Sears RoadTalker 934.38260700 Dead... :-(

[MKwrench]

Lou Franklin tells us that this chip came from four different vendors, each with its own type number.

Motorola was the original developer and called it MC145104.

National Semiconductor called theirs the MM55104

Panasonic MN6040A

And finally Nucleaonic Products Inc. (NPC) called it SM5104.

Any of those should be interchangeable.

Should.

But each will have slight differences in the physical appearance.

Any chance the slug in the VCO coil has a crack? That can make them behave in an unpredicable way.

73

Thanks Nomad, I had everything up and running and was really happy, but noticed the slug in that VCO coil was wiggly. I put a couple flakes of beeswax on top of it and heated it up with my soldering iron so it soaked down in to help hold things still. After I put the tin shield covers back on everything inside, the VCO voltage took a crap again..... I tried to adjust the voltage again and the damn slug was stuck.... I bet you know where this is going....
My plastic adjuster tool broke off in the slug. I ended up yanking the coil to find a bunch of ferrite cookie crumbles underneath. I did end up being able to remove the cracked slug and replaced it with a slotted one from another radio, which I also cracked and got stuck.....
This one didn't have any slots on the top or bottom now. So I clamped it in my mini vise and got the drill out, then ended up crushing the plastic base it when my elbow bumped into the vise handle while throwing a treat to my dog.
I'll find another one, I don't think I'll be able to rewind one like this because it's a double coil.

 

[MKwrench]

That must not have been bees wax, It may have been chunks of hot glue or hard parafin.
Bummer!
I have another Roadtalker (Base/Mobile/Slantface) that is down for the count, waiting on a donor IF xtal filter, I'll "borrow" T302 from here.
But, the bottom line is that the PLL brought the 10.24 back, the electrolytics should be replaced and we should have fire! And be careful with these hollow square ferrite slugs.
On another note, theres a .68uf electrolytic in this radio that I doubt anyone has on hand, a 1uf should work fine its what the (nearly identical) "slantface" has there.

 

[brandon7861]

Instead of wax, one thing that I find works fairly well are those little round pieces of elastic that eventually come out of the waistband of boxers/underwear. A single strand alongside the slug.

 

[VMC]

The 10.24 came back to life, but VCO voltage is still wonky. One moment I'll have the regular 40 with good VCO voltage and decent output, then voltage dissapears

Maybe check for 1) bad solder joints and 2) a bum bypass cap in there.
Living on the wet-coast, I've seen nF valued ceramic bypass caps over the decades turn into pF valued thermistors

 

[brandon7861]

With another radio, tune into the 12.32MHz oscillator (or the tripled 36.96MHz) and see if it disappears when the voltage disappears. Maybe the mixer LO is failing and throwing the radio out of lock.

 

[MKwrench]

With another radio, tune into the 12.32MHz oscillator (or the tripled 36.96MHz) and see if it disappears when the voltage disappears. Maybe the mixer LO is failing and throwing the radio out of lock.

That might be it, I cannot get the 36 MHz out of it, I'm only getting 12 something at that test point.
I believe all of that stuff is on a separate daughter board that sits above the PLL area.

 

[brandon7861]

That might be it, I cannot get the 36 MHz out of it, I'm only getting 12 something at that test point.
I believe all of that stuff is on a separate daughter board that sits above the PLL area.

Factory schematic shows a tuned transformer between the 12MHz oscillator and the mixer. Luckily the resonating capacitor is external to the can and it should be easy to test. Maybe that slug is off a little and not enough of the 3rd harmonic is getting into the mixer. Take a close look at T701

 

[MKwrench]

I think Q701 is crapping out, I had to put about 80pf from base to ground to make an even looking output after the trippler coil otherwise the output is tiny and staggered... thats next...

 

[brandon7861]

May be nothing just brainstorming, but it might be worth watching the 12MHz signal and make sure it switches cleanly between LSB and USB to verify the two crystal switching diodes. Maybe both crystals are trying to operate at the same time. Also curious if D705 was tested. If that went bad, the oscillator may be loading down the tuned input and throwing T701 off frequency.

Posting this because the sams splits the schematic in this spot. The red wire is just to fix a schematic typo.

 

[MKwrench]

Here's my old Q701, it's definitely tired...lol. I replaced it with a good used 2SC1675 I had laying around. The 12.320 and .319 seem to work fine.
Now that I have gain, it only grabs the double or the fundamental depending how I peak T701.
I think the triplets are there, but just too weak to grab. I unplugged P33 to see if is an issue on the main board, but had the same results.
The other pic is at P33 while 24.640 was showing on the counter. ... Fun! ..
.