Roland HP-860 electronic piano has HUM

Have you looked at the power supplies with your scope? Here's your chance to put those 3 years of education to good use.

Gareth.

No I haven't! Thanks for the tip, and thanks for the quick reply! I'll go and try it now. It has a bunch of different voltages out, and they're labelled on the PCB! I guess I should look for ripple on what should be DC, right?? Then trace it back to the offending cap(s). Then go buy some new ones of those only.

BTW: It has a main voltage going to all the keys of 46V which measures

You seem to be jumping to lots of conclusions for no apparent reason. This is not going to help you.

What's with this "offending caps" businsess? You seem to have decided there is a cap problem and replacing them will be the solution. Stop guessing, it will get you nowhere.

You need to investigate and gather information. You start with the power supplies, If they are OK then you look elsewhere.

Gareth.

Sounds good.

Ba-ding... Now I wonder why that voltage is low? Could it be, oh, maybe... A bad filter cap?

Thats because searching for any electronics problem on google comes up with "check all the electrolytics", ESR is the new religion. I get so many people bringing stuff to me saying, hey its probably a cap, you got an ESR meter?, can't be more than 10 bucks to fix. Yeah right. A little bit of knowledge can be dangerous. Good luck to the OP, with a scope should be an easy fix.

Er, what exactly is this 46 volts supposed to be doing? I doubt there is anything like 46 volts anywhere near the keyboard.

Please try again.

Gareth.

Yes... I checked it again. it's a common to all keys. No... there's little chance of electric shock, as there is a lot of plastic between the user and the switches.

Sorry for jumping to conclusions. It's what I do, sometimes. And... as you will know, aluminum electrolytics are prone to going dry given a few decades (or less) !

Your measurement is incorrect. No way is there 46 volts around the keyboard. Possibly 4.6 volts, a slight but possibly acceptable drop from the 5 volt supply.

You are still wanting to find the answer in dry aluminium electrolytics or something else. Stop trying to make the symptoms fit your conjoured up answer. This is not the best way to fault find. You need to establish some facts. So far you have virtually none at all. Go get some and stop guessing. Really.

Gareth.

OK, How do I do that? Thanks.

OK, How do I do that? Thanks.

OK, How do I do that? Thanks.

ESR is not a religion, it's a fact of life which, if you are a professional repair technician as your post suggests you might be, you must very well know ...

Although of course not all, many faults these days are indeed due to electrolytic caps with a poor ESR. They are the single component which I, and everybody else I know in the business, change more of per week, than any other.

That said, in this OP's case, the problem could easily be a leaky diode in the PSU, or something which may be a deal more difficult to pin down, such as a bad inter-board ground. The age of it would suggest a cap might be a firm first favourite though.

Arfa

Well, for a start, do as was suggested - and which you seemed to understand - and check each of the rails for ripple with your 'scope ...

Arfa

Make voltage measurements with your DMM. DC voltages first, then check for ripple on those DC voltage points. If, as you indicated, the voltage points are labeled, then start there... measure each voltage at its label. Check ripple by using the AC voltage function of your DMM. Beware that some DMMs are confused by a DC voltage, especially on the lowest voltage ranges. If so, temporarily connect a 1 uF capacitor (having a voltage rating above the voltages present in the organ) in series with one of the DMM leads to isolate the DC from the DMM. The DMM should now give you a reasonably correct AC voltage reading. The ripple voltage on a clean DC power supply point should be less than 0.100 VAC (ideally, less than 0.010 VAC). If one of the power supply points has much higher ripple voltage, then you can start looking at capacitor failure. Another cause of high ripple is rectifier failure, so you can't immediately blame the filter capacitors for high ripple. A bad regulator can cause high ripple, but you'll likely see a very wrong DC voltage as well.

If all are reasonably correct (+/- 5% unless you have more specific documentation), then you have to look elsewhere.

If all the power supply voltages seem to be OK, the next most likely cause of loud hum is the loss of a ground (shield) connection on an interconnecting wire. This will require that you visually inspect each wire connection where it goes off a board. If you find a broken wire, fix it and recheck to see if the hum is gone.

Those suggestions should keep you busy for a bit. When you have found something that seems out of reason and don't understand or know what to do next, then come back with your questions. I'm sure we can get your organ back in service.

Thanks Afra. I checked the rails and they look clean. The hum has a certain "spike up, down, noise, spike up, down, silence" pattern. I'm off to Jaycar now, to buy some tantalums, as it can't hurt, and I don't like aluminium electros if tantalums are available.

Where can I get a STK025 datasheet ? I find pages on Google saying such, but when I go there, it's just datasheets on STK0025 or STK0250. Even Sanyo's (mfg) site can't help me.

On Sat, 6 Sep 2008 18:39:59 -0700 (PDT), "Josh9.0" put finger to keyboard and composed:

My vrt databook lists the following:

STK025(G) = STK036: +/-30V, 20W (+22V/8 ohms) STK032 = STK036: +/-33V, 25W (+24V/8 ohms) STK036 audio amp +/-35V, 30W (+27V/8 ohms)

- Franc Zabkar

If the rails look clean, and the sound you are hearing has the described waveshape, then it is more likely that it is a buzz rather than a hum (subtle difference to the experienced ear) and not a power supply issue at all. It could be a grounding problem, but really, you need to know where the offending noise is actually getting in to the amplification chain. With items such as synths, where there are triangle and square wave generators running all the time, a simple bad ground can allow all sorts of nasty ground voltages to start appearing in amplification stages.

Try here for your data sheet

Arfa

OK, I could take a photo from the CRO, but how could I post it here ? I'm using Google. I could upload it to my site, and put a link, I suppose. Yes, a buzz it may be.

All grounding wires look intact.

If only I had another one of the same model, I could swap boards to determine where the problem is. Although, most are wired in place, it'd be a start, at least.

BTW: I replaced the caps next to one of the STK025's and it didn't help, but I'm glad I did, because these caps should last another 30 years or so.

Thanks, but I've been there b4 and all I find is other parts. EG: a 3 pin MOSFET.

Is there a sci.electronics.organ.repair or similar group?

"Josh9.0" wrote in news: snipped-for-privacy@a70g2000hsh.googlegroups.com:

Might want to try persuing some of the vintage synth sites such as the following for information etc: