Technics SA-828 Receiver from 70s- Is it worth trying to save this aging patient?

[Iceman from McMurdo]

Uh-oh... We have a MAJOR setback!

Just powered it up to get the voltage readings you requested and got an UGLY surprise - complete with smoke and crackling noises! Don't know why the fuse didn't blow (it's still intact), but something in the circuit REALLY wasn't happy.

Couldn't see exactly what it was because I was on the other side of the unit, but it appeared to come from the center power section. After unplugging, I found the pictured debris which was apparently ejected during the power-up (photo #1 below).



Upon closer inspection with a magnifying glass, I finally found the unhappy component. Q627 appears to have overheated & split in half (photo #2 below).



So what do you think caused that? I know we checked that component previously, and it appeared to give us correct readings.

Hmmm, perhaps we have other issues downstream in the circuit that still require correction??

 

[dorke]

Well,
Things like that can happen.

Take out Q627 and measure it(diode test mode).

Measure in-circuit (diode test mode)
Q629.Q631,Q635 and D615.

 

[Iceman from McMurdo]

OK, thanks, will do. Probably can't get to it until later this afternoon or evening - my CEO (wife) has other more pressing chores for me right now. Will report back as soon as I have results. Think I should re-test new bat-wing transistors (in circuit) as well, to assess whether or not they were damaged during the conflagration??

 

[dorke]

Yes,
Q635 is the relevant bat-wing.

 

[Iceman from McMurdo]

Just checked the Q635 Bat-Wing (in-circuit using diode mode) - VERY different readings from before so think it's probably toasted:

Q635:
B-C= .003 (Red probe to base)
B-E= OL
C-E= OL
B-C= .003
B-E= OL
C-E= .003

Other new Bat-wing readings appear more normal, so it may have survived the ordeal...

Will get around to testing the others you requested later tonight...

 

[dorke]

Yep,
Q635 is a goner,I would expect Q631 to be one as well.

 

[Iceman from McMurdo]

Any theories as to why? Think we tested all those previously and they were good. Is there likely another culprit farther down into the circuit we didn't get to yet?

I've got another new replacement bat-wing already on hand once we're ready to start replacing things (and have a source for more if/when needed), but can you help advise what specs I'll be looking for when searching for new replacements for Q627 and Q631 (et al) and where I might try sourcing them from?

 

[dorke]

I don't know why ,I sure hope those bat-wins you got are not Chinese fakes.
We should find all blown parts before we go replacing anything again.

 

[Iceman from McMurdo]

Well, hate to admit it, but yes, they were from he China-man.

The source was recommended to me by someone else & the seller had good reviews, so I rolled the dice... Have since found a few other sources I can try (some U.S., some Hong Kong), as well as a completely different looking transistor that some say may be a suitable substitute for the original bat-wings (see link below):

http://www.ebay.com/itm/2SC4029-2SA...rk=2&rkt=6&sd=160965253716&afsrc=1&rmvSB=true

Can you take a look and opine as to suitability? Appears they are made by same OEM vendor (Toshiba) and the source is based in the USA. Whaddya think??

 

[dorke]

These are completely different case styles will not fit physically.
It may be,just may be ,the Trs are fakes.

I would concentrate on finding all faulty devices before buying anything else.

 

[Iceman from McMurdo]

Yes, I considered that too, but have had relatively good luck with other electronic purchases from the China-man. Had a lot of difficulty finding ANY sources for the old bat-wing style transistors, so figured I'd give 'em a try. Thought about the different ones mentioned above, but didn't see how they'd mount on the heatsink given the different case style.

If the ones from the China-man were, indeed, "fakes," do you suppose that's what caused the subsequent damage to the other components? If so, lesson learned... I'll go with alternate (higher priced) sources I've since located elsewhere.

That said, I agree it makes sense to put ordering anything on standby for now until we learn the extent of the damage and develop a complete parts list.

PS - Might take me awhile to remove the burned out Q627, as that's the one that was mounted underneath that frame rail. Will have to do some major surgery to the chassis in order to gain access for desoldering.

