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

[dorke]

D619 and D621 are both blown.
Please check R679,R681,R683,R685 and Z601, all in the lowest resistance test mode range.

The Trs are all Japanese series ones.
Their full part number starts with 2S which is omitted from the writing on the case.
The full number can be found on the schematics as "notes" in various places,that should be enough for us.

I shall help you with identifying replacements(later on ,when I'll have the time).
You can start with consulting digiy-key as well,giving them the original part numbers.
If they don't have an original,ask them for a replacement part.
After you have a full list of originals and replacements, we shall check the list .

 

[Iceman from McMurdo]

Sad news on the 2 latest diodes, but I figured as much based on the readings.

Thanks for pointing out the schematic notes regarding parts identification - that helps a lot! Did some cursory searching online at DigiKey & see most of the TRs I need are now obsolete, so will have to track down suitable alternates. (I found an online cross-ref that might help)

Checked resistors you mention & they all look close to spec (based on the values I see on the schematic):

R679 = 6.0 ohms
R681 = 6.0 ohms
R683 = 2.8 ohms
R685 = 2.6 ohms

Z601 also checks good - it still reads .2 ohms ctr-to-side(s) and .4 side-to-side (as it did before)

So, I guess the casualty count to date amounts to:

3 Diodes (D609, D619, D621)
8 TRs (Q613, Q617, Q619, Q621, Q623, Q625, Q627, Q629)
2 Bat-Wings (Q635, Q637)

Anything additional to test on the left channel catastrophe?

BTW - Just thinking out loud, remember when I misidentified left channel & right channel TRs back in #116 (accidentally tested Q622/624/626 instead of Q621/623/625)? Well, the one listed as Q625 (actually Q626) clearly has at least 2 ugly numbers that would appear to mean it too is bad, correct? Wonder if that's why the supposedly "good" right channel stopped working??

 

[dorke]

After all casualties are taken of the board we need to re-chceck the ones that are left,to verify they are good.

As for the measurements of Q625 (Q626) in #116,tlooks like a bad Tr. with shorted C-E.

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

Please re-measure it .
Actually we should check all the TRs and diodes of the right channel as well.
Starting with the batwings and going "inwards".

If possible, contact digi-key and ask for replacements for the TRs and diodes they don't have.

 

[Iceman from McMurdo]

Everything is off now, except for the 3 diodes - which I will remove today.

As for confusion about Q626, one of 2 things probably happened:
1) Either a miracle occurred and it fixed itself (highly unlikely!), or
2) I mismeasured, misread, or miswrote the numbers - because I read no dead shorts now (ha!)

Here are test results of right channel Batwings, Z602, & other major TRs in center power section:

Q636
B-C= .518
B-E= .524
C-E= OL
B-C= .743
B-E= .587
C-E= OL

Q638
B-C= .708
B-E= .605
C-E= .890
B-C= .496
B-E= .506
C-E= OL

Z602 - checks good (.2 center-to-sides and .4 side-to-side)

Q618
B-C= .615
B-E= .253
C-E= OL
B-C= OL
B-E= .253
C-E= .651

Q620
B-C= OL
B-E= .253
C-E= .659
B-C= .658
B-E= .252
C-E= OL

Q622
B-C= .565
B-E= .266
C-E= .898
B-C= .835
B-E= .266
C-E= .678

Q624
B-C= .384
B-E= .585
C-E= .595
B-C= .371
B-E= .981
C-E= OL

Q626
B-C= .369
B-E= .988
C-E= OL
B-C= .386
B-E= .574
C-E= .580

Q628
B-C= .617
B-E= .636
C-E= OL
B-C= OL
B-E= OL
C-E= .761

Q630
B-C= OL
B-E= OL
C-E= .748
B-C= .629
B-E= .625
C-E= OL

Q632
B-C= .552
B-E= .902
C-E= OL
B-C= .725
B-E= .574
C-E= .568

Q634
B-C= OL
B-E= .910
C-E= .497
B-C= .558
B-E= .568
C-E= .704

 

[dorke]

There is no obvious bad Tr in the above right channel measurements.
Please check all diodes on the right channel.

