Mullard HSVT - "Reset" button woes - UK Vintage Radio Repair and Restoration Discussion Forum

Brockie · 20th Mar 2023, 3:59 pm

Hi all,
I have a Mullard HSVT (MK1) (to be fair i have one working and a couple that need "further attention" - which i will eventually get to as they are "on the list" to be fixed).

However on this one whenever to run the "HT" test card, it trips the Reset cct. If I press the reset button on the top to reset it whilst the handle is still down, all is good and I can carry on with that test no problems.
Raise the handle, Lower again and the same as above happens.

This morning i found it also happened when trying to test an EL84. Continuity test is no issues - but when i try a "Heather Cathode INS" test, it trips. Pres reset and i get the usual nice heaters firing up, and a dot firmly in the green.

I had a couple of questions if i may ask those that know far more than me...

1. What is the Reset button there for?
2. What would trigger it?
3. Anyone any thoughts as to what to look for to resolve?

Many thanks
Paul

Electronpusher0 · 20th Mar 2023, 8:03 pm

Be aware that pulling the handle down momentarily disconnects the mains from the MHSVT. As you complete the pull it reconnects.
The trip measures the current in the transformer and trips if it get too large.
Check the action of the arm and consequent actions, it involves an inhibit flag that prevents the mains being applied if there is no card inserted.
Looks like there is a momentary short as the arm is pulled down.

Peter

Brockie · 21st Mar 2023, 9:21 am

Hi Peter,
Many thanks - that makes sense... I wasnt sure what triggered the "reset" - but an excess of current draw would explain it... Will investigate further around the arm, switch and inhibit flag (on another unit i have repaired the inhibit flag, so completely understand the process).

Quick final question - Would i be right in saying that the "reset" circuit doesnt seem to appear on the Cct diagram? If it does, could someone point me to it please?

Thanks
Paul

GMB · 21st Mar 2023, 10:24 am

Look at the later models. The mk-1 diagram seems not to show it.
I think it feeds the common point of the HT winding through the relay.

Brockie · 21st Mar 2023, 10:51 am

Ahh yes... Marked as RLA, and if triggered it breaks the PRI feed into the Transformer.. Not present on the Mk1 cct diagram, however present on the MK3/4.
From what i understand (reading about), was retrofitted to a bunch of Mk1's, which would explain why it's not on the Mk1 cct diagram.

Electronpusher0 · 22nd Mar 2023, 1:01 pm

Quote:

Originally Posted by GMB

Look at the later models. The mk-1 diagram seems not to show it.
I think it feeds the common point of the HT winding through the relay.

The very first MK1s did not have the reset button and trip.
However a lot of MK1s have a Reset as it was supplied as a retro fit option.

Peter

20th Mar 2023, 3:59 pm	#1
Brockie Triode Join Date: Nov 2018 Location: Risca, South Wales Posts: 48	Mullard HSVT - "Reset" button woes Hi all, I have a Mullard HSVT (MK1) (to be fair i have one working and a couple that need "further attention" - which i will eventually get to as they are "on the list" to be fixed). However on this one whenever to run the "HT" test card, it trips the Reset cct. If I press the reset button on the top to reset it whilst the handle is still down, all is good and I can carry on with that test no problems. Raise the handle, Lower again and the same as above happens. This morning i found it also happened when trying to test an EL84. Continuity test is no issues - but when i try a "Heather Cathode INS" test, it trips. Pres reset and i get the usual nice heaters firing up, and a dot firmly in the green. I had a couple of questions if i may ask those that know far more than me... 1. What is the Reset button there for? 2. What would trigger it? 3. Anyone any thoughts as to what to look for to resolve? Many thanks Paul

20th Mar 2023, 8:03 pm	#2
Electronpusher0 Nonode Join Date: Jun 2016 Location: Bognor Regis, West Sussex, UK. Posts: 2,300	Re: Mullard HSVT - "Reset" button woes Be aware that pulling the handle down momentarily disconnects the mains from the MHSVT. As you complete the pull it reconnects. The trip measures the current in the transformer and trips if it get too large. Check the action of the arm and consequent actions, it involves an inhibit flag that prevents the mains being applied if there is no card inserted. Looks like there is a momentary short as the arm is pulled down. Peter

21st Mar 2023, 9:21 am	#3
Brockie Triode Join Date: Nov 2018 Location: Risca, South Wales Posts: 48	Re: Mullard HSVT - "Reset" button woes Hi Peter, Many thanks - that makes sense... I wasnt sure what triggered the "reset" - but an excess of current draw would explain it... Will investigate further around the arm, switch and inhibit flag (on another unit i have repaired the inhibit flag, so completely understand the process). Quick final question - Would i be right in saying that the "reset" circuit doesnt seem to appear on the Cct diagram? If it does, could someone point me to it please? Thanks Paul

21st Mar 2023, 10:24 am	#4
GMB Dekatron Join Date: Aug 2003 Location: near Reading (and sometimes Torquay) Posts: 3,099	Re: Mullard HSVT - "Reset" button woes Look at the later models. The mk-1 diagram seems not to show it. I think it feeds the common point of the HT winding through the relay.

21st Mar 2023, 10:51 am	#5
Brockie Triode Join Date: Nov 2018 Location: Risca, South Wales Posts: 48	Re: Mullard HSVT - "Reset" button woes Ahh yes... Marked as RLA, and if triggered it breaks the PRI feed into the Transformer.. Not present on the Mk1 cct diagram, however present on the MK3/4. From what i understand (reading about), was retrofitted to a bunch of Mk1's, which would explain why it's not on the Mk1 cct diagram.