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SX-838 troubleshooting assistance please

Uncle Bambi

Uncle Bambi

Funky Tut
Hello, I'm a new member of AK, and my name is John. I have a problem. :) I've been a long time "silver series" aficionado, and the first receiver I ever bought was a SX-780 that I purchased new in 1978 from Pacific Stereo in Southern California. I still have it, and am listening to it right now.

I picked up a cosmetically pretty SX-838 tonight, off of the local CL. The unit turns on, and lights up. Tuner meters deflect when the knob is spun, and I do get a "stereo" lock on FM stations. I never hear the protection relay click on, and the output is static/music for about a second before it goes quiet. All fuses are intact and ohm out to zero.

No bulged or leaking caps that I can see, and no obvious burn spots - although I do get a whiff of the suggestion of smoke. The only possibly suspicious thing I see is a light brown haze on the top (lead side) of C3.

I've got the service manual/schematic, and I'm going to print it out and go over it. Once I get familiar with the parts and their locations, I'll bust out the multi-meter and start checking things out.

I'm familiar with schematics, soldering, basic troubleshooting, and using a multi-meter - but it has been a long time since I've done anything like this. I bought this unit as a guinea pig to hone my skills and get more experience working on the vintage gear.

What I'm hoping for is a bit of guidance and helpful tips from those of you that have worked on these units before.

Any general tips, tricks, or places to start are more than welcome. I will probably ask stupid questions, so feel free to whack me upside the head if I say something bone-headed.
 
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Clean all the pots and switches 1st with De-Oxit D-5 and F5 Faderlube. Open the relay and take a folded piece of printer paper and pull thru the closed contacts. Continue to do this until the paper comes out clean. You can add a drop of De-Oxit to the paper. Don't spray the relay.

If it has Preout/Main IN jumpers pull them out and clean the pins and jacks. Try another preamp to the main in on the 838 to test the 838 amp. Then preout 838 to main in on the other amp. This will tell you whether it's the amp or further back in the unit.

Report back with results.

Larry
 
Thanks, will do!! It's Super Bowl Sunday and I'm gonna have a house full of people, so I'll tackle this on Monday.
 
larryderouin said:
If it has Preout/Main IN jumpers pull them out and clean the pins and jacks. Try another preamp to the main in on the 838 to test the 838 amp. Then preout 838 to main in on the other amp. This will tell you whether it's the amp or further back in the unit.
Report back with results.
Larry
Click to expand...

I agree.

But, the first thing I would do, is test the output transistors for shorts.

Rob
 
Robisme said:
I agree.

But, the first thing I would do, is test the output transistors for shorts.

Rob
Click to expand...

Not to disagree or discourage you from sharing (:D) some of the load, but if the main fuse ain't poppin, only one transistor per side is shorted. Thus a check of offset voltage before the protection relay tells the tale. Usually available at the protection board's pins... unlike the output transistors which take some diggin....
 
OK - 1st stupid question time........

I have the DeOxit, and have dis-assembled the receiver almost to where I can get at the pots. I cannot get the front bezel off because of the tuner knob... it looks like a hex set screw, but nothing I have fits. what size is it? The service manual doesn't specify. I assume it is metric - my smallest metric hex is 2mm.

I did take the relay cover off and clean the relay contact points. They are now nice and shiny.

The relay works - I power the unit up, and they click on. I can tune in a radio station, and it comes in with a lot of static. After many seconds, the receiver starts to pop and click through the speakers and the relay clicks off. After that, it either says off, or clicks on, then off at random.
 
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larryderouin said:
If it has Preout/Main IN jumpers pull them out and clean the pins and jacks. Try another preamp to the main in on the 838 to test the 838 amp. Then preout 838 to main in on the other amp. This will tell you whether it's the amp or further back in the unit.
Click to expand...

Larry -

It does, but while I have a spare pre-amp laying around, I don't have a spare amp.

Just to clarify - you want me to connect the pre-amp main outs to the SX-838 power ins?
 