Please check resistance from the center pin of Z601 and Z602 to GND in ohm test (not diode mode).

 

[Iceman from McMurdo]

Happy to hear right channel TRs all look good. Think the diodes do too (checked all I could find on center section of board:

D608 = .567 / OL
D610 = .565 / OL
D612 = .560 / OL
D614 = .563 / OL
D616 = .243 / OL
D618 = .258 / OL
D620 = .254 / OL
D622 = .244 / OL

Checked both Z components (center pins to GND using resistance mode):

Z601 = 24.33 kilo-ohms
Z602 = 24.34 kilo-ohms

 

[dorke]

o.k diodes look fine.

Please check voltages relative to GND on IC601 pins 4 and 8 (careful no prob slips!).

Any progress with digi-key on replacements?

I think we should find out why there is no sound from the right channel.
Fixing that issue,"cheaply" I hope, would make the all repair sensible price-wise.

The scans of schematics you uploaded are very difficult to trace from "page to page",
the traces between boundaries are not continuous.
Could you please scan these areas?.

 

[Iceman from McMurdo]

OK, the 3 offending diodes have been successfully removed, so ALL blown parts identified so far (left channel) are now off the board.

As for the right channel, I was reluctant to complicate things, but now that our left channel damage assessment looks pretty complete, I agree it would make sense to try & troubleshoot the problem now (rather than later) so if any additional parts are needed I can order them all at once.

Funny, because the right side DID produce low volume sound for awhile during the original tests - and then mysteriously quit (no bad noises or blown fuses or anything). Hoping it's something simple but, so far, everything I've checked (mostly center section TRs & diodes) appears ok...

As for the test on IC601, assume if you want "voltage" checks & "no probe slips," that means you're expecting me to try & power things up again, yes?

If so, beyond testing IC601, should I try and see what kind of sound I get out of the right channel speaker and/or headphones?

 

[dorke]

Yes,power up .
But before that ,please re-measure all surviving TRs and diodes of the left channel,
in board from Q611 and D603 "upwards numbers"

You can try a sound test with headphones.
Do you hear the clinking of speakers relay RLY1001 ,a few seconds after power-up?

BTW,
On the list at #140, Q631 and Q633 are missing.

 

[Iceman from McMurdo]

OK, think I've checked all surviving (left channel) diodes & TRs remaining on PCB - see no obvious shorts or unusual values.

As for Q631 & Q633, they were removed as requested earlier & checked, but I forgot to report values. Observed no dead shorts, but was surprised to see all the OL readings on 633. Measured it twice... Is that normal?

Q631
B-C= .574
B-E= .578
C-E= OL
B-C= OL
B-E= OL
C-E= OL

Q633
B-C= OL
B-E= OL
C-E= OL
B-C= .565
B-E= OL
C-E= OL

Will wait to power up until you advise on Q633 - in case I need to remove it.

As for checking IC601, what kind of voltage will I be looking for - AC or DC?

 

[dorke]

Q633 is blown,
Q631 looks fine but please remove it(if you didn't already).
Q631and Q633 should be a matched pair ,so if Q633 original can't be found both should be replaced.

DCV on IC601.

Please look at #147, regarding the schematics as well.

 

[Iceman from McMurdo]

Ugh, didn't wanna hear that... but just checked one last time and I only get ONE value on Q633 - everything else is OL. Just my luck...

As it turns out, just got off the line with DigiKey trying to source parts and it appears we have a mixed bag - some they were able to x-ref and some they were not. FOUR parts are listed as "obsolete" with no replacement avail. Ugh (again!). At least they found me a matching pair for Q631/Q633.

Here are the 4 part numbers they cannot x-ref to anything...

DIODE - 2OA99 (need 2 of them for D619 & D621)
TR - 2SA1124 (need 2 of them for Q613 & Q629)
TR - 2SB745 (need 1 of them for Q625)
TR - 2SC2632 (need 1 of them for Q627

Found a place called Vetco that showed 2SB745 may possibly cross to NTE234?

