Pioneer M-90 Issue- need help.

[Matt03221]

Hello, looking for some help with my M-90. I'm getting good audio out of both channels but also getting what sounds like a 60 Hz hum out of the left channel. It starts right after the safety relays kick in. It's in the headphones and at the speaker terminals and it's not affected by the volume control or input selector switch. Don't know if it's related, but I also noticed the left channel DC offset is a bit higher than what I remember- which is 4mv. Right channel still at 4mv but the left channel is now at 8mv. Any suggestions on where to start would be greatly appreciated!!!

 

[sae-7125]

original caps inside i.e. has it ever been updated/repaired?

Pioneer M-90 | hifi-wiki.com

hifi-wiki.com

 

[Matt03221]

original caps inside i.e. has it ever been updated/repaired?

Pioneer M-90 | hifi-wiki.com

hifi-wiki.com

sae-7125- I recapped it back in Oct. It worked fine for a couple of months and then I turned it on one evening and the hum was there. Story on the recap at the AK link...

Thread 'Pioneer M-90 Recap'

Aug 10, 2025

I've searched AK and see a number of posts on this amp, but nothing showing anyone replacing the main filter caps. Anyone successfully accomplished that? What did you use?

• Matt03221
• Replies: 38
• Forum: Pioneer Audio

• 

 

[sae-7125]

nice amount of photos in your recap thread.

have you tried shorting the amplifier signal input(s) and then listen for hum/noise (speakers or headphones)?

 

[sae-7125]

Post in thread 'Pioneer M-90 Recap'

Oct 15, 2025

Been running this thing for over a month now with no issues, but like I mentioned before- it does get warm. Each of the (9) heatsinked transistors on the (2) amp cards run at 100 – 110 degrees (F). The (2) main heatsinks for the power transistors run at around 100 degrees (F). With the case installed and this thing in operation that top cover hits 114 degrees easy. BTW, that's idle temps. With that, I thought I’d try to cool it down a bit to hopefully extend the life of the transistors since they are all original. Didn’t have a lot of room to work with inside, especially with that volume...

• Matt03221

• 

*custom active cooling is a nice treat vs factory disposable slow-cooker convection cooling

Completely isolated it from everything- just the amp and speakers and it's still there.

isolation is a good start. any inspection for broken solder joints while the recap was in full swing?

fractured ground connection somewhere can instigate hum. verifying input signal ground can help as well, maybe even utilize contact enhancer fluid/spray.

 

[restorer-john]

@Matt03221 Just saw this and your other thread. I noticed you had a voltage selector switch let go and you asked if it is a common issue. Yes is the answer. Plenty of them let go- too much spring pressure and nearly 40 years of plastic becoming brittle. You can either scavenge one from an old Pioneer cassette deck or something else with the same selector, but they will all be the same age and likely to fail. Your repair looks pretty good, but I would consider hardwiring the unit to your local voltage and completely bypassing the troublesome voltage selector. Bear in mind, any poor contact in that switch could lead to a fire from internal arcing.

As for your current issue, did you replace all the diodes around the old glue when you cleaned up the board for the new caps? The glue corrosion gets into the glass envelopes of diodes and can cause zeners to go crazy, other diodes to leak or fail, often randomly or due to temperature/current flow.

Basically all the small signal, zeners and other diodes where ANY glue touched them. Even if they look perfectly fine- replace them.



Your DC offset is not a problem unless it is wildly unstable.

 

[Matt03221]

nice amount of photos in your recap thread.

have you tried shorting the amplifier signal input(s) and then listen for hum/noise (speakers or headphones)?

I've not thought to try that but certainly can and report back.

@Matt03221 Just saw this and your other thread. I noticed you had a voltage selector switch let go and you asked if it is a common issue. Yes is the answer. Plenty of them let go- too much spring pressure and nearly 40 years of plastic becoming brittle. You can either scavenge one from an old Pioneer cassette deck or something else with the same selector, but they will all be the same age and likely to fail. Your repair looks pretty good, but I would consider hardwiring the unit to your local voltage and completely bypassing the troublesome voltage selector. Bear in mind, any poor contact in that switch could lead to a fire from internal arcing.

As for your current issue, did you replace all the diodes around the old glue when you cleaned up the board for the new caps? The glue corrosion gets into the glass envelopes of diodes and can cause zeners to go crazy, other diodes to leak or fail, often randomly or due to temperature/current flow.

