View Full Version : 1952 Midwest CV-20/NV-20


wdln
07-13-2017, 01:23 PM
I am looking for a schematic or service manual for a 1952 Midwest CV-20/NV-20. These were sold direct from the factory via catalogs and are therefore somewhat rare. My understanding is that Midwest was pretty successful on the radio side but by the late 50's they were out of business, after trying their hand at TV. The 1952 catalog page I found offers two models for sale... the NV-20, which was a fully-assembled floor-standing TV, and the CV-20, which was just the fully-assembled electronic chassis for DIYers who wanted to make their own cabinet. The only information I have is a tube diagram, which indicates the CV-20 nomenclature.

The unit powers up and the picture tube (20CP4) has good contrast, but there is some audible arcing in the high voltage section and the picture is not stable vertically or horizontally. I have replaced most of the paper caps underneath the chassis but I'm down to the electrolytics, and here's why I need a schematic.

There are four can-type caps on the chassis, with one additional electrolytic mounted normally under the chassis. Two of the cans are stamped with values and terminal legends so they'll be easy to replace; I haven't decided whether to re-stuff the cans or just do it all underneath. The other two are unmarked, and I can see where there used to be a paper label of some kind on the outside. One of these has 4 terminals and the other has 2. Without a schematic, I'm not sure how I can divine the values from these. I do have a meter but I doubt they will measure properly, being that old.

This set was stored in a hot/cold garage with a bad roof for many years. I'm lucky it powers on at all, but that's probably why the labels are gone.

So I'm either looking for a schematic or some helpful suggestions/tricks to figuring out what these are. I've attached a photo of the TV as well as the tube map. The caps in question are CE-20 and CE-10 on the diagram.

Electronic M
07-13-2017, 01:57 PM
Sam's shows no matches (though their older paper indexes might). https://www.samswebsite.com/en/photofact/search?t=brand&o=0&k=midwest&submit=Search&s=Search

If no one else does so before I get home from work, I can check Bietman's most often needed TV index for a schematic.

With cap testers it depends on the tester and the condition of the cap. My Heathkit C3 will usually get the approximate value of a cap unless it is open or nearly dead shorted (very leaky caps usually will test well). Some modern testers will do a poor job on failing vintage caps (and can be damaged if you do not discharge the caps first). That will probably give you the capacitance. The voltage you could obtain by measuring the voltage across each section of the original, and adding 20-30% to it as a safety margin.
Speaking of tolerance most tube era lytics had ~ -50%, +100% capacitance tolerance so anything in the ballpark will be useful (B+ value and circuit performance may be affected).

As for the arcing: With the set off, you will want to discharge the HV*, blow the dust out, then clean everything thoroughly with rubbing alcohol, to get conductive dirt off. Be careful not to break the delicate wires leaving the flyback.

If it is still arcing, try to locate the source and cover it with corona dope or sensor safe silicone. Make sure to scrape away any carbon paths before coating it, if it is arcing along a surface/through an insulator. If any HV wires are arcing they can be replaced.

*Connect the chassis to the HV with preferably a high value resistor (meg ohms) like that found in a HV meter (if you have one it will work well), or in lieu of the resistor short to ground. Note if you short the HV to ground the HV cap can recover a LOT of voltage in a few minutes and may need to be shorted multiple times to stay discharged. It will not recover charge if resistively discharged thus why the resistor is recommended.

wdln
07-13-2017, 02:50 PM
Very helpful, thank you! I actually already submitted a manual request to SAMS, first for the CV-20/NV-20, then for ANY Midwest TV, hoping the circuitry might be similar enough so as to be able to figure things out. The response was that neither SAMS nor their partners have photofacts sheets or schematics for any Midwest TV model.

As for the arcing, so far it's only something I can hear, not see. I've looked at the HV area with the shielding cover off, and also from under the chassis, and haven't seen anything. Maybe I need to turn the lights off and try again. :D

David Roper
07-13-2017, 05:17 PM
It is in Rider volume 8. That would make it a 1951 model.

wdln
07-13-2017, 05:30 PM
Sweet! Are the Rider TV manuals available online somewhere? I found these but they appear to be just radios.

http://www.nucow.com/allriders.htm

David Roper
07-13-2017, 06:00 PM
I'm lucky enough to have acquired the physical books. I don't have a scanner, but I can try to get a useable photograph. It will still require a fair measure of circuit tracing since only component values are given--there are no part numbers at all. :(

Electronic M
07-13-2017, 07:32 PM
I have the PDF of vol 8. It is a bit big for email, and I can't print it's pages to PDF, but I can take a screen cap.

Let's see if it uploaded right.

wdln
07-13-2017, 07:51 PM
Nice... I'm comfortable tracing through a schematic. I will PM you my email address so you can send a decent sized pic. Thank you in advance!!

wdln
07-13-2017, 07:54 PM
Just saw the other post... the posted schematic is a little blurry. Maybe if you email the image directly instead of posting on the forum? I know there is an image size limit here.

wdln
07-14-2017, 08:54 AM
Just to close this out, thank you to all who jumped in here, especially so quickly. I now have the schematic and can continue tinkering. I'll post progress when I have good news!