Farnell 352C 1GHz Spectrum analyser

[Wanderingfree]

Hi Jeffrey. I have -12v one side of R24 and +12v on the other so 24v. I haven't checked the volt drop across, I will do that first thing in the morning. When I purchased the unit the guy said he had to do the mod to get it to work (and he didn't have a circuit) so I was just trying to find out where the issue is. being an old piece of kit I thought it might be good to drive the tube correctly so it lasts. once they have accepted me on the Radiomuseum forum I will get access to more information I'm wondering if there was a technical update sheet. But it must have worked at some point without the added resistor.

[Wanderingfree]

Correction to above. I have just checked the volt drop across R24- zilch it's -12 both sides of the resistor so -12 on G1 sorry about that
I've obviously been looking at meters to much today. When checking the voltage either side of R24 the brightness increases!

[ppppenguin]

Never forget that the meter is part of the circuit and can affect the voltages that you measure.

[Wanderingfree]

I realise there are shunts in the meter which is what is changing the art display.

[ppppenguin]

Shunts are used for measuring current, not voltage. Here is a reasonable introduction to the subject: http://studyelectronics.blogspot.com/201...liers.html

On voltage ranges most DMMs present a 10M resistance to the outside world. Some cheap ones are only 1M. The resistors that matter around the CRT have high values, such as 1M and 2M2. A 10M meter will signifcantly alter voltages. A 1M meter will be utterly misleading. If you were using an analogue meter such as an Avo 8 you'd be using the 1000V range to measure 300+ volts. It would present 1000 x 20k = 20M at its terminals. A case where an analogue meter will load the circuit by less than a digital one.

My understanding of the 22M resistor that was added is that it's compensating for a design error. The original designer didn't imagine that current might flow into G2 of the CRT. Although the current flow may be very low, it could conceivably be in either direction. Hence a resistor is needed. I also have no idea why D10 was included. Presumably the designer had his or her reasons but I can't see them.

In any case, fitting the 22M resistor seems to make the CRT operate in a satisfactory way. So if there are no other problems with the display, just accept it. Adding the 22M certainly can't do any harm.

[Wanderingfree]

Thank you Jeffrey. A long time since I looked at that formula (back in the 70's I think) I have an AVO 8 Mk 6 so I will use that. Interestingly the previous owner did the resistor mod. It's difficult to imagine that it would have been released with the tube behaving like that unless it's down to the age of the tube but I will as you say leave it as it works.