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

[dorke]

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??

I have just noticed this.
Well ,
Some info is found in the schematics, like part numbers of devices and some voltages.
Other is analyzing the schematics and calculating/estimating.
Others are "general expected values " of devices values like Si vs. Ger
In general ,
I would say electronics experience and knowledge build over years,
in this kind of work it is mostly repair technician experience.

 

[Iceman from McMurdo]

Aha, I understand! That's how I'd have done it anyway, but I can see how folks might try and just go poking speaker wires around back there while powered up and think nothing of it... Thanks for the safety tip.

Will try it and report back soon!

 

[Iceman from McMurdo]

Well, DRATS! I connected the right channel speaker and DID get some very low volume sound out of it for awhile (YIPPEE!), but it didn't sound normal at all. Very muted sound level, even with my ear right near the speaker. Tried increasing volume to about a third as you say (no higher), and I could then hear relatively distortion-free music for a few minutes (still unusually low volume) - but then it crackled a few times and went DEAD QUIET...

Ugh, think the right channel transistors might have given up the ghost, as well? But, if so, how come the fuse didn't blow again?

Well, one step forward, two steps back it would seem. Alas, further troubleshooting will have to wait, as I must depart in the early AM for a short trip that'll take me away from this project for awhile. Hope to be back at it and seeking some follow-on advice around the 10th of Aug or so.

BTW, not sure it's relevant, but the tuner section does NOT presently work correctly (separate problems, I'm sure,, as it had already been on the fritz for a few years before the power fuse blew), but I mention this so you'll know the only way I could check for music on the right channel speaker was to hook up my phono input. Don't think that would have made any difference though just to check right channel output, would it?

 

[dorke]

O.K.
Give us a call when you are back.

The fuse will only blow if there is excessive current through it ,this isn't the case this time.
The tuner issue may or may not be relevant,needs further investigation.

When powering-up do you still hear the "relay click" ?

Please check your speaker in ohm test.what do you get?
What is written on your speaker?

 

[Iceman from McMurdo]

OK, I'm back, THANKS for sticking with me!

Yep, still get the protection relay click when powering up, so guess that's good. I did check the speaker before making original connection and got a 5 ohm reading (speaker says its rated at 4 ohms), but I will check again to make sure nothing has changed.

Where do you suggest we go from here?

 

[Iceman from McMurdo]

Just double-checked the speaker resistance - 4.1 ohms

 

[Iceman from McMurdo]

Just for grins, hooked up right channel speaker again to see if I could get anything out of tuner section, and don't seem to get anything at all - not even static (though I don't know it that's due to bad tuner section or fact even the right channel seemed to go out during previous test session). Have not retested with LP player as I did the first time.

Attached front view photo to show what appears to work, and what doesn't on tuner section. Can't recall if tuner actually "failed" on me several years ago - just think I may have started using CD player more often because it was getting hard to select the stations I wanted. As you'll see, flourescent digital freq readout is pretty much dead (just a dim light behind AM/FM part - no freq readout). Analog freq readout gauge does light up & appears to work, although sometimes erratically. Tried to switch between AM & FM using select buttons - sometimes they worked & sometimes they didn't. Signal strength meter works and does light up with increasing signal strength. Unique "Dimension Control" does still light up & appeared to work normally when I first got some volume going during the original test.

IMHO, unless something additional has failed in the tuner section, I don't think that's why I'm no longer getting sound. The unit functioned flawlessly for many years using LP phono, CD, & reel-to-reel inputs, despite having these little tuner issues. That said, I'd like to eventually fix the tuner issues too, if we can ever get the power section back up to speed...

 

[Sunnysky]

I am not sure if it has been suggested to replace all electrolytic caps yet.
Faint audio suggest near full swing output stage oscillation from snubber cap failure.

I still use my Technics for TV stereo amp and fixed it this way 10 yrs ago and still working.

 

[Iceman from McMurdo]

Thanks Sunnysky, not sure I followed all that diagnosis in your 2nd sentence, but do understand I may need to replace many of the aging capacitors which may have dried up & failed.

Not 100% sure I have the skill or patience to replace ALL of them in the entire thing though... You mention "snubber cap failure." Is that something you can elaborate on so that I might target some of the more failure-prone caps?

 

[73's de Edd]

.


