capacitor identification ? HT-37 Drifting repair

[KI4THX]

I know this will be a silly and trivial question for most here, but I cant make heads or tails of the value of this old mica cap.

I think its a .01 @ 400 V mica.



I recently bought a pair of HT-37's, and this one is drifting around quite a bit even after an hour or three warm up time. So, Ive decided to replace the old mica caps in the VFO. Tubes checked OK, Hope Im heading in the right direction.

[KI4THX]

Crickets ?

Yes I have looked at the charts, they are conflicting.

[nq5t]

I know this will be a silly and trivial question for most here, but I cant make heads or tails of the value of this old mica cap.

What does the schematic say they are???

[WD5JKO]

Yea, Like KI4THX asked, what does the schematic say?

A decent reference on those capacitor color codes is here:

http://www.tpub.com/neets/book2/3g.htm

Your cap type does not have arrows though.

As shown, perhaps it is :

1st:brown
2nd: black or blue
Multiplier: brown

Could be 100pf or 160pf.....check with schematic.

Jim
Wd5JKO

[KB2WIG]

Dots dots, IC dots.

Look at the dots....

It looks like the top cap has the 'pointey' dots......, i.e, the arrow.

klc

[KI4THX]

Don't have a schematic.....yet.

There are arrows on the top three dots.

I threw it all back together, I had read somewhere about temperature compensating caps in the VFO. The audio is splendid....even on "cough" sideband.... just too drifty for SSB. Id like to fix that.

[WD5JKO]

Went to Bama, and attaching VFO schematic here. Looks like the caps are C5, and C6. Is that where they are connected? If so, both are .001 uf or 1000 pf.

Jim
Wd5JKO

[KI4THX]

BAMA.... dang, hadn't thought about that site. Off to DL the manu-L

[KA0HCP]

http://www.radiodaze.com/PDF/COLORCOMPONCHART001.pdf

RadioDaze has a great chart for resistors, caps, wire colors.  I keep on my bench; it never gets back into the binder.

[WQ9E]

The most important thing you need to do with your HT-37 during restoration is to switch the rectifiers to solid state (with added dropping resistors and inrush current limiting).  The power transformer in the HT-37 (also the HT-30 and all flavors of the HT-32) have a well known transformer weakness where the rectifier filament winding (which also has rectified DC appearing on it) short to the transformer core taking out the transformer.  Replace with SS and disconnect and tape the 5 volt windings.  Dead transformers due to this fault are very common in this series of Hallicrafters transmitters.

[KI4THX]

Thanks for the tip. The reduction of heat in the chassis after that can only help with the drifting also I suppose.

[WQ9E]

You are welcome.  It really won't reduce the heat with the proper value dropping resistors added but it does prevent the most common failure mode for these Hallicrafters transformers.

HT-37 drift shouldn't be bad once it is warmed up for about 20 minutes, it is plenty stable for SSB.  Make sure that the VFO tuning capacitor rotor plates are properly centered between the stator plates, any offset here significantly changes the drift characteristics.

[KA0HCP]

In my experience very few of these micamold (really paper) capacitors are ever within spec, even +/- 20%.   

You can only expect drifting and poor performance from these ancient components.

I typically replace them with mica or tubular foil caps.  If authenticity is important, mica or disk caps can be embedded in casting resin in a rubber mold made from an old form.   

Casting resin and mold compounds can be found in craft stores like Michaels.  For less than $30 you can make replica parts like knobs as well.