Just picking up static
 

Thanks for all the help so far on the old sentinel 344. I changed all the bumblebee caps out this morning before work. I got it home tonight and still no luck all I get is static. The static gets louder when I hook up the antenna. As I move the tuner I get some change in noise but no channel or anything like that. The tone circuit works as well as volume. I have changed all the caps except for one mica cap. All the tubes are good. Any other ideas on what I can check? I don't have a schematic so I am kind of flying blind.
Thanks,

If this radio is new enough (late '40's+) to use miniature IF transformers, I'd say the IF's are suffering from "silver migration disease". The solution is to either replace the IF's or rebuild them. Personally, I have never rebuilt one but I've got a couple of radios that I soon will be trying my hand at rebuilding the IF's in.

I assume the IF transformers are the square cans? There are two of them on this radio. The radio is early 50's vintage. Is there any way to test the IF's? This radio is becoming a pain especially since I can't even listen to it once I do get it fixed.

Any one have any idea if this is worth a shot?

http://www.antiqueradios.com/chrs/jo...nsformers.html

We don't know yet what the problem is before blaming the IF transformers. There are a number of things that could be wrong with it.

First: did the radio work at all before you recapped it?

I could not find a 344 schematic, found a 343 on Nostalgia Air, but am guessing this is a 5 tube AC/DC radio? It should have a loop antenna and won't work properly without the loop being connected. It won't need an external antenna, just the loop.

Next CORRECTED: This has been corrected because I stated the frequencies backwards. Test to see if your oscillator is working. Put another working AM radio tuned to a dead spot between 1400 and 1500 Kc next to your Sentinal. Turn on the Sentinal and tune it back and forth around 1000Kc. If your Sentinal oscillator is working at the proper frequency you should hear a squeal or a rushing noise from your other radio as you tune back and forth.

Come back after you try these things. You'll get it going eventually.

Reece

The radio didn't work before I recapped it. The caps were bad. All I got once the tubes warmed up was a loud 60hz hum. I gave it a try. I was getting a squeal. It wasn't very loud but I could hear it.

Just for general knowledge the tubes in the radio are 2-12ba6, 12be6, 12av6, 35w4, 35c5

If you have static, it sounds like the local oscillator is not working. Have a look around the oscillaror circuit. Especially the oscillator coil.

The test that I described using the second radio would determine if the oscillator is working, provided some previous "fixer" didn't go in and tighten down all the adjustment screws which he found "loose."

Reece

So what should I check for next now that the ocillator has been checked. I haven't tightened anything and the radio hadnt probably been opened since LBJ was in office.

Whoops! I got the frequencies backwards in my previous post. Please go back and read the revised post about how to set the two radios to test the oscillator. The oscillator will be putting out a frequency about 455Kc above the frequency of the station desired.

If it still doesn't work, try another 12BE6 tube. Or check the tube you have in another working radio. Barring all that, here is some good advice on page 5 of the attached file:

http://www.hvra.org/GridLeak/GL%20Sep%20Oct%2008-F.pdf

Reece

All my tubes have been checked. I retested it and was getting a whooshing sound as I moved the tuner back and fourth about mid range of the sentinel. It is out of the case so I am not sure the specific frequency.

About halfway down my web page http://www.geocities.com/wa2ise/radios/repair.htm
I describe how to repair such transformers.

Some Zenith radio transformers are easier to take apart. The ones that use a screw shaft and nut to do the clamping of teh mica wafer. In which case I just scrape a dividing line to break the migration path between cap sections. Then reassemble.

Thanks, I also ordered a sams off of Ebay for 3 bucks so atleat I will have a schematic.

If the "whooshing sound" you describe is coming from the second radio tuned around 1400, then it sounds like the Sentinal oscillator is working. If so, this would be a good place to use a signal tracer on the radio to be sure where the exact problem lies. A tracer can be rigged from an audio amplifier, like computer amplified speaker, with a shielded lead input terminating in a cap and germanium diode in series, for the RF portion of a radio.

Reece

Thanks, I am going to wait to get the schematic in. I am not sure how much more trouble I want to go through. I won't even be able to listen to anything on it anyway. I might try to intigrate a FM tuner into it somewhere so I can atleast listen to it. I will see if I cant figure it out.

Well I just got the sams in today for it. I ohmed out the IF transformers last night. They seemed ok. I do get a change in the static adjusting them up and down. Anyone got any ideas that don't invole me making any test equipment or spending much more money? Does this thing have any collector value.

An ohm meter won't tell you if the IF has a problem with the silver mica caps.

Did you try Reece's suggestion? Although you didn't say so explicity, it sounds like you did. If not, it's a cheap and simple test.

John

Where are the caps in the IF's I took the covers off, are they inside the coil or somewhere where you can see them. I didn't try the signal tracer.

Go back to WA2ISE's post and follow his link. He describes it and has photos. I'm assuming yours are like others that have a flat sheet of mica with silver printed on it. It is sandwiched inbetween parts of the base where you can't see it until you take it apart.

I would read his description before you try to take them apart. I'd want to make sure they were at fault first because it's a bit of a delicate operation.

John

What you have is an AM radio with a RF stage (one of the 12BA6 tubes)
for added selectivity. If what you mean is that you don't care for listening
to AM radio, this unit is not for you, definitely look for a radio designed
to receive FM. If this radio has a phono input, you could possibly connect
an FM tuner, but sound quality from the 12AV6/35C5 audio amplifier
might not do justice to the FM tuner. Also, this is a hot-chassis
design, directly connected to one side of the power line, so hooking
up an external tuner might be tricky.

If you want to get this AM radio working on its own, you will have to
get some help with alignment at this point. Disturbing the IF transformer
cores without proper equipment is definitely not a good idea. Good
luck learning about tube radios, please do get back to the forum.

Thanks for the input guys, I haven't gotten to fool with it hear lately. From the lack of responces in the pricing forum, I am going to try bypassing the if tansformers and if that doesn't work it is getting new fm guts.