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 Post subject: Hewlett Packard VTVM Diagnosis
PostPosted: May Mon 03, 2021 3:22 pm 
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instead of that scope, I am now working on an HP model 410B VTVM. I should have tested it before taking it apart but I recall that on the dc measurements and probably ca the reading was off, even after adjustments. I also recall that the higher the voltage the more it was off, to me this sounds like a capacitor leaking. I have checked most of the resistors and found a trend that they have drifted but not quite out of tolerance, if they have it is not by much. I downloaded the manual but it does not have a very clear schematic. there are two electrolytic caps, two micamold capacitors and one cylindrical one with bands like a resistor.
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 Post subject: Re: Hewlett Packard VTVM Diagnosis
PostPosted: May Mon 03, 2021 3:35 pm 
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Also, I need to figure out how to read those aforementioned capacitors, the micamolds, and the one with bands like a resistor.
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 Post subject: Re: Hewlett Packard VTVM Diagnosis
PostPosted: May Mon 03, 2021 3:37 pm 
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the colors for the bottom one (from left to right) are blue, brown, white, orange, gray, brown, the second brown band is fatter than the rest


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 Post subject: Re: Hewlett Packard VTVM Diagnosis
PostPosted: May Mon 03, 2021 4:11 pm 
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acbaker123 wrote:
I downloaded the manual but it does not have a very clear schematic.

Where did you get the manual?

The one on BAMA:
http://bama.edebris.com/download/hp/410 ... 20VTVM.pdf
and the one on HPArchive:
http://hparchive.com/Manuals/HP-410B-Ma ... ix-024.pdf
both have clear schematics. (But the one on BAMA is missing the manual backdating appendix.)

As for the capacitor color codes, for the domino caps look here:
https://www.tpub.com/neets/book2/3g.htm
and here:
https://antiqueradios.com/forums/viewto ... 0#p3087430

For the Black Beauty bumblebee caps:
https://antiqueradios.com/forums/viewto ... ?p=3225484

What is the serial number of your unit? The manuals have backdating information to correct the schematic and parts list to match your serial number.

If your 410B has two electrolytic capacitors in it, it is likely a serial number 21433 and below unit, and change numbers 1, 2, 3, 4, and 5 in the HP Archive manual need to be applied.

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 Post subject: Re: Hewlett Packard VTVM Diagnosis
PostPosted: May Mon 03, 2021 4:15 pm 
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I have the HP archive manual


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 Post subject: Re: Hewlett Packard VTVM Diagnosis
PostPosted: May Mon 03, 2021 4:21 pm 
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the serial number is 13453. the chart in the manual is a bit confusing but I think you are right.


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 Post subject: Re: Hewlett Packard VTVM Diagnosis
PostPosted: May Mon 03, 2021 4:27 pm 
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acbaker123 wrote:
the colors for the bottom one (from left to right) are blue, brown, white, orange, gray, brown, the second brown band is fatter than the rest

That's brown-black-orange, 10,000 pF or 0.01 µF, white = 10% tolerance, brown-blue = 1400 volts

The domino cap is white-yellow-violet-red-silver-no color.
white = mica cap with EIA (aka RMA aka RETMA) color code
yellow-violet-red = 4700 pF or 0.0047 µF
silver = 10% tolerance

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 Post subject: Re: Hewlett Packard VTVM Diagnosis
PostPosted: May Mon 03, 2021 4:47 pm 
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I have heard before not to replace mica caps if you do not have to. otherwise, I would just replace all of the caps. also, do you think my hunch is right about a leaky cap, or can the issue be resistors that have drifted over time? the only resistors I worry about are the precision resistors.


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 Post subject: Re: Hewlett Packard VTVM Diagnosis
PostPosted: May Mon 03, 2021 4:52 pm 
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If there's an electrolytic across the meter, that's very likely to be leaky.

In my pre-serial 21433, the power supply filter capacitor was a 1 µF paper in oil capacitor in a metal can and it's still good. But yours might have been built with an electrolytic or had it replaced by an electrolytic.

