Pioneer SX-980 - Diagnosis

[cecilk]

Hi all,

I'm new to the Pioneer group - but just scored a Pioneer SX-980 (and a pair of HPM-100 speakers) for the friendly Iowa price of $100 (long story). Guy had 'em in his garage to listen to the radio and never played them too loud out of concern for his neighbors.

He said the SX-980 "quit working" a little while ago - but sometimes he could get it to work when he "jiggled the mute switch". Hoping this may just be a short in the switch that a good cleaning might fix.

I've always wanted a Pioneer system, so I really want to bring this thing back and have it live for another 50 years! Two years ago I successfully rebuilt/recapped a non-working Sansui QR-4500 and have done a lot of work on comparatively simple tube equipment. I have a decent workshop with variac, oscilloscope, signal tracer, and Heathkit IG-18. I think I can handle the SX-980 - especially seeing the knowledge and experience of the experts on this board.

My plan is:

• Clean out unit of dust/crud
• Do my homework (read service manual, study schematic, research this forum for past rebuilds, BOM, etc)
• Attempt to diagnose the issue causing it to only work intermittently (hopefully it's something simple)
• Recap
• Replace any transistors known to be noisy or problematic
• Deoxit pots, clean boards, clean RCA jacks, etc

Any general advice about this model before I start?

Thanks,

 

[zebulon1]

It should clean up and look really good.
Let us know how the evaluation goes.

 

[cecilk]

First quick test I did while cleaning was to check fuses on the power supply board for continuity.

Two fuses were blown - FU3 and FU4 (pardon my French).

The other fuses were okay.

I'm thinking that is good news - it looks like these fuses deliver power from the transformer to the 10E2 rectifier diodes that supply various voltages to the power amplifier and other boards - so this would definitely account for the unit not working.

Any thoughts on where the problem might have originated?

 

[jeffpaletz]

The question is why the fuses are blown. Something is wrong somewhere. Be sure you use a dim bulb tester to plug your receiver into while you are repairing it and testing it. Using a 100 watt bulb reduces the voltage to the receiver so when it comes on if the bulb stays bright you know you have a problem. Search for a thread that explains how to build one and use it. Also look for the thread on deoxit which is contact cleaner.

The Pioneer HPM 100 speakers are among the best in my opinion. They have great bass and clear sound and are perfect for your receiver.

 

[zebulon1]

This could be tricky to isolate.
It may end up as the PS but some troubleshooting is in order.

 

[cecilk]

I've been meaning to build a dim bulb tester - so this is a good motivation to do so.

Once I do - what's the best next step? Should I replace the blown fuses and do some voltage checks? Or should I just go ahead and recap/rebuild the power supply board first?

 

[zebulon1]

Both the +-50 and +-23 volt popped. After thinking about it yesterday, an amp board might be where the problems originate.
Still I think I would:
Build the DBT
Remove the +- 23 and +- 50 pins and isolate the Amp board supply wires from the Power Supply.
Check the isolated feeds to the Amps and see if you can find a short to chassis on one set of leads.
Replace the fuses and check the PS voltages.

 

[cecilk]

The Pioneer HPM 100 speakers are among the best in my opinion

I didn't know much about these speakers - was more excited about the receiver - but after hearing them through a working receiver, I'm a believer! Additionally, they are in pristine shape and all speakers are working fine.

 

[cecilk]

Build the DBT
Remove the +- 23 and +- 50 pins and isolate the Amp board supply wires from the Power Supply.
Check the isolated feeds to the Amps and see if you can find a short to chassis on one set of leads.
Replace the fuses and check the PS voltages.

Will do - going to Home Depot to get supplies for the dim bulb tester!

Thanks.

 

[cecilk]

Nearly done with the dim bulb tester - followed the instructions on this AK thread - just have to mount it to a secure base.



A few more questions:

• For tube amps, I was told to use a 250W light bulb in a dim bulb tester - which makes sense since tube amps have a lot higher voltages - but for this application, do I still need a bulb that large or is a 100W bulb sufficient?
• I have noticed that a lot of the board connections use wire-wrapped pins - I've never worked with these before. What's the best way to go about unsoldering these? Any trick to it?

Once I get the DMT up and running, here is my plan based on feedback above:

• Remove the leads to pins 22 (+50v), 24 (+23v), 25 (-23v) and 27 (-50v) on the power supply board and check the decoupled isolated leads for shorts to the chassis.
• Replace fuses and check power supply voltages (With leads re-connected? Or still with pins 22, 24, 25, and 27 removed as per above?)

Thanks for the help!

 

[rcs16]

Do you own a DMM?
You can check for shorts without removing those w/w connections.
You can also measure all the bjts,diodes in the power supply for obvious signs of failed devices.
You can also check the o/p bjts, on the heatsink for condition.
I like to do DMM continuity/diode/bjt junction checks as much as possible first before applying any power. You have two channels in the amplifier sections in order to compare one channel to the other channel. It makes it easy to cross check/compare.

 

[cecilk]

Yes - I own a DMM - I will follow your advice and check power supply components and output transistors.

Thanks!

