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

[Ray Fantini]

On the BC-348R that’s the old design with separate oscillator and mixer tubes, they feed the output of the oscillator into the mixer tube on the cathode. That’s a low impedance circuit so no worries there. The Q,N and J all use a more modern pentagrid converter tube that’s both oscillator and mixer and more prone to loading but think the old school R is not that delicate!
But I can always be wrong, my wife will tell you about how often I am wrong. I think observing with a scope will tell you a lot about what’s going on and what happens before the signal disappears.

Ray F/KA3EKH



From: milsurplus-bounces at mailman.qth.net <milsurplus-bounces at mailman.qth.net> On Behalf Of Mark K3MSB
Sent: Friday, August 22, 2025 1:47 PM
To: hwhall at aol.com <hwhall at compuserve.com>
Cc: milsurplus at mailman.qth.net
Subject: Re: [Milsurplus] BC-348R Oscillator Issue output Frequency is 0 for part of band 4 : 6 - 9.5 MC


You can load down an oscillator real fast by a direct connection with a probe.   I use this when adjusting oscillators....

[cid:image001.png at 01DC136F.9D0154C0]

.... attached to either a scope or frequency counter.    I have a few of them with different loop sizes and number of loops.

73 Mark K3MSB

On Thu, Aug 21, 2025 at 11:24 PM hwhall--- via Milsurplus <milsurplus at mailman.qth.net<mailto:milsurplus at mailman.qth.net>> wrote:
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<mailto: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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