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

[Iceman from McMurdo]

I have an older Technics SA-828 Stereo Receiver from the 1970s that used to work GREAT, but no longer powers up - immediately blows the 5A fuse labeled F1. Had it operating in a hot garage in central Texas last few years (yeah, I know.. STUPID!) and I suspect it suffered heat damage to the power supply and/or electrolytic capacitors. Thing weighs a ton, so don't really want to take it to a tech (plus I suspect it would cost more to fix than it's worth), but I'd really like to get it working again if I can track down problem myself & find parts. Whaddya think my chances are??

 

[dorke]

Welcome to EP.

The SA-828 is 1981, not the 70's.
It is an excellent stereo receiver and probably is worth fixing, unless the transformer is blown.

"Whaddya think my chances are?? "
That will depend on your knowledge and experience, do you have any?

You should have a service manual or at least a schematic.
For test equipment, a DMM is a minimum.

 

[Iceman from McMurdo]

Thanks for the reply (photo of the patient is attached), and yes, 1981 sounds about right (purchased overseas while on deployment with the Navy). I'm a retired military pilot with lots of mechanical experience and a solid knowledge of how to repair basic AC/DC electrics, plus I do have a DMM - however, not much experience repairing complex electronic circuits (especially if it involves having to de-solder a lot of miniature components). That said, I DID successfully track down a blown LED driver board in my big screen TV and repair it, so am certainly willing to dig into this receiver assuming I can find a little online help and troubleshooting advice. I'm hoping the problem can be quickly traced to a couple of the big electrolytic capacitors that have dried out or fried, or the large power supply that I might be able to replace with an aftermarket substitute. Unfortunately, I do NOT have a schematic (yet), although I've found a factory service manual avail for purchase on eBay ($12). Any suggestions on potential parts houses and/or other resources I might turn to for some help guiding a DIY-er like myself on a project of this magnitude??

 

[dorke]

The build quality shows.
I wouldn't jump into any conclusions about the source of the fault.

If you are willing to spend the 12$ to buy the service manual(and post it here for us to see and analyze), myself and others here will help and guide you with the repair.

The "big black box" is the mains transformer.
If that is blown it will not be economical to repair.

So, even before you get the S.M. we can try to make some tests.
Please take a good photo of the connections at the "bottom" of the transformer.
Is it a U.S model with 115V line voltage?

 

[Iceman from McMurdo]

Thanks for the offer to assist. Yes, it's a US model built for 115V, and I quickly recognized the big black box as the power supply (ugh, it is HEAVY). Made a cursory attempt to remove & inspect, but unfortunately it wasn't quite as easy as it looked - wiring bundles will need to be teased around to give me enough slack to get to the connections on bottom.

However, your suggestion is exactly the direction I had in mind - try and get to it so I can isolate the power supply & test that component by itself (hopefully with some guidance from yourself & others on the forum). As for replacing it (if it tests BAD), seems like most sites say same thing - not economical to repair... Really? Nobody makes generic power supplies these days that might substitute for the original? What about taking a chance & swapping in something similar from a donor unit I might find on the cheap at a Goodwill or Salvation Army thrift store? Do I really need an "exact" match for the power supply?? Here's a photo of the markings on mine.

 

[dorke]

It is just a transformer,not the hole power supply.
They are specially build with special requirements(like voltages ,amperage , shielding etc. ) and very unique to a brand and usually to a model as well.
It is by far the most expansive single element in a receiver/amp.
That one would probably cost(if you can find a used one) in the range 100-200$ ,
and shipping wouldn't be cheap either due to the weight .
Let's hope it isn't the transformer.

Does the unit have a voltage selector?
If it does is it in the proper 110/115 position .
How many fuses do you see in the unit?

Let's do some resistance tests with the DMM.
With the unit disconnected from the mains.
Disconnect all speakers and input/output cables to the unit.

1. Check resistance between the blown fuse holder legs and the chassis of the unit(or speaker black terminal connection) ,
what do you read ?

2. Re-insert a good fuse instead of the blown one.
Check resistance between the two pins (I assume there are only 2)of the power plug ,
with the front panel power switch in the "off" position and then in the "on" position,
what do you read?

 

[Iceman from McMurdo]

Answers to your questions:
Yes, dual voltage selector is present & it is set to correct 120 position.
There are two internal fuses (see photo below), and it's the bigger 5A fuse that blows instantly when powered-up.
Resistance between fuse holder legs & chassis is infinite (both legs).
With new fuse installed, resistance between 2 pins of power plug are: infinite (switch off) & zero (switch on).

PS - Found the original 5-page schematic n my files, so at least I do have a roadmap for the way ahead

 

[dorke]

Zero like 0.0 ohm ?
Did you check it in the 200 ohm DMM range ?

Put the voltage selector at 220V and re-measure resistance between the power cord pins.
What do you read?

Could you please scan/photo (in readable quality) and post the schematics.
At least the one that shows the Fuse F1.

 

[Iceman from McMurdo]

Hmmm, discovered voltage selector actually had 3 settings: 115, 110, & 220 (and unit has always been set at 115). Think that's how it was set when first received many years ago and it's always run fine that way.

