'49 Admiral audio problem

[kbmuri]

Thanks Einar. Just inductively taking readings from around the tube I got:

Ch2: 79.53 - 79.94
Ch3: 85.71 - 86.16
Ch4: 91.02 - 91.50
Ch5: 101.50 - 102.04
Ch6: 106.50 - 107.08

They were a lot more stable than poking probes directly onto the VHF circuits. The counter had plenty of gain to deal with the lower input level.

According to Adam's 721 thread and Steve M, the oscillator frequency should be at 87.0, so I'm currently a bit low. Now that I know what to target, I'm sure there's room on the tuning slug to get it dead-on. So I can at least eliminate that.

The "lame" fine-tuning adjustment does what I said earlier -- it only ranges the oscillator about 1/2 MHz, not a lot of room for adjustment.

[kbmuri]

I recapped B&K Analyst #1. The electrolytics only. It almost works. Probably needs a few paper caps replaced too.

Good thing about it is it broadcasts 400Hz tone, FM modulated at any IF frequency from 15 to 50 MHz. Except it doesn't work yet.

Will likely recap the other one too.

Pic shown is it running on the Magnavox. I know it has issues. After this, it should be easy to tune the Magnavox up.

[kbmuri]

Finished the B&K Analyzer. Two small 10V electrolytics on the audio-osc PCB were open too. Once replaced, the 400Hz tone is present. Actually it's at 438Hz according to my frequency counter, but for now, close enough. I'm sure there's a trimmer under there, later.

Shown working on the Admiral from Hell. Unfortunately I was mistaken, the only audio-only signals from the B&K are straight 400Hz tone, or 400 Hz tone FM-modulated at 4.5MHz. Not terribly useful for split-sound audio problems. I did inject the 4.5MHz onto either of the cathodes of the 6al5 FM discriminator, and it discriminated (i.e. I heard a pretty reasonable 400Hz (I mean 438Hz) tone at a reasonable volume).

Injecting the 4.5Mhz signal at the grids of either sound-IF 6au6 amp yielded nothing (no sound). Not sure if it would or not though, even on a working set.

In IF mode, I injected at the grid of the first 6au6 and swept up the range. There were a few places where I was able to get tone out the speaker, but not at great volume (muffled and far away, like my original "tune it by eyeball" results of some weeks ago). One point in the sweep yielded that very loud poppy 60-Hz buzz that all TVs make when you tune the fine tuning too far right. That was the only full audio level I've ever heard out of the set (other than injecting transistor radio directly into the first audio amp).

I've ruled out the LO being off, ruled out the discriminator not discriminating, and ruled out the audio amp itself. Have ruled out any power-supply issues (all plates in the sound-IF strip are at voltage spec). Have ruled out all resistors (they ohm out to spec). Have replaced all paper caps in the strip with orange drops. Have double-replaced the sound-IF tubes, have ruled them out. Leaves very little. Those caps that are buried in the IF transformers, maybe. Or the IF transformers themselves (windings shorted maybe, not open, because they ohm out ok).

The B&K is a pretty cool tool though, and will be indispensible later on other projects, I'm sure. After I toss this freekin Admiral in a dumpster.

[kbmuri]

http://cgi.ebay.com/ws/eBayISAPI.dll...m=150166963510

I gave it back yesterday because I'm expecting a couple of new projects this week, and need the workbench space back. She didn't waste any time listing it.

All the hard work's done, it's a good little set. It's not mine to sell, so I'm not pushing any of my own business. Just letting you all know it's available if you want to take a stab at it. The 12LP4 and NOS tube set is worth more than the starting bid. And 50 bucks worth of new JustRadios.com capacitors too. Not to mention the warm fuzzy of knowing the tuner and video and the audio power-amp all work. Good set for one of the "pros" out there. Just fix the sound-IF and you're golden.