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Onkyo A5 repair - preamp dropped right channel

klamsoss

klamsoss

Be willing to fail and try again.
I've got this A5 integrated amp that I picked up a while back with a T-9 tuner, knowing they both needed a little work. The Tuner hadn't been touched, gets good reception, but the stereo reception isn't working quite right. That's for a different thread, though.

The A5 integrated amp was recapped (looks like pretty good work, too... PW series for power supply parts, MUSE for the tone controls, films where they'll fit, and the solder work is competent) and it 3/4 works....

The power amp section works just fine, but the preamp is missing the right channel. I dove in and started checking voltages against the specs. The main power supply seems to be putting out voltage just fine so I went to the semiconductors.

Everything pretty much measures as it should except the collector at Q501 is at around 18.2V and it should be at 26.5V. The emitter and base are within a half a volt, and pretty much every other semiconductor pin with a voltage listed in the service manual is within reason, too.

I'm thinking it's either a bad Q501 (2sc1509) or a bad D501 (WZ-177).

Do you guys think I'm on the right track, or should I break out the o'scope probes and start chasing signal from the input?
 
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For preamp troubleshooting I usually use a signal tracer (Paco with probe) or scope. I like the tracer because you trace the sound levels by hearing the audio. going from left to right through the stages until you find the area.
 
@pwauk12 it's not the switch. I just hooked it up to the 'scope and I have waves coming through on both of the "tuner" input.
On a related note, I like working on this Onkyo... everything is labeled! I got ready to try and figure out which pins came off the input selector board but it's screened right there on the bakelite (or whatever it is).

I've traced the signal through the function buttons and it gets to the volume knob just fine. Looks nice and even on the 'scope. The mono switch works as it should, muting works like it should, and so does the balance and the loudness. I'm not sure hwo I would check the subsonic filter, but I don't think it matters for the moment because I get signal out of the volume knob.
Coming out of the volume, the wave form is distorted for one channel but not the other. That moves with the probe, though, so I'm thinking it's not the source of the problem, just a weird artifact of the probes I'm using or the Onkyo A5 interacting with the tek465b. Anyone seen something like this before? the pic is the waveform at around 50mV, as seen on the DMM

I'm not completely sure how to trace the signal from that point on. I understand tubes better than ICs. Any guidance is appreciated.

Other relevant information:
Pin 1 on IC Q302 is low. It should be around 15.8V but it's 11.2V. Oddly all the other pins 1 for Q201, Q301, and Q202 are at about 16.5V. Not high enough that I think something is up, though, and things may fall back down closer to spec once Q302 gets it's $#it together.

I'm not getting signal at pin 3 or pin 6 of Q201, but I get it at both pins on Q202, which is weird because it's the right channel that is out. According to the schematic, Q201 is for the left channel.:idea:
time for a nightcap. I'll see if I can figure anything else out tomorrow. I'm out of my depth for this hour of the night.:beerchug:
 
IC201/202 and IC301/302 have input at pin2, output at pin 6 a - so check do you have input and then output from those IC's There are a number of threads on replacing them, it will certainly sound better and Cimmarron technologies make drop in boards suitable for modern IC's

https://www.cimarrontechnology.com/

if you are using tuner input- you should put approx 150mv from your generator, which should appear at the input of IC301/302 and at the output of them. To test the phono stage (equalizer amplifier) you want about 2.5mv

The other check is to see if you have signal at pre-out ?- I think you only have on one channel.
 
Correct. I only have signal at the pre-out for one channel.
I usually use inputs about twice as hot... 5mV for phono and around 250mV for line level sources. I know it's on the high side. I can't say I have a specific reason for the voltages I use.
I'll fire the A5 up again and check pins 2 and 6 on all 4. Do you have any recommendations on where to put the negative leads for the 'scope probes? Once in a while I run into something weird where it makes a difference.
 
@pwauk12 thanks for the correction on the signal pins. I don't know if I had a typo above when I referenced pin 3, rather than pin 2, or if that was just a mistake on my part. Either way, good info, and good tip on the adapters from cimarron (more on that below).

Q 201 and 202 are the ICs for the phono stage and have good input and output for both, so they appear to be fine.
I have signal at pin 2 of Q 301 (input) and signal at pin 6 (output).
I have signal at pin 2 of Q 302 (input) but I do NOT have signal at pin 6 (output), so it would appear that the culprit is Q 302.

These are listed as TA-7136P and I assume they should both be replaced for equal sound on both sides. It looks like there are a number of DIP op-amps that will work as a substitute. I guess if I'm ordering some op-amps and constructing some adapters, I might as well do the phono ICs as well. It appears that folks think there is some improvement to be made there. Due to the position of the chips on the board, it seems like single op amps are the way to go. I don't want to get into putting together a ribbon cord wire harness for this when I can keep things more neat and tidy. I'm thinking I'll go with the OPA604 based on the thread here: https://audiokarma.org/forums/index...tor-to-replace-ta7136-chips-in-sx-650.862695/
If someone has another suggestion, I'm open to it, but I like the idea that it's a drop in fix with the cimarron adapter and the OPA604.

