Measuring HT boost voltage - UK Vintage Radio Repair and Restoration Discussion Forum

Hunts smoothing bomb · 6th Jan 2008, 11:15 am

Morning all,

In my Philips service manual fot the K70 it quotes the use of a volt meter set to the 1500vdc range to set the boost volts to 550vdc, I presume this is because the pulses are narrow and of a higher amplitude than 550v.

I have a fluke digital meter that can take upto 1000vdc.

My question is will it be ok to use the meter to measure and if not can I use my x10 scope probe connected to the meter and will it be accurate?

Cheers
Lee

ppppenguin · 6th Jan 2008, 11:34 am

The boost voltage is relatively smooth DC with some superimposed AC at line scan rate. The power level is high so normal meter loading should not be significant. The 1500V range on a normal 20k/volt analogue meter imposes a 30M load. A DVM imposes a 10M load. Both quite insignificant loads on the boost rail.

A potential problem with a DVM is that it can give inaccurate DC readings in the presence of much AC and may even be affected by the high field strengths around a line timebase. A good quality meter such as a Fluke is likely to be better than a £5 Chinese unit.

A x10 scope probe is useless here. Not only will it not do much good but at DC it's effectively a 9M resistor whhich will make the DVM read low by a factor of 10/19.

However you measure the boost voltage, beware that it's a high voltage with a lot of power behind it. If your probes are not well insulated and it reaches you it will hurt. Like mains but worse.

Hunts smoothing bomb · 6th Jan 2008, 11:53 am

Thanks for the reply, No worries about getting a jolt, I am dead (sorry for the pun!) careful around the line stage as it's the only part of the set that scares the cr

p out of me!!

Lee

MichaelR · 6th Jan 2008, 2:02 pm

Quote:

Originally Posted by ppppenguin

A potential problem with a DVM is that it can give inaccurate DC readings in the presence of much AC
.

Is it not so much inaccurate but that the meter averages the peak voltage fluctuations. The reason they ask you to use a 1500 volt range is that the peak voltages are reaching this level even though the meter will not show it due to the averaging effect.

Sorry to be asking such a question but I do not know anything at all about this TV stuff

Mike

Station X · 6th Jan 2008, 7:24 pm

I certainly wouldn't recommend using the Fluke DMM. I was measuring high voltages in a 'scope and managed to blow my Fluke's fusible resistor. The correct replacement fusible resistors are unobtainable except at exhorbitant prices. Fortunately a substitute resistor supplied by forum member HG MICKE did the job.

Hunts smoothing bomb · 6th Jan 2008, 7:56 pm

Hi Graham,

I didn't measure it in the end, I began to and had one probe in my hand and brought the other towards the high potential test point on the set and a small high freguency spark jumped across about a millimeter and also interfered with the workshop radio!!

Cheers Lee

6th Jan 2008, 11:15 am	#1
Hunts smoothing bomb Octode Join Date: Oct 2005 Location: Wimborne, Dorset, UK. Posts: 1,407	Measuring HT boost voltage Morning all, In my Philips service manual fot the K70 it quotes the use of a volt meter set to the 1500vdc range to set the boost volts to 550vdc, I presume this is because the pulses are narrow and of a higher amplitude than 550v. I have a fluke digital meter that can take upto 1000vdc. My question is will it be ok to use the meter to measure and if not can I use my x10 scope probe connected to the meter and will it be accurate? Cheers Lee __________________ Lee

6th Jan 2008, 11:34 am	#2
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: Measuring HT boost voltage The boost voltage is relatively smooth DC with some superimposed AC at line scan rate. The power level is high so normal meter loading should not be significant. The 1500V range on a normal 20k/volt analogue meter imposes a 30M load. A DVM imposes a 10M load. Both quite insignificant loads on the boost rail. A potential problem with a DVM is that it can give inaccurate DC readings in the presence of much AC and may even be affected by the high field strengths around a line timebase. A good quality meter such as a Fluke is likely to be better than a £5 Chinese unit. A x10 scope probe is useless here. Not only will it not do much good but at DC it's effectively a 9M resistor whhich will make the DVM read low by a factor of 10/19. However you measure the boost voltage, beware that it's a high voltage with a lot of power behind it. If your probes are not well insulated and it reaches you it will hurt. Like mains but worse.

6th Jan 2008, 11:53 am	#3
Hunts smoothing bomb Octode Join Date: Oct 2005 Location: Wimborne, Dorset, UK. Posts: 1,407	Re: Measuring HT boost voltage Thanks for the reply, No worries about getting a jolt, I am dead (sorry for the pun!) careful around the line stage as it's the only part of the set that scares the crp out of me!! Lee __________________ Lee

6th Jan 2008, 7:24 pm	#5
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,289	Re: Measuring HT boost voltage I certainly wouldn't recommend using the Fluke DMM. I was measuring high voltages in a 'scope and managed to blow my Fluke's fusible resistor. The correct replacement fusible resistors are unobtainable except at exhorbitant prices. Fortunately a substitute resistor supplied by forum member HG MICKE did the job. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

6th Jan 2008, 7:56 pm	#6
Hunts smoothing bomb Octode Join Date: Oct 2005 Location: Wimborne, Dorset, UK. Posts: 1,407	Re: Measuring HT boost voltage Hi Graham, I didn't measure it in the end, I began to and had one probe in my hand and brought the other towards the high potential test point on the set and a small high freguency spark jumped across about a millimeter and also interfered with the workshop radio!! Cheers Lee __________________ Lee