Mullard HSVT E7600/4 Woes

[frankmcvey]

Got this machine about 4 years ago, checked it out at the time and labelled it "serviceable-ish", then put it on the to-do pile. Dug it out a few days ago and got to work and initially it looked good. Ran through all 3 test card checks; left it on standby for a couple of hours to stabilise, then came back to it to finely tweak it and found that the spot now came up in the red zone rather than the dotted line, and did not respond to varying the mains fine adjust.

Thinking that one of the diff amp valves had died on standby, I replaced the EF91s, V2 and V3 - same problem.

OK, perhaps a cap had gone u/s. Replaced all caps in the machine one by one (I'd intended to do this anyway as a matter of course), checking each time - things became gradually sharper, but the fault remained.

Right, went through the indicator and power boards and replaced all the non-stab carbon resistors, again checking after each change. Fault still there.

Voltage measurements - checked the diff amp and CRT HTs, and found a significant imbalance in the voltages across C3 and C4: 390V across c3, -540V across C4, so replaced V9 and V10, the EY51s in the voltage double cct. Problem remained, slightly improved - voltages now 414V across C3, and -522V across C4.

The position is now, with the mains card in, the spot comes about half way up the red zone. Using the factory calibration mains adjustment pot, RV6, I can bring it up to the amber zone, then RV6 teaches its end-stop. Using the mains fine adjust switch has a slight effect on the spot - varying from end-to-end over the 11 steps, it will move the spot up or down a total of <3mm (I'd normally expect about 2mm movement each step) The brillaince and focus controls work normally, as does the x-shift. In this condition, the set zero control has significant control, raising and lowering the spot about 15mm; the reject limit control will move it +/- 3mm.

With the reject card in, and Sw posn 1, the set zero control has not effect, but the reject limit can move the spot from off-screen bottom up to the green zone.

With the reject card in and Sw posn 6 selected, the effects are as in Sw posn 1.

With the HT card in, the spot comes up to the bottom of the green zone - the set zero and reject controls work as per the last para.

I've attached a diagram of the indicator circuit show the various voltage measurements made with the mains card in and the handle pulled. Any thoughts or suggestions as to where to go next will be gratefully seized on - I'm running out of ideas!

The system won't take my drawing, probably because the file was too large. You'll find it here.

Cheers,
Frank

[frankmcvey]

Addendum to the above; RV4, the Reject Limit control also checks out as serviceable.

[frankmcvey]

Further addendum - you might find that the link I gave in post 1 needs permissions. If so this link should work for you.

[ColinTheAmpMan1]

Originally Posted by frankmcvey

Further addendum - you might find that the link I gave in post 1 needs permissions. If so this link should work for you.

Both links sort-of work in that they come up, but it takes ages to get anything to load (I gave up in the end) .
Colin.

[frankmcvey]

Hi, Colin,

Perhaps that's why I'm not getting too many replies!

I'll try again with an another version, although not too sure how legible it'll be.

Cheers,

Frank

[Freya]

I have a vague recollection someone else having this fault, it was traced to a transformer with interwinding leakage. It may have been on the VRAT forum.

[PJL]

You need to check the reference voltage on the 85a2 and the -200V supply as these are compared for the mains adjustment.
The REJECT card only uses the reference voltage (see below). What are the voltages with this card in position 6 as this should have no input. The two adjustments normally interact so if the Zero Adjust does nothing there is something wrong.

[frankmcvey]

Hi, Peter - gosh I'm glad to see you!

Attached is the diagram of the bridge with various voltages marked. TBH, it doesn't look a lot different to some I've done in the past

The green writing didn't come out too well. What it says is that the voltage at the node between R51, R53 and RV6 was at -74.2 when first measured, but varying RV6, the factory preset, varies it from -67V to -91.

Unusually, while making this adjustment, the CRT spot, which is abt halfway from the bottom of the green to the dotted line, doesn't move at all in the Y direction, but it does move a few mm laterally and defocusses!

