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

[Iceman from McMurdo]

Removal of Q631/Q633 is next up on my "to do" list. Hope to do later this evening or tomorrow...

BTW, how do you know all this stuff about exact values to expect, silicon vs germanium diodes, which components are most suspect, etc? Are you getting all that off the schematics I sent? Or is that just a lifetime of electronics experience speaking??

 

[Iceman from McMurdo]

OK, got Q631/Q633 removed - then re-tested out-of-circuit (diode mode), with following results:

Q631:
B-C = .574 (Red probe to Base)
B-E = .579
C-E = OL
B-C = OL (Black probe to Base)
B-E = OL
C-E = OL

Q633:
B-C = OL (Red probe to Base)
B-E = OL
C-E = OL
B-C = .571 (Black probe to Base)
B-E = .579
C-E = OL

Hmmm, what say ye?

 

[dorke]

Q631/Q633 are fine.
Leave them out of the board for now.

Please check Q627/Q629 in-board.

 

[Iceman from McMurdo]

Glad to hear you think Q631/Q633 are ok - At this point, I'll take all the good news I can get!

Fortunately, I was able to carefully reposition the patient and get my probes in to check Q627/Q629 without having to remove the metal frame that initially appeared to be in the way. So, here are the results:

Q627:
B-C = OL (Red probe to Base)
B-E = OL
C-E = OL
B-C = .635 (Black probe to Base)
B-E = .664
C-E = OL

Q629:
B-C = .627 (Red probe to Base)
B-E = .649
C-E = OL
B-C = OL (Black probe to Base)
B-E = OL
C-E = OL

 

[dorke]

Q627/Q629 are fine as well,
though you probably swapped the red and black writing on the previous post.

Now lets test Q617/Q619.

 

[Iceman from McMurdo]

Actually, I retested (after confirming BCE orientation and PCB markings and red/black probe orientation), and got same values as listed above. However, I DID give you the wrong IDs of the components - first one I "said" was Q627 was actually Q629, and second one I "said" was Q629 was actually Q627. (Sorry, only excuse was it was early and I hadn't had my first cup of coffee!).

Just checked Q617/Q619, with following results:

Q617:
B-C = .622 (Red probe to Base)
B-E = .248
C-E = OL
B-C = OL (Black probe to Base)
B-E = .248
C-E = .663

Q619:
B-C = OL (Red probe to Base)
B-E = .246
C-E = .652
B-C = .621 (Black probe to Base)
B-E = .246
C-E = OL

 

[dorke]

Q617/Q619 are probably fine as well.

Please re-insert Q631/Q633(in there correct location and orientation) and show a photo of the soldering work(top and bottom) .

Re-connect(good solder joint) the cut transformer wires(2 grey and one green).
Isolate them well, preferably with shrink tubing.

Don't insert Q635/Q637 yet.

Put a single screw in the following location(GND connection):




After your soldering work is "approved".
Disconnect the speakers from the receiver.
Power it up.
The fuse should not blow.
If it doesn't indeed,
carefully(no probe slips!) test the following DC voltages to chassis ground(black probe) :
Q635 collector location (66V).
Q637 collector location (-66V).
D608 Anode (2v).
D610 Cathode (-2v).

What do we get?

 

[Iceman from McMurdo]

ALRIGHT! Sounds like we may be ready for a test flight soon, eh? That's great news!

Still waiting on the postman to bring my new replacement "bat-wing" transistors, but will try and get Q631/Q633 & related heat sinks soldered back in later tonite. Then keeping my fingers crossed I get some new parts in the mail tomorrow!

BTW, in case I forget, I have an upcoming out-of-town trip coming up later this week that'll take me out of circulation for at least a week and a half. IF I'm still waiting on parts and can't finish this before leaving, PLEASE stay with me and help me see this project thru to the end. Your helpful advice & quick response times have been absolutely CRITICAL thus far! Will keep at it every day until I leave (prob Wed or Thurs) and, if necessary, will pick it up again when I return (prob around Aug 12th)...

 

[Iceman from McMurdo]

Wow, I had just finished reinstalling Q631/Q633 & reconnecting transformer wires last night, when we suddenly had some very unexpected severe weather roll thru last night. Lots of high winds & thunder-boomers, which resulted in us losing electric power & internet from about 10pm to early this morning about 8am. Got some light cleanup to do outside but fortunate not to have suffered any major damage.

Good news though, I think the resoldering job went well (pics attached below for your inspection/approval). Hardest part was resoldering the heat sink pins (guess their larger size probably warranted a higher heat setting), but they're in pretty good & I don't think they're going anywhere.






Chassis ground screw has been installed & speaker wires are all confirmed to be disconnected. Will wait to hear back from you with official "seal of approval" before attempting power-up.

PS - Additional (even more severe) thunderboomers are expected today, so if I don't respond in a timely manner it may because we've lost power and/or internet again...

 

[dorke]

Needs some rework:

Trying to read the PCB:

Blue area- are the heat-sink pads-they are bad joints(lumpy ,not shiny),for this you should set the iron to 400C(probably with a 3mm tip if you have one,or a larger iron ).
but are not very important ,as they have no electrical connections just mechanical support.

Red area-bad joints,not enough solder, device pin was not heated enough(round "black channels" around it ,void of solder).
set the iron at 350C and heat both pin and pad at the same time,add fresh solder to get a "mount Fuji" profile.

Green area- needs cleaning with rubbing alcohol and cotton swabs(these are just sample areas for cleaning.

P.S sounds like you are indeed in Antarctica...

