Restoring / recapping a Yamaha M-4 amplifier in 2026

[rupes2]

I bought a functional Yamaha M-4 that I wanted to restore / recap. First up was recapping the power supply and replacing both relays. As it gets harder to source some capacitors it seems to take a while to build a list of parts to order and where to source them. Sharing this in case it is useful to someone.

Capacitors for power supply

Capacitor	Value	LS (pcb)	Replacement Part #	Notes	Source
C609	1000µF, 25V	8mm	UHE1V102MHD6	35V	DigiKey
C613, C614	100µF, 100V	8mm	UPW2C101MHD1TN	160V	Mouser
C615	100µF, 25V	5mm	UPW1J101MPD	63V	DigiKey
C616	47µF, 16V	4mm	UPW1J470MPD	63V	DigiKey
C608	10µF, 25V	4mm	UKL1V100KDD1TA	35V, 10%	Mouser
C610, C611, C623	10µF, 16V	4mm	UKL1V100KDD1TA	35V, 10%	Mouser
C619	10µF, 50V, BP	5mm	UEP2A100MPD1TD	100V	DigiKey
C618	2.2µF, 25V, BP	4mm	UEP1H2R2MDD	50V	Mouser
C621, C622	0.68µF, 50V	4mm	MKS0C036800F00KC00	63V Wima film	Mouser
C617, C620	0.1µF, 50V	4mm	MKS0C031000C00KI00	63V Wima film	Mouser

Capacitors that were more than 20% outside their nominal capacitance value (measured with a GM328 meter):

1. C610 / C611 (10µF) -> 12.7µF (+27%) / 12.59µF (+26%). Part of the 12V regulated power supply circuit.
2. C621 / C622 (0.68µF) -> 0.862µF (+27%) / 0.867µF (+28%). Part of the LED power meter circuit.
3. C617 / C620 (0.1µF) -> 0.256µF (+156%) / 0.210µF (+110%). Part of the LED power meter overload circuit.

I did not replace the trimmers for the LED meters as Bourns part# 3386G-1-152LF was not available.

 

[rupes2]

Filter capacitors

Original specs: Nichicon CE 15,000µF, 75WV, 85°C, 50mm diameter, 90mm length, 4.5mm wide solder lugs that are 18mm apart.

When it came to selecting the filter capacitors the two best options I could find were:

1. Cornell Dubilier 383LX223M080B092VS. 22,000µF. Rated for 3000 Hrs @ 105°C so should last a long time. Almost exactly the same size as the originals (2mm taller). 5 pin snap-in so for each cap you have to cut off 3 pins and drill 2 holes in the PCB.
2. Nichicon KG Gold Tune LKG1K153MKZ. 15,000µF. Audio grade. 8mm taller than the originals. Has solder lugs like the originals that will fit after making them narrower. These were discontinued around 2023 and are hard to find.

I went with the Cornell Dubilier caps. They are not an audio grade capacitor, but neither were the original filter caps, and the specs are better than the originals.

I was a bit anxious about drilling the PCB especially since I could not find any photos online. I had AI create me a template PDF (attached) that really helped with marking up the PCB. Drilling the four 2mm holes was straightforward. I have attached photos showing where the 4 holes end up on the PCB.

 

[rupes2]

Relays

The replacements for the orginal DEC Type MS4U relays are the Omron MY4-02-DC12.

These relays have 4 contacts. The original relays have the contacts tied together internally in pairs so each connection is switched with 2 contacts. You can just remove the 6 unneeded pins and use the Omron relays as is but then each connection is only switched with 1 contact.

I had seen this post where this was addressed by drilling 2 holes in the PCB and then removing the other 4 unneeded pins. Once mounted 2 jumper wires are attached under the PCB. Instead I popped the cover of each relay and added 2 jumper wires internally to make the relay functionally equivalent to the original (see attached). If you are going to do this I suggest you remove most of the solder that is there first as I believe it is lead-free. I then removed the 6 unneeded pins.

 

[rupes2]

The amp is back up and running and I adjusted the amp module trimmers using this guide.

