Pioneer M-90 service bulletin

[rlovison]

I mentioned in another post that a service bulletin was issued for the Pioneer M-90 suggesting that the predriver transistors be replaced on both amp boards. Another member of this forum inquired about the bulletin so I called Pioneer directly. A customer service rep informed me that they couldn't release the information contained in the bulletin to a consumer so I have no idea as to why the predriver transistors needed to be replaced or whether they were updated versions. I can figure out the second part by inspecting the boards myself and when I do, I'll post the info.

 

[hennessy]

I am somewhat interested in your post cause right now I'm rebuilding a M-90 from scratch.
But there are some discrepancies in your post about "pre-driver" replacement. Maybe that's a reason no Pioneer designated heavy hitters replied.
You stated a guy replaced Q115 and Q121 on left channel A board and Q118 and Q120 on right channel B board. I'm kinda curious cause when you replace driver pairs you preferably do it with complementary pairs. In your case as stated on board "A" both Q115 and Q121 Toshiba 2SA1145 were replaced. I assume he left complementary Q117 and Q119 in place, which are 2SC2705.
Now on board "B" Q118 and Q120 were replaced, which are 2SC2705, but original 2SA1145 complementary at Q115 and Q122 were left out. Strange.:scratch2:
Were they blown?
IMHO in those positions these Toshiba BJTs (150V, Ic 50mA) are more than capable handling the designated loads: up to 82V and currents in teens. They are used by most manufacturers for this position in that era.
I am really curious about the recommended replacement in that bulletin.
On the other hand I am replacing original driver BJT pairs Q123 - Q125 and Q124 - Q126 Toshiba 2SC2235B and 2SA968B with MJE 15032/3 pairs. In my case these were blown already in one channel. Next step will be to pull all power transistors for a check-up.
On a side note, MTF :ntwrthy:mentioned somewhere about the hardships of the transistors driving three relays switching speaker and headphone circuits. The main original transistor is Sanyo 2SD438. I dropped a more powerful Fairchild BD139 there. Replace those speaker 7A JC relays with newer 10A Panasonic JCs, digikey has them.
Display ICs 401, 402, 403 you mentioned are the most abundant NJM4558DXC op-amps. I might replace them with NJM4565DD. They are not important to the sound, just have to match input resistance, otherwise the display will be out of whack.
Nice sides, though.

 

[Twestby]

Wow. Most of that went over my head. Still interested though as I have an M-90 that stays in protection mode that I may consider tackling one day. Do you have a rebuild thread started?

 

[rlovison]

I am somewhat interested in your post cause right now I'm rebuilding a M-90 from scratch.
But there are some discrepancies in your post about "pre-driver" replacement. Maybe that's a reason no Pioneer designated heavy hitters replied.
You stated a guy replaced Q115 and Q121 on left channel A board and Q118 and Q120 on right channel B board. I'm kinda curious cause when you replace driver pairs you preferably do it with complementary pairs. In your case as stated on board "A" both Q115 and Q121 Toshiba 2SA1145 were replaced. I assume he left complementary Q117 and Q119 in place, which are 2SC2705.
Now on board "B" Q118 and Q120 were replaced, which are 2SC2705, but original 2SA1145 complementary at Q115 and Q122 were left out. Strange.:scratch2:

The discrepancies are probably due to my assumptions that are probably incorrect. First off, the repairs to my M-90 were done by a Pioneer factory authorized repair shop in Litchfield Connecticut. The tech told me the bulletin recommended pre-driver replacements. That's all I was told and I gave the OK to do this. I was given all the components that were replaced in the repair. That included two C2705 transistors and two A1145 transistors. It looks as though my guess on what transistors were replaced was probably incorrect. Would it make more sense if one C2705 and one A1145 were replaced on each amp board?

