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Spec 4 with Problems

I was hoping that EchoWars would have some specific advice about this, especially with the complication of the power limiter circuitry (Q6, Q7, Q13, Q14).

I have general advice, about testing and swapping components side to side to locate the guilty party. Or wholesale replacement of transistors - which I haven't looked up yet.
 
The differential pair transistors should be replaced on both channels (the 2SA798 is fairly easy to find), as well as all the electrolytic caps on those driver boards. The 0.22µf 10V caps on Q12 is an infamous Sanyo blue-baby, and should be replaced with a Panasonic stacked film cap.

Wouldn't hurt to toss out those stock bias and offset trimpots.

When disassembling, treat those thermal diodes (remotely mounted on the heatsinks) like gold. Break the leads on these and you're screwed.

I don't have anywhere near the experiance with the Spec 4 as I do the Spec 2, but this should still be reasonably straightforward.
 
EchoWars said:
...The 0.22µf 10V caps on Q12 is an infamous Sanyo blue-baby, and should be replaced with a Panasonic stacked film cap....
Click to expand...
That is what I replace them with, but have not heard of the infamy. Fill me in. OK?

Enjoy,
Rich P
 
No big story, except the effective lifespan of those caps must be 1000 hours @ room temp. They go open, they go short, they can't be trusted. Pioneer used tons of 'em. Most Pioneer protect circuits have them, and more than a few tuners & amp input stages as well.
 
I took some readings for comparison to the service manual. The left side amp was OK.

On the right side amp here were the disparities I was able to check:

1) Q8 base should be 1.8v, my reading was -58.5v
2) Q6 emitter should be 1.8v, my reading was -58.5v
3) Q7 emitter should be -1.8v, my reading was -53v
4) Q14 emitter should be 0v, my reading was -61v
5) Q4 collector should be -3.6v, my reading was -66v, same as the emitter

I didn't check all of the transistors.

I also got a reading of -60v off of pin 11

Any suggestions as to where the base problem lies?

Thanks
 
Pull all the output transistors from the heatsink, diode-test them with a meter, and measure those voltages again with the outputs removed. I don't think one is bad, but this'll keep you from smoking one of them while you're poking around.

FWIW, Q14 emitter is simply the output of the amp. ;)

Q4 is on hard, either because it's shorted 'n dead, or being driven to saturation by a bad Q1.
 
One of the 2SD424's tested bad. Conduction between the collector and emitter.

Q4 tested OK, both by using a transistor tester and the diode test.

I did replace the electrolytics, but all of the old ones tested OK.

Q12 looked like it overheated, but tested OK.

All tests were done out of circuit.

I did order a replacement for Q1, the 2SA798. Awaiting delivery.

More to come........

Thanks for all the help.
 
Replace the output transistors. All of them. The originals are loooooong out-of-production, and anything you find with those numbers will likely be counterfeit. Use On-Semi MJ21196 for the 2SD424's, and MJ21195 for the 2SB554's. Buy them from Mouser or Digikey.

Check the emitter resistors on the board...when the output transistors go, it is not uncommon for the emitter resistors to burn up. Check carefully for other burned resistors...many Pioneer amps have many resistors covered with a fabric weave so you can't check them visually...use the ohmmeter, and if you get a funny reading, desolder one side and check again.

Would also be a good idea to replace the driver transistors (Q10 and Q11). The On-Semi MJE15032 and MJE15033 make fine replacements. Again, I can't stress enough two things:

1. Buy your parts from a reliable source, such as Digikey or Mouser
2. Check all resistors on the amp board from the bias stage (Q4~Q5) all the way to the output. Tedious, yes. But necessary.
 
Since I had it, I replaced Q1 even though the original tested OK. The previous voltages that I listed are back in line. Was Q1 the culprit, or was the bad output the problem?? I do get 0.1V instead of 0v on both bases of Q1. Is that a problem??

Based on a previous thread, I ordered Onsemi MJ21193/94's as the replacements. Are those OK or should I return them and go with the MJ21195/96's???

I will check the resistors.

With the outputs out of the circuit, the protection relays now engage within 7 sec.

Thanks for the help.
 
Looks like you got it! The outputs you have are fine and are the same as the ones Echo suggested. I don't see any differences in them. Good job!
 
All resistors checked OK, both with the ohmmeter and visually.

I'll order the replacements for Q10 & Q11.

I did replace the trim pots.

After Q10 &11 are replaced I'll see how it goes with the new outputs.
 
Nothing wrong with the outputs you bought, as long as they were bought from a trustworthy source.

