HP 404A battery operated vacuum bulb voltmeter update.
Previously I had repaired the panel meter and installed D size
batteries leak-tubes for the vacuum bulb heaters.
I've tested the panel meter, F.S.D. should be 1mA, it reads a little low, not surprizing given the abuse it had suffered.
As I didn't have the required 45V batteries, or 15 PP3s, I had to use the HP 711A to provide the HT of 135V, a tap on the original batteries would have provided 122.5V, I used a 22V zener diode in place of this.
With the original batteries it would have run for around 60hrs, before they would all need replacing.
I bought the HT up slowly to 135V, the peak current draw was about 15mA, this has since dropped a lot lower after being run for a while.
Then I switch everything off, while I went back in the house, after returning the HP 711A was misbehaving and blowing those crappy RS fuses, blew the last three of those
, then remembered I had bought some older Littlefuse fuses after this happened before and yes I went through the 711A troubleshooting guide, no problems were found, I suspect it's a transformer inrush current problem, the first Littlefuse fuse worked fine.
The 404A is working OK, a couple of the Sprague wax paper grid coupling capacitors were showing a little leakage (C10 & C17), after replacing those the leakage was gone, C8 seems OK, probably due to the lower voltage.
The free pdf manual is for a later serial number and has an extra capacitor C23 (0.047uF) in parallel with C10 (0.22uF), these are listed as 600V, but mine had a 0.25uF at 200V, it looked original.
Still to do, replace C1 (0.05uF) as it's the input capacitor, should be rated for 600V according to the parts list and take some more pictures, as I have none of the tag strips & range switch.
David