BTW, all Cs and most Rs in the bridge are new, apart from a few hi-stabs.

I'll get to the reject card checks in the morning!

Cheers,

Frank

[Chris55000]

Hi!

Do be aware that one of the electrolytic capacitors shown on that theoretical diagram has been drawn in upside down with it's negative connected to chassis in error!

Chris Williams

[frankmcvey]

Thanks, Chris. I must have looked at that diagram a hundred times and never spotted that! Fortunately, it's the correct way round on the machine in question.

Cheers,

Frank

[frankmcvey]

Hi, All,

Attached are the voltages checks requested by PJL. Nice check and good thinking, since it gives the state of the diff amp with no input, so presumably everything should be in balance.

I've also added photos of the indicator and power boards to show what work has been done.

A query was raised some time ago about how to fit new, smaller modern caps to replace the large 4uF old-style can electrolytics at C3 and C4. I find this method works quite well, although it certainly doesn't appear period correct. More by luck than judgement, the mounting holes for the old can clamps exactly line up with those on the nylon chocolate block terminals. Might have been neater to blank off the large 35mm holes with some 1mm paxolin - a piece of a broken test card would do nicely if you can work round the contact holes...

Cheers,

Frank

[PJL]

Try swapping the EF91's over and measure the grids again as I don't like the look of the difference.

[frankmcvey]

Hi, Peter,

Just to make sure, I did another confirming check with the valves as they were. Then I exchanged the EF95s, V2 & V3 and did a further series of checks. Results of all three checks appended.

There were subtle differences, the main one being that from the initial check to my doing a confirming check a couple of hours later, the grid voltage difference had reversed but the spot position and control behavior were apparently unaltered. However, it's just possible I may have had a brainstorm when noting the first checks and inadvertently entered V2's grid as being that of V3, and vice-versa. Subsequently, there was little difference in grid voltage as a result of swapping the valves

Thanks for your help, much appreciated.

Cheers,

Frank

[PJL]

Set RV4 to the mid position. Insert the REJECT card with position 6 and adjust RV5 so the dot is on the base line (as per calibration procedure). Take the voltage measurements again.
Then set to position 1 and take the same measurements again.

[frankmcvey]

I couldn't follow your instructions there, Peter, since the set zero control was having no effect at all on the spot. However, it did make me take another look at RV5 and its surrounding circuit. I was looking through the component location diagrams in the manual (not very clear b&w photos) in order to carry out further voltage checks, when I noticed that R33 didn't look correct. I had another E7600/4 sitting in the to-do pile and took the can off and it immediately was obvious - I'd made a wiring error when replacing the resistors. R33 was connected to R35, instead of its next-door neighbour R34. So your instinct was 100% correct when you commented about something being wrong when the controls weren't interacting.

Having rewired it correctly, the set zero was now working, but I still couldn't set up with the mains card. On re-checking the reference voltages, the -85 was spot on, but the -200V was down at around -146V. I replaced the EZ90 rectifier feeding the ref voltage circuits and all was looking better. It had seemed OK when we first did the bridge checks, so I can only assume it died later, while I was chasing my tail.

I set up RV6 by using the mains card while monitoring the 6.3V heater supply for the EF91s - just step thorough the fine mains adjust until the DVM reads exactly 6.3V, then use RV6 to set the spot to the dotted line. (thanks David Simpson for that dodge!)

After that, it romped through the Reject card and the HT card without a problem. I then made it test its own duff EZ90 and sure enough it confirmed that one half of the rectifer was fine, while the other half had virtually no emission. After that I checked a variety of valves and the machine was agreeing fairly closely with my bench HSVT.

Thanks for your help and suggestions, especially PJL. When working on something like this it's easy to have your mind go in circles, particularly when you know that all the components are new and serviceable!

Note to self - next time take a damn photo first!

Cheers,

Frank