Intermittent Noise in 200A with original amp

[David68]

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.

[pianotuner steveo]

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...

[David68]

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.

[David68]

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.

[pianotuner steveo]

Sounds like you found it! Great to know for future reference.

[Electrickey]

I have inspected solder joints.

As has been mentioned a number of times in other threads, reflowing ALL the solder joints is key to finding static and thunderstorms even before replacing parts.

How sure are you that replacing the resistors was actually the problem and not that the solder joint was just reflowed in the process?

Inspecting joints does not always present the problem. The joint can be defective underneath the solder where the part connects to the board and can't be seen.

Figure a 50 year old electronic instrument, things can get iffy from age and oxidation of connections.

Hence doing a complete reflow job on both the amp and preamp will at least narrow things down.

I did this with my 200A, it would sit on for an hour and then all of a sudden cough and wheeze until I did the total reflow job. Has not made a peep since.

I did use good solder though and a heat adjustable computer controlled soldering station reflowing every joint.

I hesitated assuming the parts were at fault off the bat losing tolerance as I didn't want to change the tone of the piano, preferring to keep everything as original factory parts as possible.

[pianotuner steveo]

I got my 200A for only $50 because the amp was very flaky, and the seller felt bad about charging me that much!
I took it apart, and it had maybe 2 cold solder joints. I reflowed all of them, and it's been fine ever since, roughly 25 years now.

[Tonewheel]

Steveo,

That kind of deal needs to be savoured over and over again. $50. Even 25 years ago that was fabulous. Congratulations.

Love these stories.

[pianotuner steveo]

Yeah, those deals don't happen anymore. I paid less than $100 for every Wurli I've ever owned except my 270. ( I've owned about 30 Wurlis total, most were flips)  2 were free. One was being left out for trash pickup. That was my last 120, and I wish that I kept that one, especially now that I understand the electronics a little better.