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