HP 410C repair/upgrade PCB design - Page 1
Forum section: electronics / repair

author: 1audio
author_rank: Frequent Contributor
timestamp: February 09, 2025, 11:44:45 pm
content: I picked up a distressed HP410C at a local used everything store (Urban ore in Berkeley). it was from UC berkely and had a hard live but it still mostly intact. The meter glass was smashed however once cleaned out the meter movement is OK. Getting glass or another HP meter to scavenge parts from is not too hard.The electronics are not working however. This is an early one (Oval unobtainum power cord version) and I suspect tomething is amis in the chopper. In any case after looking into repairing it I realized that its just a chopper amplifier. Looking further it seems I could replace the whole circuit and improve performance with an auto zero opamp. Suitable power is available already. There are chips that can do it all with no external components and 1 uV offset.All the active stuff is on the plug in board so making a new board seems like the most reasonable approach.I'm attaching a schematic of what I plan. The opamp is a generic placeholder not the intended part. The minimum order is 5 PCB. I'll be happy to share. Any input would be helpful because I'm not sure I thought of everything.

author: 1audio
author_rank: Frequent Contributor
timestamp: April 27, 2025, 01:09:19 am
content: Update-First pass on PCB is now working. I needed to add some parts that were not shown on schematics as part of the PCB but once figured out and adding a wire to support the negative supply (no tubes to get the HV supply working right) and grafting a new electrolytic across a dead electrolytic in the supply it now all functions.However I can't get it to calibrate correctly- it seems the precision resistors have all drifted a lot, more than the 1/2% and 1% they are marked. The design doesn't provide for correcting for that. I have not seen resistor drift like this in older instruments before. A small shift maybe but 10%? I checked with two instruments to be sure its not my references.In any case I got it mostly in spec. (+/-3%?) I'll post the specifics necessary to duplicate my efforts.

author: RichardM
author_rank: Frequent Contributor
timestamp: April 27, 2025, 09:56:51 am
content: Very interested in how your circuit and pcb goes. I have two 410C and both seem to drift as they warm up. I have never been able to get them in specification across all ranges myself.

author: 1audio
author_rank: Frequent Contributor
timestamp: April 27, 2025, 04:20:21 pm
content: So far no drift visible from cold start to 1 hour. I'm using an OPA604 which isn't even a precision opamp, just decent FET input opamp. I'll revise the PCB laypout with the necessary changes/additions and details on the other changes to switch to all solid state operation. My next goal is a Shottkey based probe for AC. But I have real work I'll need to tend to first.

author: 1audio
author_rank: Frequent Contributor
timestamp: April 27, 2025, 04:26:38 pm
content: One other problem I noticed- some internal breakdown when the voltage exceeds 600V. I think its the shielded cable between the probe and the input since its not an issue on the 500V range. replacing the probes is probably necessary on this one but I'm not sure how best to do it yet.

author: JoanBS
author_rank: Contributor
timestamp: April 27, 2025, 05:02:31 pm
content: I have an HP-410C, with the AC probe, which I bought new a few years ago from an old stock.I added a timer to prevent the meter needle from going off scale upon startup.If you need the service manual or anything else, I'd be happy to provide it.I'll be on the lookout for your HP-410C update.

author: 1audio
author_rank: Frequent Contributor
timestamp: April 28, 2025, 04:27:27 am
content: Thanks, however I have 4 versions of the manual I have downloaded. All with different clues to what is important.I found the HV breakdown location- where the shield is pulled back inside the box. The sharp bend slowly exposed the inner conductor. See pic below. I'll fit some sleeveing to address this. It now goes to 1KV without issue (except degraded accuracy).

author: JoanBS
author_rank: Contributor
timestamp: April 29, 2025, 11:02:16 am
content: I've never tested it with such high voltages.I installed gold-plated RCA connectors for the test leads.

