'49 Admiral audio problem - Page 2 - Videokarma.org TV - Video

7"estatdef · #16 06-13-2007, 05:29 PM

I got off of work a little early today so in my spare time I rewired the audio amp to use a 6SL7. Before the volume was low. In a quite room it was ok but add in kids or a wife and it was tough going. Not the case after the 6SL7 was added. Now it has pleasnt room filling sound and that's with the knob turned half way up. I did change a few values of the original caps to boost the high fq responce.
As is note in the service manual it seems as though the original has some flaws but this puts them to rest. I'll also mention that there's a good article that discribes a vertical blanking circiut for your set. I refasioned it to work on my set and it too works great.No trace of retrace! Happy to pass it along I got it off the web.
Terry

kbmuri · #17 06-14-2007, 03:00 PM

Yes, please forward me the vertical-blanking circuit mods. PM if you need an email address.

7"estatdef · #18 06-14-2007, 07:06 PM

Here you go... Read about it at:

http://groups.google.com/group/rec.a...e39.0207011623.

Diagram below

7"estatdef · #19 06-16-2007, 01:31 PM

After rereading this info I think I know why my picture is slightly smaller on the right side (abt 1/4") I'll bet I need ato adjust that shaping network.
Terry

kbmuri · #20 06-16-2007, 11:22 PM

ok, so the 6SN7 swap is really a good idea for this set, but as wa2ise hinted at, the extra stage of amplification just amplified some pretty rotten sound, louder. Hum and tinny, unacceptable.

So I did a complete IF alignment, by the book. With my crude tools, but I'm sure I nailed it. My Knight am signal generator made sine waves, and I set probe A of my tektronix dual-trace scope to monitor them, setting the sweep to 0.1 microseconds/div (or 1 microsecond per full frame of 10 divisions). Counted the waves by hand to make sure the 2nd-hand Knight generator was dead on. 21.25 Mhz where needed, 23.5, 25.3, 22.0. Then I used probe B to measure voltage deflection at test points. Had to build a detector probe per the Sams schematic and a resistor-capacitor filter test point and an insulated tube shield with test-clip loop (for injecting signals inductively into the 1st tuner tube). Quite a time-consuming affair.

Every slug in the sound IF strip was way off base. Every slug responded marvelously, every one is set perfectly now (maximum deflection where called for, minimum deflection where called for, zero-reading where called for). It was really exciting to see everything come into tune, and I was pretty optimistic about the whole process. I even got the languishing .025-volt discriminator output back up to where it belongs. Sure thought it would do the trick.

After removing the filter test point and reassembling channel 13 of the turret tuner and replacing the tube shield, I re-applied VCR signal. Lost all my sound now, and made crap out of my crystal-clear picture.

How exhausting is that?

If this set were my own property, I'm pretty sure I'd part it out now. It got the best of me. Best I think I can do now is take it to a pro, and hope he's not too judgemental of my recap and willing to work on something that's been worked on by an amateur.

Or I suppose I will sleep on it one more time...

wa2ise · #21 06-17-2007, 03:17 PM

Quote:

Originally Posted by kbmuri

So I did a complete IF alignment, by the book. With my crude tools, but I'm sure I nailed it. My Knight am signal generator made sine waves, and I set probe A of my tektronix dual-trace scope to monitor them, setting the sweep to 0.1 microseconds/div (or 1 microsecond per full frame of 10 divisions). Counted the waves by hand to make sure the 2nd-hand Knight generator was dead on. 21.25 Mhz where needed, 23.5, 25.3, 22.0.

Counting waves isn't really that an accurate way to measure frequency. But it does sound like all the circuits fundamentally are working, but at a somewhat wrong frequency.

Quote:

Originally Posted by kbmuri

Then I used probe B to measure voltage deflection at test points. Had to build a detector probe per the Sams schematic and a resistor-capacitor filter test point and an insulated tube shield with test-clip loop (for injecting signals inductively into the 1st tuner tube). Quite a time-consuming affair.
...

