#53 HP 410A vtvm repair part 1  - Page 1
Forum section: electronics / testgear

author: Radio Tech
author_rank: Frequent Contributor
timestamp: May 29, 2016, 01:52:45 am
content: In this video we go through this old HP 410a VTVM and replace all the paper capacitors.We also have to fabricate a new DC probe since the original is damaged.Manual link:http://www.hparchive.com/Manuals/HP-410A-Manual.pdfCapacitors and resistors:http://www.justradios.com/Link to Diode modification kit:http://gokarters.com/smf/index.php?topic=2579.msg18393#msg18393

author: Radio Tech
author_rank: Frequent Contributor
timestamp: May 29, 2016, 02:15:33 pm
content: So I got lucky today and was rambling through some old parts drawers. I found a pack of 20 meg resistors. I went through them and found one that read 22.1 ohms. The DC probe is now complete. I can now troubleshoot the ohms setting and complete this unit.

author: xrunner
author_rank: Super Contributor
timestamp: May 29, 2016, 02:30:58 pm
content: Very interesting I'll be following this.

author: Radio Tech
author_rank: Frequent Contributor
timestamp: May 29, 2016, 02:50:19 pm
content: Quote from: xrunner on May 29, 2016, 02:30:58 pmThanks. Since I have the DC probe fixed up I am ready to dive back in and get the ohms setting working. I really like this meter and want to fabricate a RF adaptor for it. This turns the meter into an RF power meter and should be able to read very low wattage. I saw a video from Mike at mikesradiorepair on YouTube demonstrate this. Lets see if I can find the link....

author: xrunner
author_rank: Super Contributor
timestamp: May 30, 2016, 12:32:40 pm
content: So is the tube in the probe down-converting the RF or just amplifying it for the meter?

author: Radio Tech
author_rank: Frequent Contributor
timestamp: May 30, 2016, 03:05:22 pm
content: Quote from: xrunner on May 30, 2016, 12:32:40 pmThe tube is a 2-01C. This is a low capacitance high frequency diode. Sort of like a geranium.This acts as a shunt diode circuit.

author: TimFox
author_rank: Super Contributor
timestamp: May 30, 2016, 04:21:18 pm
content: The 2-01C is literally a "hot" diode, with a substantial current at zero voltage due to the high cathode temperature. Some 410A's have a second vacuum diode inside the box to compensate for the DC offset, but others used a solid-state diode.The polystyrene end cap on my 410A is cracked, and I haven't been able to glue it successfully. Do you have any suggestions?I have a coaxial through-line adapter that the probe fits with the cap unscrewed.

author: Radio Tech
author_rank: Frequent Contributor
timestamp: May 31, 2016, 05:19:48 pm
content: Quote from: TimFox on May 30, 2016, 04:21:18 pmMine has the tube inside also. But I have the diode modification kit installed. This eliminates having to use matched tubes and the kit uses a balance tube. I worked on part two of the video yesterday and hit a brick wall. Ohm scale now works but lost AC and DC scales. Meter creeps and pegs. After checking filament voltage for V1 and V2 I found the voltage was below 3 volts and the transformer is getting very hot. So now have to troubleshoot this issue.Polystyrene is almost impossible to glue back together. Perhaps there is a way to thermal weld this. Will have to look into that.

author: xrunner
author_rank: Super Contributor
timestamp: July 02, 2016, 12:24:20 am
content: Any news on the project?

author: Radio Tech
author_rank: Frequent Contributor
timestamp: July 02, 2016, 12:45:53 am
content: Waiting on an order. Had a tube that was bad. Hope to have it next week.