OK, Time for Many Admiral 30A1 Questions

[Tom Albrecht]

Originally Posted by Kevin Kuehn

Howdy, and welcome.

I added the below blanking circuit to my 20A1 chassis about 15 years ago. Only one capacitor and a resistor, it really works well on my set. This is right out of Milton S. Kiver's-Television and F-M Receiver Servicing (great book by the way). The circuit shown is almost identical to your set. Hopefully you can see it well enough here, otherwise pm me and I can email a full size scan.

Nice to see that circuit in Kiver. I've been installing that kind of circuit in many of the TVs I restore lately to tame the retrace line problem. Each set is a little different, so you have to experiment a little to find out what works best. A few sets don't lend themselves to this kind of fix -- the available blanking signal with the right polarity and amplitude can't be found.

My copy of Kiver is the 1948 edition, which does not seem to have the figure you show.

[Kevin Kuehn]

My Kiver is copyright 1953. Here's an alternate circuit they show on the next page. But I'm pretty sure the first one is correct for the 30A1 Admiral.

[Adam]

That second circuit up there ^ is basically what I did on my 20B1. I understand why you need a positive pulse if you connect to the CRT grid, and negative pulse on the cathode. But I notice that on the first page of that book posted, it says also to connect to whichever (grid or cathode) the video does not. I think I've had it work both ways (connected to the same place as the video, or the brightness control), so what is the reason for this?

[UberVacTuber]

Originally Posted by Kevin Kuehn

My Kiver is copyright 1953. Here's an alternate circuit they show on the next page. But I'm pretty sure the first one is correct for the 30A1 Admiral.

Hmm... There are general similarities, but there's a lot of extra stuff around the Vert. oscillator:



Also, the grid and the plate in the CRT are connected to the brightness control.. (the video goes to pin #2) So, how would the blanking circuit be connected here?



In other news, I tried changing the tap on the damping resistor to see if it improved the linearity at all. I didn't see any change, so I went to reconnect it to the middle tap. Before I did, while the wire was disconnected I took a resistance measurement. My meter indicated that the resistor was open! WTF? I checked between the upper taps and got the specified 500 ohms each, or 1K together. However, when I tried to measure across the main windings (5.3K), nothing. When I really wiggled the connection around I could get a reading, but it was extremely high (like over 2 meg!)- very wierd. The schematic showing the resistor is here:



I'm wondering if this may have something to do with my width and linearity problems...

[Kevin Kuehn]

Originally Posted by UberVacTuber

Also, the grid and the plate in the CRT are connected to the brightness control.. (the video goes to pin #2) So, how would the blanking circuit be connected here?

Sorry, my mistake. I thought your set had video going to the cathode like my
20A1 chassis. Yours has the cathode going to the brightness control (pin 11). I get these Admiral model and chassis numbers all mixed up.

It looks like you need to do similar to the second circuit shown in Kiver. I don't think I have the schematic for your set here. Possibly Tom or someone can help shed some light on the correct point to pick off the blanking signal for your set.

Originally Posted by UberVacTuber

I'm wondering if this may have something to do with my width and linearity problems...

Really good chance.

[Kevin Kuehn]

Originally Posted by Adam

That second circuit up there ^ is basically what I did on my 20B1. I understand why you need a positive pulse if you connect to the CRT grid, and negative pulse on the cathode. But I notice that on the first page of that book posted, it says also to connect to whichever (grid or cathode) the video does not. I think I've had it work both ways (connected to the same place as the video, or the brightness control), so what is the reason for this?

I'm not sure, but I suspect they're worried about the added capacitance messing with the higher frequency video signals.

I was able to download the 30A1 schematic from the ETF website. It looks like the 6K6 vertical output is transformer coupled to the vertical yoke windings, rather than directly coupled using an autoformer. So as the book points out, that ends up making the pulse negative on the yoke side of the transformer. I’m not sure where the proper place is to pick off the positive going blanking pulse on this set. Possibly directly off the plate of the 6K6 vertical output, but it may need to be divided down to a lower level. I’m just not experienced enough to say exactly what will work.

[UberVacTuber]

Originally Posted by UberVacTuber

I'm wondering if this may have something to do with my width and linearity problems...

Yep, That was it. I replaced the combination wirewound damping resistor with a 5.8K (had to wire 2 in series to get the right resistance) and now I have a more reasonable width. I might need to boost the resistance up a bit more, as I'm backed out most of the way on the width coil, but I'll measure the raster and see how it fits to the tube mask. That should pretty much take care of the major issues now: