Mullard HSVT - UK Vintage Radio Repair and Restoration Discussion Forum

Radio Tech · 8th Jan 2016, 4:47 pm

I am in the process of restoring a Mullard HSVT and found the two 4uf electrolytic capacitors were shot, I did not have 4uf ones and used 10uf, in the manual it states the 4uf should be within plus 50% minus 10% but as I used 10uf is the reason why the spot is way out when the reject card is inserted? The cards I have loaned from a colleague of mine.

Best wishes

Ken

HamishBoxer · 8th Jan 2016, 4:56 pm

Possibly not the caps but 2 X 10uF in series gives you 5uF if you want to be sure of keeping a bit nearer the mark or just use single 4.7uF a standard value these days.

turretslug · 8th Jan 2016, 5:13 pm

The "plus 50% minus 10%" is likely to simply be the typical tolerance of electrolytics of this type and era, rather than any particular requirement set by the design. As a very broad rule of thumb, it's wise to not deviate too far from the value of the first capacitor after the rectifier (the reservoir) to avoid stressing the rectifier or having the HT climb excessively high but capacitors after the first smoothing resistor/choke can usually be increased with advantage. Back then, high voltage microfarads were pricey and bulky, and "what can be got away with" was the rule but now they're cheap as chips and far smaller.

All this is design dependent, of course- if (rather unusually) there's some sort of expectation of HT ripple for whatever reason (and it would be both rare and marginal design for it to be the case!), then stick to the original values.

Radio Tech · 8th Jan 2016, 5:23 pm

Thanks Hamish and Turret, I will have another look in my capacitor box and see if I have anything much nearer to the original values, I would not wish to stress the EY51 rectifiers in this. Will report back as soon as I have looked into it much deeper.

Regards

Ken

turretslug · 8th Jan 2016, 5:45 pm

As Hamish says, you could have the 2x 10uF in series for the reservoir- and "something else" that comes to hand for smoothing.

Radio Tech · 9th Jan 2016, 5:56 pm

Hi folks

I put 10uf caps in series in the end for C3 and C4, I have renewed the two 470 ohm resistors on the differential amplifier and changed the waxies on that side of the valve tester. With the pre-sets at the front all around mid way, with the mains test card in the spot flies right to the top of the screen. I haven't as yet changed any other capacitors especially those on the other side where there are four 16uf electroltyics but not to sure at this stage what is causing the spot to fly up nearly off the screen. But any help would be appreciated on the cause.

Best wishes

Ken

Radio Tech · 11th Jan 2016, 5:02 pm

Hi folks

Just a little update on the progress of restoring this valve tester, I followed Paul Stennings information on setting the pre-set mains control, got the spot around where it should be on the "A" line but noticed that the spot was very bouncy until it settled. Would this be a faulty voltage regulator valve, the 85A2, I have changed all but the 3 16uF electrolytic capacitors.

Best wishes

Ken

HamishBoxer · 11th Jan 2016, 5:35 pm

Monitor the HT rail from the 85A2 and see if it varies, of course it should be stable.

frankmcvey · 11th Jan 2016, 8:04 pm

Hi, Ken,

What model of HSVT is it?

Two of the 4 x 16uF caps (C8 and C9) are directly connected to the measuring bridge set up by the Mains test card. You need to check these.

FWIW, you're probably more likely to have problems with the carbon resistors in the circuit. The majority of these have gone high on the machines I have worked on. Check Rs 54, 55, 56 and 86; respectively they should be 8.2K, 6.8K, 4.7K and 1.5K 10% tolerance.

V8, the source of the -85V reference seems to be very reliable - I've never seen any problems with this supply, but it's easy to check.

Failing those, I would take a look at the resistors and wax caps in the indicator chain around the differential amp, V2 and V3.

Cheers,

Frank

Radio Tech · 11th Jan 2016, 9:30 pm

Hello Frank

All of the wax caps have been changed, I have yet to change the 16uF caps, but will check the resistors as I go through it. I have ordered a new voltage regulator as a matter of course just to be on the safe side. At least, its starting to look promising in its working. Will let everyone know as I progress. I did find though, that the mains selection on this machine is a bit odd, the top course voltage is 240, down from that is 220, but our mains here is or was 237V but it seems to have dropped quite a bit. The model is E7600/4

Best wishes

Ken

PJL · 12th Jan 2016, 12:55 am

The HT card compares the reference voltage on the 85a2 with the -ve rail. Mains voltages are not stable and you can expect the spot to move up and down slightly.

I have not seen Paul's instructions but you should measure the mains using a DMM and set the voltage selectors to match and then using the REJECT card follow the calibration procedure to set the offset and gain of the differential amplifier.