 

[hennessy]

No, I did not. This forum is mostly dedicated to the true vintage Pioneer which is Silver. Black 90’s Pioneer has not gained that status yet and it is understandable why. The art of engineering and workmanship of Silver Pioneer pieces extend to most of their mid tier models, which cannot be said about black face Pioneer of the 90’s, except of the Reference and very early Elite. The only silver I have is Stupidio Standard by Fisher bedroom system (CC-3000, BA-6000 and other matching components).
In addition, I do not spare expenses, with reason, sure, when rebuilding or modifying something.
Here is a freshly build up M-91 right channel board which was hacked into M-90 with some components cut out and burned out.
I do save logs and parts lists from all my projects.
What I can say else about the M-90 project - it MUST be rebuild by the condition I found the original left channel board. The reason is that "famous" Sony type brown glue. It ate and corroded jumpers, some small Zeners and diodes, ate and destroyed some small value caps, etc. Definitely would trip the protection circuit. M-91 was lucky: did not get that "treatment".

 

[hennessy]

Rlovison, your assumption might be correct. Those pairs are used twice as followers in the circuit (simply put), one before PA0016, one after, the latter driving the last pair of drivers, and subjected to a bit higher voltage and current. The most sure way to know is to open up and visualize. Would be helpful. Already intriguing.

 

[rlovison]

Rlovison, your assumption might be correct. Those pairs are used twice as followers in the circuit (simply put), one before PA0016, one after, the latter driving the last pair of drivers, and subjected to a bit higher voltage and current. The most sure way to know is to open up and visualize. Would be helpful. Already intriguing.

I'll try and get to opening the case later this morning.

 

[rlovison]

OK, just finished examining both amp boards. Here is what I discovered. The Amp A board had Q115 (A1145) replaced with A1013 and Q117 (C2705) replaced with C2383. The Amp B board had Q116 (A1145) replaced with A1013 and Q118 (C2705) replaced with C2383.

 

[hennessy]

Interesting. 1145 and 2705 are pure audio amplifier BJTs. I encounter them in almost all designs of that time, Pioneers, Yamahas etc. I am familiar with 2383/1013 transistors. But they are commonly used for switching purposes, such as relay driving. I have them in my parts stock for that purpose. I saw them on printer boards, too. They handle current up to 1A, but have parasitic collector capacitance about 20 times higher than 1145/2705. The result is Miller effect - like high pass filter at high frequencies. Maybe doesn't matter for audio applications, who knows. I think I am going to keep the 1145/2705 pairs.

 

[markthefixer]

maybe the service bulletin wanted the slower transistors to tame instabilities. it is 200 mhz 2sc2705 versus 20mhz 2sc2383 although the fairchild ksc2383 is min 20mhz with typical at 100mhz.

They already tried to tame them by tying 100pF from base to collector on the pre-drivers. imho this capacitance swamps ANY inherent junction capacitance.

I saw some familiar numbers in there too, 2sa992, 2sc1845 and so on....

but for the record, I am NOT a silver face snob. there were some good circuits in the initial blackface stuff, although IC102 the pa0016 fills me with disquiet.
for the record, it is still available from pioneer for 20 bucks each.

 

[rlovison]

Replace those speaker 7A JC relays with newer 10A Panasonic JCs, digikey has them.

Are those the two relays sitting between the large four caps up front? And if you could be so kind, what is the Digikey part number?

Nice sides, though.

It sounds nice as well although I still have an issue with the cd/line direct in. The control amp input is fine (which is the one I use) though the two direct inputs have channel in-balance as well as a non-flat response (base rise). This is something I'll take on myself at a later time.

 

[hennessy]

Mark, we know you are not.
I do have those Fairchilds in stock, I might give them a try. As I said I use them for switching purposes. Anyway, high speed BJTs in my rebuild do not make much sense, as I already installed 30Mhz MJE 15032/33 pair for a final driver pair.
I remember somewhere some highly technically inclined audiophiles were having seizures discussing Miller caps on driver stages in Pioneer C-21 pre-amp having 470pF values. Unacceptable design in theory, but the aforementioned piece sounds quite nice to my ears, mildly put! BTW, it showed up on that auction site two days ago, and was blown off in minutes for six bills.
The PA0016 is in most Pioneer Non-Switching Type circuit based amps. That covers all Black face high tier amplifiers; therefore there is no way to avoid it. IMHO it does OK job keeping bias values in check, and is pretty durable chip. I do have couple new ones (for ~ 20 bucks, sure)for spares, never needed them. A-80/90 are non-switching without the chip, and have some reliability issues. But they are Silver.
Rlovison, your cd/line problem might be bad switching relays RY301,302 on a buffer Amp board. Surprise, surprise they are the same as those “famous 13” relays on C-90 preamplifier. Therefore, the replacement is the same.
Having that in mind I shall give you part numbers at Mouser for one stop and shop. Cheaper, too.
Speaker relay RY 303,304 replacements should be Omron G4W-2214P-US-HP.
They are on a separate PCB behind the back panel close to speaker terminal block.
At the front at the big cap boards are speaker 4/8 impedance switching relays RY401,402. I would replace them with Omron G2R-24.
Buffer Amp relay RY301, 302 replacements would be Omron G5V-2-H1-DC12.

