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 Post subject: rehabing and reassembling a WV-77E: scratchy faceplate
PostPosted: May Sun 11, 2014 7:20 am 
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A photo-log of reassembling an RCA VoltOhmist WV-77E. Not to set myself up as an expert, but rather so that others might catch my mistakes! Also, because I've read a few of these myself, and they're so fun to read.

First, the workspace, just so you can see the context.
Attachment:
01_workspace_sm.jpg
01_workspace_sm.jpg [ 219.81 KiB | Viewed 2367 times ]
Yes, its cluttered.

Unfortunately, we're jumping into this in the midstream. What I'm doing is reassembling my VTVM after doing some routine maintenence. The meter was functional prior to this, although I noticed the +DC and -DC required rebiasing, which I belive means the tube is out of balance. However, I hadn't ever gone in and done a pass over all the caps. Hoping to keep this meter healthy for a nice long time, I started checking it.

The meter is a typical "balanced bridge" VTVM based on the 12AU7.
Attachment:
01_12AU7_sm.png
01_12AU7_sm.png [ 222.96 KiB | Viewed 2367 times ]


Here's the full schematic, though I doubt you'll be able to read it very well at this resolution.
Attachment:
01_schematic_sm.png
01_schematic_sm.png [ 180.08 KiB | Viewed 2367 times ]


I've just gone through and checked all the caps and precision resistors, paying special attention to the resistors in the bridge, and those critical to the differential amp, V2 (12AU7A).

Here's the board now, front and back.
Attachment:
01_board_flip_1_sm.jpg
01_board_flip_1_sm.jpg [ 254.49 KiB | Viewed 2367 times ]

Attachment:
01_board_on_clamp_1_sm.jpg
01_board_on_clamp_1_sm.jpg [ 230.73 KiB | Viewed 2367 times ]


Last edited by adicarlo on May Sat 17, 2014 5:34 am, edited 3 times in total.

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 Post subject: Re: reassembling an WV-77E
PostPosted: May Sun 11, 2014 7:59 am 
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Remember, since the meter was basically functional I was just looking to check critical components and give the board a good going over.

The big daddy capacitor to check is the rectifier's reservoir cap, C6:
Attachment:
02_c6_sm.png
02_c6_sm.png [ 168.46 KiB | Viewed 2366 times ]

Late last year I restuffed this capacitor. A pretty easy job because the canister slid right out of the cardboard cover.
Attachment:
02_c6_prior_sm.jpg
02_c6_prior_sm.jpg [ 209.61 KiB | Viewed 2366 times ]

I cut a bit of clear piece of plastic to form the top, from my recycling, and gooped in a some wood glue, and pressed in my replacement cap. The job was holding up fine. Now you see it...
Attachment:
02_cap_off_sm.jpg
02_cap_off_sm.jpg [ 242.32 KiB | Viewed 2366 times ]

Now you don't...
Attachment:
02_cap_on_sm.jpg
02_cap_on_sm.jpg [ 213.07 KiB | Viewed 2366 times ]

Cute, but not as secure as I'd like. I rechecked the cap, all good, so I gave it a daub of glue and stuck it back.

The selenium rectifier had already been replaced with an interesting can diode.
Attachment:
02_can_diode_rot_sm.jpg
02_can_diode_rot_sm.jpg [ 208.32 KiB | Viewed 2366 times ]


I checked the forward voltage drop against it, 0.54V, and made sure it was showing lots of resistance in the reverse direction, then moved on.


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 Post subject: Re: reassembling an WV-77E
PostPosted: May Sun 11, 2014 5:25 pm 
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Continuing with the recap of the electronics side, there were two problematic caps I found. I'll cover these one by one.