Sir Iceman cometh . . . . . . .where you go o o o o o o o o o ?

I certainly don't suspect their failure at all, but the HF snubber pairs on this unit would be the
R698 + C629 and the R692 + C630 R/C pairs in the AF output stages.
( In which case you would be expecting a dead shorted capacitor.)

In the use of an LP player . . .which seems to be your "bulletproof" audio input source . . . .and then in testing . . . . .by using headphones, is there being audio output on both channels ?






73's de Edd


.

 

[dorke]

Welcome back Iceman ,
Did the "bat-wings" arrive already?

Can the Receiver produce sound using phono or CD source at low volume (right channel only)?

 

[Iceman from McMurdo]

Hmmm, very perplexing... Hooked up LP player & checked for right channel speaker sound again (just in case a miracle happened while I was away in Vegas visiting my son & his family), but no such luck.

Then hooked up headphones to check for sound there and was surprised to hear low volume sound, but ONLY in the LEFT channel... Nothing in the right. What's up with that? Think the whole right channel is dead now??

BTW, replacement "bat-wing" transistors DID arrive while I was away, so I have now have two complete pairs available for installation. That's enough to replace the old ones in BOTH the left & right channels.

 

[dorke]

I would replace the bath wings on the left channel only.
Do you have the insulator and thermal paste?
Test the Tr's in diode test before assembly(just in case...)

Note:
after you install the Tr's don't connect speakers to the left channel !
First check that the fuse doesn't blow, then we should "align"
the final stage,then check sound with headphones only then try with speakers.

 

[Iceman from McMurdo]

Yes, I have fresh thermal paste and got both mica insulators off left channel heatsink without damage. Cleaned up the heatsink & plan to install both left channel transistors tonight.

Checked new transistors just now (prior to installation) in diode mode and got following values:

Q635:
B-C= .457 (Red Probe to Base)
B-E= .468
C-E= OL
B-C= OL (Black Probe to Base)
B-E= OL
C-E= OL

Q637:
B-C= OL (Red Probe to Base)
B-E= OL
C-E= OL
B-C= .569 (Black Probe to Base)
B-E= .572
C-E= OL

 

[dorke]

They are fine ,
Install them(a clear photo could help),
After you have installed the Tr's,
measure the resistance between the center pin(collector) to chassis GND,it should be very high(Mohms O.L))

Don't connect speakers to the left channel !
Power -up the unit with volume knob at minimum.
Measure the DC output voltage on the middle point of Z601 relative to GND( should be close to zero),
and the voltage from center to it's end points(two measurements).
What do you get?

 

[Iceman from McMurdo]

Got 'em correctly installed on heat sink and confirmed they are fully insulated (got an OL reading on both when checking center pin to chassis GND). Will solder in later tonight or tomorrow and send pics.

 

[Iceman from McMurdo]

OK, finally got new transistors soldered into place on the board this evening (photo below). Got big heatsinks properly reattached to chassis and think we're ready for power-up in the morning.

Doublechecked we have high resistance between center pins & chassis GND. DMM reads OL on both.

 

[dorke]

Soldering could be better.
Red - too much solder re-touch needed(remove excess with braid and re-solder)
Green- remove solder splashes, take care not to lose them,we don't want them wondering around in the receiver.
Clean well with cotton swab and rubbing alcohol .
Blue - mechanical post ,do nothing.
Please,Go over all the points you have soldered and correct if needed as above.

How do the bat- wings look on the component side?
Hope you applied enough(but not too much) thermal paste and on both sides of the insulator
,a photo please.

 

[Iceman from McMurdo]

Yep, bat-wings look great from component side. Mica insulators undamaged and just enough (not too much) new thermal paste applied to both sides prior to installation (photo provided below).

Agree soldering job could be better, but still learning as I go. Cleaned up solder spatter a bit more & reworked transistor pins to mimic OEM right channel ones as best I could.

 

[dorke]

O.K
Put volume knob at minimum disconnect the speakers.
Power -up the unit.
First thing to check is for the fuse not to blow.

After that check voltage on center pin of Z601 relative to GND (on the lowest DCV range possible).
What do we get?
Check voltages between Z601 center and side pins (on the lowest DCV range possible).
What do we get?

Do the same for Z602 (the other channel).
What do we get?