This Howard 270
http://www.nostalgiaair.org/PagesByModel/229/M0009229.pdf
was playing perfectly after re-capping and before cleaning which consisted of blowing off dust removing rust by sanding and cleaning then some washing off chassis and components very carefully with warm soapy water and drying overnight.
Getting it all back together (removed and replaced the tuning condenser and speaker) went easily and the shortwave band works fine... strong clear signals... I can see the SW osc running fine on the scope..but when switching to BC band then that BC osc is dead. I removed the leads from the coil and checked continuity on both primary and secondary of that osc coil shown by #201 in lower left area of schematic. When the sw is in BC position the B+ makes it through the primary to pin 4 of the 6a7 okay. The two trimmers are not shorted... but my scope shows no oscillations.
Any ideas?
Gotta be something simple.. but after 5 hours I cannot find it.

Hi Peter

You are doing the right things. Quite often coils open. Even windings which don't have high current. Be sure the secondary isn't open.

Normally short wave, being a higher frequency, will stop operating before broadcast band.

There are only a few components and 3 tube pins (4, 5 & 6) used in the oscillator circuit. Try increasing the 50 pf coupling cap to your 6A7. Has the grid resistor gone up in value?

Norm

:This Howard 270
:http://www.nostalgiaair.org/PagesByModel/229/M0009229.pdf
:was playing perfectly after re-capping and before cleaning which consisted of blowing off dust removing rust by sanding and cleaning then some washing off chassis and components very carefully with warm soapy water and drying overnight.
:Getting it all back together (removed and replaced the tuning condenser and speaker) went easily and the shortwave band works fine... strong clear signals... I can see the SW osc running fine on the scope..but when switching to BC band then that BC osc is dead. I removed the leads from the coil and checked continuity on both primary and secondary of that osc coil shown by #201 in lower left area of schematic. When the sw is in BC position the B+ makes it through the primary to pin 4 of the 6a7 okay. The two trimmers are not shorted... but my scope shows no oscillations.
:Any ideas?
:Gotta be something simple.. but after 5 hours I cannot find it.

You may not be dry enough. Heat oven to 150 and turn off place radio inside. Try agian tommorow.

Well so far still no success.
I did let it dry off in the oven exactly as you stated setting the oven first to 150 then turning it off.
Now it's been another 12 hours too. I agree that there may stil be something wet but what? And what could still be wet if the SW osc is working.
The only components otherwise there is the BC coil itself, the trimmer and the padder... everything else there is shared with the SW band too...no?

Norm:
I measured the grid resistor as 44k anyway I changed it with a 39k.
The coupling cap was increased to 120pf now.... still works fine for SW
I've tried checking the padder cap. I don't know what capacitance it is ... shown on schematic as #49-262
It is mounted on a hunk of ceramic block about 1 by 1/2 inch.. that mounts on the rear of the chassis w/an access hole. I examined it carefully. I unsoldered both leads and removed it. It has many plates and sheets of mica... maybe 6 or 8 or more.
I backed off the trim screw till the plates were well open and used some canned-air to blow of any possible moisture that might have collected in there. Only saw a few small flakes of mica blow away out of there... so I didn't blow too hard for fear of blowing the mica sheets all apart.
Does anyone know what the value of this padder is supposed to be? I perhaps can try a fixed substitute as a test.
The other trimmer is also in good looking shape. But what is that value?

I re-ohmed the coil and measue 2 ohms on the primary and 3 ohms on the 2ndary.

I'm really kinda confused here because it's such a simple thing yet it wont run.
H E L P!!!

Broadcast band needs more capacity than short wave. If the coupling cap is low in value this could stop the broadcast band.

Norm

:Well so far still no success.
:I did let it dry off in the oven exactly as you stated setting the oven first to 150 then turning it off.
:Now it's been another 12 hours too. I agree that there may stil be something wet but what? And what could still be wet if the SW osc is working.
:The only components otherwise there is the BC coil itself, the trimmer and the padder... everything else there is shared with the SW band too...no?
:
:Norm:
:I measured the grid resistor as 44k anyway I changed it with a 39k.
:The coupling cap was increased to 120pf now.... still works fine for SW
:I've tried checking the padder cap. I don't know what capacitance it is ... shown on schematic as #49-262
:It is mounted on a hunk of ceramic block about 1 by 1/2 inch.. that mounts on the rear of the chassis w/an access hole. I examined it carefully. I unsoldered both leads and removed it. It has many plates and sheets of mica... maybe 6 or 8 or more.
:I backed off the trim screw till the plates were well open and used some canned-air to blow of any possible moisture that might have collected in there. Only saw a few small flakes of mica blow away out of there... so I didn't blow too hard for fear of blowing the mica sheets all apart.
:Does anyone know what the value of this padder is supposed to be? I perhaps can try a fixed substitute as a test.
:The other trimmer is also in good looking shape. But what is that value?
:
:I re-ohmed the coil and measue 2 ohms on the primary and 3 ohms on the 2ndary.
:
:I'm really kinda confused here because it's such a simple thing yet it wont run.
:H E L P!!!

