Pioneer SX780 Repair

Hi guys/gals. I'm a bit of a noob to the vintage audio repair game, so please bear with me. A few months ago, I bought an old Pioneer SX780 off of Craigslist. It was in great condition and played perfectly for a few months. Then, when I moved the receiver for a work party, it stopped working. The relay click would click about every 5 seconds and a brief flash of sound would occur, with periods of silence in between. I browsed these forums extensively and have made the following changes:

-I blew the dust out of the casing and cleaned solder joints with isopropanol.

-I replaced both STK0050 Darling power packs with new units from Audio Lab of Georgia and replaced the heat sink compound on both sides of the heat sink.

-I cleaned and reapplied heat sink compound to the heat sinks for the three transistors near the power packs. It should be noted that I had a very difficult time removing Q25 from its heat sink and the transistor broke in half in my attempts to remove it. I carefully superglued it back together and reinstalled it.

-I resoldered all joints for those three transistors.

-I constructed a dim bulb tester.

Where I stand now: when I power on the unit with the dim bulb tester, the bulb flashes briefly and becomes dim, indicating that there don't seem to be any shorts. However, the relay does not click at all, the LEDs do not light up, and there is no output through either channel.

At this point I believe the problem is above my paygrade. Any and all help would be much appreciated!

Welcome Aboard!

If Q25 physically broke, as in the case broke off the metal tab, or the plastic case broke into two or more pieces, it is likely worthless.

It is apparent that there are no reservations about getting into the unit for some work, so if you have a voltmeter at your disposal a few readings could be very helpful in analysis.

The fact that the DBT only briefly lights brightly then dims could be a very good sign.

If you can, carefully measure the DC voltage at the metal tab of each of the three voltage regulator transistors, Q19, 20, and 25. Voltage readings should be taken with the unit on, volume at minimum, and no speakers connected (yet), and with the meter negative referenced to chassis ground.

Yup, also insulate all but the end of the tip of the red probe with "scotch" tape - for when slips happen. (we ALL slip)

Very good to use a dim bulb tester, and very good to look at bad solder and old grease on the three regulator transistors - a known achilles heel. In fact, that might have been your only problem. If you posted before you made a parts order, you would have probably been advised to buy three new transistors because they are dirt cheap and the old ones have been cooking themselves for many years. Just in case...

Q19 512-KSC2073TU
Q20 512-KSA940TU
Q25 512-KSC2073TU

Recommend you keep your original STK0050's safe in antistatic wrap because they might be fine. There are a lot of counterfeits floating around, and your new ones might not be as good as the old. I doubt our experts would have advised preemtive replacement. Got any pictures of the work you did?

In addition to the replacement for the broken transistor, insulating the tab and adding some heat sink area may be beneficial, at least in the Q25 position. The insulation can be something like THIS and the heat sink can be something like THIS modified to fit the space.

Wow, thanks for the prompt replies! I'm aware of the problem of counterfeits from browsing these forums, but Audio Lab of Georgia was mentioned as a reliable supplier, so hopefully they are alright. If not, I do still have the old powerpacks in a safe place. I will go ahead and just replace all three transistors and add the extra insulation and heat sink to Q25, I plan on having this old guy for years to come and a few bucks is nothing when it comes to a labor of love. Once I replace the transistors, I will give you the measurements you asked for Watthour. I've also ordered some new caps, I'm already this far down the rabbit hole, might as well do a few recaps once I get it started up again. In the meantime, here are some pictures of the receiver so you can check my work. Apologies for the poor quality, the only camera I have is on my phone. Let me know if you see anything that needs fixing. Thanks so much!

Hi all, I'm back! Since I last posted, I have replaced the following:

-Transistors Q19, Q20, and Q25
-Resistors R263, R264, R265, and R266 (I found that one was snapped right in half, so I decided to go ahead and replace all 4.)
-Capacitors C207, C208, C213, C214, C217, C218, C501, and C502

Observations: The unit turns on, the DBT is dim, the lamps are on, the tuner works and the red light comes on when the signal is good (it did not do this when it was initially broken), but there is still no output from either channel.

Watthour, here are the data you requested. For each transistor, I will give the voltages from left to right, with the unit on its back and the faceplate towards me (the same orientation as the schematic). All measurements in V.

Q25
13.07
13.64
13.78

Q19
35.4
36.7
36.4

Q20
-35.9
-37.9
-36.6

Any assistance is much appreciated. Thanks!

EDIT: I would like to add that there is no relay click. I also took the voltage measurements on the Darlington packs. They are as follows:

Left Pack
1: -1.169
2: -29.8
3: -47.2
8: -44.9
9: 29.8
10: 1.114

Right Pack
1: -1.169
2: -29.8
3: -20.1
8: -19.8
9: 29.8
10: 1.138

Something is not right with those transistors. Either they are installed incorrectly, identified incorrectly, or are already damaged.

Read the schematic carefully and identify which leads are emitter, collector, and base. Compare those to the pinouts for the replacement devices, then verify they are correctly installed. Also verify that none of the surrounding resistors are open and that the solder connections are intact.

Watthour, the transistors were installed backwards. My apologies for the rookie mistake. I flipped the transistors around and reinstalled. Now none of the needles move, including the radio signal needle, and the red stereo light no longer comes on. As before, there is no output from either channel. The new measurements are as follows:

Q25
1.8 mV
50.4 V
3.53 V

Q19
4.25 V
50.2 V
17.19 V

Q20
-1.58 V
-50.2 V
-27.9 V

Left Pack
1: -0.79 V
2: -38.6 V
3: 0.912 V
8: 0.916 V
9: 37.6 V
10: 1.115 V

Right Pack
1: -0.817 V
2: -37.6 V
3: 0.107 V
8: 0.107 V
9: 37.6 V
10: 0.116 V

My multimeter shows resistance across all the newly installed resistors (0.4 ohms) and the solder connections appear to be fine.

