Do not assume that the presense of a power transformer means an isolated chassis. Some radios and TVs used a power transformer to supply the heaters but still rectified the powerline directly. Some radios had a high voltage secondary used in concert with the powerline to feed a full wave tube rectifier. Some TVs used a voltage doubler off the powerline, some such that the chassis was hot no matter which way it was plugged in. (Line rectified to get positive 130VDC and also rectified the other way to get -130VDC (which is then designated chassis ground). Add them together and you have 260VDC to run the set on.).

Some TV design engineers I knew knew of Mad Man Muntz. "Crazy, like a fox". Back in the 50's TV was still pretty expensive compared to salaries then. So any cost cutting that could be done and still have a usable set (in high signal strength areas) would open markets to people who couldn't or didn't want to spend a lot of money on an "idiot box"....

O.K....I have seen examples of this. I have a Zenith AM/FM radio which has a plate transformer but the filaments are in series and the powerline is connected to the chassis. Also RCA CTC-71 table model 19" set...it has a power transformer for some functions but powerline connected directly to chassis. When I was about 15 I was working on one of these and saw the transfomer and thought the chassis was isolated...later found schematic and saw it was hot. Never got shocked on the ctc-71 though.

I have a Hoffman 17" TV from 1953 that uses a transformer to heat the tubes, but the DC supply is not isolated. In fact, there is a warning behind the tuner knob that the tuner shaft could be connected to one side of the line.

It is hard to believe that sets were designed so that if the tuner knob was broken, the user could be electrocuted, but that is the way it was designed.

When he sold used cars he had another slogan "I buy'em retail and sell'em wholesale.....it's more fun that way!" What a hoot. At one time he was the country's largest Kaiser-Frazer dealer.

Anthony

That sounds like a good candidate to install your own isolation transformer...have done this on sets that were a dangerous type of hot chassis design.

I rewire and polarize the plug so the chassis is ALWAYS on the neutral/gound side of the line. It's not historically correct, but a hell of a lot safer.

www.geocities.com/evilfurnaceman

I have sections on the hot chassis problem as well as the instant on thing. I welcome comments and rebuttals. If you email me with either, I'll put them on the site.

Back in the 80's, I think, we had a European set in the lab at Zenith for competitive evaluation of the design. I got elected to fire it up, and almost got literally "fired" up because it had one of those bridge rectifier circuits making the chassis hot no matter which way the plug was turned. I got my warning when the scope ground lead drew sparks.

I think I could get a picture if I could get more than 1 turn from the fine tuning knob. :thumbsdn: Well back to the work bench. :grumpy:


polaraman

Muntz Tv
 

I guess old tricks are the best tricks. In this 1949 MUNTZ, model M-159, the cabinet is plywood with a blonde verneer. The chassis, while not copper plated, sthill uses the same "ceeapo" layout, though with 1 lesss tube, I think. The tube lineup is:

6AG5 RF AMP
6J6 OSC/CONV
6AU6 1ST VIF
6AU6 2ND VIF
6AU6 3RD VIF
6AU6 SOUND IF
6T8 DET/AF AMP
6AQ5 AF OUT
6AU6 VID AMP
6AU6 DC REST/SYNC LIM
6SN7 V OSC/V AMP
6W4 DAMPER
1B3 HV RECT
6BG6 H OSC/H OUT
5U4 RECT

PLUS A 12LP4 CRT

The set work OK for the gutless wonder that it is.

I saw the chassis for the 1949 set in an odd antique store I was in a while back. (It was half of an auto body shop filled with furniture and junk, the other half was still an operating shop, the only indication of what was inside was a paper in the window that said "Antiques for sale".) I went back about a month later, and it was all gone. Maybe I should have bought the chassis, but there was no cabinet, though it would still be intersting to own a Muntz...

Is there a replacement for a IN60 Diode? Rat Shack only had IN4004. I will have to make a run to the parts store tomorrow.

polaraman

Doesn't seem too gutless to me....looks like the only features missing are a seperate horizontal oscillator tube and an agc keying stage (which may not have been in other sets of its age anyway).

Chad,
Your point is well taken. But you must admit, 15 tubes (plus CRT), is near the lower limit of cheap design, though by no means a record. A Motorola VT71 uses on 15 tubes, though it was much more common for late 40's vintage sets to 20+ tubes to do the job. I was a bit surprised when reviewing my Muntz tube lineup to see 3 video if tubes. Would have expected 2 (or less).

But I looked at a Muntz set from a mid 1950s Riders, and it STILL had 16 tubes. So maybe the Muntz reputation for cheapness is overstated...?

So.... what TV from the 1950s-60s used the LEAST number of tubes to do the job? (NOT including compactron tube sets). BTW, I don't know the answer.

Many of Muntz's sets had only two IF stages. Also, he used many fewer bypass capacitors, filters, etc. His total parts count was much lower than the major companies.

The RCA 8" portable set from the 50's uses I think only 12 tubes (not compactron) plus CRT. It has a power transformer with selenium diodes...uses a lot of 6U8's...it even uses the triode portion of a 6U8 as an audio output stage.