author: 1audio
author_rank: Frequent Contributor
timestamp: May 01, 2025, 02:26:56 am
content: You need 10 meg in the DC lead at the tip for it to work right. And don't expect an RCA to withstand 1 KV.Atached is a photo of my first attempt. Its working but I'm making a revised version with the extra parts in the layout. The new circuit and layout are below. I'll order through JLBPCB. It will be interesting to see how the tariffs are handled.

author: 1audio
author_rank: Frequent Contributor
timestamp: May 01, 2025, 02:36:26 am
content: The boards from JLBPCB are $2.00. The shipping went from $17 to $22 to cover the tariff. If I order 5 or ? who would want one?

author: coppercone2
author_rank: Super Contributor
timestamp: May 01, 2025, 02:47:47 am
content: Be careful with RCA. I know some people like it, but what you need to check is the wiring. You might have nice looking RCA cables, but if you take them apart, you may find its not really built like a coaxial cable. Its molded with a whole bunch of stranded wire bundled around the center wire. And its often skimpy. That's why it might bend nice for a shielded cable, because its not woven and severely lacking in strand coverage. I found this for a number of old RCA cables I was impressed with (from the outside). And I suspect it might not have the best insulation resistance. The only thing it does is bend nicely for a coaxial cable.I thought I was going to make a adapter for something, but when I cut it open I threw it out lol. I threw out a whole bunch of different ones when I started looking through my bag., gathered over the years. Now I don't have any, because all the wiring was crap. I got a adapter instead.If they actually wove it, or even twisted the shield, it would not be nearly as flexible. I think some people think that the flexibility is a sign of quality.

author: RichardM
author_rank: Frequent Contributor
timestamp: May 01, 2025, 01:03:21 pm
content: Is this a replacement for the entire chopper and vacuum tube assembly A3 board ?

author: 1audio
author_rank: Frequent Contributor
timestamp: May 01, 2025, 03:11:48 pm
content: Yes a complete replacement. When I looked at resusitating it I realised its just an opamp. And between unobtainium chopper components, tubes etc. I would spend less time making a replacement PCB. It seems the later solid state version is almost the same, except Jfet input opamps were not available so they used discrete Jfets.

author: RichardM
author_rank: Frequent Contributor
timestamp: May 01, 2025, 08:56:40 pm
content: Wow, that is amazing. I would love to try a board. Postage to Australia would be needed though

author: 1audio
author_rank: Frequent Contributor
timestamp: May 02, 2025, 12:00:49 am
content: At least no tariff. It would fit in a small envelope. I'll go ahead and get an order going. Any other takers?

author: 1audio
author_rank: Frequent Contributor
timestamp: May 03, 2025, 11:32:15 pm
content: More progress-I discovered that a pot is needed for the AC probe to function. Also I don't have a probe and I'm working out how to make it work with a solid state rectifier. So far I have been able to get it working pretty well with a 1n4007 (of all things) up to 1 MHz at 100V. It also seems to have OK response to 20 MHz with the 1n4007. There are still significant issues with AC zero and tracking I'm working on but they seem solvable.

author: stevebyan
author_rank: Contributor
timestamp: June 24, 2025, 07:20:46 pm
content: Quote from: 1audio on May 01, 2025, 02:36:26 amI'd like one.

author: Runco990
author_rank: Frequent Contributor
timestamp: June 27, 2025, 03:30:08 pm
content: I'll take one if you get this figured out.I'm not on here a lot, nor in any rush. So if this moves forward, you might want to PM me when ready.Thanks!

author: 1audio
author_rank: Frequent Contributor
timestamp: June 29, 2025, 05:47:43 pm
content: Im making slow progress. Too many others demands in life. Gen 2 of the layout seems mostly OK. My AC probe functions but needs more work. The free layout software has issues Im working through. However the concept using a cmos precision opamp works pretty well. In planning two AC probes. One with a 1KV diode to handle 500V which works to 20MHz in my tests and another with hot carrier diodes for extended response. Hopefully I will have a resolution soon.

