#53 HP 410A vtvm repair part 1

[Radio Tech]

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.pdf

Capacitors and resistors:
http://www.justradios.com/


Link to Diode modification kit:
http://gokarters.com/smf/index.php?topic=2579.msg18393#msg18393

[Radio Tech]

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.

[xrunner]

Very interesting I'll be following this. 

[Radio Tech]

Very interesting I'll be following this. 

Thanks. 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....

[xrunner]

So is the tube in the probe down-converting the RF or just amplifying it for the meter?

[Radio Tech]

So is the tube in the probe down-converting the RF or just amplifying it for the meter?

The 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.

[TimFox]

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.

[Radio Tech]

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.

Mine 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.

[xrunner]

Any news on the project?

[Radio Tech]

Waiting on an order. Had a tube that was bad. Hope to have it next week.