OK Norm I hear you but I increased it from 50 to 120 pf already... shall I go bigger yet... and how big is TOO much?

Alright!!! it's finally running... but not certain why exactly.

I decided to take the bull by the horns and local ize the problem. The only two things different in the BC osc are the trimmer and padder... everthing else is common.
So I swapped the trimmer from the BC osc to the SW osc and it works fine there!.. so okay it isn't that.
So put it back... After a little web search I determined that the padder might have a max of 500pf... not sure but sounded reasonable. So I decided to try something fixed in the middle of that range. I put in a Silver mica 220pf cap...and Voila!!... it started working. I increeased it to 300pf and it got a little stronger. I tried tweaking the trimmer and it peaked perfectly right at the original indicator marks I had put on there before troubleshooting.
So it seems like the culprit was the padder. I examined it and couldn't find anything wrong again. So I just snugged it down and then backed off a turn or so.
When I re-soldered it into the circuit again... all was well. It's running fine. So then I returned the 50pf coupling cap again and that too is fine!!... So what was it?
Who knows?..
Perhaps when it wasn't running initially I may have tweaked the padder out of range and then couldn't find the "sweet spot" again.... not sure.
...but right now I'm all smiles for a while... because I really love this radio so much. The tone and senistivity are so nice. And the dial is so smooth and glides along silently between stations.
This was from the days when Howard and others were doing some nice serious radio design work I guess ... way before the AA5 boom.
I've just put some time into refinishing the cabinet too... though not finished yet.
So when done I'll post some picts.

Thanks all for the helpful hints.

Alright!!! it's finally running... but not certain why exactly.

I decided to take the "bull by the horns" and localize the problem. The only two things different in the BC osc are the trimmer and padder... everthing else is common.
So I swapped the trimmer from the BC osc to the SW osc and it works fine there!.. so okay it isn't that.
So put it back... After a little web search I determined that the padder might have a max of 500pf... not sure but sounded reasonable. So I decided to try something fixed in the middle of that range. I put in a Silver mica 220pf cap...and Voila!!... it started working. I increeased it to 300pf and it got a little stronger. I tried tweaking the trimmer and it peaked perfectly right at the original indicator marks I had put on there before troubleshooting.
So it seems like the culprit was the padder. I examined it and couldn't find anything wrong again. So I just snugged it down and then backed off a turn or so.
When I re-soldered it into the circuit again... all was well. It's running fine. So then I returned the 50pf coupling cap again and that too is fine!!... So what was it?
Who knows?..
Perhaps when it wasn't running initially I may have tweaked the padder out of range and then couldn't find the "sweet spot" again.... not sure.
...but right now I'm all smiles for a while... because I really love this radio so much. The tone and senistivity are so nice. And the dial is so smooth and glides along silently between stations.
This was from the days when Howard and others were doing some nice serious radio design work I guess ... way before the AA5 boom.
I've just put some time into refinishing the cabinet too... though not finished yet.
So when done I'll post some picts.

Thanks all for the helpful hints.

Sorry about these 3 repeated posts.. It's not my fault...
I kept getting a server error or something... so each re-try .. was posting anyway...
sorry

I thought you were starting to repeat yourself. Good job on the radio.
MRO

I thought you were starting to repeat yourself. Good job on the radio.
MRO

Padders can be crap sometimes. Sometimes, too, they can short out. Taking them apart and cleaning them can help, though if you got the set working, leave it alone. I've worked on a set with such sensitive padders that it was rediculous.

The biggest problem I've had with oscillators was the tube. I've had much trouble with my Zenith 5-G-500. For a while I had to decrease the value of the screen grid resistor in order to get the tube to function at all. It was all because those 1LA6 tubes are so sensitive even when they test good on a tube tester.

Thomas

Pete, if the padders are mounted on a ceramic block (I'm assuming unglazed) that ceramic could absorb a lot of water and would not dry out for some time. Is it possible the wet ceramic caused a chenge in the capacitance? Pat

Y'know I was kinda suspicious of the same thing Pat.
Because it wouldn't run... then after 2 days it ran...
however it had been in warm oven 12hrs 1st night too. then two days in open air after that. but...Then I had tweaked it off so.. couldn't find it again... but weird... because as soon as I substituted a fixed silver mica cap that worked... right then I switched back to the padder and tweaked a little bit and it ran!
So maybe both.
The wet idea does make the most sense since it's the last thing before it stopped...
So maybe that was 1st reason.. and 2nd reason was I had it tweaked out too far maybe also.
Smart thinking Pat.

Water can stay between the mica layers. Water also stays in fibre coil forms and switch forms and cloth wiring.

Thomas