Mullard HSVT - UK Vintage Radio Repair and Restoration Discussion Forum

Radio Tech · 3rd Dec 2015, 2:19 pm

A friend of mine has asked me to look at his valve tester, the problem is that with the reject card in, and switch in position 1, the spot does not sit exactly on line C, with the switch in position 6, the spot will not fall to the zero line. Is this a case where the differential amplifier is not what it seem? I did check the 470 Ohm resistors that are on the grids of V2 and V3 these read 500 Ohms measured with an Avo meter, they were checked in situ. Voltages on the Anodes read 25V and 30V, I did not check C11 though that is tied between the anodes. Any idea please.

Regards

Ken

PJL · 3rd Dec 2015, 3:17 pm

You will find that most of the carbon resistors will have drifted well out of specification although the High Stab ones fair better. Then all the caps need to be replaced if not already done.

I am not entirely clear what your problem is but it sounds like you are trying to follow the calibration procedure using the reject card to set the zero position and C position? Are you saying that adjusting the pots/sliders does not move the spot or have they reached the end of their travel?

Radio Tech · 3rd Dec 2015, 5:12 pm

Hello Paul

The slider controls will move the spot and they haven't come to the end of travel, its just that the spot doesn't go to the zero line on the screen when switch is in position 6 and if it is possible to achieve this by using the sliders it puts the spot out even further in switch position 1

Ken

PJL · 3rd Dec 2015, 5:39 pm

Although the sliders are roughly gain and offset, they badly interact so you need to play around a bit to get it right. That is assuming the resistors have not drifted off too badly.

Radio Tech · 8th Dec 2015, 12:07 pm

Many thanks every one, problem solved now.

Best wishes

Ken

ukcol · 8th Dec 2015, 12:28 pm

Hi Ken,

Purely out of interest, what was the cause of the problem?

3rd Dec 2015, 2:19 pm	#1
Radio Tech Nonode Join Date: Dec 2012 Location: Bocking, near Braintree, Essex, UK. Posts: 2,071	Mullard HSVT A friend of mine has asked me to look at his valve tester, the problem is that with the reject card in, and switch in position 1, the spot does not sit exactly on line C, with the switch in position 6, the spot will not fall to the zero line. Is this a case where the differential amplifier is not what it seem? I did check the 470 Ohm resistors that are on the grids of V2 and V3 these read 500 Ohms measured with an Avo meter, they were checked in situ. Voltages on the Anodes read 25V and 30V, I did not check C11 though that is tied between the anodes. Any idea please. Regards Ken

3rd Dec 2015, 3:17 pm	#2
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Mullard HSVT You will find that most of the carbon resistors will have drifted well out of specification although the High Stab ones fair better. Then all the caps need to be replaced if not already done. I am not entirely clear what your problem is but it sounds like you are trying to follow the calibration procedure using the reject card to set the zero position and C position? Are you saying that adjusting the pots/sliders does not move the spot or have they reached the end of their travel?

3rd Dec 2015, 5:12 pm	#3
Radio Tech Nonode Join Date: Dec 2012 Location: Bocking, near Braintree, Essex, UK. Posts: 2,071	Re: Mullard HSVT Hello Paul The slider controls will move the spot and they haven't come to the end of travel, its just that the spot doesn't go to the zero line on the screen when switch is in position 6 and if it is possible to achieve this by using the sliders it puts the spot out even further in switch position 1 Ken

3rd Dec 2015, 5:39 pm	#4
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Mullard HSVT Although the sliders are roughly gain and offset, they badly interact so you need to play around a bit to get it right. That is assuming the resistors have not drifted off too badly.

8th Dec 2015, 12:07 pm	#5
Radio Tech Nonode Join Date: Dec 2012 Location: Bocking, near Braintree, Essex, UK. Posts: 2,071	Re: Mullard HSVT Many thanks every one, problem solved now. Best wishes Ken

8th Dec 2015, 12:28 pm	#6
ukcol Rest in Peace Join Date: May 2003 Location: Harlaxton, Lincolnshire, UK. Posts: 3,944	Re: Mullard HSVT Hi Ken, Purely out of interest, what was the cause of the problem?