BUT, before I get to that, I figure there's a reason that the existing one went bad. I don't understand how replacing the bad chip with a suitable modern equivalent would correct the issue at the collector of Q501 (2sc1509). That seems like it might be the REASON the IC failed. Now, the low voltage on pin 1 of Q302 itself might be corrected with the new chip. But still, I need to make sure I don't drop new chips in only to have them fail.
 
It looks like I can dump any of the parts hooked up to pins 1 and 5 of the existing g IC chips once I drop in the OPA604 replacements. I say this based on input provided by EchoWars, found here:
https://audiokarma.org/forums/index.php?threads/upgrade-ta7136p1-in-phono-and-tone-board.540756/

That doesn't make sense to me directly... maybe it will in the morning, but I don't think I understand how that wouldn't cause some stability problems. Can anyone shed some light?
If true, it makes the pin 1 issue moot.

But still, the main issue is that of fixing the problem that caused the original IC to fail. And I still don't understand why the voltage on the collector of Q501 is low.

The only other data point I have is that B1+ and B1- are each about 3V low. Spec says 43V and I'm only getting around 40.1V
 
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The B+/B- are OK- allowing a 10% variance is OK- it will in part be determined by your AC (110v-120v etc).

I would follow @EchoWars advice.

I would add a 0.01uf C0G ceramic across the B2/B3 lines to the IC, as close tot the IC as possible.

Replace Q501 before replacing the IC and see if it is different- do you have a transistor tester?
 
I do have one of the atlas pocket testers. Q501 doesn't test open or short anywhere that it shouldn't but I didn't compare results to the original specs. May just replace it either way since transistors like that are cheap. The zener might go, too, and the bridge cap across it. I'll try and post a photo if I can get some time.... getting ready to go out of town and tie the knot next week.

Thanks for the help!
 
FWIW - power supply problems normally affect both channels - if those components were bad you should have no channels. My guess would be bad IC loading down the power supply - (see iif suspect IC is warm.) My 2 cents...
 
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More extraneous information. Pioneer used the TA7136 in the SX-750 - later productions have a PCB installed in the TA7136P sockets, so I dugout the page from the 750 manual, if anyone wants to make a replacement.
 

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I'm back.
She said "I do" so that's pretty good. I figured I did too.

Anyway, got the chips, adapters, etc...
Question 1:
Do I need to solder this op amp chip onto this adapter? I normally wouldn't solder op-amp leads but I also usually use a DIP socket for op-amps because I sometimes like to swap different ones in and out (I've only ever messed with op-amps in cmoy headphone amps).

Question 2:
My test results for Q501 don't really add up or match what I would sort of expect based on the spec sheet for the transistor. But I also don't really know what values should match spec and what values just shouldn't exceed spec (spec sheet is posted... I just searched 2sc1509 and pulled the first spec sheet I found). The data I measure with the Atlas DCA55 analyzer are as follows -
current gain - h(FE) - 80 (I don't know what spec I should follow.... this is either way low, or perfectly fine)
Ic = 2.5mA (this seems way low... the lowest I see on the spec sheet is 150mA, and up to 0.5A)
B-E voltage V(be) = 0.700V (this is WAY off from spec, which appears to be 5V?)
I(B) = 4.237mA
Leakage current - Ic = 0.000mA

Is this the issue? My brain is dead at the moment and I just can't seem to force it to work through the logic...

I've replaced the previously mentioned diodes (D502 and D502) and the caps bridging them. I haven't tried to fire it up again because I figured I need to get all the parts back in place, having replaced the ones identified as "bad" before I give it some juice. So it's really down to the 2sc 1509, I think. If I can get that pin 1 voltage right, I'll feel more comfortable putting the new op-amps in place.
 

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I got no response so I went ahead and put the new op-amp replacement chip in and things got worse. Now I have an issue in the DC power supply. The NPN Q501 now has 9.8VDC on the base, 41VDC on the collector and 1 volt on the emitter. None of the voltages to the opamp chip are even close to right and there's no point in mentioning them since the DC power supply is the first thing that needs to get sorted.

SOOOOO things are exponentially worse.
I suspected Q501 and Q502 transistor pair were not good, but I didn't expect that the op-amp would send things so far out of whack. This doesn't actually make sense.
SO, I tested the resistors caps and diodes in the DC power supply. They all seemed good, but I replaced R503/504 and C505 because they were slightly out of spec. As previously mentioned, I replaced D501/502 and C507/508 just as a matter of course. I think one of the diodes was marginal....

B1+ and B1- are good at 41ish volts. So that's fine. The DC power supply is still F&*#ed though. So would the transistor pair there be causing everything that I'm seeing?
2sc1509 and 2sa777
Anyone want to offer up replacement suggestions? I've done the usual search and found KSA1220 replace 2SA777 and KSC2383 replace 2SC1509 but I don't know enough to know if that's appropriate for THIS circuit.
 
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