Need help troubleshooting SA-7500 II

[Cuppy]

Hoping to get some help diagnosing a problem with the protection circuit on this amp. This amp was given to me as non working. Initial power up on DBT passed indicating no major shorts. Symptoms on house power..., after initial power up, protection relay clicks after 5 secs as normal for muting function. After approx. 30 secs, relay will turn off. Then relay will continually cycle on/off with no real consistent time between cycles. Sometimes 5 or 10 secs. Sometimes longer. I have researched as much as I could find on this but not in total understanding of how this circuit works. Any help would be greatly appreciated. The one document I could find from the Pioneer Tuning Fork article on troubleshooting protection circuits, mentioned that if the relay turns on after the standard 5 secs after power up, it is likely a fault in the detection side rather than the relay driver side.

The output transistors passed testing (in circuit) for shorts and they have the correct voltage on all collectors with power on (41vdc, -41vdc).

Not sure what to include here for diagrams and the SM from hifiengine is of poor quality.

 

[bberkom]

Have you measured DC mv at the speaker terminals when the relay is on? Excess DC going to the speakers could be shutting off the protection relay. I'd suspect one of the transistors, maybe Q4 or Q5, in the protection circuit if the DC looks normal.

 

[Cuppy]

OK. I was able to take a few readings while the protection relay was pulled in. DC mv at speaker terminals measured 40mv right and 15mv left (with speaker selector set to A).

Regarding transistors Q4 and Q5, I believe these are actually Q20 and Q21 on the board. One thing I did notice is that the voltage fluctuates at the base of both even with the relay still pulled in. Base of Q21 (9 V) and base of Q20 (-1.0 V) fluctuate from 9.5vdc-1vdc and -1.5vdc-0vdc respectively. Not sure what that means or if it is helpful. I was hoping not to have to shotgun transistors or caps but will if it makes sense.

 

[Cuppy]

Still hoping for a little guidance as to what to check next. Should I replace all transistors, caps, diodes, etc. in the protection circuit? This one has me stumped.

 

[bberkom]

Not being familiar with that amp, it's hard to say. I'd pull and test all of the protection circuit transistors. Has the amp been recapped or refurbished at all otherwise? I'd imagine a recap would do it good but I understand not wanting to shotgun it.

 

[Cuppy]

Just getting back into this after a short hiatus. Really hoping for a Pioneer guru to chime in.

Same symptoms as before. On initial power up, protection relay pulls in after approx. 5 secs. It will usually stay pulled in for 30 secs-1 min before beginning to cycle on/off. Measuring for DC voltages at the test points for the outputs both sides measure approx. 35mv. Listening through headphones when the relay is pulled in I can get clear audio both channels. The one thing I do find odd is my recorded voltages at output transistors comparing to expected as listed on schematic. All output collector voltages are as expected (40,-40 vdc). All emitter voltages are reading 0 (.6,-.6 expected). All base voltages are reading .6,-.6 (1.2,-1.2 expected). Can anyone say if that may provide any clues? It would seem odd to get clear audio if the output transistors were faulty. Open to anything at this point.

 

[larryderouin]

I'd make sure the power supply is functioning correctly BEFORE anything else. If your power supply is goofy, then everything else is too. Then after getting that correct, start with voltages from pin 1 to the last pin on the protection board and list them in a vertical format in a post. This should give enough clues as to what to do. Read the Description of the Protection Circuits on Page 8 of the manual. That will give you some hints, AFTER you make sure the POWER SUPPLY is ok.

HI-RES manual here
Pioneer Manuals (lot better than Hi-FI engine) here Bookmark this for your PIONEER GEAR.

 

[Cuppy]

Larry,

I wish it were that easy for me to do as you describe. I am quite sure the power supply is solid. I am getting the steady regulated +/- 40 vdc which can be easily measured at the collectors of the outputs Q13-Q16. Unfortunately, the 7500 II does not have a separated protection board just a single integrated board. Therefore I cannot list pin voltages in a vertical format as you suggest. I do have the SM and have read the protection circuit description but it is over my head. I have also read the Pioneer Tuning Fork sheet on troubleshooting protection circuits. The only thing I can really come to is that it's probably not the relay driver section of the circuit since the relay pulls in as normal after 5 secs.

 

[LesE]

I believe that the +/- 40 V measured at the outputs are unregulated. The regulated supply is fed from separate transformer secondary windings. You will want to check the voltages in the regulated power supply as Larry suggested. You will also want to make sure that your voltmeter probe is well insulated to prevent an accidental short.

