Pioneer SX 780 Stereo Receiver Repair Part 1 - Evaluating the Receiver/No Sound

Evaluating a Pioneer SX 780 that has no sound.

[@brunoprimas1483]

I replaced the power packs with the aftermarket boards a few years ago. Cost of the boards and components came out to be about $76 back then. Excellent replacements. Plus, it allows you to adjust bias voltage because of the discrete transistors, something you didn't have the option for on the packs.

[@millervintagehifi3034]

I would do the same if I owned anything that used STK packs. I try to avoid them. The trick is convincing a customer to pony up 5x or 6x the part cost for the board vs the aftermarket STK. Same goes for shop owners trying to keep their costs down. Margins can be thin on these lower $$ units to begin with.

[@brunoprimas1483]

@@millervintagehifi3034 I added $125 to my asking price when I sold it. IMO, it was worth it.

[@millervintagehifi3034]

@@brunoprimas1483 It's nice when you have that much room to price accordingly :)

[@repairfreak]

Good luck, will be following your repair. Dealing with the same issue basically. Cannot find ANY original STK 0050 packs. After install of new packs, did a successful dc offset adjustments at 3mV. Supply voltages +/- to the pack at pins 2 & 9 in reference to chassis ground all at proper voltages and seem balanced on other pack as well. However input voltage check at input pins 10&1 reveals not 1.4V & - 1.4V but .553V & - .885 on one pack. .552V & -1.06 on the other pack. The amp turns on with relay click and sounded fine up to about 1-2W on Vu meters. When I hook the unit up to my two 8 ohm dummy load resisters and inject approx 150mV Sine Wave at 1Khz into Aux input and measure across dummy resisters with 2 chan line powered scope, I noticed an abnormality on one channel. Seemed pointed compared to other channel. Then the amplitude started to drop down on the one channel as volume of amp was increased. Heard a pfft noise and amp went into protect mode as original. Shut down amp immediately. Now the one new pack is showing shorted as .175 on diode test mode “both directions” across the one packs pins 2&3. Across 8 and 9 I measured .56 with positive lead on 8, neg on pin 9. Reversal of leads across 8&9 with positive lead at pin 9 and neg at pin 8 shows infinite resistance in this diode test mode. Why any resistance or junction resistance would measure between collectors and emitters just didn’t seem normal “if you look at the internal view of the pinout diagram of the darlington pack.” Anyways swapping in a new pack restored the relay click and brought back my audio. Now I’m just afraid to bring the volume up to high, or God forbid do a signal injection and monitor with my scope across the dummy loads. Not sure why this would cause any issues, tested this way with scope many times in past. I’m very afraid these generic STKS are just going to pop at high volume levels of music. I don’t feel confident in this repair. The unusual input voltages not being at 1.4v & - 1.4 has me scratching my head. Could the cheap STK pack be leaking voltages backwards , thus throwing these measurements off. Prior circuit to these inputs check with correct supply volts and associated resistors. I just don’t know if my problem is with these counterfeit STK modules or a problem with the input voltages. I will be very interested to see what you measure at input pins 1&10 with new pack mounted and amp in idle. If my amp keeps popping the modules I believe I will have no choice but to order the custom discrete type modules sold on ebay. Any thoughts or help is greatly welcomed. 😎👍

[@millervintagehifi3034]

So here is the weird thing. This receiver had been sitting unused in a closet for a while according to the seller we picked it up from (15 years). Sold it and the buyer hooked it up to some Advent OLAs and immediately blew the pack I'm replacing here. I only swapped one pack out (spoiler alert), returned it to the buyer, he powered the receiver on and the OTHER pack blew. I replaced that one with another aftermarket STK. The buyer wanted to swap the receiver out for a Sansui G4700 (who wouldn't)... We've been running it on some Infinity SM-112s as a demo unit in the shop and it's performed flawlessly. We don't get crazy (volume-wise) but do drive it to a respectable level. We try to steer clear of recievers/amps with STKs as much as we can because they can get a little sketchy. They seemed to have been in many makes/models. If an STK powered piece of gear was MINE, I'd swap the STK out for a discrete board kit. Unfortunately buyers (including the shop owners) aren't always willing to put over $100 into a piece of gear and margins on flipping something like an SX 580/650/680/780 make it tough.

[@bmboldt]

I got my SX-780 two years ago at Goodwill here in NW Omaha. I got real lucky that day. Its in great shape and works perfectly. I threw some new LEDs in it recently. Thinking about replacing some caps but then again why? Don't see any issues on the inside.

[@millervintagehifi3034]

Yeah, I'm typically not a recap for the heck of it kind of person...I understand the pros/cons and both sides of the argument for recapping before you need to. I think it depends on what caps/transistors are in the unit, known issues (like really hot PS boards), flaky trim pots that go bad, etc...

[@bmboldt]

@@millervintagehifi3034 I know those transistors in the power supply run hot on these. I may upgrade the heat sinks. People also suggest replacing the orange caps and some of the transistors on here can be noisy.

[@millervintagehifi3034]

I usually go by the “Top Ten Worst Transistors” thread on Audiokarma as an initial guide OR if it’s a Pioneer I’ll look for threads where MTF (MarkTheFixer) has commented on replacing specific transistors for a particular model (like the bias transistors on an SX 3800/3900 as an example).

[@bmboldt]

@@millervintagehifi3034 I go back to that TTWT thread a lot. MTF is good. So was Merlynski. I am still pretty new to this. I'm always telling people I know to let me know if they have something that needs worked on. Needless to say few people I know have anything.

[@millervintagehifi3034]

Go to my webpage and send me a message when you have time.

[@savicvladan6802]

Just modest sugestion: speak slower....

[@millervintagehifi3034]

Will do!