How to troubleshoot IF strip

[miniman82]

So the problem with the 21-CT-55 ends up being something amiss with the IF. I was able to figure out that the video looks great when injecting a composite signal, but it's all jumbled with RF. I put the B&K 415 on it, and sure enough the response curve is all jacked. Question is, why? I have a hard time believing something just went out, and I certainly don't want to go messing with the tuning slugs if there's a part that did fail. In my experience IF's are pretty simple, so I need some guidance in how to systematically go through it stage by stage.

Right now I'm providing RF to the antenna terminals on channel 4, picking up the response at the video detector. Is it possible to inject a signal by using the mod marker function, then pick up the signal with the demod probe?

[Steve McVoy]

A very simple test to tell where you may have a problem is to check the voltages on each IF tube. In particular, look at the cathode voltage. If it is right, it is a good indication that the circuit is working.

[Mal Fuller]

It will not be an alignment issue.

[old_coot88]

Instead of first going "down the throat" of the tuner, i would loose-couple the genny (at the IF frequency) to the mixer/osc tube. Just tape a turn or two of wire around the top of the tube (or slip an ungrounded tube shield over it). Clip the signal lead to that and attenuate as needed. And see if the jacked condition remains. This would confirm that the problem is indeed in the IF strip and not 'upstream' of it.
You have subbed all the IF tubes and the mixer/osc tube, right?

Mal Fuller said:

It will not be an alignment issue.

Agreed 110%.

[old_coot88]

Just a thought- could there be a broken ground at either end of the IF coax that runs from the tuner to the chassis?

[wa2ise]

Be sure you're not, say TV on channel 3 and the signal generator on channel 4. That can make the video look like trash. Or maybe the fine tuning is way off. If you are trying to tune in the lower sideband (most generators and VCRs don't bother to filter the lower sideband) your alignment will look totally jacked.

Try a VCR in place of the generator, to be sure that it's not a fault in the generator. Or try the generator on a known good TV set.

[miniman82]

Originally Posted by Steve McVoy

A very simple test to tell where you may have a problem is to check the voltages on each IF tube. In particular, look at the cathode voltage. If it is right, it is a good indication that the circuit is working.

I haven't checked in a while, but when this first popped up I remember they were all pretty well in line with what Sams called out. Gonna recheck though, just to be sure.

You have subbed all the IF tubes and the mixer/osc tube, right?

Yep, that was the first thing I tried. It's like something decided to short all of the sudden, it wasn't one of those things where it got slowly worse. Just one day while I was watching it, the screen gets all garbage and nothing seems to make a difference. I just need to figure out a way to pin down the problem to one stage or another.

[ctc17]

You need to have the factory probes for the 415 or forget the results. How is the hump screwed up? I'm sure you checked the detector diode, I have seen the bypass caps go bad and cause really screwy bandpass.

[miniman82]

I have the factory probes, thanks to John Folsom. Also, thanks to Bob G for the 415. All I had to do was fix the 45.75 pix marker, and it was good to go.

I guess what I'm asking is how do I tell which stage is the one causing the response to get all messed up? At this point the most I've done with the 415 is align a chassis that wasn't too far off to begin with (Bob's CTC-4).

The response curve I'm seeing seems to be stretched out, like someone wrote the correct curve on a piece of rubber then pulled it lengthwise. I seem to recall my CTC-4 had almost the same thing going on, which turned out to be the tuner link comensation network. I don't think this chassis has a tuner link network, so where to go?

[ctc17]

I have A bunch of articles on using the 415. Pm me your email.

[miniman82]

You have PM, thanks!

[miniman82]

Thanks to CTC17, I have some direction now.


Here's what I did tonight:

First trace is the output from the 415 right into the scope, with markers shown by the text. I don't know why it shows up reverse of what the Sams sheets show, but it is what it is.




Next in order to check overall alignment, Sams has you inject at the mixer crystal through an 'input head' which consists of a 68-ohm resistor to ground followed by a series 1500pf cap into the crystal. I don't have that, so I made the connection directly with the 75-ohm leads and the 415 function switch on IF. Doesn't look like it's supposed to: the double humped part isn't flat topped at all, and the markers on the right hand side are not where they are supposed to be at all.



Next I set the 415 function switch to channel 4, and attached to the antenna terminals. It seems like something in the areas of 41~42Mhz isn't peaking at all, or some other strange shift in overall response has occurred. Opinions?

[ctc17]

Try this:
Push the mixer osc tube shield all the way down, get an old tube shield and stick it over the uncovered part of the tube not allowing it to ground out. The ribbed split shields work best. Connect the yellow lead to the ungrounded shield and the black to ground.
Balance the tuner between two channels.
See what you get.
I wonder if this could be a tuner problem, the middle image doesn't look that bad. The mixer plate coil has a huge effect on the curve and it would almost be nice to unplug the tuner and inject directly into the IF. Just to eliminate the tuner.

Play with the bias voltage, dont go off what the sams says.

[miniman82]

Don't think that's going to work on this set, it uses a crystal diode for a mixer. That's probably why Sams injects signals right into the diode. Also I can't eliminate the tuner, the 1st IF stage is part of it. I do have another tuner I could swap, but it's a PITA and I'm trying to avoid it.

[ctc17]

Crystal diode? Is that a fancy nqme for germanium? I remember building crystal radios that used a 1N60. This is older than the stuff I'm used to so forgive the ignorance. I would change the tuner just to eliminate
. It. It will save time in the end