pioneer sx 780 protect relay shuts off with volume increase

[1rs2004a]

Hi all, I have a sx 780 that turns on plays normally until you turn the volume up to about 3. I have monitored the in put on pin 4 of the pa3004a Ic and it appears to have the out put showing up on it albeit very small. When I ground this pin the protect relay stays on and I have good strong output from both stk 0050 packs about 60 volts p-p before clipping.

Could a bad pa3004a cause this?. Or a bad stk0050?

Also found a discrepancy in voltage on the relay coil the manual says 42v and measured 47v.

Thanks in advance for any help with this.

Rick

 

[tarior]

What do you mean "turn the volume up to about 3"?
What kind of input?
Speakers?
If it's shutting down with the music kind of loud and screechy, or you are running speakers in parallel, have shorted speaker wires, and/or have damaged speakers, the circuit is probably functioning as designed. It's keeping the outputs from going up in smoke.
If PA3004 or one of the STKs were bad, it likely wouldn't come out of protection in the first place.

 

[1rs2004a]

Sorry for not being clear.by 3 I mean 3 tics up on the volume about 2 watts low volume.speakers were connected normally and there is no distortion in the music.so I connected 2x 8 ohm load resistors in place of the speakers. Then I injected a 1 v 1khz signal to the aux input while looking at the signal on pin 4 of the pa3003a chip. When I slowly turn up the volume I have an ac wave form no DC volts.when the sin wave gets to about 3 mv the relay shuts off.I have checked this on another 780 and had no sin wave on pin 4.

 

[tarior]

If you can drive 60 volts P-P (about 21 Vrms) across 8 ohms, that's about 56 watts. That would indicate to me that there is nothing wrong with the amp.
There may be something faulty between the amp output and the protection IC. I would eliminate that possibility first.
Are you seeing any DC on the outputs?

 

[markthefixer]

The sx-780 does not have any output current sensing. The load does not really matter.

Very telling that holding pin 4 grounded keeps it running.
Replace C319 47uf/16v first, then try again.
Also be sure pin 4 to ground has the R325 11k resistance working.

 

[1rs2004a]

I ckd for DC on the outputs did not have any I also ckd and adjusted the dc balance 'vr6 and vr7.I suspect the problem is between the amplifier outputs and the pa3004a just not sure how that part of the circuit works.I know there are two 75 ohm resistors that sum together to zero volts if everything is ok and sent to t he input pin of the protection IC.There are other transistors and caps that tie in also not sure what roll they play.

 

[1rs2004a]

I will replace c319 and ck R325 and report back.

Thanks

 

[1rs2004a]

Well I pulled c319 and it ckd open R325 was in spec. Replaced c319 and Whoo' Who!!! she is now playing at full power!!

Thanks again to all for help! and THANK YOU AK for providing this forum!

 

[jeffpaletz]

The depth of knowledge here by some members, especially Mark, is amazing. I had an SX-850 with low volume only on FM. Mark told me the one capacitor to change on the tuner board which fixed it.

 

[rcs16]

Chock it up to years of experience and knowledge. Hats off to all that contribute to keep these old units operational.

 

[1rs2004a]

I just recently got intrested in fixing these golden age receivers after bringing back to life my 1 st receiver I purchased new when I was a teen. It is a fisher 2004a and was put away for 15 years when it stopped working. I dug it out watched some you tube videos replaced the stk 0050 and has been working strong since. I discovered this forum recently and will be spending a lot of time here as I grow in my trouble shooting skills (with the help of gentlemen like Mark)!

 

[llwhtt]

OK now that it's fixed I'm surprised nobody, including the OP, asked why this capacitor fixed it! Other than MTF anyone care to guess or to know?

Craig

 

[tarior]

I don't have the schematic in front of me, but I suspect that it's the timing capacitor for the relay driver. It sets the delay time between unit power up and relay close.

 

[1rs2004a]

I'm guessing the cap appears as a short to that portion of the output that is on pin 4. The cap is connected to pins 4 and pin 5 of the pa3004 IC .pin 5 is grounded so that was the reason I. Was seeing the output on pin 4. As l increased the volume the account on pin 4 would increase above a threshold and shut everything down.

Am I on the right track?

 

[1rs2004a]

Should have read the amplitude would increase

 

[llwhtt]

Close enough, the capacitor shunts most, if not all, AC signals to ground leaving nothing on pin 4 in a healthy amplifier. When a high enough DC voltage comes along the speaker relay de-energizes. When the capacitor is open/missing a large enough, long enough AC will trip the protection the same as a DC level.

 

[1rs2004a]

Thanks for the explanation the problem was right in front of me just couldn't recognize it.

Question...is it always necessary to remove the components from the board to ck? Transistors, caps,resistors etc.

 

[llwhtt]

Whether to remove or not comes with experience. I use an in-circuit ESR meter to test capacitors, just have to decide whether the cap is shorted or not. Resistors depend on the surrounding circuit, again experience will come into play here. Semiconductors same thing. If I find something questionable I remove and test.

 

[markthefixer]

Close enough, the capacitor shunts most, if not all, AC signals to ground leaving nothing on pin 4 in a healthy amplifier. When a high enough DC voltage comes along the speaker relay de-energizes. When the capacitor is open/missing a large enough, long enough AC will trip the protection the same as a DC level.

GOOD explanation

To clear up something in an earlier post, R330(R) and R331(L) are 75,000 ohm resistors, both connected to R325 11,000 ohms which is also connected to ground forming a summing voltage divider. C319 bypasses the audio, while DC persists, triggering the protection IC, dropping pin 3's 0.6v to 0v, turning OFF Q26 and dropping out the relay, disconnecting the speakers. pin 8 has the timing cap. pin 7 and pin 6 are AC and DC voltage monitoring points, pin 5 is ground and pin 1 is the power to the chip.
1 power in
2 x
3 relay drive transistor base out
4 amplifier dc monitor in
5 ground
6 dc psu monitor in
7 ac monitor for fastest power OFF warning in
8 timing capacitor

vulnerability errata: if the amplifier output dc offsets are EXACTLY symmetrical ( L = +5.907v dc , R = -5.907v dc for example) the protect circuit will NOT see a problem!!!

 

[sregor]

vulnerability errata: if the amplifier output dc offsets are EXACTLY symmetrical ( L = +5.907v dc , R = -5.907v dc for example) the protect circuit will NOT see a problem!!!

Only saw this once, when one channel went to the positive rail, and the other to the negative rail. Not a good situation. One solution Ive seen is to change one of the resistors about 20 percent, so if both outputs go to opposite rails, protect circuit will still work. Not usesd here, but TA7317 protection IC has inputs for each channel to avoid this.