Yes, kinda sounds like Q1 was bad (unless I didn't follow the thread properly).
 
I bought the outputs from Mouser, so I'm sure they're OK.

I've had the amp running for several hours with out incident. Still have the original Q10 & Q11, I'll replace them when my shipment arrives.

I still have drift with the DC balance. After 2 hours, I did the adjustment procedure in the service manual. Both channels seem to drift between +25mv and - 20mv. Not sure if that's normal or an indication of problems. The receivers I've worked on seem to be more stable.

As mentioned in an earlier post, this unit has been worked on in the past. Kind of a hack job, very sloppy work. On the left channel, all of the outputs were replaced with Toshiba's of a different part number than the originals. Also several resistors were replaced. In one case, the replacement resistor wasn't soldered to the board, probably made contact with the mechanical connection. Replaced the outputs with matching Onsemi's. Replaced all electrolytics and the trimpots, and cleaned-up the sloppy work.

What's recommended for cleaning off the rosin from the PCB?

Based on the Watt meters, it seems the right channel is weaker at a given setting, it does sound OK though. Any way to adjust this?? With the exception of the DC Balance drift mention above, both channels are set the same.

Thanks for the help.
 
miklemke said:
I still have drift with the DC balance. After 2 hours, I did the adjustment procedure in the service manual. Both channels seem to drift between +25mv and - 20mv. Not sure if that's normal or an indication of problems. The receivers I've worked on seem to be more stable.

As mentioned in an earlier post, this unit has been worked on in the past. Kind of a hack job, very sloppy work. On the left channel, all of the outputs were replaced with Toshiba's of a different part number than the originals. Also several resistors were replaced. In one case, the replacement resistor wasn't soldered to the board, probably made contact with the mechanical connection. Replaced the outputs with matching Onsemi's. Replaced all electrolytics and the trimpots, and cleaned-up the sloppy work.

What's recommended for cleaning off the rosin from the PCB?

Based on the Watt meters, it seems the right channel is weaker at a given setting, it does sound OK though. Any way to adjust this?? With the exception of the DC Balance drift mention above, both channels are set the same.

Thanks for the help.
Click to expand...

I'd say the drift is acceptable and normal.
Clean the board with Isopropyl Alcohol, 91%.
Calibrate the meters per the service manual, then check the output. The volume control may not track properly.
 
YOu need to allow the amp to warm up fro significant period, before setting the bias and balance, as a final setting. Do an initial setting, athen watch as it warms up. After significant warmup, then do the final setting.

If there was something wrong, the null voltage would continue to drift as the unit is left on, or you would not be able to set the zero to service manual specs. Q1 should be suspected first, in the case of continuous drift, then Q2, Q3, Q4, Q5. Also, drifting could be attributed to D2, D3, D4, D5.

Enjoy,
Rich P
 
miklemke said:
Any recommended substitutes for:

2SC1775A-E
2SC1439-V
2SA858-V

Thanks
Click to expand...

2SC1775A-E npn (ln) to-92 120Vceo 0.050AIc 0.3WPd 400-1200Hfe 200mhz Ft
this is high gain, low noise, I would use the ksc1845 (the a992 is it's complement - just in case)
512-KSA992FBU pnp (ln)to-92 ecb 120v .05a .5w 100mhz 150-800hfe $0.06 ea
512-KSC1845EBU npn (ln)to-92 ecb 120v .05a .5w 100mhz 150-800hfe $0.06 ea


2SC1439-V npn 150v 0.050a 0.5w 150hfe 130mhz
2SA858-V pnp 150v 0.050a 0.5w 150hfe 100mhz
These are lower gain but higher power dissipation:
512-KSA910YSHTA pnp to-92 ecb 150v .05a .8w 100mhz 40-240hfe $0.07 ea
512-KSC2310YBU npn to-92 ecb 150v .05a .8w 100mhz 40-240hfe $0.11 ea

edit: HOLY SMOKES - I did that by hand, and I just went into the Fairchild cross reference site and ran the originals, and it came back with THE EXACT SAME ONES I said. A lot of times the site doesn't get any matches.
 
Thanks for the substitute list.

The drift has gotten worse, it will go up to 90mv. I'm suprised protection hasn't kicked in.

I'm going to replace Q2 thru Q5 as well as Q10 & Q11. I'll see if that helps to settle things down.
 
I'm not going to re-read the whole thread to see what it is you've got, but are you sure your meter isn't simply a POS? Or has a weak battery?
 
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