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Can't balance DC on SX-880 after repairs

Mightyhal

Mightyhal

New Member
Following up on https://audiokarma.org/forums/index...-transistor-replacements-b705a-2sa1075.961503 -- I've had some ups and downs repairing the SX-880 (I'd shorted part of the power supply with a probe slip, and one of the output chips had shorted--one which had been replaced before, along with some other parts about 20 years ago).

I've also built a dim bulb tester which saved my bacon a couple times (fun fact: the schematic has the main output chips labelled backwards--NPN/PNP swapped). Along the way, I did manage to get some other chips swapped due to carelessness, and I smoked R257 and R258 (both still measure good out of the circuit, but I intend to replace them).

My problem now is that although it comes out of protection, I still have a DC offset that I can't adjust VR5/VR6 to get rid of (I bottom out at ~0.05 Ohm, and the P/Q jumpers are in place).

Here's what I'm getting for voltages (on the DBT, admittedly) - all in volts:
  • DC offset
    • Pin 5: -0.156 (should be 0, +/- 30mV)
    • Pin 12: -0.446 (should be 0, +/- 30mV)
  • Power outputs (B should be +/- 0.6V)
    • Q1: B = -0.685, C = -43, E = -0.157
    • Q3: B = 0.362, C = 43, E = -0.154
    • Q2: B = -0.965, C = -43, E = -0.443
    • Q4: B = 0.071, C = 43, E = 0.442
  • Drivers: (B should be +/- 1.2V)
    • Q15: B = 0.84, C = 42.56, E = 0.363
    • Q17: B = -1.27, C = -42.5, E = -0.692
    • Q16: B = 0.657, C = 42.57, E = 0.071
    • Q18: B = -1.55, C = -42.5, E = -0.968
  • Pre-drivers
    • Q11: B = -39.3, C = -1.27, E = -39.9
    • Q13: B = -38.8
    • Q12: B = -39.1, C = -1.55, E = -39.8
    • Q14: B= -38.6
  • Bias and idle current regulator
    • Q19: B = -0.005, C = 41.7, E = -0.499
    • Q20: B = -0.016, C = 41.7, E = -0.509
    • Q9: B = 40.6, C = 0.93, E = 41.2
    • Q10: B = 40.6. C = 0.65, E = 41.2
    • R249: 41.8 to 0.457
    • R250: 41.8 to 0.460

I'm thinking that my issue is either my Q19 and Q20 replacements on the DC balance circuit, or my Q5-Q8 replacements (see notes below, but I used ZTX694B/ZTX795B since i couldn't source KSC1845, and the HFe seems to be about 2x-3x the originals). Before I start pulling thing back apart (the 5 pin packages are annoying to swap)--any suggestions on next steps here to get things closer?

I replaced more parts than I probably should have. I also pulled and tested all the transistors, but I may have swapped L/R on some of the parts (Q17 and Q18, though it seems the OEM parts are well matched).

Here's what I changed (I also swapped in taller heat sinks with fins): (schematic designation, original part number, replacement part number):
  1. Transistors:
    • Q1, Q2 - 2SD745 - 2SC2992Y (I might swap the originals back in)
    • Q3, Q4 - SB705 - 2SA1216Y (Q4 had been replaced before, it had failed again)
    • Q5, Q6 - 2SA979 - ZTX795A pair (complements to the ZTX694B, replaced to match Q7, Q8)
    • Q7, Q8 - 2SC2291 - ZTX694B pair (could not source KSC1845, had to replace one which had a shattered casing)
    • Q9, Q10 - did not replace (may have swapped L/R)
    • Q11, Q12 - 2SC1904 - did not replace (may have swapped L/R)
    • Q13, Q14 - 2SC1775A - did not replace (may have swapped L/R)
    • Q15, Q16 - 2SC1913 - did not replace (may have swapped L/R)
    • Q17, Q18 - 2SA1913 - did not replace (may have swapped L/R)
    • Q20, Q19 - 2SC1175A - KSC2690AYS (about half the Hfe of the 2SC1775As I have above), Q19 had failed
    • Q21, Q22 - 2SK34 - did not touch
    • Q23 - 9SC945A - did not touch
    • Q24, Q25 - tested OK, kept
    • Q26 - 2SD712 - BUT11A (original was damaged when I pulled it, could not test)
    • Q27 - 2SB536 - MJE15031G - not sure I tested this one
    • Q28 - 9SC945A - ZTX694B with twisted pin, which I did before I received a new SC945C. Original had failed
    • Q29 - 2SA904 - tested OK, reused
    • Q30 - 2SD325 - KSD363RTU - failed and replaced
    • Q31 - 2SC1384 - tested OK, reused
    • Q32 - 2S850 - tested OK, reused
    • Q33, Q34 - 2SA904 - KSA1013YTA - these were actually missing from my board, which I think may be bad sign...
    • Q36 - 2SC1914 - tested OK, reused
    • Q37 - 2SC1885 - tested OK, reused
  2. Resistors:
    • VR5, VR6 - 150k-b - 200K variable
    • VR7, VR8 - 100-B - 100 Ohm variable (board layout diagram is wrong, says 100K!)
    • R413 - 2.4K 1W - 2.4K 2W 5% metal oxide (board was toasted, lifted it up off the board)
    • R275, R276, R281, R282 - 4.7K 1W- 4.7 ohms 2W 1% metal oxide (damaged R276 removing heatsink attached by glue(?))
  3. Diodes (only D28 was bad, though):
    • D1, D2 - 1S155 - 1N4448TR (amp section)
    • D3, D4 - STV4H - did not replace, tested good (~ 1.9 V drop when cold)
    • D5-D8 - 10E2 - UF4004-E3 (amp section)
    • D9-D13 - 2-1K261 - did not change (meter assembly)
    • D15-D16 - 1S2471 - 1N4448TR (protection circuit)
    • D17 - 1S155 - 1N4448TR (protection circuit)
    • D18-D19 - 1S155 - 1N4448TR (input selection)
    • D20-D23 - SR3AM-8 - 1N540RLG (power)
    • D24-D27 - 10E3 - UF4004 (power)
    • D28 - MZ-140 - TZX14C-TR (power, I'd blown this)
    • D29 - 1S155 - 1N4448TR (power)
    • D35 - 1S2471 - UF4004 (protection circuit)
  4. Capacitors: replaced all of the aluminum electrolytics and also 2 solid tantalums-none of those are in the amp section, so I will add the details later when I have some time to pull the details from my spreadsheet.
 
