Golborne Vintage Radio

Thanks for the kind words guys!

Reason I was asking about those two transistors is that verifies that C924 and C925 are OK. These are 1nF 5kV disc ceramic and are the only thing that stands between 4kV HT and the transistors. If they were leaking the transistors would be toast.

Note that these are mislabelled on the board layout as C942 and C952.

Which makes me think of one word of warning - everything to the right of C924 and C925 is at several kV. So don't go probing around there with the scope switched on with a DVM or scope probe.

How are things going getting hold of a high voltage probe?

Played golf yesterday, spoke to one of the guys whom I thought may have one no joy just used it at work now retired.
I thought about making a voltage divider 100mohm to 100Kohm but can't find wattage value for 100Mohm resistor.

Multiple resistors in series. For my home made 25kV probe I used enough 33M resistors tomake 500M. They were Mullard now Vishay VR33 series rated at 2500V.

Keep an eye on eBay, etc. Every now and again, batches of them appear as ex-MOD lots. IIRC, my Fluke 80K-40 was about £30 delivered. It came complete with carry case and instructions, and I don't think it had ever been used. There's one on now for £90, which is still well under half the new price.

I've been down the DIY route in the past. VR33 metal-oxide resistors are the minimum you'll need - a lot of attention to detail (and cleanliness) in the construction will be needed. Although the voltage is high, the energy is low, so it's not as risky as some activities, but even so, I'm a coward so like the reassurance of the proper product.

Forget about power ratings in this application - it is the voltage rating which is important.

Although voltage and current govern the ratings of resistors in most normal applications, there will be a maximum voltage for any resistor type regardless of current.

The maximum voltage for a 0.125W resistor might be as low as 150V and will increase slightly for every doubling of power rating. The actual figure will vary depending on resistor type and manufacturer so, if you have some suitable high value resistors you would like to use, it would be wise to seek out the manufacturer's data.

Alternatively, use the chart below but derate the figures shown by 50V to be on the safe side.

[attachment=15772]

For example, if you have a 100 1M 0.5W resistors kicking around you could build a 100M chain with a maximum voltage rating
of around 300kV - but I wouldn't really recommend it!

(19-02-2017, 03:01 PM)Terry Wrote: [ -> ]...For example, if you have a 100 1M 0.5W resistors kicking around you could build a 100M chain with a maximum voltage rating
of around 300kV - but I wouldn't really recommend it!

Only if it's embedded in dry HV oil or SF6 and the layout is very carefully maintained !

I did say that I wouldn't recommend it!

Thanks gentlemen for all the advice I have been looking at this you tube video Personally I would have more insulation. RS components sell 100megohm high voltage.
I would probably be like Mark but one from eBay and use it once.


https://youtu.be/qWd9JISGYYw

I made my 25kV probe way back in the mid 1970s when I first acquired an ex rental colour TV and needed to be able to check the EHT. I had easy access to VR37 resistors (I won't say whose stores they came from but as in many cases the company concerned weren't too worried about engineers "borrowing" a few bits for personal use).

At 25kV and low energy there were no great problems in construction. The soldering was nice and neat, the whole string was in a PVC tube which in turn was in a paxolin tube. I connected a bleed resistor from the LV end of the chain to an earth clip and I think I had a guard ring part way up the probe which was about 10" long.

I wouldn't have liked to connect it to a high energy source nor to anything much over 25kV. There were 15 resistors in series, each rated at 2500V so in theory it was good to 37.5kV.

I use the probe to this day for TV EHT meaurements, including mains dervied EHT. I rebuilt it a few years ago to use a screw-in probe tip to replace the original tip which had broken in an awkward place.