Capehart-Farnsworth 3004MPA

[dobulee]

A friend of mine has been sitting on a Capehart model 3004MPA, and has recently asked me to look into doing a chassis rebuild for him.

I did a quick search for this model, and came up nearly empty on Google.
Pictures of the set and the nameplate are attached.

Does anybody know of this particular model? I'm looking for a SAMS or Beitman Manual for the schematic.

This appears to be the early years of Capeharts limited run in Television (50 or 51)

The 1951 Beitman shows Capehart-Farnsworth Chassis CX-33 and CX-33A-M used in models 320-337 and models3011 and 3012.

However, I have been unable to come up with a schematic for model 3004. There is no obvious chassis identification on the chassis itself.

Any help appreciated!
Alex

[bandersen]

It uses chassis CX-31 covered in Riders volume 4 starting on page 17 under Capehart.

The Early TV Foundation website hosts the RIders index here: https://www.earlytelevision.org/rider_index.html

[dobulee]

Thanks for this.

Not to ask the obvious question, but where might one find the Riders TV series?

I see Rider's Perpetual Troubleshooters Manuals for radio both in thr public domain and also for sale as a set on DVD, but I haven't been able to locate the guides for TV.

Any help appreciated.

Alex

[WISCOJIM]

Originally Posted by dobulee

I see Rider's Perpetual Troubleshooters Manuals for radio both in thr public domain and also for sale as a set on DVD, but I haven't been able to locate the guides for TV.

They are available on ebay, but very overpriced for what they are. They were scanned in B&W, which leaves almost every photo in them useless. And if you are familiar at all with the TV Riders, their value is in the photos.

Here is the link, but I don't recommend them at all.

https://www.ebay.com/itm/Riders-Tele...item19de183e7f

.

[dobulee]

Thanks - it actually looks like the Full Index Book (which has quite a lot of interesting information and circuit descriptions) is available on several university library websites, see attached.

Unfortunately, Penn State Shows the actual Volumes 1-4 as available online, but then does not allow them to be viewed.

They are shown as being 'on the shelf' at penn states university park library. Perhaps I can call in a favor with a friend.

[dobulee]

I put in a call to the Engineering Library At Penn State University. They have Volumes 1, 2, and 12, but not volume 4 The hunt continues!!

[bandersen]

Check your PM

[dobulee]

So this turns out to be quite the interesting little set! Thanks for everybody's help in finding some restoration aids.

This 1949 set has an integral AM/FM tuner, and a pretty hefty 12W 6V6 Push pull audio. IT has a 6AV6 phase inverter circuit set up in a configuration that I'm not sure I've ever encountered before (althouth the vast array of positive and negative voltage rails makes reading the schematic a bit cumbersome!)

Taking a page out of the RCA playbook, this is a split sound chassis, with the sound take off right at the converter oscillator! Interesting.

It's got a DC coupled 2 stage video amp, using both halves of a 12AU7, just like the RCA KCS Chassis of the era, except that it's DC coupled. If RCA's late 40's sets are any indication, that circuit leaves a little to be desired for video bandwidth, so I'm interested to see what the Capehart is like. The synch separataion and amps are pretty involved for the era...

It's likely to be some number of weeks before I get into this. But looking forward to it!

[Electronic M]

Capeheart was a high end brand that liked complexity. If Interesting mechanical designs float your boat look for a YouTube video on the capeheart turnover record changer... Just watching one opperate is a show in its self.

[old_coot88]

Originally Posted by dobulee

...IT has a 6AV6 phase inverter circuit set up in a configuration that I'm not sure I've ever encountered before...

That's a weird one. They coulda did it like this. Works great, albeit with slightly reduced gain due to no cathode bypass cap.

[dobulee]

A friend of mine has that turnover record changer! It's a mechanical wonderment to say the least!

Originally Posted by old_coot88

That's a weird one. They coulda did it like this. Works great, albeit with slightly reduced gain due to no cathode bypass cap.

Definitely! Spending some time looking at it in detail. It's sort of a quasi paraphase phase inverter? One of the 6V6 grids driven right after the audio amp/tone stack, the other being driven from the inverted signal of the 2nd 6AV6. What makes it a little bit more interesting is that capehart introduces some negative feedback around the 2nd 'phase inverter' 6AV6. I guess that makes the circuit a little more immune to gain variations in 6AV6's in order to keep the push-pull grid drives fairly balanced. A Cathodyne phase inverter would have used the same tube line-up, but been easier, better matched and fewer parts - maybe that one hadn't yet made it's way into hifi!

[old_coot88]

Originally Posted by dobulee

Definitely! Spending some time looking at it in detail. It's sort of a quasi paraphase phase inverter?

Back in the day it was called a 'grounded grid, common cathode' phase inversion scheme.
During half a cycle, say the grid of the 1st tube swings positive> cathodes go more positive, making the grounded grid of the 2nd tube negative relative to cathode.
Truth be known, the balance wasn't perfectly symmetrical but consumer grade with minimum parts count, and still be "push-pull". The balance was probably on a par with the scheme using the 6AV6 inverter. To tailor the gain, looks like they chose 220K for the plate load resistor and 220K for that series resistor going to the grid.

[AlanInSitges]

What would be the benefit of using this circuit over a plain-old, triode-in-the-middle design?

[old_coot88]

Originally Posted by AlanInSitges

What would be the benefit of using this circuit over a plain-old, triode-in-the-middle design?

The economy of one less tube and few small parts is about all. And being a novelty for some old farts to jawbone about 70 years later.

[Electronic M]

It could be the old "it's 1 tube better than the other set" argument...or a patent dodge.