Author Topic: Intermittent Noise in 200A with original amp  (Read 107 times)

Offline David68

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Intermittent Noise in 200A with original amp
« on: May 05, 2019, 01:47:50 PM »
I have a Wurlie 200A on the bench that's been driving me nuts with an intermittent crackle I've thus far been unable to isolate. For one, it's very intermittent. The piano will work fine for a while, then crackle for a bit, then go back to working. I have rebuilt much of the reed bar preamp with new transistors and new resistors. The single diode and two 100pF caps are original. The 150V supply is unchanged. The noise tracks with the volume control, so it's something upstream of it.

As far as I can tell, it's not a pickup short. I've checked reed alignment and blew the whole thing out with compressed air. I've jiggled wires and resoldered some. Playing the piano or not playing makes no difference. I've chopsticked components with the piano on. None of these produce the noise, so I'm thinking it's some PCB component failing. I know that high value resistors sometimes open up, so I'm thinking of shotgunning all 5 resistors in the 150V supply.

As for the components on the reed bar, is there anything special about the diode? Will a 1N4148 work as a replacement? (I have those in stock.) It's rare to see ceramic caps fail at low voltage, but I suppose it could happen.

Any other ideas or diagnostic procedures to try to get to the bottom of this? The frustrating part is doing the work and not being able to tell immediately if you've solved the problem. You have to wait for it.

Thanks in advance for any suggestions.

Offline pianotuner steveo

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Re: Intermittent Noise in 200A with original amp
« Reply #1 on: May 05, 2019, 09:45:29 PM »
Have you inspected the bottom side of each PCB looking for cold solder joints and shorts? Flaky traces? Is that diode either open or shorted? I don't know if that diode will work, but I do remember using those in audio boards when I was a bench tech (25 yrs ago)

Most of the time, those crackling noises are other debris or condensation on the reeds,but it sounds like you've ruled that out...
1960 Wurlitzer model 700 EP
1968 Gibson G101 Combo organ
1975 Rhodes Piano Bass
1979 Wurlitzer 206A EP
1980 Wurlitzer 200A EP
1980 Wurlitzer 270 Butterfly Grand
2000 Yamaha acoustic piano
2004 Hammond XK3
2009 73A Rhodes Mark 7
2009 Korg SV-1 73
....and a few guitars...

Offline David68

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Re: Intermittent Noise in 200A with original amp
« Reply #2 on: May 06, 2019, 12:07:19 PM »
I have inspected solder joints.

I ended up replacing all five 150V supply resistors on the main board, and, thus far, no more crackles. I'm going to run it for a few hours while I work on other things to make sure that solved the problem.

On the diode on the reed bar preamp, it's there to turn on in the event that the signal input goes too far negative, as I suppose it might as the 150V supply comes up. The 25V reverse rating might be so that it shorts to ground in the event that the input DC blocking cap fails. So I think any diode would work, but it actually has a low PIV rating because it's designed to fail in the event of a cap failure.

Offline David68

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Re: Intermittent Noise in 200A with original amp
« Reply #3 on: May 06, 2019, 07:27:47 PM »
This 200A ran all day in the shop with no more crackles, so it seems it was a failing resistor in the 150V supply.

I hate these problems that only pop up every half hour or so and then disappear again. Very hard to catch it in the act.

Offline pianotuner steveo

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Re: Intermittent Noise in 200A with original amp
« Reply #4 on: May 06, 2019, 08:40:59 PM »
Sounds like you found it! Great to know for future reference.
1960 Wurlitzer model 700 EP
1968 Gibson G101 Combo organ
1975 Rhodes Piano Bass
1979 Wurlitzer 206A EP
1980 Wurlitzer 200A EP
1980 Wurlitzer 270 Butterfly Grand
2000 Yamaha acoustic piano
2004 Hammond XK3
2009 73A Rhodes Mark 7
2009 Korg SV-1 73
....and a few guitars...