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

[Iceman from McMurdo]

Just thinking... Assuming I can successfully remove all the components you mention, and assuming they ALL need replacement (and are available from current sources), can you give me a ballpark estimate as to the total cost for the whole lot? Just want to have some assurance that I'm not getting in over my head as far as parts costs go for this "experimental surgery"...

 

[dorke]

The problem with these parts is some of them may be obsolete.
So their price may be higher then "normal",and could be that suitable "modern" replacements may be needed.

Estimated "Normal" upper limit prices :
Q631/Q633 1.5$ each.
The others should be 0.25$ each

Note: it would be better to check them individually for a more accurate estimate.

 

[Iceman from McMurdo]

OK, thanks. Was hoping to see reasonable price estimates like that, vice huge numbers based on items made of "unobtainium." Hope to start desoldering process later today...

 

[Iceman from McMurdo]

Well (sigh...), don't think it makes any difference at this point because components are clearly damaged & need to come out, but I mistakenly gave you readings from 3 "wrong" components earlier. Somehow I got my orientation mixed up when relocating everything to the garage and what I "thought" was Q621/Q623/Q625 was actually the 3 (hopefully) good ones from the right channel (Q622/Q624/Q626). I discovered this only when preparing to remove them this morning & noticed something looked strange from my earlier testing... Here are the CORRECT readings from the damaged components:

Q621
B-C= .909
B-E= .266
C-E= .804
B-C= .864
B-E= .266
C-E= .840

Q623
B-C= .864
B-E= .778
C-E= .358
B-C= .620
B-E= .552
C-E= .380

Q625
B-C= .018
B-E= .319
C-E= .308
B-C= .018
B-E= .308
C-E= .297

Good news is I've successfully removed the 3 clearly damaged components above, as well as the other 2 I'm
pretty sure got "smoke-checked" during power-up (Q627/Q629).

As stated earlier, both China-man transistors (Q635/Q637) have also been removed, so that only leaves Q631/Q633 to go - which are a bit of a pain since they're connected to those big L-shaped heatsinks I had to remove before... (ugh)

 

[Iceman from McMurdo]

And, BTW, I confirmed with DigiKey that the 2 substitute transistors you ID'd for the OEM batwings WILL work, though shipping is higher than I'd prefer ($12.50). Might do a little shopping around to see if they can be had for less somewhere else.

Good find though, and from a reputable source too, so THANKS!

 

[dorke]

Iceman,
Don't buy them yet.
Better wait till we have identified all blown parts and buy all of them together from digi-key .
Shipment of all in a single parcel will probably make the total shipping price lower.

 

[Iceman from McMurdo]

Yeah, definitely figured I'd wait to order all the parts at once, as shipping will probably be the single highest cost involved.

OK, good news & bad... First the good news - I finally got Q631/Q633 removed (those heatsinks were a royal pain!). With all requested components removed, here are the test results:

Q613
B-C= .994
B-E= .994
C-E= .006
B-C= .604
B-E= .606
C-E= .006

Q615
B-C= .587
B-E= .601
C-E= OL
B-C= OL
B-E= OL
C-E= OL

Q617
B-C= .255
B-E= OL
C-E= OL
B-C= .255
B-E= .255
C-E= .652

Q619
B-C= .058
B-E= OL
C-E= OL
B-C= .058
B-E= OL
C-E= OL

D603
.568 / OL

D605
.527 / OL

D607
.566 / OL

D609 (see note below)

D611
.559/ OL

D613
.561 / OL

Now, the bad news... The reason I have no test results for D609 above is simply because I can NOT find that stinker! It must have its cloaking device activated. I've found all ts little brothers & sisters (Q609, R609, C609, etc), but searching for that diode was beginning to give me a headache, so I took a break. Will get back to it in a bit...

What can you tell from the results so far??

 

[dorke]

Bad news,
The destruction trail goes on:
Q613,Q619 blown
Q617, probably blown
Please remove them and check them out of board.
After removing the above, check Q605,Q609,Q611,Q615 in board,please note "prob colors".

The diodes you have checked are fine.
D609 should be near one of the following:
Q615,Q619,Q621,Q629,C619 or VR601.

 

[Iceman from McMurdo]

Yep, I was afraid of that... Glad the diodes are all good though, as those look like they'd be a pain to try and replace!

BTW, I'm curious to learn, what tells you which are bad? Is it the very low values that indicate a short (like the .006s on Q613 and .058s on Q619)?

Was hoping those weren't bad, as they are on a different part of the board where access to the component side is more difficult...