Once you are sure the differential amplifier is working correctly you can recalibrate the MAINS card using the red pot.

The critical High Stability resistors are usually OK but you should check them. Most of the standard carbon resistors drift badly out of tolerance and many will need replacement.

PS: This tester uses a regulated HT supply, the MAINS settings are essentially to make sure the LT setting is accurate. The HSVT design takes considerable care to ensure the LT is within the manufacturers tolerance and as well as the calibration, it has separate LT taps for the same voltage but different heater currents.

Radio Tech · 13th Jan 2016, 4:58 pm

Yer right Peter, I started checking some of the resistors and found at least 4 of them way out of spec, these were on the same side as the 4 16uF electrolytic capacitors, I am still waiting for these to come by post, I will put these in when I get them. The resistors I am getting from Cricklewood Electronics, could not find anyone else who stocked 2W ones

More on this as I progress

Ken

Radio Tech · 19th Jan 2016, 5:39 pm

Hi folks

Put the new 16uf electrolytic caps in and all of the 2Watt resistors on that side, set all of the pre- set controls according to the manufactures procedure's and success, with a couple of valves and the cards for the test of the valves everything works fine. So all I have to do is search around for a complete set of cards. Frank, I believe has some spare ones he has offered to send me so that will be a start. A big thank you to everyone who helped and gave advice.

Best wishes

Ken

8th Jan 2016, 4:47 pm	#1
Radio Tech Nonode Join Date: Dec 2012 Location: Bocking, near Braintree, Essex, UK. Posts: 2,071	Mullard HSVT I am in the process of restoring a Mullard HSVT and found the two 4uf electrolytic capacitors were shot, I did not have 4uf ones and used 10uf, in the manual it states the 4uf should be within plus 50% minus 10% but as I used 10uf is the reason why the spot is way out when the reject card is inserted? The cards I have loaned from a colleague of mine. Best wishes Ken

8th Jan 2016, 4:56 pm	#2
HamishBoxer Dekatron Join Date: Aug 2007 Location: W.Butterwick, near Doncaster UK. Posts: 8,932	Re: Mullard HSVT Possibly not the caps but 2 X 10uF in series gives you 5uF if you want to be sure of keeping a bit nearer the mark or just use single 4.7uF a standard value these days. __________________ G8JET BVWS Archivist and Member V.M.A.R.S

8th Jan 2016, 5:13 pm	#3
turretslug Dekatron Join Date: Nov 2011 Location: Surrey, UK. Posts: 4,395	Re: Mullard HSVT The "plus 50% minus 10%" is likely to simply be the typical tolerance of electrolytics of this type and era, rather than any particular requirement set by the design. As a very broad rule of thumb, it's wise to not deviate too far from the value of the first capacitor after the rectifier (the reservoir) to avoid stressing the rectifier or having the HT climb excessively high but capacitors after the first smoothing resistor/choke can usually be increased with advantage. Back then, high voltage microfarads were pricey and bulky, and "what can be got away with" was the rule but now they're cheap as chips and far smaller. All this is design dependent, of course- if (rather unusually) there's some sort of expectation of HT ripple for whatever reason (and it would be both rare and marginal design for it to be the case!), then stick to the original values.

8th Jan 2016, 5:23 pm	#4
Radio Tech Nonode Join Date: Dec 2012 Location: Bocking, near Braintree, Essex, UK. Posts: 2,071	Re: Mullard HSVT Thanks Hamish and Turret, I will have another look in my capacitor box and see if I have anything much nearer to the original values, I would not wish to stress the EY51 rectifiers in this. Will report back as soon as I have looked into it much deeper. Regards Ken

8th Jan 2016, 5:45 pm	#5
turretslug Dekatron Join Date: Nov 2011 Location: Surrey, UK. Posts: 4,395	Re: Mullard HSVT As Hamish says, you could have the 2x 10uF in series for the reservoir- and "something else" that comes to hand for smoothing.

