Help: Accident with Speedball Testing (fried transistor) [resolved]

[JoshuaA]

My initial Crack build has been working great for a few weeks now. I felt I'd had enough of a sample to proceed with the Speedball upgrade. I had done the PCB boards a couple weeks ago, so it was an easy install. I was doing my final checks on the bigger board, with the negative probe on 12U.

I had checked OB, and it was showing 106V. Wasn't quite sure that was okay, so I glanced over to the manual, which said +/-10% is typical. So I continued and put the clip probe on OA, while glancing to check what the manual said about the OA voltage. Apparently, my unsteady hand (unsupported, because I'm using one arm only, and keeping a safe distance from the amp) made the clip probe slip off the solder point of OA, and the probe touched some(?) of the leg(s) of the TIP-50 transistor at Q2A. I'm pretty sure I read a warning about these transistors somewhere, and I thought I was being careful, but clearly not enough.

There was a quiet "pop", and the 2 red LEDs on that side no longer light up. The LEDs on the "B" side are still lit. I immediately powered off, in case it was going to cause further damage. I don't know whether I shorted the transistor legs, or if I just hit a single leg and ran some current through it straight to 12U (I didn't analyze the schematic so I don't know what effect that would have). I just know when I felt the heatsinks, the A side (blown side) was significantly warmer than the B side (good side).

I'm guessing this is clearly a dead TIP-50 transistor, but could this have also damaged other components (LEDs, tubes, etc.)? What's the best way to check? I was able to cleanly de-solder the 3K resistors, so I could probably revert the big board, and put those back in, I guess...

Thanks for any insight or suggestions.

[Edit 10/24/22] Thanks to Paul, Doc, and Eileen, I received my replacement parts in 3 days, and have a fully functional Crack + Speedball working on my desk! Appreciate all of you guys!

[Paul Birkeland]

I would measure the resistors on that side of the board to be sure they measure OK.  You can then set your meter to beep for continuity, then check each transistor looking for pairs of legs that are shorted together.  I suspect a new TIP50 and a new 2N2222 will get that board back up and running.

[JoshuaA]

All resistors on the PCBs have their designated resistance values.

Q1A:
1 to 2: 0.384k
2 to 3: 2.1k
1 to 3: 0.96k

Q2A:
1 to 2: 3-5M Ohm
2 to 3: wildly varies, 0.2k-3M Ohms
1 to 3: 50k-200k Ohms

Q1B:
1 to 2: 7.93M
2 to 3: 5M
1 to 3: 4.93M

Q2B:
1 to 2: 11.2k
2 to 3: OL (~3M when reversing probe polarity?)
1 to 3: 5.24M

A lot of these numbers fluctuate a lot, reversing the polarity of the probes produces different numbers, and a continuity mode produces no beeps. Despite being a licensed ham and having put together a few radios in my time, I've actually never tried measuring a bad transistor before, so I have no idea what's normal here.

Where can I go from here? Thanks.

[Doc B.]

Check instruction #2 under Speedball

https://forum.bottlehead.com/index.php?topic=4295.msg60044#msg60044

[JoshuaA]

Thanks, Doc, for pointing out the FAQ. I'd known I read something about shorting transistors somewhere, and spent a good amount of time re-reading both manuals and not finding it. Figures it was on the forums.

I'm guessing the two "A" side transistors that have 200-300Ω readings are the ones that are bad? Not <100Ω like you said, but those are the only two low readings I get <1kΩ, and are consistent with the side that blew.

How do I go about getting replacements? Is it possible to order them from your company? Thanks.

[Doc B.]

Transistors measure similar to diodes in that they are one way current flow devices. So if you orient the meter probes measuring across two legs of a good transistor, in one direction you will get a very high resistance reading often up into the megohms and if you swap the test leads you will get a lower reading that is typically in the 500ohms to 2K ohms range in the conductive direction. When a transistor blows in most often shorts across the junction, and thus you get a very low reading, like 200 ohms or less. So yes, the low reading you see is likely to be a short. And the way to double check is to swap the leads to see if the low resistance remains the same regardless of the test lead orientation, in other words the short is not directional like the working junction would be.

You can order replacement transistors from Eileen at [email protected]

[JoshuaA]

Great, thanks. That more or less confirms what I thought I had learned decades back. I'll email her, and hopefully I can get it figured out. Thanks again for your help.

[JoshuaA]

Thanks to Paul, Doc, and Eileen, I received my replacement parts in 3 days, and have a fully functional Crack + Speedball working on my desk!

Eileen was kind enough to suggest adding the LEDs. I figured LEDs can't possibly cost much, so I agreed, thinking it'd be better safe than sorry. Well, that "sorry" was thankfully averted thanks to her suggestion. I measured the LEDs on the "A" side, and sure enough, they let through a modest amount on both either polarity. The "B" side LEDs were thankfully intact and measured fine.

Replacing parts went well enough. Resistance checks were all good. Voltage checks were all good (AND took great care to not slip this time). Tested in the garage with a cheap DAP and $5 generic headphones that were collecting dust on a shelf, and I had sound! Tapped it, shook it, and a gentle whack later, nothing seems loose or questionable. Took it inside, and listening on some (cheaper) 300Ω headphones (HD6xx) for safety's sake for the evening. If all goes well, then I can plug my ZMF Atriums back in tomorrow. Hopefully, it's not going to blow up on me!

Sound is noticeably clearer on the HD6xx, so I hope it'll be even more enjoyable on the Atrium. Thanks again for your help troubleshooting. Appreciate all of you guys!