[Stall-U-Rated]

http://www.nostalgiaair.org/Resources/719/M0005719.htm

So i did a full recap on this radio and its  running pretty good. The original 6A7 tested marginal so i got a NOS 6A7 off ebay and the radio is not happy with it especially the SW band. Starts to "whistle" in certain areas on the SW dial..

I was going to try tuning it up so i set my Signal generator to 454khz and the tuning gang fully meshed in an attempt to peak the IF section.. I am noticing i dont hear a tone till i open the tuning gang halfway. Is this normal or is my radio way off?? 

Works pretty well in the Broadcast band hooked to a longwire.

A picture of the radio in question is located in the November finds area...

I noticed the tuneup section doesnt cover the SW alignment.. Do i have the right schematic?  The tubecount is spot on..

Thanks for your input..

Steve

[Radioroslyn]

>I am noticing i don't hear a tone till i open the tuning gang halfway.

The position of the tuning capacitor should not have any effect on the IF frequency. If you are using a service grade generator check it's frequency output w/a well calibrated receiver. If the receiver doesn't go down to 456kc listen for the generator's 2nd harmonic 912kc. You need to be sure that you are aligning the IF @ the proper frequency.

[Stall-U-Rated]

OOps, i meant 456 khz. Must have been tired last night  

According to the riders i posted, i have to inject this signal into the Morse via its ANTENNA lead with a .1 cap in line. Not through the IF tube like others do..

Heres my setup, i used the DX-390 to peak the generator to the exact frequency..

Seems like its in the ballpark, but the morse wont pick it up with the tuning gang fully meshed. I have to back the generator down to 390khz on the same scale before the Morse hears it..  

Steve

[Radioroslyn]

Hooking the the 456kc signal to the ant is fine. Set the generator for 456kc and set the modulation on. Turn the attenuator to high and adjust the IF trimmers till you hear the signal from the generator. As the signal get louder adjust the attenuator down to a minimum.
If you can't get a peak at 456kc you may have a bad IF transformer.

[Stall-U-Rated]

I was able to tune the 456kc signal, so i reccon the If transformers are fine. Starting to think this nos 6A7 may be too much for this radio. Short wave will not co operate.... Anyone have one lying around? The one i have is marginal and not much to work with as far as the top cap wire is nearly busted flush and i have a glob of solder for now..

Steve

[Eliot Ness]

For tubes I like FindAtube.com.  Bob Dobush is a good guy with good prices, although I usually try to order at least 6 to make shipping more reasonable.  eBay has some 6A7's also.

I've repaired tubes like yours before.  Solder a wire onto the stub, then use a solder sucker or wick to get the solder off the top of the tube cap to expose the hole.  Glue (super glue works well) the tube cap on with the wire sticking out of the hole.  When the glue is dry quickly fill the top of the tube cap with solder.  Be quick or you'll undo the wire that is soldered to the stub.  Try it, you've got nothing to loose and might get a good tube again.  Good luck....

   

[Stall-U-Rated]

Im surprised i even got a blob of solder to stick to the top of this tube.. I broke it flush with the nipple.. The replacement i bought off ebay tests REALLY STRONG and no shorts, but the radio cant handle it. Starting to wonder if somethings out of whack besides the caps i replaced? My DVM wont read these old resistors..

Steve

[Eliot Ness]

I'm not quite sure what you mean that your DVM won't read the old resistors.  Maybe the readings seem far off because the resistors have drifted far from their original values.

It's not unusual for some tubes to work in a circuit and other tubes, that test good, to not work.  I'd be concerned about the resistors you don't think your DVM can measure.

I'd try to get it to the original status of playing pretty good with another 6A7 and start the alignment again.  Place an ad for your tube in the classified section and I'm sure you'll get a hit.  I'd try to scare one up for you but I'm a week out of a knee replacement and can't get downstairs or drive yet.

[Stall-U-Rated]

My Digital Volt Meter doesnt have the meg range to read some resistors is what i meant to say..

The hard part for me will be translating the resistor list from the riders schematic into modern values. (im a bit rusty)

Thanks for the offer to search for a tube for me, but i agree rest your knee..  I forgot all about findatube, i think i used them to replace all the tubes in a 39 wards i did a few years back.. I may try them again.. Thanks...

Steve

[Steve Davis]

Steve, the resistors on your schematic use the old system of M = 1000. M does not = meg. 

I do not think your 6A7 is too strong, there is something else wrong, possibly a shield missing or misrouted wire. 

Steve

[Eliot Ness]

Steve, you're not the first one to be caught by the old M really means K resistor terminology.  Like Steve Davis mentions I'd check to make sure all tube shields are present and a double check of your wiring wouldn't be a bad idea either.

[Stall-U-Rated]

No worries there, it was practically unmolested (power cord changed) when i got it. All it did was hum as usual for its age. Fortunately, i had leftover new caps from the philco 16b i did prior (over ordered some).. Did them one at a time and was careful.

The only things i did off the normal, was snatch 2 line safety caps out of a directv DVR i was gutting for parts and put in a stronger 6A7 tube.. Worked great when i fired it up, just noticed SW had a squeal in part of the scale and the volume doesnt completely go silent before the switch clicks off.. Of course the SW issue clears up when i use the marginal 6A7.. Unless my tube tester isnt seeing an error in the NOS 6A7, then im presuming the radio cant handle the extra flow from a strong tube. Or i have a naughty drifted resistor fouling up the works..

Steve

[Steve Davis]

Yes, go through and check all the resistors. It sounds to me like the weak 6A7 is compensating for some other problem. 

Steve

[Stall-U-Rated]

My tubes arrived today and the used 6A7 i got performs well in the radio without that whistle. Its a philco like the one that was in there that i nipped off the tip.. The tube that was disagreeing was an RCA brand, who knows what the difference was...

So ill send it through an alignment and see how it goes..

Steve

[mikethedruid]

One thing occurs to me. You said that the Philco 6A7 tubes work well in your radio, but the RCA 6A7 does not. My question is, are the Philco tubes 6A7G, or 6A7GT type tubes, and is the RCA a plain 6A7 METAL tube? If so, that may well explain your problem. Check the wiring going to pin 1 in the chassis, not just the schematic. It should be tied to ground if the RCA tube is going to work OK, but sometimes Philco used it for a tie point for other things, since the 6A7G does not have a connection to pin 1 internally. The RCA 6A7 uses pin one to connect the metal outer case of the tube, and use it instead of a separate tube shield. Rule of thumb on Philcos, if the tube layout sticker, or the schematic call for a G type tube, use a G or GT type tube, and use the physical tube shield if there is one for that tube.