Yet another MC60 restoration

I just acquired a set of MC-60s and a Z-series MX-110 tuner. The cosmetic condition on all is excellent; the prior owner said they'd been rechromed/repainted, however, the wiring and soldering under the hood leads me to believe they are actually near-stock.

I heard them playing fine before I purchased them, when I assumed more work had been done under the hood. They sounded great!

Under the hood, a couple of caps are replaced - badly - in one unit while the other is stock except for two resistors. All the rest is original, including the Selenium diodes.

I've searched this forum a lot, but still have a few remaining questions. Hopefully I'm not retreading much ground; I want to do this right.:

1. My goal is to have the result of this restoration essentially last the next 50 years: would it be wise to replace all of the carbon comp resistors?
2. If so, I was leaning towards Vishay/Dale metal films. Trouble is, many of these are officially rated around 350V for 1/2 watt. Should I opt for the 1W versions, which are rated at 500V, or are these 1/2W resistors OK?
3. I'm leaning towards 100/100 external caps, removing the C8 socket cap, and adding a single internal 22uF. I definitely want to get the wiring correct if I do this. Tips, pictures, or schematic scribbles to confirm what I've got here would be awesome.
4. (Answered by new schematic) Both amps have a set of 220k 1/2W carbon comps soldered between pins 5 and 8 of the KT88 sockets I can't find on the schematic. Are they grid stoppers?
5. I'd like to add a thermistor to "soft start" the amps. Which pin from the wall should this go in series with - or does it matter?
6. (Answered by Brice below) The Russian K40 series PIO caps are well regarded for Bumblebee replacement. However, the K40s are also PIO - and aging. Won't they suffer the same fate?

I already have the replacement 1N4007 diodes on order, as well as new bias resistors / cap, and replacements for the 12k 2W & 220 ohm 1W resistors with Shinkoh tantalums.

(Yes, I know, needs pictures. Never fear, I'll add some later!)

I have had very good experience with the K40Y-9 Russian PIO.

Those are the military grade with glass case surrounded with a metal case (not the green glass only). They are top quality, with a fair amount of silver in it. They use paper and real aluminium foil, and not metalized paper.
The one you find will have a production date on it, the last 4 number on the case for MMYY. Usually around the 90's.

The original PIO were from the 60's, and definitely not the quality of the K40Y-9. But they bravely lasted 40 years +.

If you do the logical math at the worse possible case (K40Y-9 been used and same life span as the bumblebees) you are good for the next 30 years. But those been much better build and unused for 20 years leads me to believe that will last at least for the next 40 years.

That's why I choose them versus modern film caps. I want to keep the original sound.

I have heard that the auricap XO are also very good, but I don't like their soft leads and gluing them in place is not a proper work in such heat environment. Plus they are not cheap.

Brice.

I think I answered my question #4 by finding a different schematic which matches better. It shows the 220k grid stoppers for the KT88s, as well as a cap in series with the 0.5V RCA input that were MIA on my other diagrams.

This schematic also shows the voltage at a lot of nodes, which is useful info re: resistor voltage ratings. Still would appreciate input about Vishay/Dale resistors (CMF series) in here re:voltage ratings for the 1/2W vs. 1W variants. There is, obviously, room to use 1W resistors to be sure. Any reason I shouldn't?

Tried to attach the schematic but apparently it's 50 kB too big for AK's liking.

Pics of the MX-110 - this is very clean. The specs are dust, not pitting. Looks like it could have come off the showroom floor yesterday!

Stereo light, tuning eye, and all controls work. Zero static in the controls. Wood case is painted black with a few dings in the paint.

Before pics of MC60 amp A - Does this look like it's had a re-chrome done? Appears stock to me aside from three replaced electrolytic caps.

The cap replacements were done with poor workmanship.

Before pics of MC60 amp B - Stock aside from two resistors adjacent to the small chokes wired into the KT88 sockets. The brownish area on the chrome is a furniture reflection, not corrosion!

Oddly the replacement resistors are banded as 270 Ohms and measurements confirm this. I'm guessing a tech mistook purple for red. In any event, they'll be returned to 220R.

Wow,
Very nice pair of MC60s and MX110... Chrome looks great and lettering intact.
I can't help you regarding if original but interior certainly look so.
My pair is earlier and schematics is not same but looks similar.

I recapped all electrolytic and bumble bees... And never looked back. Only few resistors were replaced including replacing selenium rectifier with diode.
As you are thinking, I replaced 4 multi-caps with 100/100 and added 22uF (IIRC) under chassis.

These are great amps and I use them daily.

