Pioneer SX-737 audio output just stopped

[fromnowon]

I'm curious if you replaced the relay along with the recap and transistors.

 

[larryderouin]

Ideally, Relay timing is closed 4 to 7 seconds after switch thrown. It's controlled by IIRC by Q6 and C5 or C6 on the protection board. Best to rebuild the protection board along with the Amp boards. Add a 1n4004 across pins 9 & 10. Cathode side to pin 10. This mitigates the back voltage to Q7 when unit is shut down and magnetic field in the relay coil collapses, Pioneer installed in later models.

 

[mcdmgb]

Ideally, Relay timing is closed 4 to 7 seconds after switch thrown. It's controlled by IIRC by Q6 and C5 or C6 on the protection board. Best to rebuild the protection board along with the Amp boards. Add a 1n4004 across pins 9 & 10. Cathode side to pin 10. This mitigates the back voltage to Q7 when unit is shut down and magnetic field in the relay coil collapses, Pioneer installed in later models.

The caps were replaced when I recapped the unit and changed out all the problem transistors about 5-6 years ago, except for Q7, Q8, Q9 and Q10 on the amp board, which I did over the weekend. I did the diode mod also. I've let it sit overnight and the bias has stayed reasonable close to 20mV and DC offset close to 0V. I'll let it go for awhile longer before worrying about the relay.

 

[golfnut]

The drop out's sound like they are most likely heat up related in the amplifier board. #1 suspects are the 2sc1451's. These are the most problematic of the problem transistors that PIONEER used in the Early 70's to late 70's amplifier boards. The symptoms are not unlike listening to a bowl of RICE KRISPIES in Milk. WHEN the 2SC1451 finally dies, it WILL take resistors, caps and other parts including the output's with it. Prior to it dying completely you will get the symptoms stated by your initial post.

The #2 suspect(s) are the 2sa726's. They aren't as bad as the 1451 but they will present with similar symptoms.
Q1-Q2-Q3-Q4 need matching per pair. They are part of the biasing circuit and affect how close you can get.
Solution is to replace them pre-emptively, and check/replace parts immediately upstream/downstream of the offenders. Also replace any 2SA725 or 2SA726 on any boards they are present on. (about 15-18 total).

These are the transistors I replaced on my 737 amp boards in early 2014. All are FAIRCHILD transistors except for the outputs MJ21193G and 21194G which are On-SEMI. All numbers noted in this post are MOUSER Numbers.

2sa726 (ecb) > 512-KSA992FBU (ln)to-92 ecb 120v .05a .5w 100mhz 150-800hfe $0.22 ea
2sc1451 (ebc) > 512-KSC3503DS or DSTU to-126 ecb 300v .1a 1.2w 150mhz 60-120hfe $0.54 ea
2sb527 (bce) > 512-KSA1220AYS to-126 ecb 120/a160 1.2a 20w 155mhz 35-320hfe
2sd357 (bce) > 512-KSC2690AYS to-126 ecb 120/a160 1.2a 20w 155mhz 35-320hfe
------- output transistors (unlikely but for completeness) -----------
2sb530 > 863-MJ21193G pnp to-3 250v 16A 250W 4mhz 8-75hfe
2sd370 > 863-MJ21194G npn to-3 250v 16A 250W 4mhz 8-75hfe

Prices above as are 01/25/2016

Pin layout LEFT TO RIGHT (Front facing you, pins to bottom)
Q1 Q2 Q3 Q4 original 2sa726 (ecb) > 512-KSA992FBU to-92 ecb is same as the original
Q5 Q6 2sc1451 (ebc) > 512-KSC3503DSTU to-92 ecb is REVERSED B and C...
Q9 Q10 2sb527 (bce) > 512-KSA1220AYS to-126 ecb is REVERSED from the original
Q7 Q8 2sd357 (bce) > 512-KSC2690AYS to-126 ecb is REVERSED from the original

Larry

Hi Larry,
I realize this is an old post but I do have a similar, if not identical, problem. I don't know if you can answer this or if someone else could chime in but here is what I have done.
1-I have connected it to a DBT with a 60w bulb. The light comes on bright at first then dimly stays on.
2-I have spayed and worked the two tape monitor switches over and over several time with no sound result.
3-I do get a pop in the speaker here and there but no sound from the tuner. I also plugged in a source in the aux and same result.
4-all fuses are good
5-per the schematic, I get 36v respectively at the #22 and 32 as well as -36v at the #25 and 28 posts.
6-the speaker outputs show between 31 and 33v. Which I know is not right and should be mv.
So with the #6 noted above, I'm thinking it's some or all of the transistors you noted above. Being somewhat of a novice in this area, is there a decent way to test the transistors while they are in circuit? Is the power on or off? What kind of readings should I get? Your detailed explanation would be appreciated.
Regards,
Mark