Golborne Vintage Radio

Hi.  Does anyone have any information on Farnell 352C 1GHz Spectrum analyser please

I assume you've googled it and found pages like this:
http://www.technologymuseum.eu/phpwebgal...44-spectre
https://www.rescienceinc.com/product-pag...m-analyzer
https://www.radiomuseum.org/r/farnell_in..._352c.html

The Radiomuseum site has what looks like a circuit diagram.

Thank you I found some info on Radio Museum. It's a long time since I have worked on CRT equipment. I have an interesting problem
with this spectrum analyser CRT. When I got it I found it had a mod (unofficial) it had a 220 meg resistor strapped between the second grid and earth. I removed said resistor that was just pushed into place and the brightness creeps up to excess with no control. I put a meter (Avo or Digital multi meter) and it becomes correct on the screen. the voltage looks right 330v as soon as I remove meter it creeps up slowly until it's far too bright. The question is is this covering another fault or is it decoupling capacitors?

Do you really mean 220 megohms? That would be a rather special part. Would help if you can post the relevant fragment of the circuit here.

In any case, it sounds like the 2nd grid doesn't have a DC path without that resistor or, alternatively your test meter. It certainly needs a DC path. Look at the circuit diagram and see what should be connected. Always remembering that it's not unknown for there to be errors in published information.

it was definitely a bog standard resistor 220 Meg 1/2w film resistor. I will try and PM you

I don't think there's any such thing as a standard 220M film resistor. Values like that are available but they are specialist parts. What are the markings on it? If colour coded it would be red/red/violet. If standard numerical coding it would be 227. If it says 220M then that's most unlikely to mean 220 megohms. As always, photos would help.

As for PMs, this is an open discussion. I don't see any reason for part of it to become a private matter between you and me.

The OP can't post even a small section of the circuit here because it's heavily watermarked "Radiomuseum". I've sketched the relevant part.  The design can source current into G2 but can't sink any. I suppose a small current could be coming out of G2, so a resistor to 0V would be needed to stop the voltage drifting too high. It says 330V on the circuit.

I wonder why there's an extra diode in the feed to G2. I can't see what good it's doing. If it were not there, the focus pot would provide the necessary sink.

I still don't believe it's a 220M resistor.

My apologies it was a 22meg resistor. I have checked all the voltages and they are about right including 330v on G2. the voltage on go is a bit odd (+12v) but when the 22meg resistor is in or a meter applied to the second grid the brightness control functions ok. Jeffrey, R24 in the brightness circuit has 24v drop across it -12v on the junction with C21 +12v on the other side going to grid 1. I was just wondering if G1 was having an effect on G2

Interestingly the meter put either side of R29 (10k ohms) brings everything back where it should be but doesn't effect it on the anode of D10.
in circuit C23 and 24 check ok 10nF

R24 does not have 24V across it. It's a 680R resistor with one end conencted to G1 of the CRT. If it had 24V across it there would be 24V/680R = 35mA of grid current. That's clearly impossible. If you measure directly across R24 I doubt you will be able to measure any voltage at all.

Can you work out why you think there's 24V acoss it?

The 22M resistor seems to be essential, to absorb a small amount of current on G2. So leave it in place.

If the brightness control is working OK then what sort of fault are you trying to find?