Uncle Bambi said:
... I cannot get the front bezel off because of the tuner knob... it looks like a hex set screw, but nothing I have fits. what size is it? The service manual doesn't specify. I assume it is metric - my smallest metric hex is 2mm....
Click to expand...

Quoting myself. :)

Never mind - the winning number is 1.5mm. All DeOxit'ed and lubed. Waiting a bit to turn it on, in order for everything to dry.

Voltages on the power Amplifier PCB:

Pin 01 +40.6------s/b +44
Pin 02 -40.6------s/b -44
Pin 09 -41.6------s/b -45
Pin 12 -12.8------s/b -13
Pin 14 +36.2------s/b +35
Pin 17 -41.6------s/b -45
Pin 24 -40.2------s/b -44
Pin 25 +40.6------s/b +44
Pin 26 -40.2------s/b -44
Pin 29 +40.3------s/b +44
Pin 34 +40.3------s/b +44
Pin 37 -40.3------s/b -44
 
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John:

Yep. Use the separate preamp, and connect it to the MAIN IN jacks. This takes and makes the 838 an amp only. Everything else is disconnected from the amp. All your sources would be connected to the separate preamp. If the noise continues in this variation, then the problem is in the amp.


I'm not familiar with "s/b". explain please?

Larry
 
The voltages look ok.

BUT, the amp has the dreaded 2sc1451's, one per channel, as well as two 2sa726's per channel.

2sa726 pnp to-92 BCE!!!
512-KSA992FBU (ln)to-92 ecb!! 120v .05a .5w 100mhz 150-800hfe $0.05 ea

2sc1451 npn strange case ebc...
512-KSC3503estu to-126 ecb 300v .1a 1.2/7w 150mhz 40-320hfe $0.28 ea

from mouser.com.

more discussions will be needed to be sure that the transistors go in correctly orientated.
 
larryderouin said:
John:

Yep. Use the separate preamp, and connect it to the MAIN IN jacks. This takes and makes the 838 an amp only. Everything else is disconnected from the amp. All your sources would be connected to the separate preamp. If the noise continues in this variation, then the problem is in the amp.

I'm not familiar with "s/b". explain please?

Larry
Click to expand...

Larry,

Will do, tomorrow. I might be able to dig up an amp as well, to do the reverse . Yes - "S/B" = "should be"

markthefixer said:
The voltages look ok.

BUT, the amp has the dreaded 2sc1451's, one per channel, as well as two 2sa726's per channel.

2sa726 pnp to-92 BCE!!!
512-KSA992FBU (ln)to-92 ecb!! 120v .05a .5w 100mhz 150-800hfe $0.05 ea

2sc1451 npn strange case ebc...
512-KSC3503estu to-126 ecb 300v .1a 1.2/7w 150mhz 40-320hfe $0.28 ea

from mouser.com.

more discussions will be needed to be sure that the transistors go in correctly orientated.
Click to expand...

Mark,

So many questions...... :)

I don't follow - are you saying I need to buy these and replace at this time, or is more troubleshooting needed? I can call mouser tomorrow and order them, if so. They'll charge ~$8.00 to ship $1.50 worth of parts, so should I order other parts as long as I'm at it? Or, might they possibly be available at Radio Shack?

Transistor theory was a long, LONG time ago. I've forgotten most of it, but I do know how to read PNP and NPN on a schematic, and I know B = base, C = collector, and E = emitter. ::o

What do you mean by dreaded? Explain "more discussions will be needed to be sure that the transistors go in correctly orientated" .... As long as I know what the BCE are for the old parts and the new parts, I just solder the new ones in, no? Or, will identifying the the BCE of the replacements not be straightforward?

I'll do Larry's pre-amp/amp test tomorrow, and just say the word and I'll get the transistors.

In case you haven't caught on - this is my first attempt at troubleshooting and repair of a stereo receiver. I really, really appreciate you guys sharing your knowledge, and being patient with me. Oh...... and before you ask - YES, my DVM probes are insulated right up to the tip. :) I have been lurking, and I have learned a few things just by reading. Go figure. :D
 
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Looks like you are in good shape. Recognize the symbol and know it's significance.

