Motorola B12WA recap and mods
 

My brother found this radio someone tossed. It needed the usual electrolytic caps, and more unusually, needed a few new tubes (tubes are usually not the main issue in sets this old, this one dates back to 1964). Needed a new 12BE6 and some 12BA6s. But while I was in there, couldn't resist doing a few simple mods. Changed the power supply from a single silicon rectifier diode to a bridge (making it a full wave rectifier circuit), and reworked the heater string so the 12AV6 has the minimum AC waveform on it, but has half the B+ voltage on it (elevated DC on heaters helps reduce hum into the tube). Sure, the radio's internal ground is always hot no matter which way you plug the power plug in, but as the radio is packaged inside a wooden and plastic cabinet, it's not an issue. I've seen this done in 1980's era RCA color TV sets, where the entire circuit is hot.

To get a little more gain, changed the AM and FM IF stage from a 12BA6 to a higher gain 12BZ6 (which requires a shield, but this set already has a shield on this tube). And the screen and cathode pins are swapped, but here both go directly to ground, so not an issue.

And built a separate AM AVC detector circuit, to allow the detected audio to have less backfed from the AVC filter cap. And thus less audio clipping. Used a geranium diode (as the 12AV6 tube with its unused diodes was too far away).

http://videokarma.org/attachment.php...1&d=1473203637

Always enjoy your creative modding/hotrodding of orphan radios.

:hdphones:

Does the full wave rectifier raise the B+ enough to require changing the operating points of the tubes? (Specifically the audio output?) is there a meaningful reduction in hum? I would imagine the higher amplitude of 60hz hum would be partially offset by the higher sensitivity of the speaker to 120hz hum, even if the ripple voltage is lower with a full wave rectifier - curious what is the practical outcome.

The plate voltage is pretty much the same, with lower ripple. Yes, 120Hz would be somewhat louder than 60Hz with the usual AA6 speaker (this radio has a bigger than usual speaker and a wooden cabinet), but ripple contains higher frequency components which is what you mostly hear with ripple. The ripple amplitude is roughly half with the bridge rectifier, and I used bigger filter caps than the originals (which also helps). But is it meaningful? A manufacturer probably would not have spent the extra pennies for the extra diodes...

I changed out the ceramic coupling caps in the audio path with films.

http://videokarma.org/attachment.php...1&d=1472696868

Nice looking for a garbage-picked radio!

Oh, forgot to mention, first time in a very long time I had a bad ceramic cap. Became shorted. It was a bypass on the heater string. It was throwing off the current thru the heaters.

I see, the set uses a strange type 2nd AM IF circuit. It looks like a single tuned coil for each band.
BTW, what is the true function of network E2? I'm repairing a 6 tube Motorola clock radio, that has a bad 2nd IF transformer. Silver mica disease. This IF has that network built into the base and has 6 terminals. I'm just going to install a standard output IF and wire in the 2, 150pf caps externally.
I can't see why it wouldn't work as well. :scratch2:

The E-2 network is for removing any remaining RF from the audio signal. Just duplacate using standard parts. All the best,Tom

Gotta go back in, found the reason I have little sensitivity at the high end of the AM dial, the IF is off. I tuned a station at 710 with this radio, and used a 2nd radio to find this radio's local oscillator. It jams a station at 113. Which means the IF is tuned to 470KHz, should be 455KHz.

Was able to get close to 455KHz by readjusting the IF so I'd jam a station at 1330KHz on the 2nd radio when I tune on the lower side of another station at 880KHz. (peaking the IF after setting up the jamming). After that, I was able to get decent tracking across the AM dial. And decent sensitivity at the upper end of the AM band.

I find this circuit used in the higher end radios, although Philco used it in many of their models, even the five tube models. I'm going to do just that. Duplicate the circuit using the standard parts. :thmbsp:

Only reason they used the module was that it was a little quicker to install in the chassis. One 3 legged part vs 3 2 legged parts. Saves the time it takes to insert and twist and solder 3 leads. When you're making millions of sets, little things like that add up.

I have an AA5 Motorola of this receiver family. I may just go back in and change the ceramics to film like you did.
I added a 22uf bypass cap across the 50C5 cathode resistor, like with many other radios. It got louder but "screeches" at higher volume.

Was this cap normally omitted to introduce negative feedback (degenerative) to reduce distortion? Many Zeniths use an RC feedback network from the OPT.

That, and of course it cost less. :D

Both IF transformers had the silver mica disease. I tried my hand at repairing the transformers, as they're rather hard to get. They're EIA 119 sourced and have the brass rivet holding the base together. I think, I've succeeded. :thmbsp:

Hi,
sorry for pulling-up the old post.
I just started a restoration got this little radio.
I can't remove the PCB from the chassis. Please advise how to proceed. I guess that a little secret with volume knob.
Thanks