McIntosh C20 - can and should I bring it back from the dead?

[neevo]

I’ve spent some time reviewing the schematic. Looks like mine is the neater and easier to read version from the service manual:



I’ve worked this out because mine has the lamp dimmer switch on the rear which on further inspection seems to feed another 2 lamps which are not on my unit. I suspect that’s what the cut green wire is from on the front panel as it’s the same feed as the green wire for the other lamps.

You can see it here on the right of this amp. 2 bulbs and 2 plates to shield the rest of the front panel:



This seems odd. I can’t understand what these bulbs are supposed to be doing, let alone why you’d need to dim them.

 

[s-petersen]

Maybe it is supposed to backlight that pointer so there is no dark spots

 

[jkaley]

@jkaley would you be able to take a photo of the filter caps section and the RCA jacks at the end? That would be really helpful for me to know what it should look like

Requested photos attached and one for inspiration, Sir. Let me know what else you need; I'll leave it out for a few days.

 

[Archguy]

Best of luck to the OP, who is a genuine inspiration. His efforts support my contention that McIntosh is the equivalent of a classic Rolls-Royce—not a Cadillac or Lexus. Because pretty much every classic model is seen as a worthy candidate for restoration, no matter the condition. And they become genuine labors of love. Can't say that with too many other makes.

 

[neevo]

Maybe it is supposed to backlight that pointer so there is no dark spots

That makes sense for the ones behind the pointers (x6 and the one at the other side of the far right pointer), but why the bulb behind the volume control etc behind the metal plate? Let alone be able to dim those 2 on that far end?

 

[neevo]

@jkaley would you be able to take a photo of the filter caps section and the RCA jacks at the end? That would be really helpful for me to know what it should look like

Requested photos attached and one for inspiration, Sir. Let me know what else you need; I'll leave it out for a few days.
View attachment 1255299 View attachment 1255300 View attachment 1255301 View attachment 1255302 View attachment 1255303 View attachment 1255304

Thanks!!!!

 

[neevo]

Does anyone have any images of the front panel with the 2 extra bulbs on the right? My chances of finding these parts are non existant I reckon so I’m thinking I will make them from scratch and have them coated with the rest of the chassis.

I’d need to be able to get measurements and the shape of the upper part and lower part. Plus any extra details on how the bulbs attach etc would be fantastic.

I can see remnants of the plate on the markings on the front panel which can get me started but I’d need to know how far it comes out from the front panel:

 

[neevo]

Knobs have been soaking for a couple of days. Grabbed a toothbrush and paracetamol packet to scrub them clean and also (with the paracetamol packet) remove 60 years of finger grease and dirt from the slits in the metal knobs:



They’ve scrubbed up really well and the metal knobs are bloomin’ heavy. The metal faceplate has scratches on it unfortunately where it looks like someone with a ring on has used the volume control. No matter, I’m thinking I will flip it over and use the back side instead which is a lot cleaner. Just need to work out the strategy for screen printing the letters back on.

Still after dimensions on the front metal screens if anyone can help there? I’m going to make some mockups today but unfortunately pics are limited online, so I’ll have to guess a few dimensions.

I’m also on the look out for another 2 bulb holders if anyone knows some leads. Audio Classics are an option and I’ve found a reasonable option from Parts Express that looks pretty close, or even this one from surplus sales:



But I’d love the real deal, even though it will be hidden behind the face plate.

 

[neevo]

Well and interesting day today. Wasted a load of time on something, made a silly/brave move, got some wins in the back half.

First up I decided to crack on and make the bracket for the extra bulb holders. Cracked out the air shears and set to cutting some 1mm sheet:



Marked up as best I could from the poor photo above and set about bending it on my brake:



All done:



Drilled a hole for the volume pot and then went searching for some bolts to attach it:



Looking good!!!! Next up I went digging around in my parts bags for a few more bolts. And what did I find?!?



The original bracket! What a moron!!!!

So I suspect my amp is pretty original. It has the volume pot/bulb holder bracket but looks like the lower bracket has not been added. Instead a loop is attached to one of the holes for grounding the bulbs.

Next up I had the house to myself. So I thought about ways to clean the amp before I attempt to strip it down to have the chassis plated. I bit the bullet and opted for an unconventional cleaning technique:



Don’t tell the missus! We have a fancy dishwasher that allows a low temp gentle wash with a short duration. So I thought I’d try that first to see what happened.

The results? Holy cow it did an amazing job!!!!!





Here is a before shot:



It cleaned the wires up beautifully, where they looked black before they now have their proper colours. Very impressive. I used the compressor to clean the preamp after and push all the water out and make sure it was dry before leaving it. I tried to find my can of Deoxit so I could also clean the switches etc but I can’t for the life of me find it.

