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

[neevo]

Not much happening at the moment as I wait for the chassis to be finished at the chromers plus I need to sort the end caps before I make the run down to the anodisers.

I’d happily have bought them from Larry at RR-ARTifacts not only because he’s a lovely chap but because they are a perfect fit. However there were a couple of issues.

1) I felt bad that I would essentially strip the anodising off a brand new finished set of end caps and polish them.
2) the shipping to Oz was turning out to be crazy expensive. Pretty standard for us down under unfortunately.

Larry was very accomodating and I may need him in the end but instead I’m trying a different path as a gamble:



It’s a milling attachment for my lathe. I’ve wanted one for ages and the cost wasn’t that crazy for the vice, collet chuck and 2 end mills.

Considering the end panels are only 5” long by nearly 13mm square, it’s not a heavy milling operation.

I have some 6060 stock on the way too and hope to be milling as soon as possible.

Time will tell if I can get anything half decent!

 

[darkblue94]

Definitely metric.

Who understands inches

Just about every lady I've ever known understands inches.

 

[onwardjames]

Just about every lady I've ever known understands inches.

Reminds me of a joke my dear old mother (with her incredibly bawdy sense of humor) would tell.

"Why are women so bad at measurements?"

"Because their whole lives, they've been told (she holds her hands about 2 inches apart) that THIS is 6 inches!"

 

[neevo]

 

[neevo]

Safety question. How are chassis earthed with vintage 110v equipment? I’m used to having an earth lead that’s attached to the chassis but my amp only has active and neutral and also appears to use the chassis as a common ground.

Isn’t it a high risk to have a metal chassis without an earth lead?

 

[Pio1980]

Safety question. How are chassis earthed with vintage 110v equipment? I’m used to having an earth lead that’s attached to the chassis but my amp only has active and neutral and also appears to use the chassis as a common ground.

Isn’t it a high risk to have a metal chassis without an earth lead?

Depends on leakage to chassis, modern power transformers generally have low to no leakage, so no hazard. Older gear had non-polar 2 conductor power wiring, reversing plugs could reduce "ground looping" due to some leakage cross currents between pieces.
If you are more comfortable providing a chassis grouning cord and plug, you could, but do put the switch and fuse on the hot power lead and the cold side to the transformer. First, try to determine which plug orientation produces the least ground leakage and use that as a guide to determine transformer connection for lowest leakage before rewiring.

 

[neevo]

Is there risk of earth loops adding a chassis earth where there wasn’t one before?

Clear on the fuse/switch on hot side.

 

[Pio1980]

Is there risk of earth loops adding a chassis earth where there wasn’t one before?

Clear on the fuse/switch on hot side.

Possibly, if all units are not likewise wired and used on a common power source. It was always a bit of a crapshoot in the olde nonpolarized twin wire power days, reversing plugs to reduce or elimate him and noise.
Intentional design and practice has largely reduced this requirement.

 

[neevo]

Not much to report at the moment as I’m still waiting on the chassis to be finished at the chromers.

Plus my Alu came in and so I now have the materials ready to have a go at milling my own end panels. Unfortunately I’m still waiting on my die to thread the rod to attach the milling vice to the lathe, so that will have to wait a little too.

On Sunday I’m hoping to pick up a variac which I’ve had my eye on for a while so I can slowly bring the amp up to voltage and also have a way to provide 110v.

Anyone provide some thoughts on how to use it?

I understand the theory and plan to bring it up slowly over a few hours, but I have a few questions:

1. Should I bring the preamp up with tubes in or out?
2. What options do I have to test for issues where it will consume current like a DBT? Can I run the variac and DBT together?
3. Is it worth slowly bringing the pre up to voltage to protect the old components?

Does this make sense?

https://www.circuitspecialists.com/blog/using-a-variac-to-power-up-an-old-radio-or-amplifier/

I like the idea of putting my DMM in series to track current draw.

 

[smartin53]

Still loving this thread. It kinda reminds me that with the right work you can even make an ugly shell sound beautiful as long as the circuit remains intact.

 

[neevo]

I have a variac in the house. Really pleased so I can not only slowly bring the Mac up to speed but also I can use it temporarily to see if the preamp works as I don’t currently have a solution for 110v:



The chap I bought it from was delightful and I had a good chat to him. Turns out he was moving and threw a load of stuff away in a skip on his from lawn. He kindly reached in and gave me 2 boxes of stuff to take with me.

1. A box of random transitors and knobs:



2. A box of tubes:



The tubes aren’t the same as the boxes generally as I’ve found a mullard. I’m wondering if these are worth hanging on to or if they’re bin material. I was hoping there might be a 12AX7 or 2 in there.

 

[neevo]

The variac works perfectly and after a quick wipe with some simple green it’s also scrubbed up beautifully.

The tubes unfortunately I can’t find any 12AX7 or 12AU7, they’re all ones I’ve never heard of (1X2B, 1S2A, EFL200, 6GK6 etc), so it looks like I’ll try and find them a home elsewhere.

 

[Pio1980]

The variac works perfectly and after a quick wipe with some simple green it’s also scrubbed up beautifully.

The tubes unfortunately I can’t find any 12AX7 or 12AU7, they’re all ones I’ve never heard of (1X2B, 1S2A, EFL200, 6GK6 etc), so it looks like I’ll try and find them a home elsewhere.

