1962 Magnavox 23" BW theatre model 3MV330

[Motorola minion]

This is more of a thread about a unit that has been sold by a friend of mine. He got it in NC 3 years ago and finally had interest in buying it, so it got tested kind of. He was almost ready to deliver it, unrestored but functional and it did not power up.

It is still at his flea market booth and he asked me to have a look when I was in yesterday. First problem was the AC cord one conductor rip-separated at the interlock, not usable. I noticed a Bluetooth receiver plugged into the tape input on the series 100 (non mpx) preamp/tuner. He will need to power it and I suggest an AC cord connected direct behind the interlock also to a wall-wart for power to BT receiver.

Powering up like they tell us not to, radio worked fine on AM-FM, no hum but one side of the amplifier was almost dead. I swapped the original 6BQ5s, reconnected speakers that were hooked up wrong, none was the problem of course I need to pull it out for a test and partial recap if the electrolytic reformed. I even have a spare from a 1ST223, AMP 86 or 96? 6EU7 to 12AX7 convert?

The Micromatic record player had a new cartridge but the stylus was loose in its sleeve mount and needle was spun half around The motor did not spin but was getting power. I asked him to get his player repairman to give it a goin' over, so I dont have to.

Finally, the TV did come on and seemed to be bright with a full raster. It might just be OK if they don't plan to use it for long hours, certainly not left on over 2 hours. I will bring a signal source to clean pots/confirm all adjustments and no tubes are failing.

[electra225]

A 6EU7 is always a suspect when it comes to maladies where they are employed. So are dirty tube sockets and dirty function switches, and dirty speaker switches. Does that have the C36 chassis?

[Motorola minion]

A 6EU7 is always a suspect when it comes to maladies where they are employed. So are dirty tube sockets and dirty function switches, and dirty speaker switches. Does that have the C36 chassis?

There is a sticker intact that did say C36-something, figured that was it. The TV chassis is pretty close on all those early 60s mags. The design is mostly like RCA that use common tubes, easy to diagnose and repair.

[electra225]

Dave, hold that good "easy to diagnose and repair" thought until I get the C36 chassis in my Stereo Theater sorted. Last I worked on mine, nearly three years ago (!?) I had a nice raster, but no video nor audio with sync performance unknown. I have R43, the AVC resistor badly drifted, so it gets replaced first. About the only difference in the variants of the C36 chassis is how they break off the audio. In my chassis, the audio output tube serves as a cathode follower. The power switch, volume and tone controls are all in the radio tuner.

[Firedome]

Maybe depends on how the 6EU7s are used, I had several Heathkit AA-32 amps over many that use them, one was my first tube amp after 20 years of exclusively SS amps, bought from a friend, and my son ran his AA-32 hard for several years while in High School, and we never had a problem with any of them. Depends on the circuit. I have a number of spares and never needed them. My only problem with those amps were the pricey 6GW8 tubes.

[electra225]

I have several 6EU7's in Magnavox instruments that haven't caused any problems. I had also had a 6EU7 cause problems, usually requiring me to pull the tuner in order to replace one. My experience is that one half of the tube dies, or they develop H-K shorts. Dave's procedure of replacing the 6EU7 with the more available and cheaper, well-developed 12AX7 is the way to go. I don't remember the last time I had trouble with a 12AX7.

[Motorola minion]

Had to play TV man, then found a bad damper tube after it worked last week then I fixed a stripped thread on one of its legs. I could supply 100% spare tubes to go with this TV, using such common types without a second thought. TV chassis C36-23-11

[Motorola minion]

Here is an update with a picture of the C36 (U36, V36,etc) TV chassis, which I removed for some work in shop at home.
Compared to hand-wired chassis in 1960 1/2 Theatre model, this chassis employs the use of printed circuit boards, likely reducing labor costs.

The chassis had a few bad electrolytic caps, some paper caps in horizontal/damper circuits and 5 weak tubes, though many brown/maroon drop caps remain. While these circuits are very similar to RCA, built with better quality parts, still has some sections like the sound which are very different on a theatre model. The 6GC5 tube is used only for a voltage divider, not as the audio output. A plug-in RC network is installed in place of an audio tube using a 7-pin molded socket. This network is used to equalize the TV sound as a mono (low level) input to the preamp and amplifier so it comes out all the speakers. While I could not test the operation at home, I'm confident it will work even better than before.

[electra225]

That is exactly the same chassis as in my Stereo Theater. Last I ran it, the filters were all good. Who knows now, that's been almost 4 years. I need to get the chassis out of mine and replace R43, the AGC resistor. Then go from there. I'm glad to see you going thru one. I'll be interested in seeing it in operation.

Mine has remote control, not that any of that is worth much anymore. It's nice to talk about, I reckon...

[Motorola minion]

I just got this in trade for services after it was not sold after a year at the antique mall. I'm not saying what price was on it, but it was pretty high for a modest theater model. The last issue was AGC, which makes the picture unstable. I decided it had to come home after I made a few visits to the booth to tune it up which was easy, yet I was unable to hang around run for hours and observe properly.

[electra225]

I have a C36 Chassis to sort after I get the parts.