Also showed NTE32 as a x-ref for 2SA1124, and NTE399 for 2SC2632, but both NTE units appear VERY different design from originals (round flat head design vice std 3-leg transistor) so not sure they'll fit.

Any ideas??

PS - Will try some new schematic scans for you tonight

 

[Iceman from McMurdo]

Well, got brave and powered it up just now...

Good news - No immediate sparks, bad noises, or blown fuses - and it DOES come out of Protection Mode.
Bad news - Trips into Protection Mode after about a minute or so.

Tried powering up several times and result was usually the same, although one time it stayed on w/o tripping into Protection Mode for several minutes before I eventually turned it off. Not sure what's up with that?

Measured IC601 as requested and got following:
-16.77vDC (Red Probe to Pin 4, Black Probe to Gnd)
+17.26vDC (Red Probe to Pin 8, Black Probe to Gnd)

(Note - May have been coincidental, but seemed to trip into Protection as soon as I tested Pin 4?)

Tried listening for Spkr Relay but didn't hear anything loud enough to be heard over it coming out of Protection Mode. Should it be noticeable?

 

[dorke]

Yes, you should be able to hear the speaker.
When in protection mode,
please measure the voltages relative to GND on IC1002,it is a 9 pin single-in-line package,no prob slip!
This IC should be found on a little board called SUP22910B.

What is the pair they offered for Q631/Q633?
About the others, I shall try to find replacements, but not just now.

 

[Iceman from McMurdo]

K, will do.

Substitutes are Sanken 2SA1667-ND & matching 2SC4381-ND. He says they're overkill but should work fine.

No worries on timing - GREATLY appreciate your patience so far!

BTW - I had no spkrs hooked up during power-up (if that makes a difference on the relay operation). I was just gonna try headphones, but haven't gotten around to that yet...

 

[dorke]

The relay should click .
No speakers for now.

 

[Iceman from McMurdo]

Sorry it took me awhile, but finally got some (hopefully) improved schematics for you. Biggest holdup was it took me all morning trying to update the software on my continuous rotary scanner and get it to work with Windows 10. Couldn't quite get the whole 5-page schematic on one pdf file, but pretty darned close (then I scanned from other direction so you'd have a continuous view from left or right side).

Unfortunately, the file sizes are just a tad bit too large to be uploaded onto this site. Will try and see if I can reduce resolution or something to get them smaller, or I can email them to you if you're open to that option.

 

[Iceman from McMurdo]

Update on receiver status. Repositioned chassis & blew everything out with compressed air just to make sure everything was clean & nothing loose - then powered up again...

Came out of Protection Mode normally & stayed online continuously for 5-10 min while I checked for right channel sound in the headphones. Used garage CD player as source (vice previous phono input), and right channel played beautifully using the headphone. Never went into Protection Mode, so maybe that's a good sign.

Tried listening again for spkr relay & honestly cannot hear anything except the loud click when it comes out of Protection Mode. Think maybe the relay could be the reason behind the right channel no longer putting out sound to the speakers?

PS - I found the IC1002 but have not been able to obtain measurements yet. It's mounted vertically against the back & will require removing the back panel from the chassis to access the pins. If/when I do, what exactly am I checking - voltage readings from each of the 9 pins relative to chassis GND? If so, AC or DC?

 

[Iceman from McMurdo]

One more update - Had an idea to try using my mechanic's stethoscope to listen closer for proper operation of the spkr relay and it most definitely DOES make a soft clicking sound during power-up & power-down, so perhaps that isn't the problem after all...

 

[Iceman from McMurdo]

Eureka - not sure how or why but I'm not gonna look a gifthorse in the mouth! Tried right channel one more time using speaker output... and it sounds AWESOME! Much better volume than before & crystal clear. I'd forgotten what great sound this thing puts out! Maybe switching to Aux using CD as source had something to do with it, but I'm not asking any more questions about the right channel right now.

Suggest we focus on getting left channel up & operating as Priority One - just need some help tracking down those few parts DigiKey can't seem to cross-ref.

If/when we get it going on both channels, maybe we can troubleshooter the tuner section problems a bit more- but that's a secondary issue as far as I'm concerned.