Pioneer SX-1980 Hum on switch-on?

[markthefixer]

NOT a reprimand!!! Guidance. Think before acting, and if ANY doubt remains, ASK!!! That's why we ENFORCE the "there are no dumb questions" rule.

We define dumb as in blindly pushing on.

That is why this is treated as a hobby - it's slow - and it's supposed to be fun.

I am in a full contact match with a SA-9800 presently, and after as long of a lapse as my Hiatus has caused, I take PLENTY of pauses to think through my next set of actions.
This unit was ON the bench, in PIECES, when Dad passed.

 

[Jon1982]

Thanks and I hope you are coping as well as possible.

Having done the power supply, power amp, eq amp (partially) and APC boards I think it would be shrewd to recap the others. Having found problems on 3 of the 4 boards I've looked at, It seems probable that there are other duff caps lurking. Should I recap the tuner board, given the weird fault I have with it might be down to a cap? The other boards look to be bolted to the chassis behind the faceplate. Are they as difficult as they look to get to?

 

[markthefixer]

Faceplate comes off, then the 2 rows of boards get unscrewed from the chassis front and layered out while electrical connections are undisturbed.
You can see why I stand it on it's side for access, on a lazy susan base I made that it is screwed onto - to prevent tipping.
(and this picture is just before I remove the switches from the boards in preparation for cleaning first by other means, then by ultrasonic with some proprietary formulations)

 

[danrclem]

This unit was ON the bench, in PIECES, when Dad passed.

I don't want to take anything away from this thread but want to offer my condolences to you. My Dad passed away in 2008 and my Mom in 2010. I still miss them both.

 

[Jon1982]

Decided to re-assemble the 1980 for a while before I tackle recapping the other boards. All seems to be well The buzz seems to have gone and the amp seems to be noticeably more effortlessly powerful and punchy than before.. wasn't expecting that... is that possible after a mere recap of the power supply board? (I did the output boards as well although all caps there measured okay, at least from a capacitance point of view)

Caps replaced with Panasonic FR.

 

[Jon1982]

Gentlemen (and possibly ladies?!)

With the chilly months upon us I am restarting my SX1980 rolling restoration having functioned perfectly thoroughout the summer months, (other than weird tuner problem) including providing main amplification services for some pretty loud parties

My RX-1603 has been promoted to front-line use whilst the SX1980 is on the 'bench' (otherwise known as the dining room table)

You will not be surprised to hear that I do not have a significant other, otherwise I feel I would not be able to get away with such a state of affairs.

I am re-capping the rest of the amp. I am starting with the scary-looking tuner board. I have noticed that the board has been worked on in the past, but nothing looks untoward except for a 22uf capacitor which is soldered on the reverse of the board, in parallel with C69 (10 uf)

Both capacitors measure fine so I was wondering if someone could shed any light on what this extra cap might have been put in for?

 

[markthefixer]

C 69 10uf 16v is part of the touch sensor circuit. It senses the touching of the tuner knob, to take things out of quartz lock. It smooths the touch detection. I do not know if your unit has a sensor gain switch in back, so conditions may have been ripe for chatter and with no sensor gain switch in back, this was added to compensate for local conditions.

 

[Jon1982]

Having a bit of bother with the ol' 1980.

One of those notorious 35V zeners on the PSU board failed and went short circuit. I am KICKING myself a little, since having recently worked over the board, I didn't bother to replace that diode, since I could see it had already been done (believe it is a 36V which someone presumably decided was 'near enough'). Anyway, the diode also took out 1 transistor (expected), and the two 1A fuses. The current source FET and other transistors in that area appear to have survived having checked them with the multimeter.

Anyway, diode & transistor replaced and now 1A fuses are not blowing but the 6.3A fuse is. Bugger. It seems I've got some more damage to deal with then. Could anyone point me towards the likely cause?

Failure of the unit was pretty undramatic, was switched off during some pretty loud playing and just failed to switch back on, no nasty smells / smoke evident so hopefully nothing TOO serious! Bit annoying after my re-capping efforts which went very well!

 

[Jon1982]

I should add, the 6.3A fuse seems to be in the place of a 10A temperature fuse, I don't know if this was for european models or whether my unit supersedes the service manual. Anyway, the fuse is blowing immediately on power-up.

Just measured between pins 26 & 28 on the board. Dead short. That doesn't look right!

 

[Jon1982]

Curiouser & curiouser.

I desoldered what I believe to be the 'soft start relay'. Remeasured between pins 26 & 28 and no longer shorted. So I suspected relay problems.

I checked the relay though and it isn't stuck although perhaps contacts not the best. I'm wondering if my desoldering it freed off the stuck contact and let it flirt back to its N/O position before I could inspect it. Does that sound plausible? Are these relays known to cause aggro?

 

[Jon1982]

Further update... I've put the relay back in (no longer stuck closed, pins 26 & 28 no longer shorted). I have disconnected the molex connections to the outputs as I'm well aware that they are unobtainum. Anyway, unit initially powers up, but when switched off & on, one of the 1A fuses blows, sometimes.

Something is still surging as I notice the lighting in my house dip slightly when I switch on.

I have measured the voltages at the board and I have the + & - 80V and + & - 35V as expected.

Anyone give us a clue then? Thanks in advance.

 

[Jon1982]

Bit more info, the fuses that are blowing are FU3 & FU4. Looking at the diagram, am I right in saying that these provide feeds for the low voltage regulators, 13.5V, 16.8V, 5V? Interestingly, the diagram says these are 1.5A, however my unit the PCB is stamped 1A.

 

[Jon1982]

Although no-one has replied as yet I just thought I'd give some closure.
The 'Grand Old Lady' is back in action.
When fuses FU1 & FU2 blew, due to shorted zener, I also changed FU3 & FU4 and thought no more of it.
Although these fuses I removed were stamped 1A, I put these back in, and hey presto, they don't blow.
Are they 'slow blow' or actually rated 1.5A despite saying 1A on them, I wonder.
So, course of events was - zener shorts, kills transistors, blows fuses.... thicko / drunk Jon puts in 2A fuses (only ones around at the time) then 6.3A fuse blows and presumably excess current welds the relay contact in the ON position... thus any subsequent switch-ons bypass the 'soft start' and the surge again blows the 6.3A fuse.
Anyway, problem solved for now. I shall probably at some point replace those relays and any other silicon that I haven't done already.