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

[Iceman from McMurdo]

Sounds like good news for Q631/Q633, glad to hear that!

As for soldering station, looked at Weller but didn't like the prices. Prefer one with mid-level variable power (60-75 watt?) and digital readout for precision temp control - but still around the $50 price point. Appears that means something from Asia thru an American wholesaler like CSI. They advertise a nice looking 75-watt unit for$59.

Certainly willing to entertain alternate recommendations from the group if there's something in a similar price range I should consider. Thanks!

 

[dorke]

If you are going to invest in a station that is going to last for a long time,
I would try one of the following:
The Weller WES51 for 92$ here (need to inquire about the pencil that comes with it).
The Hakko FX888D should be a good buy for 97$ here

How is your soldering class progressing?

 

[Iceman from McMurdo]

Yes, I thought about the Hakko, but couldn't justify almost double the cost of the other units I've been looking at with similar specs (and I'm not a fan of the Weller as it has no digital readout).

As for training progress, soldering tutorials are helpful - but no substitute for actual experience, which will have to wait until I get the soldering station and Chemtronics Solder-Wik (hopefully this coming week). I think my soldering skills are already passable, so expect little or no problem soldering in new components (with a tool that can give me the right temp), but am anxious to gain some advanced skills in the desoldering arena...

And, BTW, another round of sincere thanks to ALL who have chimed in here and helped educate me thus far. Regardless of whether or not this receiver EVER gets repaired, I've learned a LOT from EACH of YOU, and APPRECIATE your help more than you'll ever know! THANK-YOU!!

 

[dorke]

Iceman, you are welcome.
"Nice looking and on paper specs." won't last long or do a good job!
The price difference (about 55-65%) is very likely well justified.
Can you please post a direct link to it.

 

[Iceman from McMurdo]

Here's a link to the CSI unit I'm considering:

https://www.circuitspecialists.com/csi-premier-75w-Soldering-Station.html

Also, looking at this lower-priced unit, mainly because it comes with a variety of additional tips:

http://www.ebay.com/itm/381273992130?_trksid=p2060353.m1438.l2649&ssPageName=STRK:MEBIDX:IT

Understand there are higher-quality units out there, but they're at or above the $100 price point and I'm just kinda reluctant to spend that much - especially when I see positive feedback reports from others that say the lower-priced overseas units work just fine for their limited needs. Understand long-term reliability may be somewhat "questionable," but at my age (early 60s!), who knows how long it really needs to last, eh?

That said, I do understand the value of having the right tool for the job, and I guess I could justify spending a (little) bit more if you guys (the experts!) really think the above units are unsuitable. Did go out over the weekend to see what was avail in local hardware & computer stores, but price markups were simply unreal! - so eBay will most likely be my primary shopping source (unless you can suggest some other internet vendors that might offer better stations at a super discount).

Eager to get something ordered today so I can start moving on this again... Wife is starting to wonder how long her dining room table is gonna be used as an electronics repair station!

 

[dorke]

The 937 (and 936) are lower quality Chinese replicas of the Hakko models (obsolete by now,they were replaced by the 888).
It will do the job,I guess...tip quality is unknown but you could get a Hakko tip if it isn't good enough.
How much will it cost including shipping?

The CSI is a total unknown to me.

 

[Iceman from McMurdo]

Well, took your advice and looked around a bit more for a good mid-power soldering station... Turns out when shipping & taxes were added, the Hakko FX888D really wasn't that much more expensive than the CSI unit I was considering. Lucked out & got it for only $82 (Jet.com, using my 15% off coupon), which appears to be a really good deal. Hopefully it'll ship quickly so I can get to work soon. Thanks for steering me toward the Hakko!

 

[dorke]

Sounds like a good price for a good product ,
and it comes with a 1.6mm chisel tip as well.

 

[Iceman from McMurdo]

OK, good news! Used my new Hakko soldering station & Solder-Wick to successfully remove both bad (left channel) transistors. (New replacement transistors were ordered last week & will hopefully arrive soon.)

Once new parts arrive, I presume the plan should be to remount them to the heat sink (using same screws & new thermal paste), and then resolder legs to PCB - but then what? Recheck the FWB again (with new transistors in place) to confirm values there are now correct? If good, would we then be ready to reconnect the transformer & attempt power-up with a new fuse?

 

[dorke]

Now it is time to re-check(in circuit) transistors Q631/Q633 ,
and check Q627/Q629 as well.

Verify that the resistance between +/- DC of the bridge is high(much higher than the 005 that was there before).

What do we get?

 

[Iceman from McMurdo]

Hmmm, I rechecked Q631/633 and got values below when testing in diode mode (some of which you can see were not very stable, but at least the readings appear significantly higher than before).

Q631 / Q633:
B-C = 637 / approx 1200 at first, then rising to 1 (red probe to Base)
B-E = 642 / approx 1200 at first, then rising to 1
C-E = 761 / 1683
B-C = approx 1500 at first, then rising to 1 / 625 (black probe to Base)
B-E = approx 1500 at first, then rising to 1 / 633
C-E = 1688 / 630 at first, then rising to 700+

Is it normal for some of those readings not to stabilize?