TV tubes, possibly some useful.

 

[neevo]

Chap was in to Ham radios but also an electrical engineer. Pity there aren’t any that I can use in the preamp.

Better in my garage than a skip bin either way.

 

[neevo]

Today I managed to crack on with the end plates. I was waiting on my collets for the end mills (I have the chuck and draw bar already) but couldn’t be bothered to wait anymore and put the end mill in the 3 jaw chuck instead. I know it’s not proper but I took minimal cuts anyway, that I thought it would be fine. For the second stage (removing material from the inside), I’ll use the collets hopefully if they arrive in time.

Here it is all setup:



As I was running the lathe for a long time my DC motor was really heating up. I’ve actually been modifying the motor to be water cooled and today I set about connecting it all up. I’m happy to report that it works flawlessly and manages to keep the motor at 37c or below the whole time... and not one leak



I need to tidy up the tubing a little bit that will be a job for the future.

I managed to reduce 16mm square Alu stock down to 13mm x 14mm x 130mm. The surfaces are a little rough but that doesn’t matter as I will be sanding them and polishing them anyway prior to anodising:



I couldn’t resist a quick test fit to see if they’ll look ok after:



Yep! Looking good so far. Nothing that looks out of place, so I’ll power on.

Next up I marked the back sides in preparation for milling out the material in the centres:



I will need to setup my dial indicator for the horizontal travel of my lathe so I can get the required depth on the steps. Likely will make a carriage stop at some stage.

That’s it so far. A few hours work for not a lot of result. But hopefully another session on the lathe and I’ll have them done.

If anyone in Sydney has a milling machine they need storing somewhere let me know. I’d love one

 

[smartin53]

That's an interesting reuse of PC water cooling. I like repurposing stuff outside of its intended use so I'll have to keep that in mind for later uses myself.

 

[neevo]

That's an interesting reuse of PC water cooling. I like repurposing stuff outside of its intended use so I'll have to keep that in mind for later uses myself.

Correct. It’s a 2 fan radiator with a little pump and reservoir. The cooling on the motor is done with 5mm copper tubing wrapped around the motor body and I also soldered copper tubing to the brushes inside the motor too, so it circulates through the brushes then around the outside of the motor, before running through the fan and back to the reservoir. All of the system is run off a temp sensor mounted in the motor clamp against the body of the motor which has a relay in it and auto runs/shuts off the fans and pump.

It’s been highly effective at managing the temps, more than I imagined!

The motor was up to 49c before I set up the tubes and would have climbed further if I hadn’t noticed. Now it runs 37 - 39c all day.

 

[neevo]

Set about milling the backs of the end plates today. I carefully milled out the ends with a 4mm end mill, then did the step and started getting rid of the material in the centre.

Got this far:



Then disaster struck. Bang!

I knew exactly what it was as I’ve done it before. I’d melted one of the wires in the motor and it had shorted. This was the reason I built the water cooling but it looks like I have a flaw in my design. Whilst it works perfectly at keeping the motor body and brushes cool, it does nothing for the internals and so I’ll be looking to add a fan for that too.

It’s only really a problem when milling as the motor runs for so long and fast. Water cooling alone will be fine for lathe work.

The benefit of using a DC motor from a treadmill is that they are readily available. Whilst I could have picked one up for free I took the opportunity to see a chap who had a selection for not much money. Given I had specific dimensions I was working with I took my old motor and went and saw him.

He had a coupe of likely candidates and I took a Chinese made one over a higher HP American made motor, mainly because the brush housing was metal and this meant I could water cool the brushes again.

It didn’t take me long to strip the new motor, add the tubes for the water cooled brushes and had it up and running before dinner.

So I’m ready to try and finish off the end plates tomorrow. This time I’ll use the compressor and add some air to keep the motor cool.

 

[neevo]

Finished off the milling on my end panels today. I wasn’t too unhappy with them considering it’s my first ever time milling but there were a few niggles that I’d prefer weren’t there.

Either way I had them done and ready for sanding/polishing.



Started with 180 grit to get out the machining marks:



Then on to 240 grit.



Then I realised I was making a grave mistake. I was tidying up the non visible sides and realised I’d taken too much material off at an angle:



Poop!

Well it looks like these are a proof of concept and I will remake them. I’ve already ordered the Alu. At least this time I can try and remove the errors from the first lot. The Alu is also only $9, so no cost other than my time.

 

[neevo]

So much progress... just none on the amp.

I hated milling on my lathe so I took a chance on an old piece of heavy machinery with a view to using it as a mill. It’s a very old Taylor Hobson Pantograph that is heavy duty! I was told by the chap I bought it from it was 350kg but reading up on it some more it’s listed as closer to a tonne

It’s not finished yet as I’ve spent the last few weeks stripping it down and cleaning off years of grime and baked on grease, but I have it back up and running and I’m hopeful I can be making chips soon. All for the princely sum of $250 and a milling vice.

I haven’t figured out the collet situation yet as it takes an odd size but I have the ability to mill at least and I have no doubts it will be a significant step up from my lathe.

The other exciting bit of news is my chassis is complete!!!! I can’t pick it up until after Xmas as the chap is closing for the festive season but I asked how it turned out and his words were “They look great, I’ll send you some photos”.

So fingers crossed they live up to the 3 month wait