[morzh]

Well, the reason was as I expected it a stupid one: when I reconnected the tuning cap, I soldered back the pentagrid's cap but forgot the antenna coil.
I noticed that when I connected the generator - it was coming through but extremely weak....

Well, it sings where it's supposed to...mostly, and the alignment seems to be where it should be, with a single problem: at the Short Wave band when I use the generator the beep disappears right about between 8MHz and 7.5MHz. I knew the tuning cap is OK as at the BC band it works in the same position.

So I theorized the oscillator is not oscillating starting about 7.5MHz and connected the scope to the IF coil output...I was correct, the oscillation sharply decreases at about 8.5 and fully disappears after 8MHz.

6A7 is my pentagrid. Since it oscillates otherwise I think the coils are OK (I think I checked the continuity but will check again just for s..ts and goggles).
The pentagrid was the single weaker tube: the pentode section is supposed to test at about 1000uMho and it gives me about 700. Although I don't see how that can kill the oscillation.

Have to see if there are old micas there...not sure they can do that though.

Anyone has experience with something like that - part of the band is bad? The other part is OK and is very well aligned.

Oh...hadn't checked the voltages yet. Have to do that to see if the regimes are OK.

[morzh]

Checked the voltages - seem OK. Not right on the money but then I am using a variac plus a modern Fluke DMM, plus my speaker's field coil is 500 Ohm and not 1.5kOhm, so.....I don't sweat it, they are within the ballpark.

All coils are OK and the DC resistances are within reason to what the schematic names for them.

The 6A7 would be the easy usual suspect except somehow I feel that when I buy a new one it will behave the same. OK, will spend 10 bucks......
Sadly none of my other radios has a 6A7 in it; I do have a 6A8 but the pinout is different.

Oh, I even pulled a couple of stations around 10 MHz on the FBC, and one around 6.5MHz (where the oscillation does not work) which tells me one of the tubes was acting as an attenna all on its own.

[morzh]

Some photos of the chassis.

   
   
   
   
   


As you can see I tried to preserve conspicuous Philco parts. Resistors are the only parts I usually don't get too finicky about.

[Ron Ramirez]

Had a problem recently with my Philco 650. When I turned the set on, it would not play after warm-up until I moved the band switch to LW, then back to AM. Then it played fine.

Oscillator trouble, right?

Starting with the easiest thing to try, I pulled the 6A7 and put in another.

Problem solved.

Morzh, great job restuffing those caps.

[Radioroslyn]

Nice job!!! Everything looks as it should!!
Terry

[morzh]

yeah, I need to go to Michaels or Arts and Crafts and get me a colored hot glue. The light brown one. Then the ends will look better.


Ron, yes, I just have to buy another 6A7. I am watching the auctions. Don't want to pay 20 bucks for NOS. I'm cheap.

[BrendaAnnD]

I'll look this evening to see if I have a 6A7G in my stock. I know I have metal ones, but don't think those will be suitable for your purpose.

[morzh]

Brenda,

Thanks, appreciate the offer; it may or may not be as if the GND case pin is used for something else as a convenience mount point then i would have to reshuffle stuff around a bit to accommodate the "G" type, otherwise it should work OK.


But let me try this auction today first. A shouldered type also will not look out of place as it is the original type used.
Will see tonite.

[Ron Ramirez]

The 6A7 was only available in the ST shape, as far as I know. Being a standard base (7 pin) tube, it was not made in metal.

Brenda, you may be thinking of the 6A8 - electrically identical to the 6A7, only with an octal base?

[morzh]

I do have 6A8 (bought one - it was used in the Westinghouse I fixed for my Old Man) and it so happened the old one was working jusr fine despite it being a bit weak, but the pinout is very different though functionally they are close. I thought of that but then looked at the pinout and it made me sad

[morzh]

Oh! Ron, I may know what Brenda is talking about!

There IS a metal 6A7. But it is a Russian pentagrid tube, and is NOT pin-compatible with American 6A7.
This may confuse some falks - they are sold same way, as 6A7 on auctions.

It has that characteristic "ruffling" on top.

http://www.radiomuseum.org/tubes/tube_6a7~2.html

[morzh]

Won that auction, was the only bidder, NOS 6A7 $1.99 plus 3.50 shipping. if it works and is indeed NOS - very good, I see them at 15 bucks.

[morzh]

Antenna: does anyone know how it really connected for this 38/39 series? I have a pair of cut wires in not so good shape.

[morzh]

Well, I was right about the pentagrid.
I received the one that I bought on the eBay.
First I repeated the experiment opnce more where I could not get the generator tone below 7MHz.
Then I replaced the pentagrid and I was able to receive the tone on the correct frequencies all the way down to 5MHz.
Scope also showed the oscilaltion all the way down, though really small aplitude.

I was also able to receive some stations down in that region. More than one, I think three.
Interesting is that with the old pentagrid, when I put it back, I was able to hear one of thopse stations. Also interesting that at least in some of the "dead" region I was able to receive the tone but instead opf going from the 7MHz down on the generator I would have to go in the opposite direction.

Last thing is, the tube was listed as NOS. It did not come with the original packaging.
The chart for Hickok shows 1000 for pentode and 300 for oscillator.
This tube shows 700 and 330.
My old tube shows 750 and 350.

So I wonder: is this thing (although working) really NOS?

Does anyone have data for 6A7 or 6A8? (they are more or less the same for Hickok measurements).


UPDATE:

I went back to the Hickok and checked the new tube on English: it is right in between Bad and Good. on the Question mark.
I also found a 6A8 that I replaced (it was working though) because id did show about 650 and it was also borderline bad on English.

Also by looking at the tube I think the soldering on top the Grid cap has been redone and then polished off - it looks as if it was smoothed with grit paper.

Well, maybe for 5 bucks it is not a bad tube but then the auction was for an NOS.

Let me talk to this guy first.

[Mondial]

On my Hickok 539B, a new 6A7 or 6A8 will read above 1400 for the mixer and above 500 for the osc, so it seems your NOS tube may actually be used.

Do you have a good 6A8 to compare with? 6A7 and 6A8 are electrically identical as to readings.