Recently I bought Thermo Electra ratemeter (Thanks Joseph!) and have some thoughts to share.
0. Overall device looking very good, a nice example of brutal engineering from 1990s. Lots of free space on PCBs, very reliable 80c32 microcontroller with external ROM for firmware, this device will survie after nuclear war.
1. Original connector can be easily replaced with BNC, there is a lot of space there. Original bracket with original connector can be easily removed unscrewing 4 screws and separated for installing BNC connector.
2. It's very strange but Electra controls current drain of connected detector and shuts off HV converter if current seems to be higher than expected. Lowest divider chain resistance for Electra is 66 MOhms and can be selected from settings. In amateur practice we need to work with lower divider chain resistances often, so a little modification should be made.
At least: remove jumper LK3
Put a jumper wire between pins 9 and 11 of IC7
This modification will remove signal proportional to HV converter current from the compoarator and microcontroller will not get any overload signal. However, HV converter power will be insufficient to deliver 1400 V to 10 MOhms load, only around 950 V can be delivered and Electra will indicate Error 6 if I try to set more than ~1000V while working with 10 MOhms chain.
Step 2: Increasing power.
In HV converter locate 3 resistors: R6 = 390 k, R2 = 3.3 M and R10 = 5.6 Ohms.
This is a flyback converter working with a very low frequency, around 6 kHz.
Replace R6 with 510k, R2 with 2.2 M and short R10. It will increase frequency to ~8 kHz and pulse length, maximum power of the HV converter will be increased in ~2 times. All done, now it can deliver up to 1400V into 10MOhms load and you are beautyful!
That's all with Electra for today :)
