Heathkit W-5M Red Plate Issue

[Trout]

I found this old Heathkit W-5M at a sale recently. I finally got around to looking it over and plugging it in.

During a visual inspection, I noticed a couple of leaky electrolytic caps, Specifically C7 and C8 and C4 and C5.( Circled on schematic below)

I replaced them with the exact values shown on schematic. Plugged it in and all seemed ok. I quickly checked the Zero Adjust, Saw it was about .2V, reset it to zero per Heathkit spec.

I hooked it up to my old laptop as a source and it played nicely for about 5 minutes. Just about the time I thought all was good, I glanced at the 6L6 and noticed rather severe red plating on 1 tube. Needless to say, I pulled the power FAST!

The Amp originally had KT66's as standard, But when I got it, It had RCA 6L6GC's.
The Tubes all tested good prior to power up.

Where do I start to locate or solve this type of problem?

W-5M schematic

[anatech]

Hi Gene,
Neat find! I have one, not two.

Did you check for 50VDC on the cathodes of the outputs? How about C14 and C15 for leakage?

At this point you should check your resistor values as well. They could be all over the place by now.

RCA 6L6GC's are good, you may want to try 7581A's in there. They are the US equiv. to a KT66. I find with Fender twins that I need to go to these, most 6L6GC's can't handle the twin otherwise.

Sorry for diving into the basics Gene, just in case you forgot something.

-Chris

[Trout]

I'll pull of the voltages later today, I bet your hitting it on the head with C14 & C15. If memory serves, Thats a common problem. Sadly, I have nothing that big here so I will have to order a couple in. The ones in the amp are these big black enormous pyramids! The only 1.0 caps I have are sprague 100V which just wont cut it.

Cool amp though, Very Heavey. That Peerless/ Altec 16458 Opt is very popular.
If I remember right, I read that the resistor on the input might need altered also. I have to search around a bit to locate the exact info.

Sorry for diving into the basics Gene, just in case you forgot something.

NO Problem, If I could remember everything I have read and learned at this site, and had 100% recall. I wouldnt need my computer LOL

You Guys Rock!
Trout

[anatech]

Hey Gene,
Use a smaller value coupling cap to check before you order the proper ones if you need to.

You should only have to measure the grid voltages to see if they agree with the schematic. Or lift one leg and tack a couple smaller ones across.

-Chris

[Trout]

Did you check for 50VDC on the cathodes of the outputs? How about C14 and C15 for leakage?

Oh Boy, Cathode voltages are terribly high.

I powered up just long enough to get sound from the speaker, Quickly hit the cathode with the volt meter 1 tube at a time. Then Killed the power.

Cathode voltage continued to climb and It was clear at Tube#1 80V is not a good thing. Tube 2 tested at 88V. I have no idea exactly how high they may have gone, They were going up fairly fast.No sense frying stuff further.

Clearly the C14 & C15 also have issues, I do not think 18V, and 66V is correct.

Looks like this thing might just end up in the sell parts on eBay bin.

Trout

[anatech]

Hi Gene,
Wow, must be leaky coupling caps then. For you, this should be an easy fix. Want to try some smaller coupling caps and give it a whirl?

-Chris

[Trout]

Originally posted by anatech
Hi Gene,
Wow, must be leaky coupling caps then. For you, this should be an easy fix. Want to try some smaller coupling caps and give it a whirl?

-Chris

Probly a good idea, But the biggest cap I have in the house here is .1/630V at the moment.
I need to look in my old chassis pile and see if I have anything bigger I can salvage.

Trout

[anatech]

Hi Gene,
0.1 uF is fine. You just want to fix the problems to see if it's worth rebuilding. Once you know those are problems then get the proper ones.

One of those "make it go" things. You may want to check the resistor values in the cathode circuits.

-Chris