 

[neevo]

Lastly I spent a few minutes going through the preamp to find any issues. I found a couple that I’d be keen to hear people’s thoughts on.

1. On V7 and V3 there appears to be a capacitor between pins 7/8 (red drop) that has a floating leg. I think I can see the remnants of the leg on each tube sockets pin 8 but they are both floating which is odd:





I checked the schematic but struggled to see a capacitor between pins 7/8. I’m going to need some help on this one.

2. There are 2 resistors that don’t look original:



These appear to bridge the tag strip like the resistors below (original ones). Again I’m not sure what’s going on here. Any ideas from anyone?

After I had a good look around I got stuck in to pulling the chassis apart. I’ve gotten it all done and I’ve drilled out a few of the rivets which I’m going to have to find a solution to:



I really wanted to remove the central frame piece but the work to unsolder all those wires is not really worth it. I’m not confident in putting that all back together and to be honest you’ll never see that part anyway as it will be hidden under covers. So it stays for the moment along with the rear panel:



I’m also keen to pull off the back panel as it will allow me to also have that plated and I can remove all the rusted solid rivets. The sticking point is the screen printing. I’ve messaged a screen printers in Sydney to see how much it would cost to redo. If it’s reasonable the whole external chassis will be redone.

I also pulled apart the transformer. I’m going to chemically strip the paint off. Some gentle wire brushing, then it will get a fresh coat of satin black paint/primer.

 

[s-petersen]

I think I got my bulb holders from Radio Daze. I don't think I installed the bulbs behind the volume knob.

 

[neevo]

I think I got my bulb holders from Radio Daze. I don't think I installed the bulbs behind the volume knob.

Do you have the dim switch on the back? Did you not wire them up at all? I’m struggling to see what they do, but plan to install them anyway.

 

[s-petersen]

My unit does not have a switch

 

[jkaley]

On V1 and V2 there is a 39mmf cap between pins 7&8. C10 and C50 on schematic. 2.2k resistor on pin 7.

 

[clinic-audio]

I’ve spent some time reviewing the schematic. Looks like mine is the neater and easier to read version from the service manual:

View attachment 1255082

I’ve worked this out because mine has the lamp dimmer switch on the rear which on further inspection seems to feed another 2 lamps which are not on my unit. I suspect that’s what the cut green wire is from on the front panel as it’s the same feed as the green wire for the other lamps.

You can see it here on the right of this amp. 2 bulbs and 2 plates to shield the rest of the front panel:

View attachment 1255088

This seems odd. I can’t understand what these bulbs are supposed to be doing, let alone why you’d need to dim them.

Your schematic is coming from SAMS photofact . This is not a MCINTOSH original C20 schematic .
Be carefull to use schematic with same serial number or you will get big differences

 

[Pio1980]

Your schematic is coming from SAMS photofact . This is not a MCINTOSH original C20 schematic .
Be carefull to use schematic with same serial number or you will get big differences

Be mindful of mistakes on Sam's schematics. Check against actual examples.

 

[neevo]

Your schematic is coming from SAMS photofact . This is not a MCINTOSH original C20 schematic .
Be carefull to use schematic with same serial number or you will get big differences

Yeah ok. I’m a little confused now as the only schematic I’ve found with the lamp dimmer switch has been that one. I’ll keep looking.

 

[BillWojo]

neevo, I'm cheering you on! Most folks would have walked right on by that Mac, just shaking there heads at the condition it's in.
By the time your done with it, it will look very presentable but more important it will be a great addition to your system.
Glad to see your not afraid of the dishwasher, handy machines!
Keep us updated, this is a really great thread.

BillWojo

 

[neevo]

This preamp continues to confuse me. The faceplate I have appears to be a late model and yet everything about the preamp screams early model:

1. Solid rear panel with RCA’s set in to the panel not an insert panel
2. Serial number (3LXXX)
3. Dim switch
4. Non potted transformer

Does anyone know if it’s possible to fit the later model larger glass on the early models?

I’m wondering if the original plastic face broke and it was replaced with a later one.

Interestingly enough though I have all the side panels, top/bottom glass trim pieces to hold the later glass.

This is all based on the assumption that the 4 7/8” glass is from the later models which is what I was reading off a site that has replacement parts.

Also interesting; my amp has no bumblebee caps in it. Obviously getting the model locked is essential for parts purchases and understanding which schematic to work from.

 

[s-petersen]

Measure the volume plate to get the proper height for the replacement glass. The early models had a solid cast frame around the faceplate, See the link I posted earlier.
The potted transformer was only on the later model. The standard transformer was on the early and mid models. The additional bulbs are in a position that would possibly light an area of the faceplate that has some graphics.