Dynaco ST120 weird issue

[Raccoon1400]

The original transistors are currently installed. I put them back in after finding out the new ones didn't fix the problem.
I'd tried a KSC945C in Q1.

 

[BSST]

I imagine you’ve already confirmed no drop across the output inductor.

It would be a tedious chore, but you could swap Q5 and Q6 across channels and see if the problem moves.

 

[rayma]

Signal does indeed clip at Q2 collector at same point it clips in the output. And in both places, as soon as it clips, the point of clipping gets pushed down more.

That's just the bootstrapping capacitor discharging, it's normal. You replaced that?

 

[Raccoon1400]

Which cap is that? I replaced all the electrolytics.

 

[rayma]

That's C3 in your schematic.

 

[Raccoon1400]

I suppose I'll try it with a new set of outputs and drivers and see what happens.

 

[Raccoon1400]

It was the outputs.
I used a pair of 2N3055s, I have a bunch of those. Didn't want to waste new onsemis on this thing.

 

[wg_ski]

So whose 2N3055’s were in there, and whose did you replace them with?

 

[Raccoon1400]

2N3055 is what I put in.

 

[wg_ski]

So what was in there before, presumably with very poor gain if one channel was falling short on power?

 

[rayma]

Were those eBay fakes, or from good vendors?

 

[Raccoon1400]

Here's what was in it.
The 3055s are RCA. Definitely look real. They are from a box of old parts someone threw at my feet.
Amp meets power spec in both channels now. (I changed the good channel too)

 

[rayma]

Here is the factory Dyna Stereo 120 TIP modification. Note the higher output beta specs.

Q1 replace 40233 (100-250 beta) with BC108A (130-180 beta)
Q2 replace 2N3053 (100-200 beta) with 2N5320 (160-260 beta)
Q3 replace 2N3053 with TIP31C
Q4 replace 2N4037 with TIP32C
Q5, Q6 replace 2N3055 (17-25 beta) with 2N3772 (60-90 beta @ 1A)
D1 replace with 1N4733
D2,3 replace with 1N4004

C1 replace with 5 µF 10V tantalum
Add C13 68 pF from Q2C to Q2B, both channels
Add C14 0.01 µF from Q3C to GND, both channels
Add C15 27 pF from Q2C to Q1E, both channels
Add C16 .01 µF across xfmr secondary at rectifier bridge on PC-15
Replace C6 (30 µF, NP) with .47 µF mylar
Add 3.3 Ohm ½ watt emitter resistor to Q4, both channels
Add 1 k Ohm 1.2 watt resistor across the red/black output jacks

 

[wg_ski]

Old RCAs are usually good. Even the hometaxial 3055’s. RCA was the king of the single diffused process. Those Je ne sais quoi things were probably running out of beta. I’ll bet it was less than 17 at the 5A you could be drawing with an 8 ohm driver. At 4 ohms all bets are off. It might be a TO-3, but might not even be a 15A part. A 5 amp part might have a beta of 5 or 10 at 5 amps. I’ve had MJ15015’s (high voltage 3055’s) fall to the single digits at 7A and they’re supposed to be 15A. The parallel bank in an old QSC I was rebuilding with them just fell apart and I gave up and stuck in 15024’s the 2nd time around. But they still met spec……

I’ve been tempted to get some of the RCA1B01’s that electronicsurplus.com still has. Just tempted, not that I really need any more 2N3055-oids. Which is why I haven’t pulled that particular trigger. I still have a whole box full of 2N3772’s that were house numbered for TRW. Yes, they’re good.

When doing “the TIP mod” it’s just as common to go to the MJ15003 these days. One could only dream about them when the ST120 came out.