Basically all the small signal, zeners and other diodes where ANY glue touched them. Even if they look perfectly fine- replace them.

View attachment 3678841

Your DC offset is not a problem unless it is wildly unstable.

Thanks for the comments! I did not replace or even check any of diodes during the recap simply because the unit was working fine prior. However, probably best to go back in there and do that. That was lazy on my part.

 

[Matt03221]

Post in thread 'Pioneer M-90 Recap'

Oct 15, 2025

Been running this thing for over a month now with no issues, but like I mentioned before- it does get warm. Each of the (9) heatsinked transistors on the (2) amp cards run at 100 – 110 degrees (F). The (2) main heatsinks for the power transistors run at around 100 degrees (F). With the case installed and this thing in operation that top cover hits 114 degrees easy. BTW, that's idle temps. With that, I thought I’d try to cool it down a bit to hopefully extend the life of the transistors since they are all original. Didn’t have a lot of room to work with inside, especially with that volume...

• Matt03221

• 

*custom active cooling is a nice treat vs factory disposable slow-cooker convection cooling


isolation is a good start. any inspection for broken solder joints while the recap was in full swing?

fractured ground connection somewhere can instigate hum. verifying input signal ground can help as well, maybe even utilize contact enhancer fluid/spray.

I tried shorting the inputs with an RCA cable between L and R channels- no change. Can you please share how I can verify input signal ground?

 

[sae-7125]

RCA cable between L and R channels

i should clarify, shorting plug across + and - of signal input on each channel individually. some older mcintosh and marantz systems used such things for unused signal inputs like phono to reduce system noise consumption from the environment and/or inner chassis stray emf.


a visual example of an electrical modification. chances are the noise will still exist but it helps to reduce all variables.

to verify signal ground with a meter, measure resistance between signal - (rca outer shell) and chassis frame/metal. most vintage gear and a large portion of modern stuff use a common grounding system via the chassis.

 

[Matt03221]

i should clarify, shorting plug across + and - of signal input on each channel individually. some older mcintosh and marantz systems used such things for unused signal inputs like phono to reduce system noise consumption from the environment and/or inner chassis stray emf.

View attachment 3679305
a visual example of an electrical modification. chances are the noise will still exist but it helps to reduce all variables.

to verify signal ground with a meter, measure resistance between signal - (rca outer shell) and chassis frame/metal. most vintage gear and a large portion of modern stuff use a common grounding system via the chassis.

Thanks do

i should clarify, shorting plug across + and - of signal input on each channel individually. some older mcintosh and marantz systems used such things for unused signal inputs like phono to reduce system noise consumption from the environment and/or inner chassis stray emf.

View attachment 3679305
a visual example of an electrical modification. chances are the noise will still exist but it helps to reduce all variables.

to verify signal ground with a meter, measure resistance between signal - (rca outer shell) and chassis frame/metal. most vintage gear and a large portion of modern stuff use a common grounding system via the chassis.for

i should clarify, shorting plug across + and - of signal input on each channel individually. some older mcintosh and marantz systems used such things for unused signal inputs like phono to reduce system noise consumption from the environment and/or inner chassis stray emf.

View attachment 3679305
a visual example of an electrical modification. chances are the noise will still exist but it helps to reduce all variables.

to verify signal ground with a meter, measure resistance between signal - (rca outer shell) and chassis frame/metal. most vintage gear and a large portion of modern stuff use a common grounding system via the chassis.

Thanks for the additional information! Tried shorting per your instructions but didn't notice any real change. The resistance between the chassis and outer shell if the input terminals settles in at about .5 ohms. CD direct and line direct settle in pretty quick, but the control amp input takes a minute to settle down and has a pretty radical spread rapidly jumping all over the place until it does settle- reading as high as 400+ ohms. I've also poked around a bit with a small diameter wood dowel, but no luck pinpointing a change in the hum. When i get a minute i plan to jump and start checking those diodes john recommended.

 

[restorer-john]

CD direct and line direct settle in pretty quick, but the control amp input takes a minute to settle down and has a pretty radical spread rapidly jumping all over the place until it does settle- reading as high as 400+ ohms.

That also needs investigation. The control amp input grounds should all be at exactly the same (0V- ground) as all the others RCA outers.

 

[Matt03221]

That also needs investigation. The control amp input grounds should all be at exactly the same (0V- ground) as all the others RCA outers.

0V or 0ohms? I only tested the inputs between RCA outer shell and the copper chassis with an ohm meter and power off. Looking at the attached it does look like the outer shell is tied direct to ground so odd that the ohm meter doesn't go right to 0.

