Mullard HSVT Refurbishment learning exercise... (Any help gratefully received!)

[Brockie]

Having worked on an E7600/3 before, with success (i think more by chance than anything else!), I have picked up an E7600/4 to have a play with and try to get working again. Also i find that it's a great learning exercise to understand how things work by having a play and learning by experience.

Figured i would leave all as is, and slowly wind up on a Variac. You never know, those Caps may well reform meaning i can check all works before going to re-cap (I like to be sure that i am working form a known position).

First thing i noticed was that the Control Grid button wasnt able to be depressed... Eventually found to be caused by a blob of solder on the rear of the Metal Sliding switch mechanism. Definately placed there on purpose. I have left alone for the moment... I wonder why this would be done..

However even at lower voltages (50V) and with the lever actuated (No card in - is this advisable?), I am getting some lovely loud buzzing from the Transformer - even after leaving 30 mins to see if it settles! It's pulling around 0.7A @ 50V. Momentatarilly raising to 60V and the buzzing gets louder... So have backed off for the moment until I a little more confidence in it

1. Any advise from those that have done this more often than me please?
2. Does anyone have a list of Caps/modern equivalents please that they would be willing to share? I could use this as a guide so i can order replacements ahead of time.

Im sure this will be the first of many requests - i will keep all in this thread just for the sake of completeness - and also it should document a Journey... hopefully..

Thanks
Paul

[Brockie]

Just to add on the "buzzing" front - If i put any of the test cards in the buzzing (when actuated) stops....

[Ed_Dinning]

Hi Brockie, With no card inserted there should be no problem closing the gate switch as power will not be applied to the system.
I would caution against running with a buzzing transformer as it is a sign of stress in that part and they cannot easily be rewound.
It would be best to first do a full internal clean and inspection before applying power again. Then try without valves and take some voltage readings.
Note that you will never reform caps with only 50v applied as the rectifiers will not be operating

Ed

[Brockie]

Hiya Ed,

Many thanks for getting back to me

Originally Posted by Ed_Dinning

Hi Brockie, With no card inserted there should be no problem closing the gate switch as power will not be applied to the system.

Oh ok - i believed that, with no card in position, this would apply "operating" power to the pins. Checking the Cct Diagram (Maybe i should have checked beforehand!) I can see now
If i put the "Mains" card in then (meaning the Gate and Card safety switch would be operated) would this mean that the majority of the Circuit would be energised then??

I would caution against running with a buzzing transformer as it is a sign of stress in that part and they cannot easily be rewound.

Exactly whay i stopped

It would be best to first do a full internal clean and inspection before applying power again. Then try without valves and take some voltage readings.

Completely understand - Just running at 50v wasnt the intention - The idea was to run for 60 mins at 50v (to make sure there was no moisture remaining in the transformer windings at all), then up to 100v for 30 mins, and then up to 230V...

Internal clean is really needed... so i think that will be my next step. Nothing obviously damaged, however Grid definately needs a clean (of course - 70 years of dust and things dropped inside!!) alongside, and Switches will need a once over too... plus the mysterious riddle behind the Control Grid Button!

[m0cemdave]

You will certainly need to replace all the paper capacitors. At this age they will all be leaky to some extent, and even a small amount of leakage will upset the DC amplifiers.

You will also need to check all the resistors. Some of them will have drifted high in value, preventing the correct test voltages being applied when testing particular valves.

Hopefully the PSU regulator valve will be OK, otherwise you will be in for a nasty surprise when you look for a replacement. But a previous thread suggests that it can be replaced with a modern EL34 with minor modifications to the tester.

[Brockie]

Originally Posted by m0cemdave

You will certainly need to replace all the paper capacitors. At this age they will all be leaky to some extent, and even a small amount of leakage will upset the DC amplifiers.

You will also need to check all the resistors. Some of them will have drifted high in value, preventing the correct test voltages being applied when testing particular valves.

Hopefully the PSU regulator valve will be OK, otherwise you will be in for a nasty surprise when you look for a replacement. But a previous thread suggests that it can be replaced with a modern EL34 with minor modifications to the tester.

