Zenith IF adjustment strangeness - Printable Version
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Zenith IF adjustment strangeness - BDM - 07-26-2008
Question, I've noticed while adjusting the IFs on both my Zenith chassis 5905s, adjustment "B" of the IF never peaks with a signal generator. In fact it's strange, it actually tunes down. Meaning the lowest reading you get on the DVM is the peak. I've confirmed this several times on actual stations. I've learned to peak A C & D first, then peak B on a station. Thought I was nuts until I noticed the same thing on the 1204 chassis. Same front end but the 12 tube version. Anyone have an idea as to why? They function fine, but it's always been in the back of my head.
http://www.nostalgiaair.org/PagesByModel/503/M0025503.pdf
- exray - 07-26-2008
A guess...
Where are you placing your lead or probe? If you are using the AVC line you would want to be where there is some isolation via a resistor. The grid of the eye tube would be a good place.
Or alternatively you could simply measure via a resistor (~4.7M for example) on the tip of your probe.
- etech - 07-26-2008
Brian,
I have not noticed that on my 1204 chassis. As xray mentioned, I would question how you are measuring this. I would be curious to know if you get the same results using an analog ac voltmeter (or multimeter) across the speaker voice coil.
Ed
- BDM - 07-26-2008
Guys, I'm measuring AVC voltage off the eye tube. Can't recall the pin. Anyhow, I just got done checking my work. Sure as heck, if you peak IF adjustment "B" to it's lowest negative voltage setting, it's peaked! I know this sounds stupid, but that's what's happening. The other three, peak high(negative) as they should. Like I said. I can peak adjustments A C and D. then peak "B" on a station, and I get the same result. B is at it's lowest voltage(negative). Now, I'm injecting the signal at the IF amp. I'm not coupling in, just getting the sig gen lead next to it. Enough to pick up the signal. Strange, it's gotta be me. Ed, how are you injecting your IF signal?
- etech - 07-26-2008
I usually inject the 456kc signal at the 1st detector grid (grid cap of 6L7G). However, I get the same results by injecting at the antenna terminal for that matter. If you are looking at the avc signal on the grid of the eye tube, you could be loading the avc down. That is very high impedance at that point. I do not usually tune the radios with the avc voltage. I usually use the Tektronix 465 scope and probe the plate of the 6H6 2nd detector and peak for maximum 456kc signal. As an alternative, I probe the 6F6 plate with the scope and peak for maximum demodulated audio (1000hz tone from signal generator). The radio peaks pretty close to the same both ways. I think you are getting deceptive readings because you are loading the avc circuit with your meter. If you don't have an o'scope, put your multimeter across the speaker voice coil and peak for maximum demodulated audio that way. Let me know what happens.
Ed
- BDM - 07-27-2008
Ed, appreciate the help very much. You're right! One IF adjustment was off, way off. Using the scope with the probe connected to the 6H6 2nd detector, pin 3 plate. I was able to peak all 4 adjustments spot on. This improved dial calibration. More on that later. Bottom line, it had to be effecting the AVC as you stated. Funny, using the grid of the eye tube has been suggested many times by many folks. I'll have to check this against my Philco's, and see if I get the same results.
Again many thanks
RE: Zenith IF adjustment strangeness - Texasrocker - 07-27-2008
there is some good info posted on this same Zenith IF subject on the HVRA ( Houston Vintage Radio Association) website. ( "repair notes", link on their website!) Those old fella tube-tech radio restorers know their stuff also in Hou-Tx area!! click below to navigate their website. Those guys even restored the orig radio room of the old Battleship Texas that served in WW1 & WW2. Most probably retired NASA techs as well? ... heres the link:
http://www.hvra.org/
enjoy!!
RE: Zenith IF adjustment strangeness - BDM - 07-27-2008
Thanks Randal. Interesting tech notes there. Here are a few links to them.
http://www.hvra.org/znthshop.htm
http://www.hvra.org/polarize.htm
Also, a nice photo section
http://www.antique-radios.net/radpix2/radpix2.html
- Texasrocker - 07-27-2008
... always tryin to help if I can!!
- etech - 07-27-2008
Brian,
On the 1204 chassis, the grid of the eye tube is actually tied directly to the avc line. That is a very high resistance circuit and any metering at that point would upset the avc. On the 5905 chassis, the eye tube grid is only isolated from the main avc line by R3, a 100K resistor. That is not enough. I'm afraid metering at that point would upset the avc also. I am not an expert and I'm sure there are some sets where metering at the grid of the eye tube would not upset the avc and therefore be a valid technique. As exray suggested earlier, you might try metering through a 5 or 10 meg resistor to the grid of the eye tube. That may help. Or, you could just use the scope and be done with it.
Ed
- BDM - 07-27-2008
Ed, I'm using the scope from now on. Easy to do, more accuracy. No harm from using it. A win win situation all around.
- Doug Houston - 08-09-2008
That's a strange phenomenon, and I've run into it a few times. I use a Hickok 209(?) VTVM to read AGC voltage. I should be more suspicious about it, but I'm always in a hurry.
When these sets were new, there were no VTVM's, so they had you use an output meter on the speaker as an indicator. Tune for strongest output, and there you were. Doing that, you never had any notion of what the AGC was doing. But tou weren't putting any load on the AGC bus that way, either. The audio level wasn't a true indication of the detector's DC output, so that method isn't very good. The VTVM on the AGC has it all over the output meter.
Funny tale about one of my sets. It's an Airline (Wells-Gardner) set, about 1936 vintage. Very nice radio, as all of those W-G jobs were. Re-capped and aligned, for some reason, I accidentally shorted the AGC bus to ground. There was a small spark!
On the Hammarlund HQ 120, as I recaLL, THE S-meter is pretty sensitive, and reads AGC voltage directly off the detector. You don't see THAT, very often.
Think about it. If a set has an eye tube or a tuning meter, you have a built-in resonance meter. Just tune to max meter swing or minimum eye closure. You don't need a VTVM at all.
I once re-capped a Scott Imperial Allwave. It has 2 or 3 AGC systems in the RF-IF channel of the set. It also has possibly 3 I.F. stages. I did the alignment, and the set was so hot, you couldn't hold it down. A bit of checking, and the AGC tube for the I.F. strip was totally dead; just drawing filament current. A new one got the set going just fine.
Buit, reading the AGC bus is the best way to read your peaks, bec ause you're reading the true output of the detector. I seem to favor reading it at the AGC feed to the RF or mixer grid, and everything works just fine.
Please excuse my use of "AGC" in lieu of "AVC". It applies to radio and video either way. It saves brain wear and tear.