What tube did you get with yours?
I re-tubed them with Russian Gold Lions (KT88) and RCA, Sylvania & Telefunkens NOS; these sound very sweet and warm. Very melodic and I'm happy with them.

Good luck

TR6man98 said:

...
I recapped all electrolytic and bumble bees... And never looked back. Only few resistors were replaced including replacing selenium rectifier with diode.
As you are thinking, I replaced 4 multi-caps with 100/100 and added 22uF (IIRC) under chassis.

These are great amps and I use them daily.

What tube did you get with yours?
I re-tubed them with Russian Gold Lions (KT88) and RCA, Sylvania & Telefunkens NOS; these sound very sweet and warm. Very melodic and I'm happy with them.

Click to expand...

At first I was just going to replace out of spec resistors, as well as caps/diodes obviously, but then after finding about half of them close or out of spec I decided I was going to do this exactly once and never worry about them again. It also bothered me replacing a different subset of resistors in each amp. When a full set of Vishay/Dale metal films costs under $40, I decided it was best to just do it right.

These had an odd, mixed set of tubes. An unmarked diamond-base 12ax7 was in each, and the rest of the smaller tubes were International C rebrands from Japan, Yugoslavia, China. One amp had a set of EH KT88s and the other a new set of McIntosh KT88s (SED Winged C rebrands). I've got a more uniform set ready (new production Gold Lion KT88s, Sovtek 12ax7LPS, NOS Mullard 12au7, NOS Sylvania 12bh7) once repairs are done.

I haven't yet ordered the 100/100uF caps, but that's what I intend to do. If you have a marked up schematic - or just wisdom about that mod - to confirm the wiring, I'd appreciate it!

I've got NOS Russian K40y-9 PIO caps on order to replace the bumblebees, thanks for the reassuring info @Brice. I may leave the 0.22uF 160P DiFilm Black Beauties.

Wasn't able to source 12k 2W Shinkoh tants, so I'll be using Amtrans carbon films there. The majority of the remaining resistors will be 1W Vishay/Dale metal films.

Today I replaced the selenium diodes with 1N4007s - which barely fit with minimal end crimping - and the bias resistors to 5.1k.

Yeah,
I had to stretch those diodes as well.
It seems you are doing a good job.

There is a picture in my post just over a year ago showing the added cap (22uF) under the chassis. I just followed the schematics and used plastic clamp to support it...
I think you'll be fine just to do similar.

TR6man98 said:

Yeah,
I had to stretch those diodes as well.
It seems you are doing a good job.

There is a picture in my post just over a year ago showing the added cap (22uF) under the chassis. I just followed the schematics and used plastic clamp to support it...
I think you'll be fine just to do similar.

Click to expand...

Did you use all four of the 100uF sections in the new cans?

The original schematic has the two 80uF sections in parallel for a total of 160uF and the two 40uF sections in parallel for a total of 80uF. I assume you tie two sections of the new cans together for 200uF to replace the 160uF node, and a single 100uF replaces the 80uF node (summed from 40+40). That's only three 100uF sections, though.

Does the last 100uF section in the new cans go unused, or should/can it be tied in to one of those sections to add a little more capacity to the PS?

Yes,
all four sections of two cans 100/100uF.
Can sections A (80 uF) were replaced by one section 100uF each and sections B (40uF) are connected now to the other 100uF of the new cans.
Remaining C11C & C12C sections of the old cans got their own 22uF electrolytic each under chassis.

I've kept wiring close to original but it could be done the other way, off course.

Use as much capacity as you can. The more the better for the power supply filtration.

You won't break the power transformer for an additional 100uf at startup.
Even of you added much more.

One thing you could do is to add a thermistor for a nice slow start in the primary winding of the power transformer. I did not see one in your picture but I could be mistaken.

Maybe the expert here could give us their opinion.

Brice.

SilverTpt999 said:

Did you use all four of the 100uF sections in the new cans?

The original schematic has the two 80uF sections in parallel for a total of 160uF and the two 40uF sections in parallel for a total of 80uF. I assume you tie two sections of the new cans together for 200uF to replace the 160uF node, and a single 100uF replaces the 80uF node (summed from 40+40). That's only three 100uF sections, though.

Does the last 100uF section in the new cans go unused, or should/can it be tied in to one of those sections to add a little more capacity to the PS?

Click to expand...

Brice said:

Use as much capacity as you can. The more the better for the power supply filtration.

You won't break the power transformer for an additional 100uf at startup.
Even of you added much more.

One thing you could do is to add a thermistor for a nice slow start in the primary winding of the power transformer. I did not see one in your picture but I could be mistaken.

Click to expand...

