Admiral 30A12 restoration

[wa2ise]

Originally Posted by bandersen

I now have a nice little burn mark on the side of my thumb.
... and I think I was overdue.

Years ago, when our beloved sets were just old, the horizontal output tube cap in a CTC19 I was trying to fix nailed me. I seem to remember seeing a black line running up my arm afterwards...

[kvflyer]

Originally Posted by wa2ise

Years ago, when our beloved sets were just old, the horizontal output tube cap in a CTC19 I was trying to fix nailed me. I seem to remember seeing a black line running up my arm afterwards...

Are you sure it wasn't brown and on a different appendage?

[Kevin Kuehn]

Ohmite makes 25W @ 2k, 2.5k, 3k, 3.5k, 4k - bolt a couple end-to-end and you'll be in the ballpark. Wattage divides between the two, and I bet the original 50 watt rating was plenty conservative. http://www.mouser.com/ds/2/303/res_270-231448.pdf

[bandersen]

Good idea and I did consider it, but they are about $5.50 each or $11 per set.
Instead, I went with some 7.5K 50W Ohmite from Allied on sale for only $4.68 I'll add a 20K 10W resistor in parallel to reduce the value.

Since I need six of these the price difference really adds up.

P.S. I agree about the power rating because it really doesn't really get all that hot.

[bandersen]

The new resistors have arrived. It turns out the modern 50W Ohmite resistors are about 1/2" shorter and smaller in diameter than the old ones. Still, they should fit fine.


I also popped out the tuner and gave it a good cleaning and lube. It's operating much more smoothly now

[bandersen]

I reinstalled the tuner, installed the new power resistors, replaced the electrolytics and cleaned up the chassis a little. I'll try powering it back up in a few.

[bandersen]

The picture is good and the sound is getting better, but still weak. Also, the reception on channel 4 is terrible, but pretty good on 3

I really hope to get this sorted out over the weekend

[kvflyer]

Originally Posted by bandersen

...

I really hope to get this sorted out over the weekend


...

No doubt in my mind...

[bandersen]

I tried doing an alignment and discovered a broken slug in the IF can on the tuner. I suspect it broke when someone tried to adjust it because it's an odd design in that it performs two functions. The top slug is for video and the bottom for audio. You need to use a very slender alignment tool passed through a hole in the top slug to adjust the bottom.


It's L106 on the schematic. I think every other split carrier set I've encountered has the audio trap on the main chassis and the tuner has a single output. Admiral made this change in later models too.


Luckily, I had a spare tuner and popped it in and proceeded with the alignment. That solved my problem of lousy reception on channel 4, but the sound was still bad.
So I proceeded with the alignment and ran into another problem. Some of the IF coils don't peak. In fact adjusting the slugs seems to have no effect at all.

I decided to pop out the first audio IF can (T201) for a peek inside. Sure enough, it has the type of exposed mica cap that so often go bad in radios.
I carefully disconnected one end of the coil in parallel to the the cap. I measured no capacitance and it's very leaky on even the lowest voltage range!
Unfortunately, I don't have any 30pF micas on hand so I'll have to order some. Might as well get some for the other IF cans as well.

[bandersen]

I replaced the bad mica cap in the audio IF can and that did seemed to do trick
Then, I moved on to the video alignment and discovered that none of the three traps would peak. So I popped one open and discovered another bad exposed mica cap. I'm sure there's a bad mica in the other two IF cans too.

[bandersen]

I replaced the mica caps in all three video IF cans and they are peaking properly now However, I'm still not all that happy with the sound and I noticed that the audio trap on the tuner wasn't peaking properly either.

So rather than screw around with it anymore I decided to replace it yet again but with a slightly newer version.

That's the newer 94C8-2 tuner on the left. Instead of having the video and audio coils stacked inside of one can, they're mounted side-by-side.

[Phil Nelson]

Originally Posted by bandersen

I replaced the bad mica cap in the audio IF can and that did seemed to do trick

I had to replace those in my Admiral 24C15, as well. Here's another of those funny bare caps.





Not a common problem, but something to be aware of in these Admirals, I guess.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

[7"estatdef]

Hey Bob
Tnx for the tip on the caps! The audio was always kinda low so I cheated on my 20A1. It used a 6SQ7 as the pre amp in the audio. It got swapped for a 6SL7 which gave me another stage of gain. Works really well it can really rattle the speaker with this setup.
Terry

[bandersen]

I now have clean audio but the volume still stinks. My 30A1 chassis is an early production run and I've noticed a few production changes in the service literature to improve volume.
Perhaps I'll try implementing some of them or try a 6SL7 too

[bandersen]

After trying several tuners, testing/replacing every component and doing multiple alignments it finally occurred to me to try different RF sources.

Our local LP OTA channel 6 pseudo station comes booming in, but I know their audio carrier is much stronger than normal.

My trusty OTA ATSC converter box I use with all my sets has solid video but weak audio.

Next, I took the composite output of the converter box and fed it to a cheap stand-alone RF modulator. That definitely improved the audio It's also much better if I use a Blonder Tounger modulator.

So I'm concluding that it's working as designed and it's just more sensitive to the audio carrier than other sets.

I had so much fun with this set, I decided to plunge right into restoring another one Electrically anyway. I figure that will allow me to compare the performance of the two. It should also go a whole lot faster since it's fresh in my mind.

I already have the lower chassis rebuilt and it seems to work fine.

However, I noticed a blue flash over in the 5U4 on power up. I tried a few different 5U4G and 5U4GB and all had flashover too and with both power supplies.

The original design has a 40uf + 2Hy choke + 40uf for a filter. I replaced them with 47uf. I wonder could that be putting too much of a strain on the 5U4 ? Perhaps I should knock the 1st cap down to a 33uf ?
Or maybe throw a power resistor or thermistor in series