Re: dreaded 2sc1451's....

They fail, reliably... the biggest pita I have run into. Other manufacturers used them too, with the same (long term) results. Bad....

Best thing to say is that if they haven't failed, to get them out before they cause damage - and they are positioned to do the most damage, in the bias section. At worst their failure can take output transistors, driver transistors and resistors with them.

I too chafe against the shipping costs, but the potential costs WHEN (not if) they fail are far more than the shipping.

It's a 160v t0 300v 100mA lower noise transistor with a lot of gain - not that common... and for pity's sake, DON'T get NTE.....

This is the best preemptive advice I can give. I didn't even mention upgrading the protection relay driver transistor, the protection's current dump transistor adding a flyback diode across the relay's coil, beefing up the power supply transistors, recapping the p.s , protection or other circuits.
None of those problems are more than annoyances. Even the 2sa726's aren't "dangerous" enough to other circuits when they fail.

But to spread out the shipping costs, other parts can be ordered and used later. There are lists, and if you post the lists before finalizing the order I will vet them for the most up to date understandings.

But the 1451's....

I "widlarise" them, which involves an anvil and sledgehammer, making sure that component never harms another innocent circuit.

http://en.wikipedia.org/wiki/Bob_Widlar
American electronic engineer and a pioneer of linear (analog) integrated circuit (IC) design.

THE BEST OF BOB PEASE
What's All This Widlar Stuff, Anyhow?
http://www.national.com/rap/Story/widlar.html

We still have a sign around our lab, "This is not a black-smith shop." But there were times when Bob would discover he had wasted a day or two, just because one bad part had screwed up his circuit. He would bring this bad part -- a capacitor, a pot, a transistor, an IC, or whatever -- over to the vise and lay it on the anvil part. Then he would calmly, methodically beat it with a hammer until the smallest remaining part was indistinguishable from the dust on the floor. Then he would go back to work and get the right answer. He explained that it makes you feel much better if you do this, and, you know that bad part will never come around again and goof you up. He was right. And I recommend that you join me in doing this "Widlarizing" when a bad component fools you. You will feel a lot better.
Click to expand...
 
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markthefixer said:
.........It's a 160v t0 300v 100mA lower noise transistor with a lot of gain - not that common... and for pity's sake, DON'T get NTE.....
Click to expand...

I don't understand this. Will I not be safe ordering the part #'s you specified?

markthefixer said:
But to spread out the shipping costs, other parts can be ordered and used later. There are lists, and if you post the lists before finalizing the order I will vet them for the most up to date understandings.
Click to expand...

Ok - will look. tried a forum search on the SX-838 and it didn't work very well. Perhaps bad search string - I'll keep trying.
 
Uncle Bambi said:
<snip>
I don't follow - are you saying I need to buy these and replace at this time, or is more troubleshooting needed? I can call mouser tomorrow and order them, if so. They'll charge ~$8.00 to ship $1.50 worth of parts, so should I order other parts as long as I'm at it? Or, might they possibly be available at Radio Shack?<snip>
Click to expand...

markthefixer said:
<snip>

I too chafe against the shipping costs, but the potential costs WHEN (not if) they fail are far more than the shipping.

It's a 160v t0 300v 100mA lower noise transistor with a lot of gain - not that common... and for pity's sake, DON'T get NTE.....

<snip>
Click to expand...

Uncle Bambi said:
I don't understand this. Will I not be safe ordering the part #'s you specified?<snip>
Click to expand...

It sure looked like you asked
Or, might they possibly be available at Radio Shack?
Click to expand...
for an alternate supplier...

YES YOU WILL BE SAFE ORDERING FROM MOUSER.com ....