9th Jan 2016, 5:56 pm	#6
Radio Tech Nonode Join Date: Dec 2012 Location: Bocking, near Braintree, Essex, UK. Posts: 2,071	Re: Mullard HSVT Hi folks I put 10uf caps in series in the end for C3 and C4, I have renewed the two 470 ohm resistors on the differential amplifier and changed the waxies on that side of the valve tester. With the pre-sets at the front all around mid way, with the mains test card in the spot flies right to the top of the screen. I haven't as yet changed any other capacitors especially those on the other side where there are four 16uf electroltyics but not to sure at this stage what is causing the spot to fly up nearly off the screen. But any help would be appreciated on the cause. Best wishes Ken

11th Jan 2016, 5:02 pm	#7
Radio Tech Nonode Join Date: Dec 2012 Location: Bocking, near Braintree, Essex, UK. Posts: 2,071	Re: Mullard HSVT Hi folks Just a little update on the progress of restoring this valve tester, I followed Paul Stennings information on setting the pre-set mains control, got the spot around where it should be on the "A" line but noticed that the spot was very bouncy until it settled. Would this be a faulty voltage regulator valve, the 85A2, I have changed all but the 3 16uF electrolytic capacitors. Best wishes Ken

11th Jan 2016, 5:35 pm	#8
HamishBoxer Dekatron Join Date: Aug 2007 Location: W.Butterwick, near Doncaster UK. Posts: 8,932	Re: Mullard HSVT Monitor the HT rail from the 85A2 and see if it varies, of course it should be stable. __________________ G8JET BVWS Archivist and Member V.M.A.R.S

11th Jan 2016, 8:04 pm	#9
frankmcvey Rest in Peace Join Date: Jan 2015 Location: Cottesmore, East Midlands, UK. Posts: 858	Re: Mullard HSVT Hi, Ken, What model of HSVT is it? Two of the 4 x 16uF caps (C8 and C9) are directly connected to the measuring bridge set up by the Mains test card. You need to check these. FWIW, you're probably more likely to have problems with the carbon resistors in the circuit. The majority of these have gone high on the machines I have worked on. Check Rs 54, 55, 56 and 86; respectively they should be 8.2K, 6.8K, 4.7K and 1.5K 10% tolerance. V8, the source of the -85V reference seems to be very reliable - I've never seen any problems with this supply, but it's easy to check. Failing those, I would take a look at the resistors and wax caps in the indicator chain around the differential amp, V2 and V3. Cheers, Frank

11th Jan 2016, 9:30 pm	#10
Radio Tech Nonode Join Date: Dec 2012 Location: Bocking, near Braintree, Essex, UK. Posts: 2,071	Re: Mullard HSVT Hello Frank All of the wax caps have been changed, I have yet to change the 16uF caps, but will check the resistors as I go through it. I have ordered a new voltage regulator as a matter of course just to be on the safe side. At least, its starting to look promising in its working. Will let everyone know as I progress. I did find though, that the mains selection on this machine is a bit odd, the top course voltage is 240, down from that is 220, but our mains here is or was 237V but it seems to have dropped quite a bit. The model is E7600/4 Best wishes Ken

12th Jan 2016, 12:55 am	#11
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Mullard HSVT The HT card compares the reference voltage on the 85a2 with the -ve rail. Mains voltages are not stable and you can expect the spot to move up and down slightly. I have not seen Paul's instructions but you should measure the mains using a DMM and set the voltage selectors to match and then using the REJECT card follow the calibration procedure to set the offset and gain of the differential amplifier. Once you are sure the differential amplifier is working correctly you can recalibrate the MAINS card using the red pot. The critical High Stability resistors are usually OK but you should check them. Most of the standard carbon resistors drift badly out of tolerance and many will need replacement. PS: This tester uses a regulated HT supply, the MAINS settings are essentially to make sure the LT setting is accurate. The HSVT design takes considerable care to ensure the LT is within the manufacturers tolerance and as well as the calibration, it has separate LT taps for the same voltage but different heater currents. Last edited by PJL; 12th Jan 2016 at 1:03 am.

13th Jan 2016, 4:58 pm	#12
Radio Tech Nonode Join Date: Dec 2012 Location: Bocking, near Braintree, Essex, UK. Posts: 2,071	Re: Mullard HSVT Yer right Peter, I started checking some of the resistors and found at least 4 of them way out of spec, these were on the same side as the 4 16uF electrolytic capacitors, I am still waiting for these to come by post, I will put these in when I get them. The resistors I am getting from Cricklewood Electronics, could not find anyone else who stocked 2W ones More on this as I progress Ken

19th Jan 2016, 5:39 pm	#13
Radio Tech Nonode Join Date: Dec 2012 Location: Bocking, near Braintree, Essex, UK. Posts: 2,071	Re: Mullard HSVT Hi folks Put the new 16uf electrolytic caps in and all of the 2Watt resistors on that side, set all of the pre- set controls according to the manufactures procedure's and success, with a couple of valves and the cards for the test of the valves everything works fine. So all I have to do is search around for a complete set of cards. Frank, I believe has some spare ones he has offered to send me so that will be a start. A big thank you to everyone who helped and gave advice. Best wishes Ken