Zenith X940 restoration project

[stbasil]

Fixing it up for my daughter and son-in-law. I've got two projects going, this one and the Motorola SK29! Garage is too hot for cabinet work right now, so I'm working on the electronics and chargers in my upstairs shop.

12"woofer, two tweeters and a midrange(?) horn per channel, 25ZT120 chassis. It's got the VM 1297 Microtouch 2G changer. Pretty much finished refurbing the changer, and it had some interesting issues which I'll describe later below.

[TC Chris]

What are you doing about the faceplate trim piece? Since you're aiming for "lookin' good" and not "passing Greg's inspection at the Buick club," I would be inclined to fashion a substitute and not aim for perfect historical accuracy

Chris Campbell

[stbasil]

Chris, I agree completely! Unless I get lucky and find a parted-out panel, I'll work out something else that looks OK.

When I began disassembling the changer, I found that the little flipper doohickey (the older service manual for the 1200 series that I have does not actually show that part) was bent over and rotated around and the spring was also deformed.

I was able to straighten everything out and reform the spring, now after cleaning, it appears that it's going to work okay. Fingers crossed. Can't figure out how somebody managed to accomplish that destruction!

I was interested to see that this version of the changer has several improvements to lower the tracking force requirement. One major difference is the little ball bearing slider that is under the trip link, unlike the two other 1200s that I have serviced. You can really feel the difference in the way the trip link friction is reduced!

Trip link bearing, before cleaning

As soon as I have a decent idler wheel, I'll see how well this thing works!

[stbasil]

Finished recapping the chassis this afternoon. Looks like the power supply cap cans had been replaced sometime ago, and one separate 1000uf/50v was replaced with 1500uf/75v connected with lamp cord (the blue one on the left).

Prior to recapping

I didn't trust any of those, so I replaced them, and several other 'lytics in the amp section, except the output coupling caps tested fine, so I left those in for now (the tan ones).

After recapping

Amazingly, it works, and nothing is overheating! FM and AM need alignment, and I think perhaps one amp channel is weak, but it's hard to tell because this strange volume/balance control. I had tested the output transistors, and one looks like it may be bad, I'll look into that. It's all germanium, so that will further complicate replacement parts.

[danrclem]

Nice looking cabinet.

[stbasil]

Anybody been successful in clearing the yellow in these plastic knobs? I've tried soaking in oxiclean, but no luck.

In other news, I've been working on getting the FM stereo to work, good news is that the FM is really sensitive, back to back stations across the dial with only a short length of unterminated twinlead, and the AFC works well, but no stereo yet. The 19 khz pilot is not getting through all the way to the indicator circuit, but I'm getting close to finding the cause. I'm grateful for the socketed transistors, it's so much easier to troubleshoot. Imagine troubleshooting a vacuum tube design that had all the tubes soldered to a PCB!

Once I figured out the volume/balance control, both amp channels appear to be working OK, but I plan to check them out more thoroughly after I get the radio section sorted.

When I hooked up the amp to my 8 ohm Optimus speakers in my shop, it immediately started oscillating, both channels, at about 42 hz! I paralleled some 15 ohm resistors across the speaker leads, and that stopped it. I saw that same kind of instability in the Maggie portable, also germanium transistors, that I recently restored. Hopefully this Zenith will be stable with the factory speakers!

[TC Chris]

Zenith was good about those socketed transistors. My first portable radio has them.

I've read about stereo threshhold pots getting dirty. Mark the wiper and move it some and see what happens?

Chris Campbell

[stbasil]

Well, turns out this is a Z940, for what it's worth. I think the models for the 940 started with X, then Y, then Z.

This stereo mpx circuit is driving me crazy! Here's a portion of the schematic:

TR13 is supposed to have the 19Kz pilot at about 35V P-P on it's collector, it was zero, but the DC was there, at about 16VDC. The base had the required 1.6V P-P 19 Khz signal. The "Mute Control" is supposed to adjust the threshold at which the transistor allows the pilot signal to pass, so for a weak, noisy signal, it will block it, and thus go into mono mode. Well, that pot was very dry-sounding, Deoxit Fader did quiet it, mechanically, but no effect on the problem. The 0.8 VDC on the emitter is present, and varies with the pot setting as indicated on the schematic. For whatever reason, the transistor refused to turn on and pass the pilot. Over a few hours, I swapped it with other equivalent transistors, checked all the resistors in the circuit, looked for shorted leads, etc, All transistor leads and sockets were deoxed. BTW, the low end of that mute pot goes to the -12 VDC supply, which checked out. I did notice that all the supply voltages were a couple of volts low, possibly because I still had a 100W dim bulb in the mains. So, even though I didn't expect it to do anything, I removed the dim bulb tester and plugged straight into the 124VAC wall outlet (no polarity change on the cord). The 35V P-P pilot signal magically appeared at the collector of TR13, and the stereo light began working as expected! I seized the opportunity to tweak the 3 transformers in the stereo circuit for maximum channel separation using my scope and a stereo FM transmitter, feeding a tone in one channel at a time. The mute pot works, adjusting the stereo threshold as expected.

I had a hard time believing that the slightly low line voltage was the cause, so I hooked up the dim bulb again, using a smaller bulb to get the line down to about 105 volts, and of course, everything continued working, pilot signal, stereo light, etc. , even at the low voltage. So, there is something strange going on--I poked around to see if any mechanical knocking about would cause an issue, but it didn't. So, I called it quits for today, and I'll see what this thing decides to do tomorrow!

I would really like to have an actual Zenith service manual instead of the Sam's, which I do have. The excerpt schematic above is from a photo that someone published in one of the forums years ago (I cleaned it up in Photoshop), I think he took a photo of his Zenith service manual schematic. It actually has helpful troubleshooting information, such as informing us what the P-P voltages should be in some of the signal circuits. That was invaluable information for me, but the Sam's stuff doesn't have any of that.

[William]

It is worth noting, the letters indicate the year.

X-1967
Y-1968
Z-1969

Bill