Lets start with the AC input reservoir cap, C3. Here's the relevant part of the circuit:
Attachment:
03_ac_input_schema_sm.png
03_ac_input_schema_sm.png [ 196.34 KiB | Viewed 2342 times ]


I think its important to understand electrically how things are being used. V1, a dual diode, is being used here as a full-wave rectifier, with C3 playing the role of a reservoir cap. Basically, C3 is providing charge storage for the AC "valleys". It will charge up to the peak of the AC input circuit, and then drain until the next peak.

Parts list of the manual has the relevant ratings and such:
Attachment:
03_parts_list_upper_sm.jpg
03_parts_list_upper_sm.jpg [ 226.82 KiB | Viewed 2342 times ]


For C3, we're expecting 20nF +- 20%. After lifting a leg of this cap, I read it at 15.5nF. Now, we're pretty close here, because (20 - 15.5) / 20 = 0.225, meaning we're 22.5% off spec. Normally, I would call this "close enough". However, given that this is not a radio but rather measuring equipment, and given the role this cap plays in the circuit, I decided to replace it. My reasoning is that that a lower C3 implies problems near the lower bound of the AC input the meter is able to handle. In short, a lower capacitance value means the capacitor has less charge storage, therefore we're going to see more "ripple" on the lower end of the frequency range. This will effectively raise how attenuated lower-frequency signals are.

As an side note here, the positive side of C3, which is also the plate for the first diode in V1, we see goes through R4. This will get routed through an input voltage divider circuit, based on S1 and S2 settings. We also show a connection to R5, which goes to the AC balance calibration, R18. As I understand it, the function here is to offset the DC contact potential offset of the rectifier, V1. I won't go into depth here, but rather cite Rhys Samuel:
Attachment:
03_samuel_cover_rot_sm.jpg
03_samuel_cover_rot_sm.jpg [ 221.93 KiB | Viewed 2342 times ]

Attachment:
03_samuel_text_sm.jpg
03_samuel_text_sm.jpg [ 223.14 KiB | Viewed 2342 times ]


Accordingly, I replaced it with another mica cap, but one very close to spec, it measured 19.5nF. Here's a shot of the new cap in place. Placement in the circuit is rather awkward, its right between the power transformer and the battery.
Attachment:
03_c3_replacement_sm.jpg
03_c3_replacement_sm.jpg [ 192.38 KiB | Viewed 2342 times ]


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 Post subject: Re: reassembling an WV-77E
PostPosted: May Sun 11, 2014 5:27 pm 
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BTW, its really quite a lot of work to these photo-journals in this depth. I don't know how 'badrestorer' did it on those huge long threads.

If you find any of this useful, please post and let me know. I'll probably trail off posting if I don't see enough interest here.


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 Post subject: Re: reassembling an WV-77E
PostPosted: May Sun 11, 2014 6:32 pm 
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Moving on to the second problem cap I found, C7. The relevant bit of the schematic is pretty self-contained. Pay attention to the common (black) input, and where that signal goes.
Attachment:
03_gnd_to_case_schema_sm.png
03_gnd_to_case_schema_sm.png [ 161.84 KiB | Viewed 2338 times ]

R35 provides the DC path between common and "earth", that is, case ground. Likewise, C7 provides an AC path between common and earth.

Why not just connect common to earth, directly? In the hopes that someone pipes up to confirm or deny the account, here's my stab at an explanation.

First, the whole meter is floating. That is to say, there's nothing to tie the rectified DC voltage to earth ground or anything. The common probe, through the black connector on the face, is tied directly to the circuit ground. In short, its expected and required that circuit common will be floating either above or below earth ground, in some cases, by quite a bit. Tying circuit common to earth would radically limit the use of the meter. You'd only be able to probe circuits tied to common!

However, we want the case to be safe to touch, in particular, we want the case to be safe to touch, assuming the toucher is at zero potential, earth ground. R35 provides a DC path between circuit common and earth. Suppose the case is earthed, but the circuit is floating up at 100V or so. Since R35 is such a high value (1Mohm), we've got I = E/R = 100 / 1e6 = 1 / 1e4 = 1e-4 = 0.1mA.

