HMV Pitch Block Capacitors - UK Vintage Radio Repair and Restoration Discussion Forum

PsychMan · 24th Jun 2020, 1:31 pm

I've recently started work on a poor old HMV 501 table radiogram. It had languished in a garage for many years, but despite less than perfect storage, it is still very restorable.

Upon powering up, audio stage works but low output, radio dead on both bands. Anode voltage of the output valve too high, other anode voltages way too low, all screen voltages low - Not a happy radio.

Quite a lot of the higher value capacitors are encased in a pitch block. I've read about these a little on here and decided to get the block off and see just how bad they were. I'd hoped as they were encased in pitch they might have survived a little better - like the oil ones. Nope!

I tested each one that had been used in the circuit in 3 ways, purely because I thought it would be interesting - a Chinese component analyser, a decent metra multimeter, and the high voltage leakage tester / reformer found on this site (all tested at 250v).

(Cap/Value) - Analyser - DMM - Leakage @ 250v
----------------------------------------------------------------------------------
1/ 0.1uf - 1uf - 1uf - 15ma
2/ 0.1uf - 36pf - 140pf - 10ma
4/ 1.0uf - 16uf - 11uf - 24ma
5/ 0.5uf - 10uf - 10uf- 22ma
7/ 1.0uf - 35pf - 5uf - 20ma
8/ 1.0uf - 36pf - 29uf - 20ma
9/ 1.0uf - 14uf - 12uf - 23ma
10-11/ 8.0uf - 242uf - 307uf - 26ma

Pretty atrocious!

Id read on here about freezing the block as opposed to boiling it to remove the contents. That seemed to work very well and it was completed and tidied up within 30 minutes.

I've shared the pictures of how it came apart and what came out of it, this might enable someone else to do a neater job of it. I used a flat blade screw driver and tapped with a hammer in various places to remove the bottom plate.

I plan to wash the can out, bend back into shape and spray it before adding new poly caps to the terminals.

PsychMan · 24th Jun 2020, 1:31 pm

Last pictures

Hartley118 · 24th Jun 2020, 1:44 pm

Nice job - I'd never thought of freezing to access the contents of a capacitor block. I'll remember that.

In passing, I've often wondered why there was a preference back in the day for encapsulating all the paper capacitors in a single block. It's difficult to believe that it was cheaper than fitting individual components - and the wiring was necessarily more complex.

Does anyone know the thinking behind the capacitor pack/block approach?

Martin

PsychMan · 24th Jun 2020, 2:15 pm

Id be interested too!

One more point actually, to start removing the innards, I found it helped to drive a screw into one of the capacitors and use grips to pull it out. Once one was out the rest came out very easily.

24th Jun 2020, 1:31 pm	#1
PsychMan Octode Join Date: Jun 2013 Location: Fleet, Hampshire, UK Posts: 1,764	HMV Pitch Block Capacitors I've recently started work on a poor old HMV 501 table radiogram. It had languished in a garage for many years, but despite less than perfect storage, it is still very restorable. Upon powering up, audio stage works but low output, radio dead on both bands. Anode voltage of the output valve too high, other anode voltages way too low, all screen voltages low - Not a happy radio. Quite a lot of the higher value capacitors are encased in a pitch block. I've read about these a little on here and decided to get the block off and see just how bad they were. I'd hoped as they were encased in pitch they might have survived a little better - like the oil ones. Nope! I tested each one that had been used in the circuit in 3 ways, purely because I thought it would be interesting - a Chinese component analyser, a decent metra multimeter, and the high voltage leakage tester / reformer found on this site (all tested at 250v). (Cap/Value) - Analyser - DMM - Leakage @ 250v ---------------------------------------------------------------------------------- 1/ 0.1uf - 1uf - 1uf - 15ma 2/ 0.1uf - 36pf - 140pf - 10ma 4/ 1.0uf - 16uf - 11uf - 24ma 5/ 0.5uf - 10uf - 10uf- 22ma 7/ 1.0uf - 35pf - 5uf - 20ma 8/ 1.0uf - 36pf - 29uf - 20ma 9/ 1.0uf - 14uf - 12uf - 23ma 10-11/ 8.0uf - 242uf - 307uf - 26ma Pretty atrocious! Id read on here about freezing the block as opposed to boiling it to remove the contents. That seemed to work very well and it was completed and tidied up within 30 minutes. I've shared the pictures of how it came apart and what came out of it, this might enable someone else to do a neater job of it. I used a flat blade screw driver and tapped with a hammer in various places to remove the bottom plate. I plan to wash the can out, bend back into shape and spray it before adding new poly caps to the terminals. Attached Thumbnails

24th Jun 2020, 1:31 pm	#2
PsychMan Octode Join Date: Jun 2013 Location: Fleet, Hampshire, UK Posts: 1,764	Re: HMV Pitch Block Capacitors Last pictures Attached Thumbnails

24th Jun 2020, 1:44 pm	#3
Hartley118 Nonode Join Date: Aug 2015 Location: Cambridge, Cambs. UK. Posts: 2,198	Re: HMV Pitch Block Capacitors Nice job - I'd never thought of freezing to access the contents of a capacitor block. I'll remember that. In passing, I've often wondered why there was a preference back in the day for encapsulating all the paper capacitors in a single block. It's difficult to believe that it was cheaper than fitting individual components - and the wiring was necessarily more complex. Does anyone know the thinking behind the capacitor pack/block approach? Martin __________________ BVWS Member

24th Jun 2020, 2:15 pm	#4
PsychMan Octode Join Date: Jun 2013 Location: Fleet, Hampshire, UK Posts: 1,764	Re: HMV Pitch Block Capacitors Id be interested too! One more point actually, to start removing the innards, I found it helped to drive a screw into one of the capacitors and use grips to pull it out. Once one was out the rest came out very easily.