2226B Rebuild Help

[OakBayou]

First, let me thank everyone who participates here past and present. This forum is the primary thing that got me into vintage gear, and built my knowledge and confidence to try to conserve the Marantz magic for future generations.

I've gotten myself into a pickle with this unit and hoping I can get some help and advice. I'll write up some backstory in another post, but for now, I'm trying to find a short or bad device that is causing me grief.

While testing with a scope and the heatsinks off and volume up to near clipping running on a dummy load--probably not my best idea--the unit blew the main fuse.

After replacing the fuse and starting on a dim bulb, I get nothing. Measurements of various transistors, diodes, and some resistors like R776 (100Kohm) on the power supply seem to fluctuate all over the place, off power and on the dim bulb. I had built a snubber and had it installed across the main power wires on the new switch, but then realized that there was already a snubber in the back of the unit across the two power outlets, so I removed it. Tested continuity from mains in to switch, and from bridge rectifier to power supply. Looks good.

I decided to at least finish recapping, so got that done. I "practiced" soldering on this thing years ago, using crappy Radio Shack lead-free solder. Two traces had lifted due to higher heat with a cheap iron, and I noticed a cold and broken joint. I have better tools now. I removed all the lead-free with wick, carefully scrapped away some solder mask with a razor, and soldered the new caps in here with the leads bent down onto the freshly exposed copper trace. The connections look very good to me.

Next I pulled out the main filter caps and tested the bridge rectifier. Looks good. Tested Q750, the large power supply transistor (2sd 330). The measurements off-power, in-circuit looked bad, but I pulled it out and it seems to test fine, about a half volt drop.

The lamp dial has a different supply from the transformer that passes through a fuse and a diode, so I started to wonder why at least the dial didn't power on. I tested Q749 (WO6B), off-power in-circuit it looked bad with only reverse readings of 0.96v and OL forward biased. Out of circuit, it tests fine at 0.556v forward. Is this even possible? Is my DMM bad?

I haven't tested output transistors yet, but I've already got replacements for them (the infamous batwing 2SB617 and 2SD587).

OK, where should I go from here? After the diode test, I'm a bit stumped.

 

[OakBayou]

A bit of backstory:

I've been working on this receiver on and off for a few years now. I originally salvaged it while helping to move my aunt's stuff out of her apartment which was destroyed by Hurricane Katrina in '04. She had no idea if it still worked, but she bought it new in 78 or 79. I thought it looked cool and liked the way the tuner dial felt. I kept it in the hopes that one day I could have it fixed up. It was in sub-optimal storage in non-climate controlled environment in southern Louisiana for 9 years (bad move #1).

I brought it back from the dead in 2014 when I moved to Wisconsin with some LED upgrades and a few key capacitors (in hindsight, it was probably glue that was worrying me and not leaking electrolytes). I had never done any electronic work before this, and have no background in electronics. I have a liberal arts degree, my dad is an auto mechanic. I've tinkered, but it was this site that led me to try my hand at restoring the Marantz.
This unit was my primary stereo system until a year or two ago, when I started to notice some distortion in the right channel. This inspired me to attempt a full recap, and also started my journey with learning schematics and using an oscilloscope.

I traced the distortion to a driver transistor, Q702 (2SA 750). My plan was to recap the entire unit with Elna Silmic II's and WIMA's, clean and replace thermal grease on outputs and diodes, and replace the faulty transistor, as well as several others in the main amp board. I worked backwards, doing the phono/selector board, tone board, and then started on the PS/main amp board (it is integrated on this unit). Then I got busy and put everything away and told myself that circuit work was going to be my winter projects, without reassembling the parts of the unit (bad move #2).

This winter came around, and I pulled everything out again. One day, we had some family over (our household all got COVID in November so concerns about infection weren't an issue), and I thought it might be fun to show some of them how to use a scope to trace a test signal.

I had the unit powered on, with the volume up pretty high and output on a 8ohm dummy load. I was looking at the signal from the tone board, and comparing it to the output, to practice my "demonstration," but I had not paid attention to the fact that I removed the heatsink fins and possibly had some varistors screwed down to it, too (really bad move #3). I'm guessing I blew the output transistors and quite possibly some other components.

 

[bobdod]

Those outputs get hot

I suspect they did not survive. What are you planning to replace them with? Have you tested all of the diodes in the driver circuit out of circuit?

 

[OakBayou]

Replacing outputs with MJE15030G and MJE15031G. I haven't tested the diodes out of circuit yet, that's next on the plan.

Could someone explain how/why a diode would test OL forward-biased and full-volt reverse in circuit, and out of circuit test OK? I thought diodes failed open or short, with OL in both directions or similar readings in both directions.

 

[bobdod]

Replacing outputs with MJE15030G and MJE15031G.

Consider wattage of the original vs proposed.

 

[OakBayou]

100v 6A vs 150v 8A. Not perfect but the originals are long obsolete. I got recommendations on replacements from this site.

 

[bobdod]

There are more appropriate options. Those original Batwings are 80W I think. Your suggested replacements are 50W.

 

[OakBayou]

I'll do some more digging, but I think this may be the closest fit left in stock on DigiKey.