 

[cecilk]

Update from last night:

• I individually unplugged and tested the output transistors - they check out fine.
• I checked most of the resistors on the power supply board - all are within tolerance - or pretty close. Some could not be tested in circuit. No open resistors.
• Tried to test transistors in circuit on PS board - this was difficult as my DMM leads are the large pointy kind (not the insulated kind with the mini hooks). Going to try to get some today from our local electronics store.
• Tested diodes in circuit on PS board - got weird readings from some of the diodes - but because they are in circuit is hard to tell. No "open" diodes as far as I could tell.

Is it best to unsolder one leads of diodes with strange readings to test them outside of circuit?

Tonight:
* Test continuity and test for ground shorts on PS board and PA boards
* Test components on PA boards

Thanks for all help/advice.

 

[zebulon1]

I thought it might be easier for you to isolate the short by splitting the circuits up.
Most likely the short(s) are in one of the amps. Slim but possible PS issue.
The outputs would of checked good. The problem is in the pre or driver stage of one amp.
First do a good visual check with a magnifying glass of both amp boards and PS. This might be all you need to isolate the board in question.
This part is for your better judgement:
Remove the pins for the +-23, 50 volt feeds from the PS or the Amp boards. Whatever is easiest to remove. Pins 22, 24, 25, 27, of the PS or Pins 1 and 4 of the Amp boards.
Check for continuity from the pin 1 and 4 traces to chassis ground on the amp board. See what gives a low ohm reading.
With the PS's +-50 and 23 volt supplies isolated from the set you could put good fuses in and operate it on the DBT checking for a dim bulb and reading the PS voltages.

When removing pins:
I heat the pin from the top with my wide tip or Hakko FR300. Wait for the solder to melt and push the pin to the board.
Never twist the pin while trying to remove it. Just pull gently. The pins are square as is the board hole.
Some pins are shaped like an "F" which makes for a more difficult removal. Best to find another path to success.

 

[cecilk]

Sorry for the long delay - life got busy and the SX-980 had to wait a while.

UPDATE: I had started meticulously testing the power amplifier boards - every resistor, every diode, every transistor. Both boards checked out A-OK. That got me thinking: I hadn't checked the caps on the power supply board - and only checked diodes in-circuit. So I went back to the power supply board and pulled it up to give it a more thorough check.



As you see, I missed something obvious - the two black 4.7 uf caps (C19, C20) are so bloated that the wrapping is coming undone. Pulled 'em out and they were both toast. Also - C17 and C18 were both bad - tested slightly out of tolerance with very high ESR and vLoss. C17 and C18 also had some brownish marks on the side - might just be flux but wondered if it could also be some kind of heat damage?

D8 and D9 both test OK. The other electrolytics on the power supply board were all within spec - though slightly higher ESR and vLoss readings than new caps. Since I had the board out I went ahead and re-capped all the electrolytics (using the most recent AK component list for this unit).

Tonight I'm going to re-test Q1 - Q4 (got some new pincer leads for my meter to better get at those little critters!), and will also re-test D1 - D4 with one lead out of circuit. If any of the diodes test funny, I'll also test C1-C4 in the same manner.

If anyone has any thoughts based on what I found so far - let me know! Thanks!

 

[cecilk]

One more update:

D1, D3, and D4 all check out fine and as expected. But D2 tested bad (it read 0v).

Also - on closer examination, I noticed some brown discoloration around D8, D10, and D11 that may indicate overheating - but these diodes all test fine.



The scorching also appears around Q1 and Q4:


Making progress. But learned a hard lesson - sometimes a visual check can save you a LOT of time!

 

[cecilk]

Am I on the right track here? Should I remove Q1-Q4 from circuit to test?

 

[buffdriver]

I think you are on the right track. If you don't replace Q1-Q4 (recommended), clean off and replace the thermal compound and check for cold solder joints.

 

[cecilk]

Thanks - So I proceeded to remove Q1-Q4 to test out of circuit - they all tested GOOD. Used both the diode test mentioned often on AK Pioneer boards and also used a transistor tester.

So, to recap (pun intended):
Q1-Q4 test good
C1-C4 test good
D1,D3-4 test good
D2 tested bad and was replaced
D8-D11 test good
C17-18 tested bad and were replaced.
C19-20 tested bad and were replaced.
C11-C13, C15-C16, C23-C25 tested good but were replaced as a precaution.
Resistors all good.

So - I have replacements for all transistors - should I just go ahead and replace 'em? Or put the originals back in? What are pros/cons?

Also - should I replace other diodes too, for safety?

Once done, my plan is to replace fuses that blew and turn power on with unit plugged into DBT.

Thanks for everyone who is helping with this! Hope we can have this up and running soon!

 

[larryderouin]

The replacements are a bit more robust and shed heat a lot better than the originals. If the originals are out, just replace them. If not, up to you, although I would just go ahead and replace them for peace of mind. If the diodes are working and within spec, your choice. Any resistors and diodes replaced should be lifted up about 1/4" away from the board. PIONEER Power supplies have a history of blazing hot operation and the more you do to alleviate this, the happier the unit is. Larger heatsinks on individual transistors (where applicable), lifting diodes, and resistors, etc. It'll still be hotter than the hubs of hell, but the modifications and higher rated parts will offset this. Capacitors with a 105*C rating are a must here.