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just skimming through and its missing some positive voltage . i have trouble reading long posts but that's what i spotted for now .
might have some transistors backwards ..i will check the schematic shortly .
 
I'm fairly sure I have the transistors in the right way, working from the markings on the board (and matching my schematic).Though who knows, I might be staring right at the problem...I've done that before.

VR7/8 are both set at their minimum (about 0.5 Ohm), reducing the idle current (following the service manual, setting them to the lowest setting to adjust the DC balance). Increasing VR7/8 doesn't noticeably affect the DC balance; it does shift the values on the base of the pre-drivers (Q15-Q18) in the wrong direction though (sends them all more negative, when they mostly need to be more positive).

Q9 and Q10, best as I can tell, regulate the current to the base of the drivers. The values on the base (40.6V, spec: 40.3V) and emitter (41.2V, spec: 40.9) are about where they should be, but the collector voltage is low (Q9 is 0.93V, Q10 is 0.65, should be 1.2V). Meanwhile, the emitters on Q11 is -1.27V, Q12 -1.55V, when both should be -1.2V.
 
I might have fixed it….

First, since I had one good pair of original Q5 and Q6 (2SA979 and 2S2291) I put those in on one side, but that didn’t make a difference (which is good news, since I’m hoping my replacements are a close match).

Next, remembering that I had originally tried a KSC3503DSTU for Q19 and Q20 but my dc balance was much further off than with the higher Hfe KSC2690YAS, I decided to scrounge around to see what I had available that had a higher Hfe… I ended up trying a ZTX694B with a Hfe of 1230, and that worked! Luckily I had one more with a Hfe of 1090 (just a spot measurement at room temp)… and that put both channels in range for me to adjust the dc balance down to under 30mV.

The ZTX694B chips vary a LOT for Hfe, so I’m glad I had a couple dozen to sort through. I’m not sure that it’s a great replacement, though, since it’s only rated for 150mW power dissipation when the 2SC1775A sheet I found says the original would be rated for 300mW…. The question is whether 150mW is sufficient?

I’ll be doing some more testing tomorrow, it this is definitely encouraging!
 
The operation of the SX-880 Q19 & Q20 are "bias stealers" that are hopefully close matches for what's in the 2sa979 / 2sc2291 positions.

What directed you to use a ksc3503 or ksc2690 for Q19 or Q20?

IF they were ksa992 & ksc1845, I would recommend the ksc1845.
It is NOT a power or heat issue, you want them to be dissipating about what the other four are doing. So the base currents are close.
You do notice that the PNP input transistor's base is being balanced by an NPN transistor?
As heat affects the pnp, the npn connected to it is reacting in the opposite direction.

vr7 & vr8 are only for idle current, really shouldn't affect 0.000v output.

It will be interesting to see what the stack of R239 (&R240) @69k, Vr5 (&Vr6) @150k , and R241 (&R242) @ 130k does to this.
R241 is shorted, but the jumper can be cut if the adjustment needs MORE resistance to get the base near 0.000v
more resistance is less current and less power dissipation.

The hfe in the Zetex will vary by temperature DIFFERENTLY than the hfe in the c1845 - which will give you future headaches.

Basically, don't fool with that transistor, keep it like the others as the "standard" and adjust the resistance / current to get things squared away.
Get the pot to operate above and below (pos and neg) through it's range.
 
I don't have any c1845 transistors, Mouser/Digikey have them with delivery dates in 2022...

And I didn't have the original 2SC1775A to get a measure of what the Hfe would be--whichever source I built my list from had it from 400-1200, which is a fairly wide range. So I tried what I had on hand, not quite comprehending how Q19/20 interact with the 2sa979/2sc2291 pairs.

At the moment, I have the Zetex in for Q19/20, one pair of Zetex ZTX795A/694 one channel and then the original 2sa979/2291 pair on the other--though, I think I'd better swap in the Zetex for those for the thermal response as you point our (the Zetex Hfe are all over the map, so that won't be a ton of fun to match up). With the current setup, though, I can just balance the dc (about +12 mV on one side, and +2 mV on the other)--though the trim pot I have is a single turn and touchy. I' ordering a multiturn to replace that (I'd gotten the wrong part before--couldn't use it).
 
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