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Old 2nd Jul 2020, 8:53 pm   #1
Brockie
Triode
 
Join Date: Nov 2018
Location: Risca, South Wales
Posts: 48
Default Mullard HST - Advice please!

Hi all,
So playing with a Mullard High speed tester... Hasnt been fired up for a while, so i'm in the process of tidying up the internals, changing all the capacitors, and general inspection. Please bear in mind i'm more of a modern day engineer than an older discipline engineer... so every day is a vertical learning school day where valves and older sets etc are concerned!

So i come to those with knowledge to learn again!

Managed to pick up the selection of Capacitors needed (Many thanks to those involved for their updated component lists (UKCol).

Easy to identify...

https://www.vintage-radio.net/forum/...1&d=1593719081

However when it comes to replacements, there are challenges...

Original 4uF 500/550v Electrolytics :
https://www.vintage-radio.net/forum/...1&d=1593719296

New 4uF 600V Electrolytics:
https://www.vintage-radio.net/forum/...1&d=1593719296

Firstly, am i missing something here?? There's a MASSIVE size difference...
Secondly - if i didn't miss anything, how on earth would the new caps be mounted (Any advice welcome here from someone who has done it please??).

Finally... There's an AWFUL lot of caked on dirt in here... Short of stripping every part out (unless i have to) any advice on how to give both a function and a general tidy up please??.....

https://www.vintage-radio.net/forum/...1&d=1593719495

Many thanks!
Paul
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Old 2nd Jul 2020, 8:55 pm   #2
Brockie
Triode
 
Join Date: Nov 2018
Location: Risca, South Wales
Posts: 48
Default Re: Mullard HST - Advice please!

Also to follow up - any advice on how best to clean the matrix please? It's not dysfunctional dirty, however verging on it!
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Old 2nd Jul 2020, 11:23 pm   #3
frankmcvey
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Join Date: Jan 2015
Location: Cottesmore, East Midlands, UK.
Posts: 858
Default Re: Mullard HST - Advice please!

Modern caps are a lot smaller than the original cans. F&T do a range of can caps in the same 35mm size; not cheap though, and I don't know if they go as small as 4uF.

Couple of things you can do:

1. Remove the original caps and restuff them with your new smaller caps, if you value the original appearance. You'll find info all over the internet. Some varieties are easier than others. Some are waxy inside and the guts come out easily. Others use something like pitch and they're rock hard and are well-nigh impossible to empty out. OR:

2. Remove the original caps, making careful note of the wiring and the resistors (change them while you're at it) - take a couple of photos. Buy yourself some nylon chocolate block terminal strip - this stuff. Cut a length suitable to bridge the two large holes left by the old caps and mount the strip using the original screw-holes left by the can cap clamps, if possible; if not you may have to use one ccc screwhole and drill a new one to match the connector strip. You may have to source longer screws. Wire your new small caps and the resistors to the connector strip; sleeve the component leads if necessary. Doesn't look original, of course, but you can make a neat job and it works.

As for dirt, blow out as much as you can using an airline, then it's a fiddly bum-ache using a small paintbrush (1/2" or so), cottonbuds etc and IPA and slowly work your way round. Don't use acetone, which eats some plastics, and watch you don't scrub the colour codes off the resistors.

The manual is quite clear that you shouldn't use solvents on the gate switch matrix - the springs are steel and the only thing protecting them from rusting is the factory light grease coating. It recommends a soft brush or cloth. Use DeOxit or Servisol (other switch lubricant/cleaners or available) on the front panel switches and exercise them.

Most of your resistors will have gone high-value and will need replacing. Not a bad job on the E7600/4, but a bit of a mare on the earlier machines like yours. Don't bother to change any of the hi-stab resistors (the Eries with a pink tolerance band, which usually looks more white than pink), they're usually fine and this will save a lot of work.

Cheers,

Frank

Last edited by frankmcvey; 2nd Jul 2020 at 11:38 pm.
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Old 3rd Jul 2020, 6:42 am   #4
Electronpusher0
Nonode
 
Join Date: Jun 2016
Location: Bognor Regis, West Sussex, UK.
Posts: 2,287
Default Re: Mullard HST - Advice please!

I used modern non- electrolytic capacitors for the 4uF capacitors which were much smaller, I left the original ones in place and used the solder tags as mountings for a tag board which housed the new caps, I attach a photo, sorry its not too clear but its difficult getting in there.

For the switches I used lead free flux cleaner, its magic at removing the black layer on brass contacts, followed by servisol to leave a protective film.

The EY50 and EY51s needed replacing on mine, the EY50 was down to air.

Not sure which model you have but I found the schematic for the /3 was very difficult to read compared with the /4 so I redrew the /3 schematic based on the /4 layout and annotated it, attached.

Peter
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