 

[Iceman from McMurdo]

Yeah, was using 700F for both pin soldering & the heat sink pads and could tell from dull appearance of solder I needed more heat for the latter. Bumped it up to 750 but still probably needed more heat (guess they were doing their job as "heatsinks" pretty well, eh?

Didn't notice the other joints you mentioned (red areas), as I didn't do any work on those - only components I removed & resoldered were the two triangle-shaped pin arrangements in the middle (Q631/Q633). However, I do see the black channels you mention where those nearby resistors obviously need some help.

Will reattack & let you know when ready for reinspection!

 

[Iceman from McMurdo]

OK, tweeked OEM resistor pin soldering, wicked out some remaining solder left over after removing the bat-wing transistor, and cleaned up entire area using some rubbing alcohol. Played with heatsink pads a little bit, but since they're mechanical (not electrical) connection, think I'm gonna leave well enough alone for now...

 

[dorke]

Looks much better.

If you can ,
1. Go over the entire board and inspect for bad solders,like the one you fixed.

2. Go over the entire board and gently remove all the solder "splashes "(red),
especially near bare solder points,use a fine screwdriver or similar tool.
Take care for them not to wonder anywhere.

3. Remove the remaining cotton(blue) with fine tweezers.

After that, you can power-up the receiver as per #67.




.

 

[Iceman from McMurdo]

Ugh, bad news... Cleaned everything up & inspected PCB for other bad pin solders using magnifying glass - all looked good & ready for test. Even looked up how to identify cathode & anode of those diodes so I'd be ready to check voltages once powered up, but then... Fuse blew IMMEDIATELY upon applying power. (Expletive deletive!)

Hmmm, what do you think we missed? Think maybe there could be other bad components unrelated to the left channel power? How about some of the electrolytic capacitors in the circuit (especially those two huge ones in the middle)?

Frankly, I fully expected one (or more) of those to be the main problem since I'd always heard they were prone to drying out & failing when operated for a long time in an excessively hot environment (like my garage here in central Texas, where temps routinely top 100F in the summertime).

Sad, sad times right now as I contemplate this setback. However, I'm now more determined than ever to track down the problem and FIX it! Can I impose upon you for more troubleshooting help??

 

[Iceman from McMurdo]

Arghhh, Mea Culpa!!!! (

Yes, I'm a dummy... Upon closer inspection of the burned-out fuse, turns out my elderly eyeballs played a nasty trick on me...

The fuse I'd quickly installed for the test was woefully undersized. What I thought was a 5A fuse was actually a milliamp fuse! Ha, no wonder it blew so quickly! (Yes, there's a reason I should be wearing my glasses more often for closeup work!)

At any rate, after installing the CORRECT (5A) fuse, WE NOW HAVE POWER & THE FUSE DOES NOT BLOW! Even better, ALL FOUR VOLTAGE TESTS YOU REQUESTED CHECK GOOD!!

Hooray! Time for a celebratory cup of coffee with EXTRA cream & sugar!!

Now where's that nice postman with my replacement transistors??

 

[dorke]

That is good news indeed.

When you power up the receiver you should hear a " relay click"
after about 3 seconds or so.
Does that happen?
If so,put the volume knob at it's minimum ,
check the DC voltage(lowest range possible) on the "good "right channel speakers terminals(back panel).
What do we get? Note down that voltage.

Check the DC voltage(lowest range possible) between the center pin of Z602 and both it's edge pins( on the "good" right channel )
What do we get? Note down the voltages.

If the DCV on the speaker terminals is less than 200mV ,
and you have a good speaker that shows resistance of 4 ohms or more(actually check it in ohm test).

You could connect the speaker to the right channel output (only to the right channel,not the left!!!,receiver off) and hopefully get sound, don't crank the volume up too much, just a bit to produce sound at low level.
Do you get sound?Is it clear with no distortion?
If so you can crank the volume higher but not beyond about 1/3 .

When you do get the bat-wings transistors don't install them immediately.
First check them in diode test mode.

Then,you should remove the old thermal grease from the heat-sink .
Was there any insulator(mica) for the transistors?
Carefully(easily breakable) remove it and clean the old grease.
Apply grease to the heat-sink and Trs and install with the mica insulator ,insulator bushing and screws.
close the screws well ,careful not to strong so you don't break the threads!

Now check resistance between the chassis and the TR's pins it should not show a short or low resistance!(means the TR's are well insulated from the chassis ).

At this point please take photos and post them.
After that you can solder the TRs and clean well.

 

[Iceman from McMurdo]

Yep, can hear the relay click as it comes out of protection mode, so that's good.

Also checked DC voltage at the speaker terminals on the good right channel and get 0.0 mv with volume at lowest level possible (digital up/down control vice rotary knob). Also confirmed DC voltage DOES increase as I increased volume level slightly.

Before I test Z602, could you please clarify which probe (red or black) goes on center pin? Then I shall check two voltages - center to side1, and then center to side2, correct?

 

[dorke]

Not important,you can put the black probe in the middle.

 

[Iceman from McMurdo]

OK, checked Z602 and get 10.6 mV DC one way, and -10.7mV the other way.

Before I connect a speaker to the (good) right channel to see if we get any sound, can you please clarify meaning of your comment "receiver off" in your earlier post? Presume receiver has to be on to check for sound - do you maybe mean the "Tuner" input (vice Aux or Phono) should be off?

 

[dorke]

When connecting the speaker wires, turn off the receiver to prevent accidental shorting with the speaker wires.
When the speaker is already connected ,power the receiver up.