After removing the filter test point and reassembling channel 13 of the turret tuner and replacing the tube shield, I re-applied VCR signal. Lost all my sound now, and made crap out of my crystal-clear picture.

How exhausting is that?

Been there at times... VCRs usually create a signal on either channels 3 or 4. I presume you didn't try to use channel 13 (I've made similar silly mistakes myself... :-) )

kbmuri · #22 06-17-2007, 07:53 PM

I wish I had a professional's lab, but for most things what I have is adequate. This is the first job I've done that's required some real precision. I admit counting waves on my scope assumes that when the scope says it's sweeping at 1 mHz, and I count exactly 22 peaks from leftmost grid to rightmost grid, that my generator is broadcasting at exactly 22Mhz. A leap of faith, no doubt. So ok, I probably perfectly aligned the set to the wrong specs. This is a learning deal for me, this one.

I've got this coming in the mail next week: http://cgi.ebay.com/ws/eBayISAPI.dll...m=170122318458 which may or may not be calibrated either so I guess I still need some absolute reference. But at least it'll be another opinion. The price was very right, and nixie tubes are cool, but I may want to look into buying something newer and more likely to be using modern technology. Anybody out there have a strong preference/recommedation?

Sorry, I guess I didn't mention the the Sams alignment procedure called for removing the channel 13 turret coils and then tuning to channel 13, to disable the local oscillator "to prevent erroneous indications". Reinstalling them was the last step after the IF alignment procedure. Of course I tuned back to channel 3 to view the VCR output.

I'll give it another shot before parting it out...

wajobu · #23 06-17-2007, 08:03 PM

The images in the first post are very, VERY cool

kbmuri · #24 06-17-2007, 09:13 PM

Yeah, and I got there after only one evening. This audio thing has been nothing short of miserable, and it's weeks later. And right now, even the cool images in post 1 are gone. Like I said, this one's giving some tough lessons. If I beat it, I'll be more of a "pro" for later on. Worthwhile in that respect, but I wish the dam thing would go away.

wa2ise · #25 06-18-2007, 10:00 PM

Quote:

Originally Posted by kbmuri

I've got this coming in the mail next week: http://cgi.ebay.com/ws/eBayISAPI.dll...m=170122318458 which may or may not be calibrated either so I guess I still need some absolute reference. But at least it'll be another opinion. The price was very right, and nixie tubes are cool, but I may want to look into buying something newer and more likely to be using modern technology. Anybody out there have a strong preference/recommedation?

Sorry, I guess I didn't mention the the Sams alignment procedure called for removing the channel 13 turret coils and then tuning to channel 13, to disable the local oscillator "to prevent erroneous indications". Reinstalling them was the last step after the IF alignment procedure. Of course I tuned back to channel 3 to view the VCR output.

I'll give it another shot before parting it out...

A way to test the counter is to get a crystal oscillator using a known frequency crystal (like a color subcarrier crystal, 3.579545MHz). And see what the counter says. But counters are a bit fussy to work with, they need a minimum level to count accurately. Another and easier way is to connect your signal generator to an antenna and the counter, mess with the levels so you get a reading that looks about what you'd expect. Set the frequency of the generator to match that of a local mid signal strength AM radio station of known carrier frequency and zero beat the signal generator as heard on an Am radio in the room. And see what the counter says. AM radio station carriers are quite precise to something like 0.1 part per million. If it yields a correct (or very close) number, it should give accurate readings for the alignment frequencies for the TV work.

Please don't part the TV out, someone here at AK will probably be interested in it. As it looks like all the major hard to find replacements for parts look to be working well (the CRT, flyback, yoke, power transformer and such).

kbmuri · #26 06-19-2007, 03:06 PM

It isn't mine to part out. A friend was very excited to have found it for 50 dollars at a flea market in Shipshewana, Indiana. She knew I had some experience in repairing them, so I said ok. It would have scored me some points to get it back to her in 2 or 3 days. As it is, I'm starting to look bad.