Will try and remove them tomorrow and report off-board values.

PS - Once again, THANKS for hanging with me on this. For what it's worth, I AM learning some new things and broadening my soldering/desoldering skills.

 

[dorke]

Yes,
the low values are indeed a sure indication of a bad Tr.

The way we measure a Tr. with a DMM in diode test mode is looking at it as composition of 2 diodes.
The Base of the Tr. is common to both diodes,polarity changes with the Tr. type.
So, a good Tr. should have each diode O.L + about 0.6(for Silicon).
To that we add the test of C-E in both directions which should be O.L+ O.L

note :you can not make a Tr. with 2 individual diodes like that,it is just a model.

like so:

 

[Iceman from McMurdo]

Ha, wouldn't you know it... Finally found that last Diode (D609) and it's the ONLY one (at least so far) that tested BAD! Shows a direct short both ways (.000/.000). Ugh...

Had a power outage this morning, so couldn't pull the blown transistors (Q613/Q617/Q619), but will try & do that this afternoon.

 

[Iceman from McMurdo]

OK, offending transistors removed - all checked bad off-circuit & several had obvious signs of heat damage. Ugh.

Here are latest test results (first 3 values are Red Probe to Base, second 3 are Black probe to Base):

Q605
B-C= OL
B-E= OL
C-E= OL
B-C= .600
B-E= .608

Q609
B-C= .000
B-E= OL
C-E= OL
B-C= .000
B-E= .582
C-E= .582

Q611
B-C= OL
B-E= OL
C-E= OL
B-C= .607
B-E= .605
C-E= OL

Q615
B-C= .588
B-E= .602
C-E= OL
B-C= OL
B-E= OL
C-E= OL

And I think I know what those .000 readings on Q609 mean, right? Ugh...another one in a particularly difficult location to get to.

 

[dorke]

Normally that .000 would be a bad Tr.
but Q609 is wired with it's B and C shorted(acting like a diode) so this is fine.
All the above look fine.

One more device we need to test is Q607.
This is a dual NPN transistor in a single 5 pin package with the center pin being common E for both Trs.
Please test it like testing 2 Tr.s(A total of 12 measurements) in board.


Have you found D609?

 

[Iceman from McMurdo]

Hmmm, interesting (and WELCOME!) news about Q609.

Wish I could be so lucky about results of D609 (found & reported earlier), but since it's a simple 2-pin diode I'm pretty confident 000's BOTH ways means it's a goner, eh?

 

[Iceman from McMurdo]

OK, think Q607 is good. Tested both sides using std methodology - Red probe to outer bases first (for initial 3 readings), then Black probe to base (for second 3 readings); Then same thing for other end of component:

Q607
B-C= .615
B-E= .621
C-E= OL
B-C= OL
B-E= OL
C-E= OL

B-C= .615
B-E= .622
C-E= OL
B-C= OL
B-E= OL
C-E= OL

 

[dorke]

I missed the D609 in #131,yes it is blown.

Please test D603 and D605.

 

[Iceman from McMurdo]

Already tested them in #127 (checked good) - do I need to test them again?

 

[dorke]

No need,I missed those diodes as well .

Please pull out all identified bad parts and re-test them out of circuit to verify.

Then make a list of all of them.
Try finding them at digi-key,and let us know what is available and what isn't.
If they don't have them ,contact digi-key for a suitable replacement.
You should have the total price including shipment.
Please let us know what replacements are offered.

note:
some Tr's should be compliment pairs,if they don't have a single original from a pair,
then the all pair needs to be replaced.

 

[dorke]

Not sure we tested D619,D621.
If we didn't they should be checked as well.

 

[Iceman from McMurdo]

Don't recall checking D619/D621 earlier, but results don't look good...

D619
.000/.000

D621
OL/OL

Whay say ye?


As for replacement parts, how do I go about identifying replacement transistors? I can see most had some identifying numbers & markings, but not sure what they all mean (and some are difficult to read due to heat damage). Here's what I "think" they read - is this enough info to source them?

Q613 = A1124
Q617 = C1815
Q619 = A1015

Q621 = D661-5
Q623 = D661-5
Q625 = B745-1

Q627 = C2632
Q629 = A1124

Thanks to you (and DigiKey tech rep), believe the bat-wing substitutes I need are:

Q635 = SB755 or Sanken 2SA1215
Q637 = SD845 or Sanken 2SC2921

Lastly, I have no idea at all how to source any diodes that may be needed (D609, & prob D619/D621)? Are there specs I need to obtain somehow from the schematic or what?