Still to do:
- Recap the amp modules (+ replace trimmers and 2SC458 transistors)
- Upgrade the speaker terminals
- Replace the button lamps

 

[rupes2]

Components for Pre Drive Sheet (left)

Component	Value	Replacement Part #	Notes	Source
C225	100µF, 10V	UPW1H101MPD	50V	DigiKey
C203, C205, C217, C219	10µF, 80V	UKL2A100KPD1TD	100V	DigiKey
C231	1µF, 100V	MKS2D041001K00JSSD	100V Wima film	Mouser
TR227, TR229	2SC458C	KSC1815YTA	Same pinout	DigiKey
VR201	100Ω	3296P-1-101LF	multi-turn, DC balance	DigiKey
VR203	2.2KΩ	3296P-1-202LF	multi-turn 2KΩ, 65V adj	DigiKey

Both Pre Drive sheets completed

On the right board FR202 had been replaced with a regular resistor and FR204 measured 42.2 ohm (+27.9%). I could not find fuse resistors so replaced them with 0.25W metal film resistors with flame retardant coating (part # RNMF14FTC33R0).

 

[rupes2]

Components for Drive Sheet (left)

Component	Value	Replacement Part #	Notes	Source
VR401	4.7KΩ	3296P-1-502LF	multi-turn 5KΩ, Bias	DigiKey
C401	10µF, 16V	RFS-25V100MF3#5	25V Elna Audio SILMIC II	Mouser

Both Drive sheets completed

 

[rupes2]

The recap is complete. 31 electrolytic capacitors, 6 trimmers, 4 transistors, and 2 relays.

Cost of the capacitors, trimmers, transistors, relays, and mica thermal pads in 2026 was about $140 (without shipping and tax).

 

[rupes2]

Speaker terminals

I was looking for speaker terminals that matched the aesthetic of the M-4 and were good quality. I saw listings on EBay that were made of a zinc alloy or did not specify what they were made of.

In the end I went with the Mueller 6883 from Digikey (same as the Pomona 6883). The other thing I like about these is they are keyed into the plastic piece that holds them together in pairs. So it is not necessary for the adapter plate to be keyed. Someone created a 3D printed adapter plate that works perfectly with these. I also ordered some Keystone Electronics 7340 solder terminal lugs. Total cost was about $20 (without shipping and tax). I am happy with the result.

 

[rupes2]

Button lamps

Since the lit buttons are an important part of the look of this amp I decided to go with incandescent lamps (14.5v, 80mA, 4mm). If I still have the M4 when they die I will try LEDs next time.

The orange silicone boot for the power switch was missing and I only had one good green silicone boot for the speaker buttons. I got replacement lamps and silicone boots from @dgwojo who was great to deal with. The dark green boots he has match the color of the originals. I don't have the an original boot to compare the orange to but based on brochure images the power button might be a bit too orange (the original was more amber?) but it looks good. I got some grommets from the hardware store to use with the new boots and that worked well.

This was a fun amp to work on. My FB marketplace ban did not last and I think a tube amp project will be next.

 

[satpaul]

How many original filter capacitors did you measure? Pavel

 

[rupes2]

How many original filter capacitors did you measure? Pavel

@satpaul I measured the capacitance of all the electrolytic capacitors I replaced with a cheap GM328 meter. The only ones that were outside a 20% tolerance were the 6 I mentioned in my first post. The original 15,000µF filter capacitors measured 13,830µF and 13,520µF.

 

[satpaul]

I think the filter capacitors are still fine. What was the ESR measured?

 

[satpaul]

The measured values of the original capacitors are still within the norm.

The capacity drop is -7.8% and -9.87%, which reliably meets the usual manufacturing tolerance of ±20% for large filter electrolytic capacitors. The capacity alone does not necessarily mean that the capacitor is in 100% condition. The GM328 multitester shows two other key parameters:ESR (Series Resistance): For such a large capacity (15,000 µF), the ESR should be extremely low, ideally below 0.02 to 0.05 Ω. If the tester shows higher values (e.g. above 0.1–0.2 Ω), the capacitor is already aging and filtering current peaks more poorly.Vloss (Voltage Loss): It expresses the leakage current (self-discharge). The value should be below 2% (ideally around 0.5–1%). Higher Vloss indicates internal degradation of the dielectric.Pavel

 

[rupes2]

@satpaul I am aware of the other measurements my meter takes when measuring capacitance and know what they mean.

It is possible that the original filter capacitors were ok. But testing with a GM328 won't tell you about things like ripple current rating and thermal behavior, leakage current under full rated voltage, or dielectric absorption. Just because they measure fine out of circuit on a bench meter doesn't mean they'll perform reliably under real operating conditions or indicate how much life they have left.

