Roland KC-350 Preamp Problem

[RWood]

This is the smaller of the KC550/KC350 keyboard amp line. It came to me from a friend who reported that it worked fine one night, next night turned it on and it did nothing - no pilot light.

I did some investigation and saw that the two fuses, 500mA, on the low voltage side were blown. Replaced them and tried the amp on a dim bulb and although the bulb didn't light up noticeably, the two fuses again blew right away. The main fuse, 4A, was and is intact.

Then I tried it on the variac with more new fuses and by going slowly I was able to get the amp up to voltage, +24v from Bridge Diode 2, enough to check some other things out. The pilot light works off this lower voltage rail, so it lit up. The speaker has a trace of a hum, so it seems the power amp might be okay. The speaker thumps when I press the contour switch in or out. I hear a trace of pot noise when I raise and lower the input level pots.

However, I get nothing at all from any signal going into any of the four channels. Total silence, although as I said I can hear trace of pot noise regardless of anything plugged in. I checked the connectors from the input board to the preamp board and don't see anything amiss. No bent tines on any of the connectors. No visibly damaged parts on any of the boards.

What would you recommend I check next? In the service manual I read about a Mute circuit but I don't see any Mute buttons so not sure what's going on there. Also no effects send /receive as far as I can tell.

I'm at a bit of a loss. Most of what I do is with tubes, and not circuit boards. I'll attach the schem for the KC350

Thanks,

RWood

[The Dude]

Check your + & - 15V rails at Q1 and Q2.

[RWood]

Thanks, Duder.

The + rail looks good on Q1 but the - rail Q2 not good at all. Only about -.5vdc on the emitter.

Obviously a problem but more of a symptom or a cause?

[DrGonz78]

Well it could be a faulty Q2 for starters. However, I would suspect C9, C11 or even D2 zener diode. Time to test some of those parts to see if they are the true suspects.

[Enzo]

Yes. Id check them all, but the zener would be first on my list. However it is blowing the fuses, so the load may be the issue.

Mutes don't blow fuses. You need to get power supply straight before anything else has any significance. Yes, mutes can cause loss of signal. They are just there as power up mutes, they mute the amp for a brief moment at power up, to prevent big speaker thumps.

The 9 pin connector on the power supply connects over to the preamp board. Unplug that connector, now does it still want to blow fuses?

[RWood]

With the 9-pin disconnected there is -0.015 vdc on D2's anode (0 on it's cathode). I presume the anode should read -15v. Replace?

I did not check C9 or C11 but if I need to flip the board to replace the zener I will just replace them.

Could renewing these parts bring Q2 back into action?

Thanks for the info about mute - I get it now.

[DrGonz78]

Sounding like a dead zener to me. Test it both directions with a diode checker if you have that on your meter. If not test it with a good ole fashion resistance test.

[RWood]

Yes, D2 was shorted. I replaced it along with C9 and C11 but there still seems to be a problem with Q2.

On Q2 I am seeing the -24v and the -15 from the zener but not the slight voltage drop from the -15 on the emitter. 0 volts there.

Replace Q2?

[The Dude]

Is the transistor shorted or open? Check it. If it's bad replace it.

Edit: That wasn't meant to sound condescending. I have no idea what your skill level is. The point is, it's normally quicker to check suspect parts rather than just start replacing things.

[RWood]

I thought I was checking it, in circuit.

Are you suggesting I pull it out test it?

[The Dude]

Q2 is a PNP transistor. Does your meter have a diode check function? If so put the negative meter lead to the base of the transistor and positive lead to first collector, then base (power off, of course). Each should read the same as a diode junction- about .7 or so volts. In this case, you can test with the transistor in circuit. Normally, you would also test collector to emitter and make sure that no shorts exist. I'm guessing that the base/emitter junction is open from the voltages you posted, but it's easy enough to check to be sure.

[Enzo]

I test parts in circuit first, but if there is a doubt, I pull it and test it by itself.

[RWood]

Thanks.

Ok I pulled it, tested OL on all combinations.

Toshiba SB1642 ... of course it's not available except from a couple sellers on eBay.

[The Dude]

MCM has them in stock here:

Toshiba Transistor To 60V 4A 2W BCE | 2SB6142 | Distributed By MCM

[Enzo]

I am sure someone in Richmond has either it or something like it. A plain old very common TIP42A would replace it just fine. (or TIP42B, TIP42C)

There is nothing special about this part. Meet its specs with most any PNP TO220 type, and there you are. It isn't in your signal path, it is just a pass transistor in a voltage regulator.

And much as I dislike NTE stuff, I bet someone sells that line in Richmond, and it will have a sub for the part. If you order it from somewhere, you pay shipping, so if you pay an extra couple bucks more for it locally, you are still ahead.

In fact amp repair shops or TV repair shops ought to have similar parts. Walk up to any competent electronics tech and ask if he has a 60v 4A PNP TO220 he would sell you, and I bet he does.