Some Vintage Test Gear Madness

[mayfly]

Gentlemen!

Update!  Modified with new content!

I was fortunate enough to be gifted a pair of vintage Vacuum Tube Volt Meters.  Meters of this type are famous for their sensitivity and are especially useful for tracking down noise and degraded components.  They also have 'built in' Peak-Peak to RMS conversion, and the better ones also have db metering.

This pair came from a work friend who's father used them extensively.  Here they are!

Gonna open them up, tweak them, then put them to use!

 

[mayfly]

So!

I pulled one apart.  Unfortunately the 1.5V D-cell in there was old and leaked over everything.  In addition, the metalwork was badly bent, and the power transformer was so corroded it measures as a short.  Crap.

To put the nail in the coffin, it was loaded with ancient wax/paper caps, which likely went out of tolerance 2 decades ago.

BUT!!

The second one was a lot cleaner, so now the plan is to use the first one for parts and get the second one running.

Looks Like I have a project!!

 

[mayfly]

A little research revealed the schematic for the 'good' one (a PACO V-70):

https://www.nostalgickitscentral.com/paco/schematics/v70.pdf

One of the tubes is a good old 12AU7, but the other one is a 6AL5, which is a new one on me.

The other thing that I noticed about the schematic (Besides the selenium rectifier), is the lack of a mains fuse!!  Hmmm...

I guess this was back in the day when they didn't worry too much about UL approvals and stuff 

 

[Logrinn]

Approvals?

Moahahaha!

 

[Cagey]

Interesting. Too bad about the one that got ate up, but the other looks quite recoverable.

Sometimes I miss my Simpson 260. Digital meters are nice because they're smaller and lighter, but there's nothing like watching a needle sweep. Lotta information in that.

 

[mayfly]

Here's something for you Cagey!!

 

[Cagey]

Nice! Can't buy 'em, but if anyone's bored out of their skull, here's how it works...

[youtube]0gSPxQY-dOk[/youtube]

 

[mayfly]

So we got some parts from Digikey, the schematic, and the meter.  Let's go!

 

[mayfly]

First victim is a 0.02uf wax cap.  These old things are junk - just replace them

out with the old in with the new

 

[mayfly]

After a couple of those, it's time for the electrolytic.  It shows the tell tale signs of a leaky cap with a dried out dielectric.

Another size comparison.  Golly! it's amazing how small they can make those things these days

 

[mayfly]

Next in line is a 0.02 uf cap rated at 1600V.  Took me awhile to find a suitable replacement!  But in the end Digikey came through again.

here's all the dead solders.

Here's the new electrolytic and that new 0.02uf 1600V thing.  Sure hope that selenium rectifier works!!

And the two smaller 0.02uf 600V caps

 

[mayfly]

I also replaced the power cord (original's insulation was dried out and cracking).  I fired it up on the dim bulb tester and it didn't explode.  Power supply came up and the lamps lit.  Full voltage was ok as well.

So... I got my function generator, set it for around 0.5V AC rms, and took a measurement!

Holy crap!  I really didn't expect it to actually work out of the gate. 

 

[Cagey]

Wow! If I'm reading it right, it looks like it's spot-on, too!

 

[mayfly]

Turns out it was a fluke.  I tried changing the amplitude but the voltage on the meter didn't track it very well.

Now I guess I need to figure out calibration!!

 

[Cagey]

Turns out I was reading it wrong. I was on the wrong scale. Still, the fact that it works at all is huge.

 

[mayfly]

oh yea!  the 1.5 scale is below everything else!  I just noticed that too.  :tard:

 

[amigarobbo]

First victim is a 0.02uf wax cap.  These old things are junk - just replace them

out with the old in with the new

Put it on eBay, vintage caps for your new Gibson Les Paul.  :evil4:

 

[rgand]

:icon_thumright: :icon_thumright: :icon_thumright:

 

[mayfly]

ok! Let's talk calibration.  I looked high and low for a manual for this thing and came up empty.  BUT!  There is a Heathkit VTVM that has almost the exact same schematic.  The calibration manual is readily available for that, so I snagged it.  The manual is pretty funny:  It says to calibrate the DC using "the specially calibrated D-cell supplied in the kit" and to calibrate the AC using "the 117 Volt AC line.  Caution!  117 Volts AC can be dangerous!" 

Following the procedure (using a 'non-calibrated' energizer D-cell that I had lying around) it turned out that I could not calibrate it - the pots ran out of range before I could get there.  Same with the AC.  So I pulled the 12AU7 and replaced it with the one from the other meter.  Worked better - I could calibrate it using those sources.  But then measuring other stuff (like the signal generator output) afterwards showed up a general lack of accuracy in the meter.  Several options available:

1 - order another 12AU7.  You can get NEW ones!
2 - check all the resistors and replace ones out of tolerance. 
3 - both 1 and 2
4 - say "ah heck - it's just for debugging anyway.  Forget it!"

Leaning towards #4.  But I might check the resistors anyway just for the heck of it.

 

[Cagey]

Since you already have a meter(s), the temptation would be to let it go. VTVMs used to be desirable due to their very high input impedance making them suitable for testing things you didn't want to load at all, but I don't know if that's still an advantage today. Since it's old, it may be full of carbon comp resistors, which as you know are sloppy little buggers that will change value over time. Replacing those with metal film parts may make it better than new, but there's undoubtedly a lot of them in there. Then, while you can get a new tube, you know how inconsistent those things are. You may already have two paws on a tar baby with no briar patch in sight while you're contemplating kicking it.