Here are resistance readings with DMM set at 200:
115 = 02.8
110 = 02.8
220 = 05.2

Will do my best to upload schematic, but document is a large single accordion-style sheet equivalent to 5 pages. Not sure how many photos I can upload at once so will have to spread it out & hope for the best. Here's a closeup of the line power input, dual fuse assy, and transformer circuit (other photos to follow):

 

[Iceman from McMurdo]

Schematic Part 1 (Reads RIGHT to LEFT from Power Input)

 

[Iceman from McMurdo]

Schematic Part 2 (Reads RIGHT to LEFT from Part 1

 

[Iceman from McMurdo]

Schematic Part 3 (Reads RIGHT to LEFT from Part 2)

 

[Iceman from McMurdo]

Minor CORRECTION to earlier post about input voltage selector & power cord resistance. Upon closer examination the 3 possible voltage settings are 120 (Instead of 115), 110, and 220. Unit has always been set on 120 & operated just fine.

A re-test of resistance readings on the power cord pins (using a better DMM) are:
120 = 02.2
110 = 02.2
220 = 04.4

Sorry for any confusion...

 

[dorke]

The DC resistance is rather low ,but probably is ok. that means the primary of the transformer is probably o.k.

The schematics are barley readable.
It would be easier if you can scan them in a higher resolution.

Please open and remove the bottom plate.
And a photo please.

Is it easy to disconnect the two grey wires coming from the transformer?
If is is, disconnect them (de-solder) them .
Then measure the resistance between them ("to the transformer") and between each grey and the green wire coming from the transformer(amp chassis as well).
Do the same on the board connection itself.
What do we get?

 

[Iceman from McMurdo]

Thanks for continuing to help troubleshoot this thing... Sorry about poor resolution of schematics, will reattempt to post better quality scans.

In the meantime, I pulled transformer out and confirmed no obvious damage to underside (all wires intact & no burn damage to epoxy sealant). Also took bottom plate off (photos attached below) as you requested, but other than confirming absence of any obvious damage, doesn't look like it will offer much help.

Ready to check resistance on the 2 grey wires & the 1 green wire you mention, but rather than desolder them I think it would be easier for me to just snip them where they are most accessible, take the resistance readings you want, and then reconnect them later if/when needed. Any problem doing it that way instead? Will wait for your response before proceeding...

Underside Circuitry:




Another Photo of Underside Circuitry (Closeup of Center Power Board):

 

[Iceman from McMurdo]

OK, here's the schematic scans (Pages 1-5)

 

[dorke]

The new schematics scans are workable,thanks.

Before we continue,
I forgot to ask an important question:
How good are you at soldering?What equipment do you have?
Can you please show a soldering work you did and a photo of your equipment.

Good soldering is essential to the repair,
if done the wrong way may cause more damage/reliability problems.
I would prefer to de-solder wires instead of cutting them (unless absolutely essential or needs lots of work to access).
Can you please show the grey wires connection spot on the PCB.

The opening of the bottom plate will serve us in measurements
and removing /replacement of parts .

Could you please remove the black plate and take a photo of the parts below it.

 

[73's de Edd]

.


Sir Iceman from McMurdo . . . . . . . .
(Don't your turbines get iced up a lot ?)

If the Gods are with you and all planets are lining up precisely . . .testing all of the diodes within
YELLOW boxed markup probably will result in one or a couple of ZERO ohm dead shorted units being your problem.

Next . . .100 comparative degrees . . . LESS likely would be the separate PINK boxed markup units with the same fault (s)


EXTRACTED REFERENCING:





73's de Edd
f16 thunderjockey

 

[Iceman from McMurdo]

Thanks to both dorke & 73's de Edd for helping here. (And no, turbine icing isn't as big a problem in Antarctica as one might think - we pre-heated the engines of our C-130s and UH-1N helos prior to start & then avoided unnecessary shutdown until daily ops were complete - VERY low humidity down on the ice & precip of ANY kind is actually quite rare.)

OK, I've attached photos of the main power board (w black cover plate removed), as well as a closeup of the grey wire connections.





Based on my novice internet search, I wondered if the primary suspect might be one or more shorted diodes, as you suggested... I found D701 right next to where the grey wires enter the power board (closeup photo below) - it's marked as such just under the right side of the large heat sink. However, the other diodes mentioned (D702-D705) are MUCH smaller (no heat sinks) and mounted close together on a completely separate board (2nd photo below).





PS - Soldering skills are probably average (have basic Weller gun plus smaller fine tip soldering pencil for lighter work). Have successfully repaired minor electronic boards associated with automotive circuits, but desoldering always presented more of a challenge for me than soldering itself. If we can track down probable components, I will attempt desoldering - but for now I will snip the grey & green wires to get your resistance readings just because it's so much more convenient at this stage:

Grey to Grey (Transformer Side) = 05.5
Grey #1 to Green (Transformer Side = 05.5
Grey #2 to Green (Transformer Side) = 05.6
Grey to Grey (Board Side = INF (1)
Grey #1 to Green (Board Side) = INF (1)
Grey #2 to Green (Board Side) = INF (1)

Where do we go from here??

 

[dorke]

There is something wrong with the last measurements.

Grey to Grey (Transformer Side) = 05.5
Grey #1 to Green (Transformer Side = 05.5
Grey #2 to Green (Transformer Side) = 05.6

The Grey to Grey(Transformer Side) should be the sum of the greys to green.
i.e should be about 11.1
Could you please re-measure the transformer side .

After that we should do some voltage measurements like so:
Make sure the wires are disconnected and not touching anything .

Connect the receiver to power,does the fuse blow?
If it doesn't,
check AC voltages (coming from the transformer)between each grey and green ,
and grey to grey.
Careful high voltages !use well isolated probes and no hand touching!
you should get about 48VAC each grey to green and 96VAC grey to grey.