[Milsurplus] BC-348R Oscillator Issue output Frequency is 0 for part of band 4 : 6 - 9.5 MC

[jphutch60bj]

Ray,  all -

All thanks for the feed back.

Testing oscillator output using a Fluke Frequency Counter, 1953 A, 
hooked to the cathode of the 6J7, sniffing the oscillator output.

I'm just going to do brain surgery on the RF deck, anyway as it is past 
due.  It may be a weird switch issue?

Yep all the resistors that I did not replace some 20+ years ago, -ish,  
are now north of +20%  of their values.

R 56-1 is 1270+ on the ohm meter, should be no more that 1100 ohms.

Just odd that the one band, band 4,  would go dead mid band,  as all the 
other bands just trucked along...

I had checked for tin whiskers on the Tuning cap.  Shot some Deoxit on 
all the switches, and tuning cap.

Did not change, swap out,  the tube or tubes, as it is working on all 
the other bands just fine.

Just checking my six for a Got-U, as it has been a while.

Thanks for the feed back.

JPHutch



On 8/22/2025 9:18 AM, Ray Fantini via Milsurplus wrote:
> Wow, going back and pulling the schematic on the "R" makes me realize just how much I like the  "Q" version over that series. Not trying to be ignorant but you did change the tube and confirm that its not that? Otherwise it's a fairly straight Armstrong oscillator with trimers on the grid side of each coil for each band. Would assume that if its working on all the other bands and craping out on half of band 4  that the oscillator itself is somewhat good but you do want to check that your B+ to the tube itself is good, maybe R 56-1 increased in value and there is not enough B+ and it affects that one band?
> Pin 8 on the mixer is a good test point, I would hook a scope at that point and look at the amplitude on a good working band and then band 4, also may be helpful to see if the amplitude decreases as you approach center of the band or just suddenly stops at some point? Have no reason other than my own self-satisfaction but I would swap out the mixer  and see if that had an effect.
> And last but not least did you clean all the switches in the oscillator compartment? That old style has a ton of contacts to clean.
>
> Don't know if that helps or not.
>
> Ray F/KA3EKH
>
> -----Original Message-----
> From: milsurplus-bounces at mailman.qth.net <milsurplus-bounces at mailman.qth.net> On Behalf Of hwhall--- via Milsurplus
> Sent: Thursday, August 21, 2025 11:25 PM
> To: milsurplus at mailman.qth.net; jphutch60bj <jphutch60bj at gmail.com>
> Subject: Re: [Milsurplus] BC-348R Oscillator Issue output Frequency is 0 for part of band 4 : 6 - 9.5 MC
>
>
> That's an unusual approach to aligning the tuning.  Are you quite sure the freq counter isn't loading down the oscillator output & sometimes just killing it?
>
> Wayne
> WB4OGM
>
> On Thursday, August 21, 2025 at 05:47:35 PM MDT, jphutch60bj <jphutch60bj at gmail.com> wrote:
>
> Reading the Manual TO-12R2-3BC-112, Revised 15 April 1957,  Page 4-3 paragraph 'e'.
>
> On the four lower frequency tuning bands the oscillator frequency is higher than the desired signal by the intermediate frequency. On the two higher frequency ranges, bands 5 and 6, the oscillator is on the low frequency side of the desired signal. The latter results in a more uniform tuning ratio over these bands and increases the image rejection ratio.
>
> Before I start doing Brain surgery on BC-348R by pulling the RF Units .....
>
> I was Checking Oscillator output frequency with a frequency counter, tweaking the outputs from the oscillator on each band.
>
> Band 4, frequency range of : 6 - 9.5 MC,  reading the manual the Oscillator High Side output should be between 6.915 - 10.415 MC  for band 4?
>
> My Oscillator goes dead no output,  between DIAL Frequency reading of:  7.2  to 8.8 MC - which equates to an Oscillator output frequency of  "+.915"  should be between  8.115 - 9.715 MC.
>
> Issue the output Frequency goes to 0 as measured  on Pin 8, VT-91, 6J7.
>
> If the manual is not wrong,  why is the oscillator output dropping to 0, on this one band?
>
> It aint the tuning cap!-} , as there are no problems with other bands.
>
>
> Thanks for you thoughts in advance.
>
> JPHutch
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