[radiorich]

Hello Mr Fixr,
You are right it is a model 7H820
Sincerely Richard

[462ron]

Here is my Zenith 7H920, similar to your H725. It is a great sounding set with more volume than I would expect from a SE output design. I tried doing a sweep alignment of the FM section but could never get the marker blip (10.7Mhz) to show up to set the discriminator properly. You can see the results in my post about Eico 368 Sweep generator in the “Test Equipment” section of the phorum. I was able to get aligned on the money using the crystal marker only and it sounds great. When you get to aligning yours it should be doable with a non-sweep generator. Looking forward to your progress!

Ron    

[Paul Philco322]

Yes Mr Fixr you are right on local hardware stores I much prefer. I have a few good ones. I'll even buy paint there. I'm still looking for the 10 dollar a gallon exterior at Sears?. The speaker here is almost an eight inches, the selenium rectifier right under speaker is weird, but that's a packed chassis. The Foster Seeley FM circuit is used here I think. Once done they are really nice.

[EdHolland]

Alignment will have to be the simple method here as I don't have much test equipment. When I did the Zenith AM/FM console I had no problems and it worked out well.

Paper caps to sort out first!

[radiorich]

hello Ed,
I am looking forward seeing the progress on your Radio.

Ron , It looks like your model only has two bands Mine is the model with 3 bands .

Sincerely Richard

[462ron]

Yup, the current FM band and that’s fine, less complicated to work on. This is the only Zenith I have other than a TO and I am impressed with it, great performer!

Ron

[radiorich]

Hello Ron,
I bet with they good size speaker and well design circuit .
One of my best sounding table models is my Philco 610b .

sincerely Richard

[EdHolland]

Lunchtime today brings me to about 1/4 through the paper cap replacements, starting at the RF end. Everything seems to be in hard to access locations, and the old solder is not terribly cooperative, but we're getting there.

I also sorted out the tuning mechanism. Everything had become extremely stiff, and even after cleaning and lubrication it was impossible to get a good drive from the cord. The tuning cap itself was very stiff - I read about a similar problem on one of these sets. With no bearing adjustment, the only recourse was to "ease" the frame ends apart very gently, so I tried the same. This did the trick - although I disliked taking such drastic action. It now works properly through the range, and has what I would describe as the right feel. The cord for the dial pointer has also been replaced.

Hoping to get the caps done today, and give the set another test on dim-bulb power.

[EdHolland]

Cap replacement is completed. (Excepting PSU filters for now). A cautious power up got the expected behaviour of the dim bulb, but atrocious FM reception and nothing at all on AM. I looked around the areas I had worked, and eventually found some components disturbed near the last IF can for AM.... I'd pushed them causing leads of these components to short out the IF transformer Fortunately no lasting damage done, and rearranging the cap and resistor positions brought everything back to life.

[radiorich]

Hello Ed,
That is wonderful news and far as dim bulb tester I need to build a new one .
You, are lucky that nothing got damaged !

Sincerely Richard

[EdHolland]

Well something isn't right, there's a lot of crackling, particularly on AM, but it seems also to affect FM to an extent. It is most noticable when tuned into a station, and only just discernable when tuned to a quiet spot. It sounds like the descriptions I have read of silver mica disease - static crashes - and it seems to get worse as the set is run for longer periods.

I'll have to pull one of the IF cans and take a look. Meanwhile I'm thinking about the best way to track down which of the transformers is causing the problem, presuming for the moment that it is just one of them misbehaving - I'd like to get that one first, just to be sure I'm on the right track, and to minimise the risk of causing damage.

One thought is to inject 455 kHz starting at the detector and then working back up the IF chain. If necessary, the upstream stages could be disconnected to prevent noise coming through.

Any thoughts from the experts would be welcome at this point - this is not something I've encountered previously.

[Arran]

Ed;
Before getting into the IF cans I try to make sure that the tube sockets, and tube pins are clean, those 7 and 9 pin miniatures are as bad as loctals for experiencing intermittent contact this way, particularly on cheap sockets. Incidentally this is part of why I wash most of my chassis down with a solution of TSP and hot water, it does a good job of not only removing dirt but oxidation, particularly from aluminum (or as you say aluminium) parts, it seems to prevent a lot of intermittent contact issues that way. Brand Z transformers do experience silver migration as well, but the construction is different from the Automatic style cans, and others, the mica sheets are not buried in the bases of the IF cans, and I believe that brand Z sometimes also gave the values of the caps, but maybe not this time.
Regards
Arran

[EdHolland]

Arran - thanks, I'll chase those easier things first. The sockets and pins were dirty, and there was a bit of crackling at first that was provoked by wiggling the tubes, but it seemed to have settled down after a couple of removal and replacements (tubes out when working on the chassis etc... Still worth a shot. Ditto another freshening up of the bandswitch.

Alas, there is no value given for the IF caps, but I did find this thread:

https://antiqueradios.com/forums/viewtopic.php?t=395731

And it may have some helpful info. Need to check that the transformer part numbers match what I have. There seem to be a number of chassis variants. I finally found a very feint stamp on this one declaring 7K01. Some of the sets have AM and FM transformers combined in one can (as does my console), this set has separate cans for each band, 7 in all, including the discriminator!

Cheers

Ed