You need to also worry about drifted resistors in the vacuum tube circuitry. If the halves of the bridge are too unbalanced, the response gets too non-linear. So the cathode and plate resistors must be within spec.

In my unit, many of the precision resistors had drifted out of spec. It's hard to measure the high-value precision resistors with enough accuracy to assess whether they are out of spec. What are you using to measure your resistors?

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 Post subject: Re: Hewlett Packard VTVM Diagnosis
PostPosted: May Mon 03, 2021 5:30 pm 
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i am using a simple digital multimeter to measure resistors.i took them out of circuit to test them. the cap across the meter was a micamold cap.


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 Post subject: Re: Hewlett Packard VTVM Diagnosis
PostPosted: May Mon 03, 2021 5:36 pm 
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acbaker123 wrote:
i am using a simple digital multimeter to measure resistors.i took them out of circuit to test them. the cap across the meter was a micamold cap.

What is the spec’d accuracy of your DMMs resistance function? What are its limits for the resistance ranges - does it go up to 47 Megohms?

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 Post subject: Re: Hewlett Packard VTVM Diagnosis
PostPosted: May Mon 03, 2021 5:47 pm 
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i think i will test and/or replace the caps, then see whether it can be calibrated.


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 Post subject: Re: Hewlett Packard VTVM Diagnosis
PostPosted: May Mon 03, 2021 6:29 pm 
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That capacitor with the color bands should be replaced without a second thought. Yes, I agree, mica caps generally do not require replacement. Neither do discs as a general rule.

As for the resistors, since you said they are "close" I'd see if it will calibrate, or even function, before you start replacing those. As Steve noted, you need an ohmmeter with enough precision to well exceed the precision of the 410 in order to get resistors you can trust for some of the range selectors... make it work with what's there, and deal with those as phase 2 if still needed. Note that no matter what you do, you're not going to make the meter exceed its original spec. Since it's what, 60 years old, be happy if it meets it's published spec :)

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 Post subject: Re: Hewlett Packard VTVM Diagnosis
PostPosted: May Mon 03, 2021 6:36 pm 
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acbaker123 wrote:
I also recall that the higher the voltage the more it was off, to me this sounds like a capacitor leaking.
...
also, do you think my hunch is right about a leaky cap, or can the issue be resistors that have drifted over time?

If you mean that the error was higher on the higher voltage scales, then no, there isn't any capacitor in the circuit that could cause that.

If you mean that the error was higher at the upper end of each scale, then maybe, although it's more likely that the non-precision resistors have drifted enough to throw the differential vacuum tube circuitry out of whack.

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 Post subject: Re: Hewlett Packard VTVM Diagnosis
PostPosted: May Mon 03, 2021 6:41 pm 
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ok, i will re-check the resistors


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 Post subject: Re: Hewlett Packard VTVM Diagnosis
PostPosted: May Mon 03, 2021 7:15 pm 
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would these resistors be on the large board of resistors, or elsewhere? the manual labels them and tells where they go.


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 Post subject: Re: Hewlett Packard VTVM Diagnosis
PostPosted: May Mon 03, 2021 7:17 pm 
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Follow the schematic.

In my unit, some were directly attached to the tube sockets.

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 Post subject: Re: Hewlett Packard VTVM Diagnosis
PostPosted: May Mon 03, 2021 8:39 pm 
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the three resistors attached to the socket are in spec. I am tempted to replace all the resistors on that large board, but I think I will hold out for now. I just would not like to pay mouser shipping twice.


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 Post subject: Re: Hewlett Packard VTVM Diagnosis
PostPosted: May Mon 03, 2021 9:20 pm 
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acbaker123 wrote:
I am tempted to replace all the resistors on that large board, but I think I will hold out for now. I just would not like to pay mouser shipping twice.

You've lost me there. Why would you replace all of them? Why not just replace the ones that are out of spec?

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 Post subject: Re: Hewlett Packard VTVM Diagnosis
PostPosted: May Tue 04, 2021 4:33 pm 
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because if i missed one it would be an inconvenience to, pay 8 bucks worth of shipping for maybe one 10 cent resistor. just a thought.


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