Pioneer M-90 Issue- need help.

sae-7125 said:
some circuit and chip variances will make things less than perfect (hence +/- 5% or 10% is a tolerable tolerance). resistance measurements on the chip pins helps with looking for unusually low resistances (shorted internally or low resistance) and considered to be a crude/low-level component test (still useful though). in terms of overall chip functionality, only in a circuit with power applied and design spec load connected will it show if it works or not (multi-pin mystery box).


are the numbers with or without dim bulb and is there a load connected?

the numbers in post-119 are the original outputs (power transistors)?
Click to expand...
The numbers are "without" the DBT and "with" all the original transistors. I've not put a load on the speaker terminals yet.
 
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sae-7125 said:
some circuit and chip variances will make things less than perfect (hence +/- 5% or 10% is a tolerable tolerance). resistance measurements on the chip pins helps with looking for unusually low resistances (shorted internally or low resistance) and considered to be a crude/low-level component test (still useful though). in terms of overall chip functionality, only in a circuit with power applied and design spec load connected will it show if it works or not (multi-pin mystery box).


are the numbers with or without dim bulb and is there a load connected?

the numbers in post-119 are the original outputs (power transistors)?
Click to expand...
I put 8 ohm resistors on the speaker terminals. No DB tester and no signal running thru the amp. DC voltages on pins 8 and 14 were slightly higher and pins 10, 11, and 12 were about the same for both channels. Reading DC voltage across the output resistors: -.02mV on the left and +3mV on the right.
 
sae-7125 said:
some circuit and chip variances will make things less than perfect (hence +/- 5% or 10% is a tolerable tolerance). resistance measurements on the chip pins helps with looking for unusually low resistances (shorted internally or low resistance) and considered to be a crude/low-level component test (still useful though). in terms of overall chip functionality, only in a circuit with power applied and design spec load connected will it show if it works or not (multi-pin mystery box).


are the numbers with or without dim bulb and is there a load connected?

the numbers in post-119 are the original outputs (power transistors)?
Click to expand...
Took a look at the overload detectors- Q127 L Ch and Q128 R Ch. L Ch E= -.3mV, C=11.4V, B=24.8mV; R Ch E=4.4mV, C=11.73V, B=30.1mV. Also fired it up on some speakers and it's sounding pretty good. L Ch heat sink running at 105 degrees (F) and right at 106 degrees
 
sounds like things are reasonably balanced, nothing going into high stress mode nor pulling too much power at idle.

i am guessing that the rear panel impedance switch is staying at 8r for most of the tests and measurements up to now?

not sure if i have gone down the LCR meter rabbit hole in regards to chip resistance measurements (at either 100hz or 1khz). probably best not to rock the boat when things are stable.
 
sae-7125 said:
sounds like things are reasonably balanced, nothing going into high stress mode nor pulling too much power at idle.

i am guessing that the rear panel impedance switch is staying at 8r for most of the tests and measurements up to now?

not sure if i have gone down the LCR meter rabbit hole in regards to chip resistance measurements (at either 100hz or 1khz). probably best not to rock the boat when things are stable.
Click to expand...
The overload detectors are 2SC1845s. I have some KSC1845s- do you think it's worth swapping those to check and see if it makes a deference on the numbers or are the differences not enough to worry about? I know the reading on pin 11 of each chip, which appears to be roughly the same as the DC offset for each channel, is not caused by the chips since that reading didn't change when I swapped them in post 119. Also, considering trying the replacement power transistors again now that I know they were not the cause of the protection issue earlier. Don't expect them to clear up anything, but for longevity they still might be a good idea. Thoughts?
 
Matt03221 said:
The overload detectors are 2SC1845s. I have some KSC1845s- do you think it's worth swapping those to check and see if it makes a deference on the numbers or are the differences not enough to worry about?
Click to expand...
should be fine, the newer components and slight variance in hfe or other electrical parameters might help as well (less np or pn junction leakage). the ksc1845 is a drop-in replacement for 2sc1845.

there are some sites with substitution lists, ak has more than a few versions plus other random sources.