 

[Matt03221]

So I'm thinker around with it a bit and noticed the heat sinks warming up fight away, especially the left side that has the hum. I know this thing gets warm, but don't recall it heating up that quick and that left side is definitely warner.

 

[sae-7125]

measuring dc bias for each channel might shed some light on the situation.

 

[Matt03221]

I pulled the left channel amp card out so I could start checking components. I jumped to that vs checking a few voltages as I was afraid to burn up the power transistors. How would I have checked bias as there's no adjustment procedures or checks in the service manual. I notice on the schematic it shows 24mv on the emitter of one of the power transistors- would that be it? Sorry, my circuit theory is minimal.

 

[sae-7125]

Post in thread 'Pioneer M-90 draws 100W+ when idle !'

May 6, 2024

Mine pioneer m90a draws 300 watt righ after switching on, and around 111 watt on idle.

This is due to the way the amplifier circuit works, which sets a high bias current and stabilizes it to avoid distortion.
pioneer m90/m90a/m91 operates close to class A amps .Its Non switching type 2 circuit.
It runs hot and its normal.

• Pioneer.user

• 

*this explains a few things. relying on a chip is easier for the design and manufacturer until it ages in an unpredictable manner and the amp channel nukes itself.

Thread 'Pioneer M90 restoration'

Feb 7, 2019

Even if there is another similar old thread from @hennessy, I decided to still share my work on the restoration of a Pioneer M90 that I recently bought for parts.
The manuals online are quite bad, and I really hate the way Pioneer at that time did not list the parts as every other manufacturer; there are also not a lot of detailed pictures of the internal boards and components online.
Hopefully this thread will help others who decide to embark on a similar project with plenty of pictures. As this is an ongoing project, I will update the thread as I work on the amplifier.

This is...

• MattiaM
• Replies: 15
• Forum: Pioneer Audio

• 

*nice amount of photos

measuring across the output emitter resistor is the usual way of evaluating amp channel bias.

 

[sae-7125]

Pioneer M-91 on thevintageknob.org

vintage Pioneer Elite high-end audio power amplifier with flashy red meters

www.thevintageknob.org

*interesting notes about m-90 vs m-90a/m-91

having lots of schematic voltage labels helps. interesting to see load specific voltages for the q129/q131 circuit directly above bias control ic101 pa0016. more than a few rd39eb zener diodes as well.

https://ia804508.us.archive.org/2/items/manual_M90A_SM_PIONEER/M90A_SM_PIONEER_text.pdf

 

[Matt03221]

sae-7125, thank you for the additional info. I was working on the card and checked a number of diodes affected by the glue, but all checked good. I did notice R159 did not look right. Looks like it's a 100 ohm and it's got a lower-case f in a circle which I think means it's fusible? It actually reads 136 ohms. Could this be my problem- causing the hum? I have a pick of the actual resistor as well as what one of the undamaged 100 ohm fusible resistors looks like- there are several of the same resistor in there. I checked a couple and they read 100 ohms. Ethier way, I often read fusible resistors should be replaced, but what should I replace these with? Their part number code deal isn't very helpful.

 

[sae-7125]

M

Thread 'Question: Fusible resistors'

Mar 23, 2010

I've seen varying advice (or maybe I missed the specific contexts) regarding fusistors/fusible resistors.

When dealing with old fusistors, what to do? Replace them when out of spec? Replace them regardless of whether they're in or out of spec? Replace with new fusistors? Replace them with a specific other type of resistor?

• Martin.Ca
• Replies: 15
• Forum: Solid State

*info should help with the topic of fusible resistors, it can be messy depending on circuit design. other info around ak and other audio forums can be helpful as well.

if there is a resistor somewhere that is an order of magnitude higher and beyond, it could reduce a circuit grounding strategy (high resistance vs low resistance) and in turn cause noise to propagate within a sub-circuit.

on vintage gear with wire wrap connections even the slightest wire/metal fatigue can reduce conduction therefore introducing higher connection resistance which is a pain to locate... until a wire mechanically breaks off, another headache to inspect for.

 

[Matt03221]

My plan at the moment is to check a few more components on the card while it's out and then order a metal film resistor to replace the R139 that's out of spec- and anything else I might find. I'd hate to put this all back together and find out that resistor isn't the only issue. It takes some effort to get the card in/out and it really needs to be out to swap components. I'll report back on progress/results.