As for Q627/629, they are in a more difficult to reach position on the board & will require some additional disassembly of things in order to test. However, if you think they're essential to test, I'm certainly willing to dig deeper....

 

[dorke]

In general,it is not normal for the transistors readings to be un-stable.
When testing in-circuit they can be ,usually due to capacitors charging/discharging.

It is not clear which are Q631 readings and which are Q633 ones.
When you write "then rising to 1" it is ambiguous.
Do you mean 001(shouldn't it be falling to...)?
Or do you mean "reading as in open probes"?
Please post a photo of that reading on the DMM.

Did you check the FWB rectifier -/+DC resistance ?

As for Q627/Q629,
No need to disassemble,you should have easy access from the PS(print side,bottom).

IMPORTANT:
When the parts arrive don't install them!
until we will complete all tests needed,more are pending,we just go gradually.

You should also get some thermal white paste.

 

[Iceman from McMurdo]

Sorry for confusion with the readings (formatting looked good in reply box but got screwed up when they posted):

Q631: (First 3 readings are Red probe to Base; Second 3 readings are Black probe to Base)
B-C = 583
B-E = 589
C-E = 580
B-C = 590 (increasing to OL)
B-E = 940 (increasing to OL)
C-E = 750

Q633: (First 3 readings are Red probe to Base; Second 3 readings are Black probe to Base)
B-C = 961 (increasing to OL)
B-E = 709 (increasing to OL)
C-E = 860 (increasing to OL)
B-C = 574
B-E = 583
C-E = 550 (increasing to OL)

As for the problem of testing Q627/Q629, they are too small to be tested from above and there is a metal frame residing directly over the pin side of the PCB where they're mounted, so to check I have to remove the entire PCB.

Also retested the FWB rectifier (diode mode) & got following values:

Red (- to +) = 856
Red to AC1 = 506
Red to AC2 = 500
Black (+ to -) = 500 (rising to OL)
Black to AC1 = OL
Black to AC2 = OL

PS - New supply of white thermal paste arrived in mail today!

 

[dorke]

The tests of Q631/Q633 are inconclusive.
They don't appear to be "obvious bads" .
I would take out Q631/Q633(carefully and mark which goes where),
test them out of circuit.
What do we get?

The bridge output is no longer shorted.

About testing Q627/629.
Better test them after Q631/Q633 are removed from the board.
Don't remove the entire PCB ,try to reach under that frame with the probes.
It looks like there is enough space for that.
If not,
maybe the removal of that side frame alone is possible(hard for me to see from the pics)?

Please test in (low ohm range) Z601(the 3 legs "white block"").
Center pin to the 2 edge ones, and center to point -97 ,the point where the transformer green wire enters the board.
what do we get?

 

[Iceman from McMurdo]

Easy stuff first... Tested Z601 (white block) and got following values:

Center to outboard leg 1 = 0.0
Center to outboard leg 2 = 0.0
Center to green wire entry point = 24.88 k-ohms

Agree, removal of that metal side frame by itself may permit testing of Q627/Q629, but will concentrate on removal & testing of Q631/Q633 first...

 

[dorke]

Z601 should read 0.22 ohm center to edge ,and 0.44 edge to edge.
Could you please check Z602 for comparison.

 

[Iceman from McMurdo]

OK, I rechecked Z601 on what I think is lowest resistance range my new DMM will go and I get 0.4 from center to each edge, and 0.8 from edge-to-edge.

Different values than you mentioned, but at least the ratio appears similar.

Also checked Z602 and got roughly same readings.

 

[dorke]

That is probably O.K .
What is written on Z601,Z602?
When you short the probes of your DMM (lowest resistance range)what do you get?
Could you please post a photo of the DMM or a brand and model number.


Another set of tests diodes D615,D617,D619,D621 -in diode test mode.
What do we get?

 

[Iceman from McMurdo]

DMM is a Centech 98674 (see photo below). Admittedly, a budget test instrument, but supposedly capable and much better than the little $5 thing I was trying to use before. However, it's accuracy at very low resistance readings does take a long time to stabilize... As you can see in photo above, meter still reads 0.2 after probes had been shorted for awhile. After longer wait, reading DID eventually fall to 0.0, but fluctuated now and then 0.1 to 0.2 Ohms.



I did check the markings on Z601 and you're absolutely correct about the values I should be seeing, as Z601 is clearly marked 0.22 Ohm (see photo below). Now knowing the DMM takes more time to stabilize, I retested Z601 and eventually got 0.2 from center to each side leg, and then 0.4 between side legs themselves - so presume that component is probably good, eh?

Checked diodes you requested and got following:

D615: .260 / OL
D617: .260 / OL
D619: .267 / OL
D621: .271 / OL

Sounds like those are working ok too, right??

 

[dorke]

Yes Z601,Z602 are fine.

The diodes D615-D621 reading of .26 is low for a silicon diode,
but these are OA99 germanium diodes,they are fine.

Any progress with Q631/Q633 and Q627/Q629 ?