author: 1audio
author_rank: Frequent Contributor
timestamp: July 13, 2025, 11:11:40 pm
content: The replacement board seems to work fine. My efforts to make a replacment AC probe have been mixed. 5V up it works great- 100V RMS to 1 MHz, 10V to 30 MHz just fine. The 1.5V range and the .5V range not so good. I'm not sure how to tweak (hack) the low voltage compensation bias to match a solid state diode. Open to any ideas. I have tried several biasing tricks and switched between a standard silicon rectifier and a hot carrier diode but still no progress. The basic circuit is below. The second diode is now a jumper. When I zero the probe the gain and tracking are way off. Maybe I'll give up on the .5V and 1.5V ranges.

author: 1audio
author_rank: Frequent Contributor
timestamp: August 07, 2025, 01:04:12 am
content: Another update-I have now managed to make a probe for the .5 and 1.5V AC range. Its accurate above 1 MHz. I need a bigger cap (s) for lower frequencies. I'm using Shottky hot carrier diodes in a voltage multiplier as a detector. I have 120 MegOhms in series to get the range correct. This should have a pretty extended bandwidth. The 1.5V version works fine to 20 MHz and up to 120V at 1 MHz. I'll relayout the boards to work with these parts properly (and fix my other mistakes)I will post updated schematics for this and a revised layout (for JLCPCB) later this week. There are still some aspects to packaging the AC probes to resolve but I think it will fit inside a generic probe. Not as elegant as HP but useful.

author: 1audio
author_rank: Frequent Contributor
timestamp: August 18, 2025, 03:49:09 am
content: I think I have all the detals worked out. I was baffled by the AC zero control until I found a reference that a vacuum tube diode would have a current with zero volts- its from the electrons boiling off the cathode. I think I can get useful and extended AC with the probes I have worked out. I have revised the PCB here:https://oshwlab.com/1audio/hp-410c-ampand have ordered some. They should arrive in about 2 weeks. I'll post more details on assembly when they get here. EasyEDA (and most other EDA software) is not tolerant of sloppy component creation, something I learned the hard way.Hopefully some will get a benefit from this. Its been an interesting effort even though I don't really have use for the instrument.

author: quadtech
author_rank: Regular Contributor
timestamp: August 18, 2025, 05:43:32 am
content: Quote from: 1audio on August 18, 2025, 03:49:09 amIn your schematic, in the 1500V calibrate, R5 value is incorrect.

author: 1audio
author_rank: Frequent Contributor
timestamp: August 18, 2025, 03:30:42 pm
content: Almost everything in the schematic is a placeholder. Once I have an assembled and calibrated board I'll share an updated version. There are issues getting suitable pots so I created a footprint that will support 6 different styles.

author: TimFox
author_rank: Super Contributor
timestamp: August 18, 2025, 04:15:14 pm
content: Quote from: 1audio on August 18, 2025, 03:49:09 amThe vacuum diodes used in the 410 series are "hot", to achieve good performance at high frequencies, and therefore have higher current at 0 V bias than the more common diodes (6AL5, etc.) used in other VTVMs.My elderly 410A uses two Eimac 2-01C diodes in exotic packaging, where one is used to compensate for the zero-bias current of the one in the probe.The datasheet shows approximately 1 V into 100 M\$\Omega\$ at zero signal, which is the reverse voltage required to essentially stop the thermal-electron current.Later 410s used other tubes, since the 2-01C was discontinued.https://frank.pocnet.net/sheets/140/2/2-01C.pdf

author: 1audio
author_rank: Frequent Contributor
timestamp: August 27, 2025, 09:56:39 pm
content: Boards arived today. Unfortunately somhow I missed on the solder mask and the edge connector is all masked. I'll figure out the fix and I can test the AC probes anyway. Total risked $10. No addl. tarriffs yet. Not sure about the next batch.