Les.

 

[Cuppy]

Les,

Thanks a lot for the input and correcting me on the regulated power supply. Based on that input I have taken measurements and compared to the schematic you posted.

Readings as follows; Measured (expected)

Q22- B=43.7vdc(43vdc), C=54.9vdc(51.4vdc), E=43.26vdc(42.4vdc)
Q23-B=14.02vdc(13.8vdc), C=43.93vdc(43vdc), E=13.42vdc(13vdc)
Q24-B=-20.48vdc(-21vdc), C=-45.7vdc(-47.6vdc), E=-19.93vdc(-20.4vdc)

AC volts from transformer secondary on pins 51 & 53 =42.3vac

Does this confirm the power supply is ok?

 

[LesE]

Those voltage measurements look reasonable to me. I assume that the amp was in protection mode when these were taken.

I'd look at the protection voltages next.

Les

 

[Cuppy]

Thanks Les, Actually, the protection relay was cycling on/off when the readings were taken but readings remained rock solid regardless. I will take readings on protection circuit and report back. I do know that these readings are fluctuating, especially near Q21.

 

[Cuppy]

OK. Here goes. Measured readings stable except where indicated. Hopefully this will help point to the culprit. Readings taken with relay cycling on/off.

Q17-B=-.009vdc, C=22.75vdc, E=-.6vdc
Q18-B=-.007vdc, C=22.7vdc, E=-.624vdc
Q19-B=23.2vdc, C=-1.2vdc (fluctuating rapidly), E=18.9vdc
Q20-B=-1.2 vdc(fluctuating rapidly), C=2.7vdc (fluctuating rapidly), E=-1.2vdc (fluctuating rapidly)
Q21-B=2.7vdc (fluctuating rapidly), C=36.6 vdc(fluctuates rapidly 24vdc-37vdc), E= 1.2 vdc (fluctuates rapidly 1-4vdc)

Voltage measured at either side of R87, 37.56 vdc & 43.0vdc (stable)

Any suggestions from here greatly appreciated.

 

[LesE]

Q19 looks a little suspicious to me. With the base being more than 4 volts higher than the emitter and stable, Q19 should be solidly turned off. Instead, Q19 appears to be turning off and on rapidly which is causing Q20 and Q21 to toggle.

Les.

 

[Cuppy]

OK. Thanks Les! I'm guessing you are recommending replacing Q19. Q19 is a 2SA733 at present. Can this be replaced with a KSA992? Also, since I will be placing an order, are there any other items in this circuit you would recommend replacing in this protection circuit as insurance?

 

[Awesomeaudio]

The symptoms you describe was exactly what my SA 7100 used to do.
The unit is aging and the components are simply tired. I'm not familiar with the 7500 but it was explained to me that in the 7100 there are some ticking time bomb transistors that always go bad.
I did a full recap and replaced all the problematic transistors including a few diodes and has been rock solid for years.
Thanks to Mark the fixer, patfont, Larry and a few others for the guidance
I can understand if you're not inclined to do the full refurb, it is time consuming as you know, but if you plan to keep it I'd definitely go down that path.
All the best

 

[LesE]

OK. Thanks Les! I'm guessing you are recommending replacing Q19. Q19 is a 2SA733 at present. Can this be replaced with a KSA992? Also, since I will be placing an order, are there any other items in this circuit you would recommend replacing in this protection circuit as insurance?

I'd try replacing Q19. I believe that a KSA992 would be adequate for this application although a KSA1013 is a little beefier. I'd also order a few KSC2383's just in case Q20 is causing the problem. You may also want to consider changing Q21 with a KSC2690 since Q21 takes a beating from the relay coil back EMF.

Les.

 

[petehall347]

i would also replace c47 whilst in there

 

[Cuppy]

Thanks all for the tips. I will replace Q19-Q21 and C45-C48. I have already replaced the relay. I am also thinking of replacing the 2 failure prone 2SA798's in this unit with matched KSA992's.

 

[Cuppy]

Update and sincere thanks to everyone who provided guidance so far. Q20 was apparently the culprit. I first replaced Q19 with same result. Once I replaced Q20, protection relay pulled in and held as it should. I ran this little amp hard (12 o'clock on volume) for 2 hours driving some rather inefficient KLH 4 ohm book shelves. My plan is to go ahead with a full recap and replace the 2SA798's with gain matched KSA992's as seen on another thread. If anyone doesn't think this is a good idea please chime in.