Roland HP-860 electronic piano has a BUZZ!

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Hi.
I recently bought a 2nd-hand Roland HP-860 electronic piano (88-key,
analogue).
It has a louder hum/buz than the piano notes.

I'd like to fix it myself. I have been learning electronics most of my
life & I'm in the 3rd year of Electronics Trades at TAFE NSW
(Australia).

I've removed the plug from the final stage PCB (Revo board), and the
amplifier is quiet. This makes me think that somewhere before the amp,
there is hum being made.
I could poke around with my CRO, but I wouldn't know what to look for.

This piano has heaps of boards, for which I've worked out most of the
features.

I could try replacing all the aluminum electrolytic caps, but that
could just be a waste of time.
It was made some where between the late 70's and early 80's.

I looked for date codes, and only found 1, I think.

This piano uses a wiring loom design, with some PCBs using plugs, but
most are wired up (soldered).

To see pics of it, search Google images for
"roland hp-860"
use quotes and thaisecondhand.com has the pictures.
NOTE: I have the 240VAC version.

Please help!
TIA

BTW: I have checked the PSU board's voltage rails, and all of them look clean.
I have replaced the electrolytics next to one of the STK025 amps, and it didn't help.

[Maschinengeist]

I'd check around for a transistor or op-amp.

[Maschinengeist]

I just found the HP-600/700/800 service manual in my stuff. Let me know if you want a copy (for cheap, excellent, complete pdf). Maybe that could help you out.

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I'd check around for a transistor or op-amp.

there are hundreds of transistors & about 2 8pin op-amps

[Maschinengeist]

That I know. But you go by elimination.

Where the buzz can come from, the more naturally? Power supply, from a power transistor. And from the output module.

You can use some Deep Freeze to cool 'em down. When they'll heat, you can hear the buzz. Go from section to section, methodically.

Inject a signal and look for distortion on your scope. You know the drill.

Usually, the power supply is the suspect. I cannot say for sure since I've never worked on your model. Check the transformer too.

[Maschinengeist]

http://cgi.ebay.com/ROLAND-HP-KEYBOARD- ... 1769r33887

The guy only ships to USA... maybe you can change his mind. I'm willing to share the bill for clean, readable copies.

Or maybe there is a USA member here who wants to help me out buying them and I'll do you clean PDFs

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The guy only ships to USA... maybe you can change his mind. I'm willing to share the bill for clean, readable copies.

It doesn't mention my model. But thanks for finding it anyway!

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You can use some Deep Freeze to cool 'em down. When they'll heat, you can hear the buzz.

If it happens as soon as I turn it on, from cold, could that still apply? Do they heat up in a millisecond?
They aren't getting hot, but maybe ambient temperature.

Inject a signal and look for distortion on your scope. You know the drill.

Yeah, but I haven't had enough practice in that, and I don't have a signal injector. A sound source is limited in it's use here.

Usually, the power supply is the suspect.

Would you say I should look at the DC rails with my CRO on AC coupling, and increase the v/div to see if there is any ripple ?

Check the transformer too.

Seems ok, it should be giving out just a sine wave, right?

[Maschinengeist]

The guy only ships to USA... maybe you can change his mind. I'm willing to share the bill for clean, readable copies.

It doesn't mention my model. But thanks for finding it anyway!

Odd... last night, in the description of the auction, there was a HP-860 service manual in the lot... Am I going insane or the guy sold it off auction??? Call the X-Files!

[Maschinengeist]

Not that I don't want to help you out but it I do the complete troubleshooting, this is work. And as a small business, I cannot troubleshoot everybody's instrument for free. Especially working without any schematics nor any references. That's why in here I give general pointers for the person to check.

If I do a complete tutorial, this is not only time-consuming but I can't pay my bills. You'll see if you go solo that money is unfortunately an object. Sounds like greed but that one of the ways I earn a livin'. Of course, if I wasn't on my own and was working for a shop at 50K/year, I'd pass my free time working everyone for free. But it's not the case. You understand?

Before worrying about the scope, do simple voltage tests on the power supply. Test the key components for accuracy. Transformer: is it putting out the right voltage? Track down his specs by its model number and compare.
Power tranformer: is it testing right? (you know how to test transfomers, right?)
Electrolytic caps: if they are the original ones, they are no longer filtering anything. Check them out.

Rule of thumb: if the buzz is on the right on the startup, you know that the power supply is wrong. Otherwise,there would be a buzz after warm up.

I am not quite sure that there is only one problem here but a few. An vintage piece of gear is like a car: when something starts behaving, chances are there are more than just the heard cling-clang.

Hope that helps

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OK, I understand.
Thanks for your help thus far.

[Maschinengeist]

Let me know how it goes...