 

[dorke]

Please double check that you have mounted Q631,Q633,Q635,Q637 the correct parts.

Please Remove Q635 from the board,
and measure ,all in board,in diode test mode:
Q621,Q623,Q627,Q629,Q631.
D615 ,D617.

 

[Sunnysky]

After repairs before inserting Batwing, note the caution symbols on Collectors of Q631,633. The current thru the collector R's must be low with the 2 POT's before installing the batwing next time. I.e. voltage drop equiv of 10-20mA

 

[dorke]

The warning symbols have nothing to do with current adjustment.
Only to use same type and rating of resistors((non-flammable etc.)

Although it is a very good idea to start with drive at minimum,we should do that next time.

 

[Sunnysky]

Yes the resistors survived without flames but the other details were overlooked.

 

[Iceman from McMurdo]

OK, finally got around to performing requested tests (kinda bummed by the setback)... Have NOT yet removed damaged Q627, but DID remove both offending China-man power transistors (Q635 & Q637). Upon removal of the heatsink these parts are mounted on, I was able to see additional physical damage to Q621 & Q625 (tops appeared blown off - see photo below).



Went ahead & tested following components (in-circuit using diode mode), as requested. (Note that many values did not immediately stabilize, so I held probes in place for awhile until the rise in value slowed dramatically and/or stopped). Here's what we're working with at this point (as usual, first 3 readings are red probe to base, second 3 are black probe to base)...

Q621
B-C= .730
B-E=.247
C-E= .795
B-C= .740
B-E= .247
C-E= .247

Q623
B-C= .381
B-E= .553
C-E= .631
B-C= .363
B-E= .770
C-E= .863

Q625
B-C= .295
B-E= .307
C-E= .017
B-C= .000
B-E= .318
C-E= .018

Q627
B-C= .921
B-E= .995
C-E= OL
B-C= .860
B-E= .906
C-E= .737

Q629
B-C= .002
B-E= .006
C-E= .005
B-C= .002
B-E= .006
C-E= .005

Q631
B-C= .570
B-E= .575
C-E= .593
B-C= OL
B-E= OL
C-E= OL

Q633
B-C= OL
B-E= OL
C-E= OL
B-C= .540
B-E= OL
C-E= OL

D615
.246 / OL

D617
.242 / OL

Appears we have multiple casualties, eh?

 

[dorke]

Yes,
A chain of destruction that probably started with the breakdown of the batwings and penetrated inwards.
You need to take out Q621,Q623,Q625,Q627,Q629,Q631,Q633,Q635,Q637

After that please measure, in board, Q613,Q615,Q617,Q619
and D603,D605,D607,D609,D611,D613 .

 

[Iceman from McMurdo]

OK, will do. Might take me awhile, as the wife has made me relocate my repair operation from her dining room table out to the garage. Gotta get things setup out there on the workbench & figure out best way to access all the necessary pins.

 

[dorke]

About batwings replacements.

I did find the 2SA1215-ND and 2SC2921-ND both from Sanken, they are the same case style.
If you buy them from digi-key you will not get fakes,it will be more expansive,but worth it though.

http://www.digikey.com/product-search/en?keywords=2SA1215

http://www.digikey.com/product-search/en?keywords=2sc2921

You can contact digi-key ,they should help with making sure these are indeed suitable replacements.
In any case don't buy them yet(you need just one pair).
We still need to find all damaged parts...

 

[Iceman from McMurdo]

Yes, thanks, that's a much more reputable source if they are indeed suitable replacements -and the price actually seems very reasonable. Hope they don't scalp me on shipping...

I'd been looking around for the original Toshiba bat-wings from a US source and could only find the ones below, which are more pricey:

http://www.ebay.com/itm/160965253716?_trksid=p2060353.m1438.l2649&ssPageName=STRK:MEBIDX:IT

Think I have a plan to facilitate removal of the other damaged parts, and hope to do that tonight or tomorrow.