Transistor Substitutions – Hudson Valley HiFi

hudsonvalleyhifi.com hudsonvalleyhifi.com

Matt03221 said:
Also, considering trying the replacement power transistors again now that I know they were not the cause of the protection issue earlier.
Click to expand...
now that more variables have been identified that were influencing the circuit things should be stable now. the new outputs should have no issue handling 4r/8r loads and do so much easier. final listening tests will be need though to critique the new outputs and their dynamics, +/- sonic flavor.
 
Put the new power transistors back in (2SC5200/2SA1943)- no other transistors were swapped. Didn't want to change multiple things at one time. Full up and running there's no notable change at all in the 119 post readings. Running at the same temps and I'd have to say there's no notable change in sound quality- good or bad. I'm happy those are working! Trying to decide if it's worth changing the overload detectors. Would like to swap out the pre-drivers, at least Q123-126 since the run at over 100 degrees F with a heat sink. Thinking they could use a refresh but have to wonder if the replacements are legit. Does that lower line of transistors look legit to you? Originals are above and are currently installed.
 

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Screenshot 2026-06-11 at 19-08-58 2N3055 - 2SC2238B.pdf.png

the "new" 2sc2238b has an "o" suffix, not sure if it would be an issue assuming the new components are 100% up to spec vs fakes.

a drop in replacement for 2238 and 968 would be:


and can be sourced fresh from mouser or digikey or other reputable primaries.

Matt03221 said:
Does that lower line of transistors look legit to you? Originals are above and are currently installed.
Click to expand...
difficult to critique a 2d image of a 3d object. even if a gram scale shows new and original being almost identical weight, its just easier not to deal with potentially catastrophic ebay sourced components in the first place. ebay/others are sometimes a source of last resort sort of like using nte transistors.

Screenshot 2026-06-11 at 19-35-27 2SC2705 - 2SC2705.pdf.png

an alternative to consider might be ksc2690a-y and ksa1220a-y even though they are slight overkill which is why i spec-shop for such things.


shopping criteria used to find the 2690:

 
sae-7125 said:
View attachment 3781096

the "new" 2sc2238b has an "o" suffix, not sure if it would be an issue assuming the new components are 100% up to spec vs fakes.

a drop in replacement for 2238 and 968 would be:


and can be sourced fresh from mouser or digikey or other reputable primaries.


difficult to critique a 2d image of a 3d object. even if a gram scale shows new and original being almost identical weight, its just easier not to deal with potentially catastrophic ebay sourced components in the first place. ebay/others are sometimes a source of last resort sort of like using nte transistors.

View attachment 3781119

an alternative to consider might be ksc2690a-y and ksa1220a-y even though they are slight overkill which is why i spec-shop for such things.


shopping criteria used to find the 2690:

Click to expand...
Much appreciated!!
 
sae-7125 said:
View attachment 3781096

the "new" 2sc2238b has an "o" suffix, not sure if it would be an issue assuming the new components are 100% up to spec vs fakes.

a drop in replacement for 2238 and 968 would be:


and can be sourced fresh from mouser or digikey or other reputable primaries.


difficult to critique a 2d image of a 3d object. even if a gram scale shows new and original being almost identical weight, its just easier not to deal with potentially catastrophic ebay sourced components in the first place. ebay/others are sometimes a source of last resort sort of like using nte transistors.

View attachment 3781119

an alternative to consider might be ksc2690a-y and ksa1220a-y even though they are slight overkill which is why i spec-shop for such things.


shopping criteria used to find the 2690:

Click to expand...
Couldn't find an available source for the KSA1220 to go with the KSC2690. I see on here (AK) a number of posts saying the following are suitable subs for those two, but looking at the specs they fall short on HFE and Ft. Would you say the same?

2SC2705 --> 2SC2383 or C3503
2SA1145 --> 2SA1013 or A1381
 
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