NTE is a bubble packing company that repackages cheap transistors that others make, mark up the costs by a factor of 10 , print a "cross reference" for them and shoot them out in a blizzard of cardboard to be stocked nationwide for the unsuspecting all over the USA. Sometimes they are adequate parts - close enough of a match to the cross referenced original, sometimes..... double whammy - they don't make their parts, but they make a cross reference....

looks like I only did the phono amp caps, and I (or someone else) need to add the transistors and diodes for the protection and power supply.


phono amp
awf-011 c1 1 25 cssa 1 50 667-ECQ-V1H105JL5
awf-011 c2 1 25 cssa 1 50 667-ECQ-V1H105JL5
awf-011 c7 330 6 cea 330 10 647-UPW1A331MPD
awf-011 c8 330 6 cea 330 10 647-UPW1A331MPD
awf-011 c11 3.3 25 ceanl 3.3 50 647-UKL1H3R3KDDANA
awf-011 c12 3.3 25 ceanl 3.3 50 647-UKL1H3R3KDDANA
awf-011 c17 100 25 cea 100 25 647-UPW1V101MPD
awf-011 c18 220 16 cea 100 25 647-UPW1E221MPD
awf-011 Q1 2sa725 0 0 0 0 512-KSA992FBU
awf-011 Q2 2sa725 0 0 0 0 512-KSA992FBU
awf-011 Q3 2sc1313 0 0 0 0 512-KSC1845EBU
awf-011 Q4 2sc1313 0 0 0 0 512-KSC1845EBU
awf-011 Q5 2sc1313 0 0 0 0 512-KSC1845EBU
awf-011 Q6 2sc1313 0 0 0 0 512-KSC1845EBU

mic amp
awm-064 c2 220 35 cea
awm-064 c3 0.47 16 cssa
awm-064 c6 220 6 cea
awm-064 c7 0.47 16 ceanl

control amp
awg-029 c3 47 35 cea
awg-029 c4 47 35 cea
awg-029 c7 10 25 csza
awg-029 c8 10 25 csza
awg-029 c13 100 10 cea
awg-029 c14 100 10 cea
awg-029 c15 4.7 25 ceanl
awg-029 c16 4.7 25 ceanl
awg-029 c29 10 25 csza
awg-029 c30 10 25 csza
awg-029 c31 10 25 csza
awg-029 c32 10 25 csza
awg-029 c35 100 6 cea
awg-029 c36 100 6 cea
awg-029 c37 47 35 cea
awg-029 c38 47 35 cea
awg-029 c39 2.2 25 csza
awg-029 c40 2.2 25 csza

power amp
awh-031 c1 1 25 cssa
awh-031 c2 1 25 cssa
awh-031 c3 0.47 25 cssa
awh-031 c4 0.47 25 cssa
awh-031 c7 33 16 cea
awh-031 c8 33 16 cea
awh-031 c11 330 10 cea
awh-031 c12 330 10 cea
awh-031 c13 100 50 cea
awh-031 c14 100 50 cea

protection
awm-025 c1 0.22 10 cssa
awm-025 c2 0.22 10 cssa
awm-025 c3 330 6 cea
awm-025 c4 330 6 cea
awm-025 c5 4.7 25 cea
awm-025 c6 100 16 cea

power supply
awr-055 c8 470 80 ach-038
awr-055 c9 470 80 ach-038
awr-055 c10 100 50 cea
awr-055 c11 100 50 cea
awr-055 c12 220 16 cea
awr-055 c13 2200 10 cea
awr-055 c14 220 50 cea
awr-055 c15 470 16 cea

OR
power supply
awr-064 c8 470 80 ach-038
awr-064 c9 470 80 ach-038
awr-064 c10 100 50 cea
awr-064 c11 100 50 cea
awr-064 c12 220 16 cea
awr-064 c13 2200 10 cea
awr-064 c14 220 50 cea
awr-064 c15 470 16 cea
 
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Thanks for that!

Should I be concerned about replacing the three big caps on the chassis - ACH-001-C and ACH-015-0? In the Service Manual, page 35, under "Miscellaneous Parts List".
 
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