Thanks for the tip! Indeed, we're thinking along the same lines. I have a couple of Ametherm MS22 75004 NTC resistors ready. These might be a little over spec, but by my calculations they would drop the voltage seen by the amp ~1-2 volts when warm. Since my line voltage here is a consistent 120v I figured I'd use those with the 117v transformer leads.

Still working on an optimal mounting solution, though. They get toasty (169C)! I am not drilling any new holes; my best idea right now is to fab or 3D print a bracket that would mount on top of the nearby transformer screw. There's enough threading left to secure another item there.

This is exactly what I did on my custom MC225.
I added a custom stainless steel plate mounted on the transformer screws.
And I attached to that plate the thermistor the regular way.

It shields the chassis from direct heat and no additional holes.

On mine I also mounted on that plate an optical relay to switch on/off the amp from my preamp automatically. But this is an addition and not a permanent modification as I did not alter the schematic.

SilverTpt999 said:

Thanks for the tip! Indeed, we're thinking along the same lines. I have a couple of Ametherm MS22 75004 NTC resistors ready. These might be a little over spec, but by my calculations they would drop the voltage seen by the amp ~1-2 volts when warm. Since my line voltage here is a consistent 120v I figured I'd use those with the 117v transformer leads.

Still working on an optimal mounting solution, though. They get toasty (169C)! I am not drilling any new holes; my best idea right now is to fab or 3D print a bracket that would mount on top of the nearby transformer screw. There's enough threading left to secure another item there.

Click to expand...

Interesting findings for the night:

One amp had been converted to use 117v from 125v. I know this because the "serviceman" apparently found desoldering too difficult to bother with. Instead, he CUT the red/black 125v transformer lead at the auxiliary AC jack, laid the yellow/red 117v lead on top of the joint, globbed on a little solder, and called it good.

I got that joint barely warm and it literally fell apart. Yikes. Then I found the cut-off stub from the other lead. The only real bright side is that the 125v lead remains long enough for use, and it stayed together long enough to make it to my bench so no lasting damage done.

Also, it appears that almost every capacitor in amp B is installed in the reverse orientation compared to amp A - aside from electrolytics, of course. The main phenolic board is upside-down in amp B too (all the numbers are on the opposite side from the turrets/resistors). Was it opposite day at the McIntosh factory? Inner/outer foil discussions aside, this shouldn't matter - but interesting.

Finally, two completely wrong fuses were installed. One labeled 32V and the other 5A slo-blo. They've been replaced with the correct 250v 3.2A slo-blo from the service manual.

Good thing these are getting a top-to-bottom refresh!

Had to take a long break from this, but tonight the refurb is finished! I've tested one amp under power and it passed voltage checks, and made beautiful music. The other one will be powered on tomorrow.

In case anyone else is interested, here are a few photos of the rewired power supply including the bracket I designed and 3-d printed with ABS plastic to mount the thermistor and the new 22uF cap, since the new cans are 100/100 instead of triple section. Bumblebees replaced with tested, matched, NOS Sprague Vitamin Q's and Russian PIOs. Original Black Beauties retained, and all original wiring. I deleted the 10w resistor arm to power an attached preamp, but otherwise left everything else intact.

More pictures to come of the beautiful polished and waxed exteriors once they're off the bench later this week! If anyone likes, I'd be happy to share the .obj file used to fabricate this bracket.

Nice ! Very well done. I hope it sounds as good as it looks.

The thermistor gets quite hot, I don't know hot hot it gets but ABS plastic is rated for 105C. Something to look at?

Nice to see K40-Y9 in place. And those nice resistors, Dale CMF 65?

Did you test those Black Beauties for current leakage under max rated voltage? I had bad one by the past.

The resistors are all Dale, mostly CMF 65 with a few from the 55 series thrown in because Digi-Key didn't stock the exact resistance I needed. Tried to stay within 1% of the schematic values. Exceptions are the twin 12k 2W ones, which are Amtrans AMRG carbon films.

The two rightmost caps are K40-Y9; all others are matched NOS Vitamin Q's (which are getting hard to find).

Hopefully the ABS holds up. I'll use these for a couple days and check it. The thermistor is mounted over an inch away from the bracket, and the bracket is on the opposite side from the KT-88s, so I hope this isn't an issue. That output transformer usually runs quite cool on top of there.

I did not test the Black Beauties under max voltage. Not entirely sure I have the proper test equipment to do this; I didn't want to remove them from the board due to thick leads (concern for the turrets). However, if it's easier than I think (power up, check current across the cap?) that would be entirely doable.

All right, You are all good to go then.
Congrats on your rebuild.

Please let us know how it sounds !