What's to prevent large amount of AC current between case and ground? AFAICT, nothing. So -- be careful!

OTOH, if nothing at zero potential is touching the case, it will float up to match circuit common. The whole purpose here seems to be to allow the case to be at the same potential of circuit common in most cases, but still maintain safety in the event that the case is grounded or at some other potential. I'm really not sure why its important to link case to circuit common, perhaps to prevent arcing between the case and the circuit?

The cap I found in place was a 1nF mica cap, but the readings were quite odd. It was reading 1.53nF, with the capacitance drifting around quite a bit. I decided to replace this with a film cap, reading 0.96nF nice and stable. Here's the shot of the replacement:
Attachment:
03_c7_replacement_sm.jpg
03_c7_replacement_sm.jpg [ 207.53 KiB | Viewed 2338 times ]

Its possible this was a completely unnecessary replacement, but... better safe than sorry, right?


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 Post subject: Re: reassembling an WV-77E
PostPosted: May Sun 11, 2014 6:59 pm 
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Wrapping up the circuit maintenance side, the last component I replaced was the fuse, F1. This fuse is part of the Resistance reading part of the meter. You'll see it near the bottom here:
Attachment:
04_fuse_schematic_sm.png
04_fuse_schematic_sm.png [ 165.18 KiB | Viewed 2336 times ]


The circuit calls for a leaded glass fuse here. A previous restorer, faced with a burnt fuse, opted to shunt in a replacement:
Attachment:
04_fuse_shunt.jpg
04_fuse_shunt.jpg [ 160.35 KiB | Viewed 2336 times ]

If you look closely, you can see even more history here. Prior to the shunting, someone replaced the fuse with another leaded fuse by clipping the leads and tacking it on. I decided to clean this mess up.

The parts list calls for a 3AG 0.5A leaded fuse. Mouser had them in stock, so I got a few and soldered one in:
Attachment:
04_fuse_sm.jpg
04_fuse_sm.jpg [ 210.08 KiB | Viewed 2336 times ]

The board layout for a leaded fuse is pretty awkward, but you don't really have to worry about the top end of that fuse shorting with the other components, they're all linked to ground.

Speaking of fuses, I note that this devices has no fusing on the AC primary circuit at all. Sadly, the specs for the meter make no mention of the power requirements. So, I could fuse it but I'd have to try to make an educated guess at what is required here. No current plans to make that modification, but I could be swayed.


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 Post subject: Re: rehabing and reassembling a WV-77E
PostPosted: May Thu 15, 2014 6:33 pm 
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Good afternoon, thanks for the great pictures and the write up!!
Lynn


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 Post subject: Re: rehabing and reassembling a WV-77E
PostPosted: May Fri 16, 2014 10:46 pm 
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Quote:
the selenium rectifier had already been replaced with an interesting can diode.


I noticed in the parts list that CR1 was a silicon diode/ not selenium. Your measurement of .54v is also in range for a silicon diode - so maybe this is just an early form-factor silicon diode and is original to the unit.

BTW - thanks for the photo-step-by-step. Nice Job!

Best regards,
Grid2


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 Post subject: Re: rehabing and reassembling a WV-77E
PostPosted: May Sat 17, 2014 5:12 am 
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grid2 wrote:
I noticed in the parts list that CR1 was a silicon diode/ not selenium. Your measurement of .54v is also in range for a silicon diode - so maybe this is just an early form-factor silicon diode and is original to the unit.


I concur completely!

Quote:
BTW - thanks for the photo-step-by-step. Nice Job!


Ah thanks, I was getting lonely with the lack of responses!


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 Post subject: Re: rehabing and reassembling a WV-77E
PostPosted: May Sat 17, 2014 5:13 am 
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lbarnica wrote:
thanks for the great pictures and the write up!!


My pleasure, and thank you for the thanks!


