SX-780 Protection Relay Chatters on Startup

[cgtuttle]

Hello,

I have a Pioneer SX-780 that I bought new in 1980 and worked well through the late 90's when it was relegated to a closet. I have since dug it out and I am using it in my office where it works beautifully except for one issue that is more bothersome than anything. On startup the protection relay which in the past delayed pulling in for several seconds now pulls in immediately then lets go, and then may or may not repeat that process two or three times, over a second or so, before finally pulling in and working fine.

I have done the following to try to diagnose/fix the problem:

1. Replace C319 which appears to be the timing capacitor for the protection circuit
2. Check Q26 with a diode tester and re-install it
3. Re-flow the three transistors attached to the heat sinks - Q25, Q19, Q20
4. Re-flow R325 and check its resistance
5. Check voltages of several components (in order of BCE):

Q26: +0.642; +2.342; 0.0mV
Q25: +13.91; +25.60; +13.30
Q19: +40.16; +50.16; +39.58
Q20: -39.81; -50.33; -39.18

These are all close to spec, but the collectors of Q19 and Q20 are off by 1.5 and 3V respectively. Is this a problem?

I also checked the pins of the protection module Q27:

1: +13.38
3: +0.639
4: -13.0mV
5: 0.0mV
6: -7.20
7: -40.1mV
8: +7.21
These numbers appear close to spec, but also slightly off, especially pin 7 which is supposed to be 0, and pin 3, which is supposed to be +0.9V.

First, have I looked in the right places and does any of this point to a solution? to me nothing is obviously causing the problem.

Second, if none of this is a concern, where might I continue to look? The fact that the system sounds great leads me to believe that the musicy part of the circuit is in decent shape.

If you have read this far, thank you for taking the time. I welcome any thoughts.

Thanks,
Carl

 

[Watthour]

Welcome Aboard!

If the voltages you reported at Q26 are B-C-E, Q26 is on (conducting) and the relay should be pulled in.

C317 is the delay timing for pull-in, not C319. C319 (along with R325) is the delay for voltage detection in the speaker outputs.

 

[cgtuttle]

Wow! Five minutes to replace C317 and problem solved! Thank you Watthour, my SX-780 is like new again.

 

[Watthour]

Nice save!

 

[Watthour]

Since you are already under the hood, you might want to consider some routine maintenance.

The output packs can usually benefit from a fresh layer of thermal paste.
The solder joints at the power supply regulator transistors should be inspected closely for signs of heating, and re-flowed if necessary.
Check the offset (DC balance) of the output devices, and adjust as necessary to get the offset in each channel as close to zero volts as possible.
Glue a strip of aluminum foil to the inside of the top bonnet, just above the location of the dial illumination lamps to prevent further heating of the wood veneer.
Lubricate the dial cord guide rollers, and apply some beeswax to the dial cord where it passes over the tuning spindle.
Clean out dust and debris as best you can.
Clean and exercise the switches with a de-oxidizing cleaner, and apply a potentiometer/switch lubricant afterward.

I can come up with more if you get bored.

 

[cgtuttle]

And now looking through the service manual I see that if I had read more carefully I would have seen the diagram pointing out that the timing capacitor comes off of pin 8 of the protection module to ground, and is thus C317.

 

[cgtuttle]

Thanks for the maintenance list - I see symptoms relating to a lot of those tips (squeaky tuner dial, slight scorching of the veneer on the case above the lamps). I'll work through the rest.

 

[Watthour]

I've done this to all the 80 series and some of the 50s I have: