Mystery Component - Page 1
Forum section: electronics / beginners

author: stormdog
author_rank: Newbie
timestamp: January 24, 2018, 03:20:27 am
content: Greeetings!I'm beginning to attempt repairs on an old Lafayette VTVM. Though I've read some excellent stuff on theory of operation of VTVMs (the write up at Fun With Tubes is really excellent!http://www.angelfire.com/electronic/funwithtubes/VTVM.html). However, I haven't been able to dig up a schematic anywhere for this particular VTVM and there is a component in it that I can't recognize. I showed it to a few friends with more electronics experience than me, and they're not sure what it is either. I thought I'd ask here and see if anyone can help. A picture is below.I know that power transformers in VTVMs typically have two secondaries; one provides power for the heaters in the vacuum tubes, the other powers the rest of the machine. Is it related to that?The yellow wire on the top is set off from the body by some kind of insulator and is junctioned to what looks like a big filter cap, a secondary from the transformer, and the adjustment pot for the triode balance formed by one of the tubes. The red wires are soldered on at the top and bottom of the body and look like they connect to the other set of transformer secondaries and another big filter cap.[iLafayette VTVM InteriorbyChristopher Allen, on FlickrThanks for your help!

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: January 24, 2018, 03:28:12 am
content: I'm betting onhttp://www.rfcafe.com/references/radio-craft/copper-oxide-rectifiers-july-1933-radio-craft.htmYou probably won't be able to test these as a "diode" on a modern DMM since the forward voltage drop is in the several volts.Also, apparently they can fail spectacularly.Perhaps it's best to put this VTVM on a bookshelf.

author: stormdog
author_rank: Newbie
timestamp: January 24, 2018, 03:32:02 am
content: Huh; that's fascinating. Thank you!I'll definitely keep that potential safety issue in mind. I'm not even plugging the thing in until I make sure I thoroughly understand the thing. The person I bought it from said that when turned on, the meter either pegs high or low. When I first opened it up, I noticed that someone jumpered around a blown fuse too.

author: amspire
author_rank: Super Contributor
timestamp: January 24, 2018, 05:33:19 am
content: Those rectifiers can be replaced with a modern silicon diode like a 1N4007, but they did have a significant voltage drop in the forward direction, so when you switch to a silicon diode, the supply voltage for the meter electronics can rise by 5 or 10V. It might effect the circuit. You can disconnect the meter movement and connect a multimeter in DC volts mode in its place until you sort out the other problem. Make sure the leads are really safe - the voltages in this meter are dangerous.

author: drussell
author_rank: Super Contributor
timestamp: January 24, 2018, 06:12:36 am
content: It is a metal oxide rectifier. Probably selenium.As mentioned above, you may need to account for any extra needed voltage drop, etc.See Bob Andersen explain the process while changing the one in a Motorola Y14P3-2 television to a silicon diode:

author: stormdog
author_rank: Newbie
timestamp: January 24, 2018, 04:24:06 pm
content: Thanks again for all the help and information! I'd never heard of these before and they're really interesting. I can get back to work on trying to draw out a schematic. (Man, the ten-position, six-wafer range switch is all sorts of...fun...to map out.)

author: drussell
author_rank: Super Contributor
timestamp: January 24, 2018, 04:49:07 pm
content: What model is this VTVM?

author: stormdog
author_rank: Newbie
timestamp: January 24, 2018, 05:08:27 pm
content: It's a Lafayette 174.Front PanelbyChristopher Allen, on Flickr

author: drussell
author_rank: Super Contributor
timestamp: January 26, 2018, 09:35:04 am
content: It is probably really close to this one:Does that seem to match?

author: stormdog
author_rank: Newbie
timestamp: January 27, 2018, 03:17:34 am
content: Thanks so much for that!I have to admit that I'm not good enough with schematics to tell by sight comparison. I only started getting into the idea of working with electronics hands-on a month or so ago. Tomorrow I'll hopefully have time to start tracing out a schematic, and if it looks like it's coming out to be pretty similar to this one, it'll probably help a lot!What's this one from? I did manage to find schematics for some of the big makers, but nothing for Lafayette. I found one forum post where someone said it might be a Superior Instruments design, but I didn't turn up anything down that path either.

author: stj
author_rank: Super Contributor
timestamp: January 27, 2018, 03:39:57 am
content: if a selenium rectifier fails, you WILL smell it!!

author: CatalinaWOW
author_rank: Super Contributor
timestamp: January 27, 2018, 03:48:21 am
content: Be careful with selenium rectifiers. While I laugh at the current extreme fear of mercury and lead as hazardous materials, selenium is nasty enough that it worries me. Not quite as bad as thallium, but up there.

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: January 27, 2018, 08:09:26 am
content: Some of the early TVs had selenium rectifiers. It was the one fault you could diagnose as soon as you walked in the house. If repairing vintage gear it's tempting to just bypass them with a silicon diode, leaving them in for the sake of appearance -but don't. Remove them as they can go short.

author: drussell
author_rank: Super Contributor
timestamp: January 27, 2018, 08:31:03 am
content: Quote from: IanMacdonald on January 27, 2018, 08:09:26 amWell, if you leave them there for appearance, obviously it should at least be disconnected!

author: drussell
author_rank: Super Contributor
timestamp: January 27, 2018, 08:57:17 am
content: Quote from: stormdog on January 27, 2018, 03:17:34 amI think that one is likely pretty close, except for the added low-voltage AC ranges. You will have to trace that out and add it to the schematic if the rest of the values look close.QuoteLafayette didn't actually make stuff, they were a mail-order outfit and hadRadio Shack-type stores, selling re-branded items as their house brands as well as other big names of the time. My Lafayette"Tube and Transistor Tester"is actually a re-badged EMC model 215, for example. Most of their gear was manufactured by companies like Kenwood, Planet, Superior (SICO), SECO, EMC, etc.That is the schematic for a SECO model 208 VTVM. The only VTVM I know of made by Superior is the 77, which I don't think is what your Lafayette is based on but let me know if that schematic doesn't seem to match very well and I'll take a closer look.If it does end up being somewhat similar, a copy of the manual for the 208 is here:http://bama.edebris.com/download/seco/208/208.pdfMost VTVMs are relatively similar since the general concept is usually the same. Some values will change in dividers, etc. but they are relatively straightforward instruments. It is just really nice to have a schematic that actually shows most of the correct values and routing for the particular unit you're working on. Especially for a beginner.According to other forum posts, supposedly there is a SAMS Photofact that covers the 174 but I couldn't find it in the index. You could always call SAMS and ask, if you get really stuck and want to pay the $20 or whatever for the SAMS, assuming it really does exist and match.

author: stormdog
author_rank: Newbie
timestamp: January 29, 2018, 04:24:19 pm
content: Things have moved fairly slowly so far. I wrote up my experiences over at my blog (https://stormdog.livejournal.com/1198447.html), but to make a long story short, I've been learning largely by making faulty assumptions and then correcting them. Which is fine; at least I'm getting somewhere!So far, the bits I've mapped out do seem similar, especially once I realized that tube socket pins are numbered as viewed from the bottom! I'm going to poke at it more this week. Thanks again to everyone who offered help!

author: james_s
author_rank: Super Contributor
timestamp: January 29, 2018, 08:31:26 pm
content: Quote from: stj on January 27, 2018, 03:39:57 amI had one burn up when I was a kid, to this day I don't think I've smelled anything else so horrid, it was like a sulfur fart multiplied an order of magnitude. Seems like the smoke it made was colored too but that may be my imagination.

author: drussell
author_rank: Super Contributor
timestamp: January 30, 2018, 12:40:15 am
content: Quote from: stormdog on January 29, 2018, 04:24:19 pmWell, everyone needs to start somewhere...Many days, I feel like I've pretty much been doing that continuously, every day, since the '70s... LOL...QuoteYeah, tube pins are numbered clockwise from the gap or the keyway, as viewed from thebottomof the socket. That can trip you up if you're used to modern IC pinouts, etc. which are generally specified as the top-down view, not as "how to wire the socket" from the bottom...