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 Post subject: Re: rehabing and reassembling a WV-77E
PostPosted: May Sat 17, 2014 5:29 am 
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I wonder if anything can be done for the faceplate for my RCA WV-77E VTVM. Its got some pits and scartches:
Attachment:
face_close_sm.jpg
face_close_sm.jpg [ 196.18 KiB | Viewed 2232 times ]


It seems to be a brushed and polished steel. Its funny how the digital camera brings out the scratches at the right angle. Here's another angle:
Attachment:
face_lower_side_sm.jpg
face_lower_side_sm.jpg [ 207.62 KiB | Viewed 2232 times ]


I don't know of a way to clean this up that doesn't seem to be overly aggressive. It really shows every mark. So far I've opted to leave it alone.

On the back, here's an attempt to used a high speed polish brush. As you can see, very intrusive:
Attachment:
face_back_2_sm.jpg
face_back_2_sm.jpg [ 198.63 KiB | Viewed 2232 times ]


Even steel wool or emory paper leave fine scratches, it doesn't look good.

Should I just leave it alone?


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 Post subject: Re: rehabing and reassembling a WV-77E: scratchy faceplate
PostPosted: May Mon 19, 2014 7:14 am 
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I don't think you can do much. Any aggressive brushing will take off the paint.


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 Post subject: Re: rehabing and reassembling a WV-77E: scratchy faceplate
PostPosted: May Mon 19, 2014 4:24 pm 
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The front panels of these instruments were sprayed with lacquer to help them last longer. Given the ravages of time and use, that coating can get pretty beat-up. What you can do is give them a light sanding with 600-grit paper on a hand sanding block. Do not try to remove the scratches or the old lacquer, just take the surface sheen down a little. Clean the surface off with a lint-free rag moistened with a little lacquer thinner. Buy a new rag from a paint or finishing place; never use old clothes rags. Not only are they guaranteed to get lint all over everything, but they have soap residue which can interfere with finishes. Then re-spray with a light, even coat of new lacquer. It should fill in and hide most of the light scratches.

As with any finishing project, test on something you can afford to throw away before trying it on something you want to keep.

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 Post subject: Re: rehabing and reassembling a WV-77E: scratchy faceplate
PostPosted: May Mon 19, 2014 10:48 pm 
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Chris108 wrote:
The front panels of these instruments were sprayed with lacquer to help them last longer. Given the ravages of time and use, that coating can get pretty beat-up. What you can do is give them a light sanding with 600-grit paper on a hand sanding block. Do not try to remove the scratches or the old lacquer, just take the surface sheen down a little. Clean the surface off with a lint-free rag moistened with a little lacquer thinner. Buy a new rag from a paint or finishing place; never use old clothes rags. Not only are they guaranteed to get lint all over everything, but they have soap residue which can interfere with finishes. Then re-spray with a light, even coat of new lacquer. It should fill in and hide most of the light scratches.

As with any finishing project, test on something you can afford to throw away before trying it on something you want to keep.


Very useful information! I don't have any spares I can play with, other than the back of the faceplate itself. Hmm. Will have to consider, might just leave it scratchy for now.


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 Post subject: Re: rehabing and reassembling a WV-77E: scratchy faceplate
PostPosted: Sep Wed 08, 2021 2:22 pm 
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That lettering is sunken well under the face. You could lightly sand to rid of the ā€œbumpsā€ and smooth them a bit, then Scotchbright pad the entire face in one direction for that brushed look. It will remove the lacquer finish, but not sure how much that matters. you could re-spray it with lacquer i suppose.

But when I do older gear like thisā€¦or my other hobby being outboard motors I try to decide how far to take the restoration. I figure this is part of the units story and history. As long as it works as designed.

Great job on this piece!

Greg


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 Post subject: Re: rehabing and reassembling a WV-77E: scratchy faceplate
PostPosted: Sep Wed 08, 2021 6:48 pm 
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This thread is now seven years old. Hopefully Adicarlo got some good use out of his meter.