Also others please feel free to chime in. I welcome any and all assistance.

Just a little bump. Does anyone know what to make of these new readings or what my next step should be? All help is greatly appreciated.

It's an open forum, so anyone can post at any time.

50V at the collector of Q25 indicates a problem with its power supply. There should only be about 25V there.

Check the DC voltage at both ends of the large cement resistor on the heat sink (R1) to measure its voltage drop - I'm guessing there is none.

Also check the voltage across zener diode D4.

Reporting the E-B-C voltages from Q21 and Q22 could also be useful.

Thanks for your continued help Watthour. The voltage drop across R1 is 0.5 mV. The voltage drop across D4 is -376 mV. Here are the transistor measurements:

Q21
E: 3.58 V
C: 17.18 V
B: 3.04 V

Q22
B: -0.00 mV
C: -27.8 V
E: 1.306 V

EDIT:
Wow I could have sworn that I included this in my original post, but I see now that I didn't. When I was initially attempting to fix things, I accidentally shorted the left Darlington pack due to shoddy soldering. There was a puff of smoke and the 4A fuse blew, hence my replacing the Darlington packs and that fuse. I'm sorry for omitting this information initially.

My apologies, polite bump. I don't mean to be a pest but I don't want to let this thread die. I'm a novice, but I am learning a lot from this project and appreciate the assistance.

No need to apologize.

The voltage drop at R1 is not right. I should have asked for the voltage measurement at each end of that resistor instead (referenced to chassis ground).

Since the power supply is producing the expected 50V at Q19, it is probably O.K.

The fact that the collector of Q21 is only at 17V and the zener is nowhere near the expected 14V is a problem.

The incorrect installation of the regulator transistors may have damaged them, or may have caused collateral damage. Testing those regulators and Q21, 22, 23, and 24 may be helpful. Perform the "6-way" diode test on each of them if you don't have a component tester available.

http://www.audiokarma.org/forums/showthread.php?t=43186

Also, in reviewing the images you posted earlier, it appears that the heat sink for Q25 is severely distorted.



That needs to be made to fit the transistor so that the heat generated there can be effectively dissipated.

Watthour, as always, thanks for taking time out of your day to help out a budding hobbyist.

Since the last post, I found a cold solder joint on one of the capacitors I replaced and reflowed it. As a result, a few measurements have changed. I’m going to err on the side of giving you a lot of data, so please bear with me.

The voltage drop across R1 is now 0.2 mV. The voltage at each end of R1 referenced to chassis ground is 49.1 V.

Here are the new voltage measurements for the transistors that you have already asked about.

Q25
E: 586 mV
C: 49.5 V
B: 13.46 V

Q19
E: 23.1 V
C: 49.4 V
B: 36.0 V

Q20
E: 1.91 V
C: -49.4 V
B: -24.4 V

Q21
E: 13.47 V
C: 36.0 V
B: 12.89 V

Q22
B: 0.1 mV
C: -24.4 V
E: 12.35 V

The voltage drop across D4 is now 13.47 V. The previous measurements were made in diode mode instead of voltage mode. After doing some more reading on the matter, I now see why Zener diodes must be tested in voltage mode. Again,apologies for the novice mistake.

I removed the transistors and performed the diode test on them. Q23 shows a voltage of 170 mV across the C-E junction, meaning that that transistor must be replaced, yes? Do you have a Mouser number for an appropriate replacement? Also, while all of the other transistors were intact, I believe Q22 was installed backwards. The rightmost pin is the base, which should be pointing up according to the schematic, but it was pointing down in the receiver. Please see the attached picture.

Yes, the heat sink was distorted in the midst of my difficulty removing the original transistor. I have moved the warped heat sink to Q20 since Q25 is the most prone to overheating, but if you have a Mouser number for a replacement heat sink I would gladly replace it.

BD - Thank you for the updated readings. Your information is helpful.

It now appears that Q21 is closer to normal, and is trying to bias Q19 on at about the 40V design - Your 36V is close for now.

Q19 C and B are reasonable now, but the emitter of Q19 is still quite low at only 23V. There may be something still dragging down that supply or a poor connection somewhere in the power supply area.

Similarly, Q22 now appears to be operating but will never get close to normal until the emitter voltage is 0V. Verify the 12.35V measurement you reported and if it is true, check all the circuit grounding. That emitter lead should be a chassis ground potential. If that can be repaired, the other circuit voltages may come around.

I will reinstall the transistors that I have verified as good and ensure that Q22 is installed in the proper orientation, but I still need to replace Q23 based on its failing the diode test, do you have a link to a suitable replacement? Once all transistors are reinstalled, I will check the grounds, take new voltage measurements, and report back.

bdaniels said:

...if you have a Mouser number for a replacement heat sink I would gladly replace it.

Click to expand...

Given the fairly cramped surroundings in that area, selecting a heat sink will require a bit of measurement. You might want to check the dimensions of this type for suitability:

http://www.audiokarma.org/forums/showpost.php?p=6434652&postcount=1

Q23 shows a voltage of 170 mV across the C-E junction, meaning that that transistor must be replaced, yes?

Click to expand...

Q23 (along with Q24) is a field-effect transistor (FET) instead of a bipolar. There is a reasonable test procedure for that explained here:

http://www.audiokarma.org/forums/showpost.php?p=1070957&postcount=2

Thanks for the help on the heatsinks, I will look into it. I used that test procedure and both Q23 and Q24 seem fine, they both have resistances of 314 milliohms and the voltage drops are 640 mV for Q23 and 605 mV for Q24. So it seems now that I should reinstall all of the transistors, taking care to mind the orientation, check the grounds, and report back.