Thank you.

Can the resistors be tested in Circuit, or do they need to have at least one end disconnected for a proper reading?

[GMB]

with the lever actuated (No card in - is this advisable?)

Absolutely not. Doing this shorts everything which is why there is a safety cut-off switch that the actual card has to operate. If this switch is faulty then you have your explanation.

As resistors more often go high in value with age then you can test that in circuit - a high reading means it is definitely bad, and a low reading means you must consider the circuit and maybe disconnect (or use an in-situ bridge to make the measurement).

[Brockie]

Just wanted to update with some pics of the Grid, and a possible issue....

Highlighted in the corner is what seems to be a snapped actuator of some description.... Can anyone advise what this does please?? Am guessing i need to fix this first off




Also, a pic of the offending solder "addition" to prevent the "Control Grid" button from being pressed...
The microswitch bottom right of the Pic seems to be operating correctly.



Next up I think will be to give the Grid a good clean (Any suggestions?? - i was thinking either of a general clean up and then Isopropol Alcohol and a tooth brush, or Contact cleaner)...

Slowly working my way through the resistors.. A lot seem in spec... a couple seem way out, so will re-measure and check.

I have a fair few Replacement Caps anyhow, so the 01.uf paper caps will be up for replacement tomorrow...

[Brockie]

Ahha... it seems that the broken piece of metal is indeed the Safety Switch... (Taken from the CT60 Manual)



Begs two questions...

1. The Manual states above that it's an insulator.. on mine it is a piece of metal.. so is the opposite.... thoughts?

2. If this is the Safety switch.. what is the Microswitch for on the bottom right? Cant find a mention of that anywhere..

[GMB]

From everything you have said it seems to me that your tester has had a few modifications. You will just have to assess whether the fixes are working for the required effect.

[Brockie]

Just a quick update.

Cleaned the matrix from all the congealed 60 year old grease - looking a lot better now.
Above where i mentioned the Safety switch was a piece of metal - After removing the two constitiuant pieces and inspecting, it seems this is plastic, however was really heavilly coated with dirt... Cleaned, glued the arm back on and re-connected.

Next is to re-assemble the Grid, and replace all the old Caps..

[Brockie]

All caps replaced other than the 2x 4uf, since i need to put together a new assembly on a tagboard to replace them.....

Figured i would wind up on a variac and see what happens...

Having given it 30 mins at 60v, then another 30 at 90v... made sure it was set at 240v and slowly wound up. Put the "mains" test card in, and pulled the handle. The moment of truth. A dot appeared... and then as quickly as it did, it disappeared again. A matter of milliseconds.

Looking at the state of the two large Caps (4Uf) they arent looking too happy...So i guess it's on with C3 and C4 replacements

With any luck this might being some life to the CRT...

[frankmcvey]

Here's one method for replacing C3 and C4, using chocolate block. Simple, neat, quick and effective and keeps the replacements clear of the outer can, with just 3 solder connections to the assembly. If you're as lucky as me, the mounting holes will line up with the original cap mounting holes. Some purists might say that it's not really in keeping with the vibe, but hey, it works!

Cheers,

Frank

NB - I know - 500VDC is pushing my luck a little. Higher would be better.

[Brockie]

Thanks basically what i did, however on a tag Strip. I found some 4uf 650v available in the UK, so used those
Yours deffo looks quicker

Well the magical Dot appeared... about 5mm south of where i wanted it to be... So checked the X-Adjust and centred... however whatever i tried i couldnt get it up to the line. Then it magically disappeared - I found it was only visable to the left of centre... then then more left of centre... then more... then off the screen to the left!!!.
And now i dont know where it has gone, so i gave up for the afternoon
Also to top it off... the 2v mains adjust didnt seem to make any difference either

Does anyone have any known good Test voltages internally i could check please?? just feeling like i need to break this down a little to progress.

Feel so disapointed... i was elated that it worked and the dot appeared... until it didnt any more