why my relay isn’t latching. Should’ve waited on the amp board but found several bad caps.
My schematic is on one 8x11 sheet of paper and I can’t see it! Getting old.
I have supply voltage on pin 1of each module and about 150-160mV on pin 4. It appears the module is good.
150-160mV is uncomfortably high for my altecs but I don't see it tripping protection, happy to be corrected.
The usual next step is to look at the protection circuit, it's fed by the power amp rail so relay power should be ok.
Suggest work back from the relay drive transistor Q9, measure dc voltages at base then emitter, want Vb = Ve + 0.6V (ON state).
If Q9 is ON then look at R51 (open?)
If Q9 is OFF then check Q8 b, e voltages,
wanting Ve > Vb (OFF state)
Also check Q7 b,e voltages
wanting Vb > Ve (OFF state)
I may have found the problem. The tuner quit working too so it and the protection relay are feed from a dedicated 43vdc from the power supply and then regulated at 13.5-15.5vdc. I need to see if my solder joint on the power supply board is bad or I have a bad regulator circuit. This is the only post I desoldered. I bet I wiggled it loose.
Thank you so much.
Q7 is bad. It’s not passing voltage. I need to see if I have one. This is the voltage regulator I suppose. It’s a C1398.
Did you try the 2SC2528?
The MJE 15032G is a heavy duty replacement with the same pinout. That’s what I ordered from Digikey.