I bought these items on Monday. I should have them by Friday.
http://cgi.ebay.com/ws/eBayISAPI.dll...m=200119280910
http://cgi.ebay.com/ws/eBayISAPI.dll...m=200061373963

I'll put the Heathkit online too, so between the generator, counter, and Heathkit counter, I'll have 3 opinions of my signal frequency. If they all match, hooray.

I'll zero-beat 1190 kHz (WOWO Fort Wayne) for an absolute reference. I would like to try to do the same for WWV Fort Collins Shortwave at 20.00 mHz because that's a lot nearer the frequencies I'm targeting. 20.00 doesn't come in too often around here though. 10.00 often. I think WWV also transmits some reference audio, accurate to some parts per billion. It'll be a fun "science project" late-night insomniac event.

Just glad I'm not being paid by the hour. I'd have been fired some time ago.

kbmuri · #27 06-25-2007, 10:49 AM

Got my test gear. This is a cool photo. Both counters are extremely accurate. Was able to zero-beat WWV on the digital radio at 15mHz (20mHz doesn't come in often). Both counters were dead on. Also did WOWO 1190 Khz, same results. The radio's top shortwave frequency is 21.75 mHz but no broadcast there. Still was able to tune everything to that and tone-modulate the generator and hear it there on the radio and see 21.750 on the green counter. The Heatkit scrambles its brain around 20.000 so I suspect that's its advertized maximum. So I'm glad I got the heatkit for show, for cheap, and the new one for doing useful work. Even if I can't fix this audio problem, it will be nice to know how to maximize performance on my other sets. Fun stuff.

kbmuri · #28 07-01-2007, 11:00 PM

First thing I did was check my 'scope against the new tools. At 15.00 MHz and zero-beating WWV, my 'scope set at 0.1 microsec/div, expecting 15.0 sine waves to be displayed from left grid to right, I got 16-1/2 instead. At 25 MHz I had 27-1/2. So ok the scope is out of calibration and the previous IF alignment was a waste of time. It's a Tektronix 465B that I paid pretty good money for & works perfectly otherwise. I'll probably get the motivation to recalibrate it sometime. Now that I have a good reference.

I repeated the Admiral's IF alignment procedure at the accurate frequencies. Got the crystal-clear picture back. Audio is dead silence again. Right back to post-recap square-one.

I snagged an apparently-working replacement Sencore PS148 so I can get my sweep setup going. http://cgi.ebay.com/ws/eBayISAPI.dll...m=250136626040

Hopefully the sound isn't making it out of the tuner, somehow. Next week maybe...

wa2ise · #29 07-02-2007, 12:47 AM

Quote:

Originally Posted by kbmuri

Hopefully the sound isn't making it out of the tuner, somehow. Next week maybe...

To get a rough idea of where the bad part of the sound IF chain might be:
Do you get a fair amount of "FM hiss", like that you hear on an FM radio tuned between stations? If I recall correctly (IIRC) you were getting some detected audio, but very weak. If the sound IF is operational but on a wrong frequency, you would be able to get good sound but lously or no picture, or good video and no sound. Do you get much FM hiss with the TV tuned to an empty channel? If not, there may be a not working late sound IF stage that's losing you the sound signal strength. Or a weak link from the TV tuner to the sound IF, as you mentioned.

With your generator set to the proper sound IF frequency, do the alignment. But you should be able to set the output of the generator level to lower levels as you step thru the IF from the detector to the tuner. (you'd do this by injecting the generator output, via a small capacitor like 100pF @ 300V or higher rating, at various points (tube grids, tube plates) in the sound IF stages. You should be able to make it to the plate circuit of the tuner mixer tube (thru a capacitor) with a very low output setting on the generator. If you lose it, or have to jack up the level, you've found a problem area.

kbmuri · #30 07-10-2007, 07:59 PM

No hiss at all on empty channels. I'm sure it's down there with the rest of the missing audio. I only got weak audio by drastically misadjusting a couple of the tuning slugs at the very end of the IF strip (nearest the 6AL5). I have no audio at all, presently.

I don't know if the new signal generator is modulating FM. One would think that would matter. I'll play with it some more and find out.