4270 - P800 P/S: No 35V or 14V DC post rebuild...

[jd3rdcoast]

HI all, hoping can get some direction on this power supply issue that I'm not getting DC voltage out of after rebuilding. Stated problem from my collector buddy: "was playing a record at low volume, it ended and he was not in room for for 30 minutes when he heard a pop in speakers. Ran into room and smoke was coming from top of 4270. Immediate powered down". So he brought it to me.

Inspection: I did not see any seriously burnt components or strong smell. Unit did power up on DBT 100W bulb very dim lite and Relay closed. So drawing some current. R808 (150/5W cement) measured 150 ohms but is visibly dark on end and very hot with burnt heatshrink on legs. 1A Fuse intact. Also lot of old glue from caps in that area.


Started checking caps (most looked pretty stressed) and found C801 (very high), C805 (Bad), C816 (Bad), C808 (corroded leads).
H806 calls for a 2SC945 but a 2SC1318 was in it's place which is same as H807. (Protection circuit)
C806 (33/50) has a 22/63 which I have read here is factory change. There is also a 100/35 cap between J814 and ground not seen in schematic that looks factory installed with lug board screw.
Non of the BJT's or Diodes tested bad.

Since the board is difficult to work on (requiring de-soldering wire-wound pins and H801/H804 from their pins to flip it) I decided to replace all Caps, Diodes and transistors with the following gleaned from AK threads.
R808 150/5W >>> Vishay 150/5W/5% CP0005150ROJE14.
Caps are all Nichicon UPW equivalents with a bump in voltage if was available. The 100/35 violet cap on J814 I used a 100/35 UKA.
BJT's
H801, H804 - 2SD331 >>> MJE15030G w/new mica insulator to heatsink. (BCE)
H802, 803, - 805 2SC945 >>> KSC945CYTA (ECB)
H806, 807 - 2SC1318 >>> KSC2383YTA (ECB)
Diodes
H808-H811 SIB01-02 >>> 1N4004 standard rectifiers
Zener H816 WZ-071 >>> 1N5236B 7.5v Zener
Zener H817 WZ-140 >>> 1N5244B 14V Zener
This board has a Bridge Rectifier separate from the P800 board that feeds the amps and is working.

After getting it all back together it powered up on DBT with no problem and no excessive current draw. Lamps are lit. Amps have decent Bias 7-16 mV and DC Offset on left side is below 10mV Right Side is -13mV and -70mV. Yet to be adjusted.

But, as usual for me, something is not right and I am not getting any DC voltage from the P800 board. 35V is zero as well as 14V for FM section. I have gone back and checked all caps, diodes and tranny's are in proper location and original orientation. I made notes and pictures of all the wire locations so pretty sure got it back as it was. But most likely something I got wrong.

Here is where it gets weird:
I am getting 75 Vac from transformer to the J801/J802 (feeding H808/H809) these are jumped to J806/J805 to feed H810/H811 diodes since the bridge rectifier is not on this board. What is strange is I am also getting -50 Vdc riding on the same AC points. I do not get any DC voltage on the cathode ends of either sets of the 1N4004 rectifiers or from the zeners.
VR-807 is good but has no effect.
H801 & H804 Collectors have -86mV DC.
Fuse F801 if good 1A/250V.

Any thoughts on next steps to isolate this anomaly would be appreciated. Wondering if the MJE15030 was not enough gain to operate properly as replacement for 2SD331? Should I put them back in?

Pics: Original R808 cooked Cap orientation

finished P800


P800 schematic:

 

[jd3rdcoast]

Update: Took yesterday off for the 4th and to ponder. My guess was a wire connected wrong. I pulled the P800 to have a look under and found the ground wire from main caps had broke loose during re-install and was hiding loose under the board. Lucky it didn't contact something and POOF!. Got a chuckle out of that as lately most issues I run across immediately after a repair are of my own doing or oversight even though I try to take meticulous notes and pictures. Used jumper wires to hook back up and test before proceeding (making sure nothing making contact with board lifted up) and immediately got my 35V back and dialed in with Relays closing (although very quick - 2sec). Will re-seat the board and go back to dial in the offsets and bias which were close already. Fingers crossed.

 

[jd3rdcoast]

All Bias and DC dialed in very solid after treating variable resistors on amp boards...
Except Right Rear DC is unstable jumping around 0.5-1.0 volt intermittently but consistent.
Here is pic of scope with RR on top and LR on bottom.


I’ll try to isolate to see where this drops into the circuit. Suspecting perhaps pre-drivers. Edit: I mean Input Differential Pairs H701/H703.

 

[jd3rdcoast]

Pulled the H702/H704 (2SA763) for Right Rear differential pair. They tested good. Hfe was mismatched slightly 184/190. Enough to cause flaky DC? I don’t know and don’t have curve tracer to test under load. I changed them out with a matched pair of KSA992FBU. Left Rear was very well matched but changed them while had the board out. Checked all diodes and caps measured within spec on board, outputs tested good so put the amp back together. Tested and DC is now stable as well as Bias. System sounded great in 4 ch and 2 ch modes.

Im sure there’s other issues lurking within but owner not ready to spring for full service. I’ll probably get to have more fun with this one in the future.

 

[rBuckner]

The gain mismatch wouldn't cause the noise - the old fellas just went noisy on ya. That won't show up on a tester usually.

 

[jd3rdcoast]

The gain mismatch wouldn't cause the noise - the old fellas just went noisy on ya. That won't show up on a tester usually.

Thanks Randy for the info.
I also had Stereo light not coming on. Checked the lamp and someone had put a non bi-pin bulb in rather poorly I might add. It showed good via ohmmeter but no voltage reading at bulb. H804 putting out correct 8.3V. Pulled bulb and get 13.3 to pin. So put in correct 12v, 40ma Bi-Pin and viola! This also seemed to get rid of small pop at power off on Rt. Ch. strange. Anyway. All is well.
Pic of offending stereo bulb which was still good.

 

[rBuckner]

^^^ Good detective work jd!