This amplifier is about 45 years old and I know nothing about its history. Like many others on AK who have recapped an M4 I decided to replace all electrolytics (and upgrade some to film). My hope is I will not need to go back in and replace any capacitors in the future and that it will last another 45 years with its original output transistors. I only detailed what I did here in case it is useful to others.

 

[satpaul]

Yes, it's okay, the replacement is good. You can throw away the small old capacitors. But I would save the filter ones for another project. The Nichcon 15000uF/75V ones were quality things. Write down how much ESR they have, they are definitely within the norm. The replacement you made will last you another 50 years. Best regards, Pavel

 

[nakmandan]

Nice work! These are great amps! Built right with serviceability in mind. Not like the MX series amps that are a nightmare to work on. I rebuilt my eBay "special" M-4 about four years ago, doing most of what you did, although I did not replace the main filter caps, nor the relays (I just cleaned them). If I remember right, I think the seller said it was working but the sound was cutting out and there was maybe some smoke! I don't mind a good restoration project when it's my own and the price is right (and it was). Here's the write up from my technician's worksheet:

eBay purchase. Good condition. Two dents on lower rear panel. Original power cord plug has been replaced with hardware store plug. Power lamp is out. Left channel output intermittent due to dirty relay contacts. Input pots dirty. Straightened bent rear panel. Replaced power cord and all lamps. Discovered several chipped power resistors, one burned resistor, two open fuse resistors, one very out of tolerance resistor and one shorted drive transistor. Two of four TO-3 output transistors on the right channel have been replaced with generally accepted substitutes. Replaced all defective components, installed 25T trimmers for fine adjustment of voltage, bias and DC offset, recapped all electrolytic capacitors except for the two large, main filter caps. Replaced all fuse resistors for reliability. Cleaned both output relays. Cleaned unit inside and out. Performed all factory settings. Working well. Amp rated at 120W/ch, doesn't hit clipping until about 170W. Into 4 ohms, clipping comes in at about 230W.

It's been running and sounding great for the last four years, running a pair of Infinity RS-6000's I LOVE this amp! I like the speaker terminals you used on yours. MUCH better than the spring clips originally fitted! I will probably do something like that on mine too.

 

[ivandezande]

Speaker terminals

I was looking for speaker terminals that matched the aesthetic of the M-4 and were good quality. I saw listings on EBay that were made of a zinc alloy or did not specify what they were made of.

In the end I went with the Mueller 6883 from Digikey (same as the Pomona 6883). The other thing I like about these is they are keyed into the plastic piece that holds them together in pairs. So it is not necessary for the adapter plate to be keyed. Someone created a 3D printed adapter plate that works perfectly with these. I also ordered some Keystone Electronics 7340 solder terminal lugs. Total cost was about $20 (without shipping and tax). I am happy with the result.

Looks killer. I love when binding posts get added to vintage gear and still look good.

 

[FockeWulf190D]

Great work, rupes2! I have two questions, if you would be so kind... (I´m just blowing dirt and waiting for Caps and things coming) 1. Why did you replace TRs 227,229 2SC458C? 2. Do C401 10mf/16v need to be Audio specialty one? I found one channel (Predrive) in mine, with TRs 232,244 2SC1624, replaced by 2SC2073, so I ordered MJE15032 to replace all of them. Cheers!

 

[rupes2]

@FockeWulf190D
1) 2SC458 transistors are known to go bad. See Top Ten Worst Transistors.
2) C401 / C402 are the only capacitors that are ever in the audio path on the M4. And only when the AC / DC switch on the back is set to AC. The Elna Audio SILMIC II was a popular choice that was available so I went with that.

 

[FockeWulf190D]

Excellent, rupes2! Being there... will see if I find some KSC1815YTA! Waiting for components I feel obsessed with cleaning, I thought to go saving time taking out the elements to be replaced, but I know it´s a good recipe to fail... Maybe I could post some pics in these days... Btw, I have and listen everyday an A-1, original less the Speaker connectors that are and always were a PITA... It was my revelation to the Hig End sound, no joke (I used to sell HIFI - HI End Audio at some point of my life) Is something apart as a 1 piece wonder (call it integrated if you want, but it don´t have a Pre stage!!!! That is part of it´s magic)...