 

[markthefixer]

I have had Vbe issues using the on-semi 15032/33 because their Vbe "loss" (i.e. 1.8v to 1.2v to 0.6v through the stages ) is less than found in the usual transistors, which results in a higher minimum idle current. In fact I have had times where the idle current was not able to adjust DOWN far enough to hit spec.

Just sharing my past "gotcha's"...

 

[hennessy]

Thanks, Mark. What would you suggest for Toshiba 2SC2238B / 2SA968B replacement?
I have some Sanyo 2SC2344 / 2SA1011 pairs from BDent. But while 1011 show proper hfe on my tester, 2344 looks like fakes, and hfe is 6-8. I do not want to buy those there anymore.
Toshiba C3298B / A1306 ? I have some of those, but they are old, hfe are scattered all over.
Hitachi 2SD1138 / 2SB861 - I have no idea where to get them.
Toshiba has replacements C4793 / A1837. Mouser carries them, just their power dissipation is less than C2238B / A968B. But probably that's the best choice.

 

[rlovison]

Speaker relay RY 303,304 replacements should be Omron G4W-2214P-US-HP.
They are on a separate PCB behind the back panel close to speaker terminal block.
At the front at the big cap boards are speaker 4/8 impedance switching relays RY401,402. I would replace them with Omron G2R-24.
Buffer Amp relay RY301, 302 replacements would be Omron G5V-2-H1-DC12.

Sorry to be such a bother with this as I'm a newbie when it comes to repair.

Are relays 303, 304 DC12 versions?

There are quite a number of Omrons with the G2R-24 designation. Again, for relays 410 and 402, which version?

The buffer amp relays I found.

Rick

 

[hennessy]

All 12 VDC. Be careful; this piece is not the easiest to mess with.

 

[rlovison]

Thanks.

 

[agronk]

I`m repairing same blown channel, I changed output transistors they were blown: A1516,C3907,all resistors on output board,also Q117,Q115,Q123,Q125
and all resistors around them! after a careful check on every component and when everything was OK I powered ON it works!! but when I measure voltages on IC leg 11-8,14 where it has to be around 3V I got +1.5V on leg 8,-0.05V on 14!!!.
I`m not expert but it will be non-symmetrical! if I left it like that, I checked all again everything seems to be OK, I don't know what should change or do?

I read this topic and I need advice and help?

 

[hennessy]

You should measure PA0016 voltages @ pos. 8 and 14 referring to the Ground and not to pos. 11.
Position 11 is a feedback for PA0016 to monitor the DC offset of the amp. According to it the chip adjusts the DC through changing bias values @ pos. 9 and 13.

Position 11 to the Ground measured is the DC Offset of one of the other channel of the amp. The same value can be obtained between the speaker positive and negative terminals when/if protection relay is closed.

Please, measure and post the DC measurement between those terminals (right and left) after the amp had been switched on and idling for a while - up to half an hour. Input RCAs can be left disconnected or shorted.

 

[agronk]

You should measure PA0016 voltages @ pos. 8 and 14 referring to the Ground and not to pos. 11.
Position 11 is a feedback for PA0016 to monitor the DC offset of the amp. According to it the chip adjusts the DC through changing bias values @ pos. 9 and 13.

Position 11 to the Ground measured is the DC Offset of one of the other channel of the amp. The same value can be obtained between the speaker positive and negative terminals when/if protection relay is closed.

Please, measure and post the DC measurement between those terminals (right and left) after the amp had been switched on and idling for a while - up to half an hour. Input RCAs can be left disconnected or shorted.

I measured DC-offset in both channels it`s about 3mV.