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.
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=1570018)
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.
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=1570024)
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=1570030)
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
![Evil >:D](https://www.eevblog.com/forum/Smileys/default/evil.gif)
, 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.
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=1570036)
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.
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=1570042)
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=1570048)
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.
![Face Palm :palm:](https://www.eevblog.com/forum/Smileys/default/facepalm.gif)
David