One minor technical point for anybody reading this in the future. The 6AL5 is not actually in a full wave rectifier circuit. The left diode on the schematic has its cathode grounded, not returned to the pick-off point for the AC voltage measurement. The right diode is the only one that provides voltage to the rest of the circuit, so this is a half wave rectifier.

So what are they doing with the right diode? Well, tubes have what is known as contact potential. The elements are made of dissimilar metals so there is a potential difference between them. When the tube is in operation the electron stream between cathode and plate is across this potential difference. So what the left diode is really doing is adding about one volt to the reading, because the right diode is going to subtract a volt. This improves the linearity of the rectifier.

The downside of this arrangement is, the meter only responds to the negative half-peaks of the AC waveform. So it is okay for symmetrical AC waveforms like sine and square waves, but not for asymmetrical ones. Later meters corrected for this with different rectifier designs.

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 Post subject: Re: rehabing and reassembling a WV-77E: scratchy faceplate
PostPosted: Sep Wed 08, 2021 10:07 pm 
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Chris108 wrote:
This thread is now seven years old. ...........
One minor technical point for anybody reading this in the future. The 6AL5 is not actually in a full wave rectifier circuit. The left diode on the schematic has its cathode grounded, not returned to the pick-off point for the AC voltage measurement. The right diode is the only one that provides voltage to the rest of the circuit, so this is a half wave rectifier.

So what are they doing with the right diode? Well, tubes have what is known as contact potential. The elements are made of dissimilar metals so there is a potential difference between them. When the tube is in operation the electron stream between cathode and plate is across this potential difference. So what the left diode is really doing is adding about one volt to the reading, because the right diode is going to subtract a volt. This improves the linearity of the rectifier.

The downside of this arrangement is, the meter only responds to the negative half-peaks of the AC waveform. So it is okay for symmetrical AC waveforms like sine and square waves, but not for asymmetrical ones. Later meters corrected for this with different rectifier designs.


Some VTVM's are indeed setup as you describe, however in the WV77E the 6AL5 is setup as a voltage doubler to allow peak-to-peak readings. Contact potential is bucked out by feeding a small voltage from the power supply through the 91 meg (R5) resistor shown in the upper right hand corner of the partial schematic. The full schematic is attached.

RRM


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WV-77E schematic.pdf [250.6 KiB]
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 Post subject: Re: rehabing and reassembling a WV-77E: scratchy faceplate
PostPosted: Sep Thu 09, 2021 3:15 pm 
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It's one of those slippery slopes where it could be anything depending on how you look at it. RCA themselves referred to it as a full wave rectifier in the WV-77E manual and since the diodes do conduct on both halves of the AC waveform, that is true. But it is also true that the left-hand capacitor (C2) will charge when the left hand diode is conducting on positive voltage excursions, and that charge will be added to the negative voltage excursions when the right hand diode conducts so voltage doubler is right too. And it will respond peak-to-peak on symmetrical waveforms. But it is possible to envision some asymmetric waveforms--demodulated analog video with sync tips positive for example--where the sync tips will be added to the negative-going video information to give some kind of average value like a half wave rectifier. If the signal was identical but inverted, i.e. sync tips negative as they are in baseband video, the meter would give a somewhat different response.

This is not a criticism or critique of the design, BTW. RCA and its competitors sold millions of VTVMs using essentially the same circuit and they were highly satisfactory for most purposes. Just trying to point out that understanding an instrument means understanding its limitations too.

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"Hell, there are no rules here--we're trying to accomplish something!"

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 Post subject: Re: rehabing and reassembling a WV-77E: scratchy faceplate
PostPosted: Sep Thu 09, 2021 4:29 pm 
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You made a mistake, most humans do.

RRM


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 Post subject: Re: rehabing and reassembling a WV-77E: scratchy faceplate
PostPosted: Sep Thu 09, 2021 4:56 pm 
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No I did not make a mistake, I learned something that I thought I understood but did not fully comprehend. Isn't that what this is all about?

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