Speedball issues [resolved]

[Paul Birkeland]

Can you give me all of your voltages on all of your boards? 

[Droo]

OB:82
1B:278
BAB:0
OA:89
1A:278

This is with the big board removed.
The big board voltages are on the previous page.

[Paul Birkeland]

I suspect you either have a very poorly soldered TIP50, or a toasted transistor.

Bad transistors will show very low DC resistance between pairs of pins, so they are easy to check with your meter.

[Droo]

Yes, I found that one is fried. Where would you recommend going to replace it?

[Paul Birkeland]

Mouser usually has them all in stock.

-PB

[Droo]

I replaced the transistor, but the LEDs on the same side of the board still won't light up.

[Paul Birkeland]

You can use your meter's diode check function to check the LEDs as well.

[Droo]

Good call, I swapped out the LEDs and now they light. The right channel still turns on first though (immediately after plugging in). I get

250 on B, 130 on OA, 150 on OB and 0 on G.

Oddly enough, I get 0.6 ohms across both resistors in the headphone jack. Might that have something to do with it?

I've tested all the other circuit elements, and everything else seems to be working properly.

[Paul Birkeland]

There's either a miswire from the board to the circuit, or there's still a toasted part in your kit. 

The resistors on the headphone jack will read 0 ohms with nothing plugged into the jack; that's normal.

[Droo]

I'm almost absolutely positive that there's no midwife. How do I proceed from here?

[Droo]

*miswire lol.

[Paul Birkeland]

Post some photos of the top and bottom sides of the PC board, as well as the build as it sits now.

Also check the resistance of the R1 resistors on the big C4S board.  You can measure their values in circuit.

[Droo]

All joints https://imgur.com/a/tEgLPw1.

R1 resistances are both 32 ohms, and the resistances of every single other resistor is within 10% of what it should be (except the ones in the headphone jack). Base to collector and base to emitter are between 655 and 740 on every single transistor, probing the joints from the underside of the circuit boards. Diodes are all around 500-600 on the transformer.

All 5 capacitors charge (20KOhm setting used to check). RCA resistances are 92 and 99 KOhms (L and R respectively). Tubes glow on both sides, and neither of them seem particularly bright or hot.

I know one of the TIP50 transistors looks a bit janky and seems like it has a poor mechanical connection, but the leads won't fit in the cutouts, and the resistances from the bottom seem about right. How likely is it that the tubes or the transformer have gone bad? Thanks again for your patience.

[Deke609]

Droo -

If that were my amp I'd do three things before trying to troubleshoot any resistance or voltage issues:

(1) Reflow every solder joint in the amp by (a) placing the soldering tip so it makes good contact with both the lead and the terminal (i.e., both surfaces to be soldered), and then (b) waiting until the solder suddenly flows like water around, and gets sucked into, the joint. From your pics there look to be a fair number of sketchy joints - too many to circle or list. Even if they're working now, I'd be concerned that they'd fail in the future after repeated heating and cooling from use. PB has said he can reflow a Crack in 30-45 mins. So I figure an ordinary human can do it in under 2 hours.  Even if this doesn't solve your issue, it will help ensure that the amp, once you get it working, stays working for a long time.

(2) Fix the diodes.  You've got all 4 on the same side of the terminal strip. That's a short waiting to happen - either by the leads of two diode making contact or a diode grounding out to the chassis. I'd reinstall them as per the manual or at least insulate them from each other and the chassis (but the latter fix will take more time that simply reinstalling them two-a-side.)

(3) Fix the small 270K resistor in the power supply near the big white 5W 270R resistor.  This also looks a short to chassis waiting to happen.

I figure you could do all of the above in less than 3 hours.  And who knows, maybe that will solve your problem. And if not, it will make tracking it down a whole lot easier.

cheers and good luck, Derek

[grufti]

Most of your work looks good to me. Derek mentions fixing the diodes. I wouldn't necessarily do that. Unsoldering them will be very difficult and putting two back in on the other side will be even more difficult. You could slip some electrical tape between them for a minimal level of protection.

Many of your wires still need clipping with a good sidecutter. Those are about $20 with shipping and tax. It's going to be difficult to get to some of the wire ends with everything assembled. You would have been better off clipping them sooner.

I'm with PaulB when he suspects another toasted part. Tubes and transformer are almost certainly still good. They are robust.