Roland KC-350 Preamp Problem

[The Dude]

^^^^^^ Agreed and I'll add: If you use something other than the original part, make sure the basing diagram is the same or reorient the part if necessary.

FWIW: The NTE cross is NTE2571.

[Enzo]

I have never seen a bipolar transistor in a TO220 that wasn't BCE across the front. But always good to check.

[The Dude]

Agreed in this case, Enzo. I was making more of a generalization for those who might use this information in other circumstances (particularly with TO92's).

[RWood]

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)

Well you are right.... on my fifth stop I talked to a very nice guy at Metro Sound in downtown Richmond. He came out with a bunch of baggies from Mouser and one of them had a Fairchild TIP147T. In fact it was the only PNP that he had in a TO220 case. The specs look like they exceed what I need here. Good to go?

[Jazz P Bass]

The original transistor, 2SB1642, appears to be a plastic enclosed package.
As well as it is a simple power transistor.

How is the transistor mounted on the printed circuit board?
If it stands up in the air, the TIP147T should be o/k in that respect.

If the original transistor is mounted to a heatsink, then you will need to isolate it with a mica strip & a plastic washer.

My only beef with the TIP147T as a replacement is that it is a 'darlington' type wheras the 2SB1642 is not.

2SB1642.pdf

Not too sure if this is an issue.

The datasheet for the TIP shows 0.5 A of Ib (continuous base current) where the 2SB1642 indicates that it can handle 1 A.

[Leo_Gnardo]

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.

Some shops will, some won't, glad our OP found a cooperative tech.

I tried that around here 35 years ago, needed a TL074, local shop would NOT sell me one even though I offered $10 for a $1 part. "Oh no, we have to look at the piece, that's $80 minimum." It was at that moment I got a lot more motivated to find vendors that would not give me a line of BS and just sell me the part. Now they're long since out of business, and I'm as busy as could be, so a left-handed thank you to the bozo who wouldn't take my money back in 1980. That's when I found Mouser and a couple other outfits, it's been smooth runnings ever since.

[nsubulysses]

Don't let that bozo get you down

[Leo_Gnardo]

My only beef with the TIP147T as a replacement is that it is a 'darlington' type wheras the 2SB1642 is not.

Not too sure if this is an issue.

Oh whoops, it IS an issue, you can't swap darlingtons for single transistors & verse visa. Something's likely to go bang, or just plain not work at all. Better search for a more appropriate substitute.

- - - - - -

nsubulysses, thanks & I didn't let bozo get me down for long. There's only a couple things that can be done dealing with such an attitude. You get discouraged and give up, or you get discouraged and find a way around the obstacle and resolve to *try* and treat other people better than that. And what happened to that bozo and his shop, I'll quote the 2000 year old man "That's why I'm here and they . . . not." Justification enough.

[Jazz P Bass]

A TIP42C will sub the 2SB1642 very well.

Although it does have the metal collector tab.

https://www.fairchildsemi.com/datasheets/TI/TIP42.pdf

[DrGonz78]

The KC350 that I worked on had those particular transistors mounted upright and not attached to a heat sink. The one I worked on might have been a bit older perhaps so I am curious to know if they have them installed the same.

[Enzo]

The tab will be OK, it just sticks into the air.

Glad you found a helpful guy, they are around even if everyone is not one. But TIP147 is not a good sub, TIP42C is. The reason being the darlington thing mentioned above.

[RWood]

Yep; I will place an order. I did try the TIP147 and it passed negative voltage on the emitter but it didn't take much voltage to realize there were problems. Noises from the speaker, faint whiff of something heating up on the preamp board. I'll get the right part and report back.

Thanks, everybody for the help.

[52 Bill]

Yep; I will place an order. I did try the TIP147 and it passed negative voltage on the emitter but it didn't take much voltage to realize there were problems. Noises from the speaker, faint whiff of something heating up on the preamp board.

Those might be the signs of the problem that took out the original regulator circuit.

[Jazz P Bass]

For sure the smoke is telling you that there is an underlying problem that took out the original parts.

The downstream (of the -15V rail) path should be checked with an ohmmeter to see how far it sits from ground.

My feeling is that it is not that far, hense the high current draw (smoke).

I would also recommend that the speaker be disconnected until you sort things out.

[RWood]

Good advice and yes, with the cable disconnected at CN13 on the pre-board, the -15 pins measure only 115Ω from ground (compared to the +15 pins that are upwards of 3Meg from ground.)

So......how do I go about finding where its got this near-short to ground? Any likely suspects? The only thing I can think to do is to systematically remove the power supply jumpers along the -15 line, starting from the end of the line. Then see where it gets back to normal and find out what is on that part of the line. What would you guys do?