BC-348-Q Repairs - Looking for advice
 

I recently acquired a BC-348 Q in fairly decent shape and have set about
getting it into useable condition.

First, I found a dead tube (open filament). Since some of the others
tested weak on my tester, I collected a full complement of NOS tubes
and repopulated the set. No go.

Next, I found a wiring error made by somebody who had attempted the 28v
to 6 volt filament conversion. The error left both sides of one of the
tube's filaments hooked to A+, i.e. no way it could light up. This was
easily corrected.

Once I fixed this, I could hear stations...albeit faintly. While poking
around the 1st RF section, I found an open (literally cracked open)
plate resistor. With that replaced, the radio generates enough volume to
drive a small speaker to comfortable levels.

I cannot seem to get the BFO working, however. The resistors around the
BFO seem to check out ok. The condition of the caps are unknown at this
time, though I have not run across bad silver micas or the pF dogbones.

I know the larger uF caps are prone to dry out and deteriorate, but I
tried shunting a few of them with alligator leads and known-good units.
It didn't seem to make much difference.

What are my options? Is there a common failure mode for the BFO in this
set? Short of removing parts and testing them out-of-circuit what are my
options? Does anyone have a functional BC-348 Q who could give me
nominal voltage readings around the pins of the VT-233 that I could
compare with mine?

Side note: The crystal filter also appears not to work. Basically, it
behaves like an open...nothing gets past it, unless bypassed by the
switch. Do these particular crystals tend to deteriorate and die with
age? What do you replace it with, a crystal ground to the IF freq?

I'd be interested in corresponding with somebody who has been successful
in restoring one of these radios. My email address is

hpf atsign gainbroadband dotcom

Thanks,
Pete
AC7ZL

BC-348-Q Repairs - Looking for advice
 

H. P. Friedrichs wrote:

I cannot seem to get the BFO working, however. The resistors around the
BFO seem to check out ok. The condition of the caps are unknown at this
time, though I have not run across bad silver micas or the pF dogbones.

Congratulations on getting this far....The BC-348 is a good, classic
military radio and quite a good listener when fully restored.

You need to examine every resistor in the set. Some of them can and will
drift upward in value quite substantially as they age under the best
conditions, and most of these radios were used under conditions that are
far from the best. If any resistor measures more than about 10% off the
marked value, I would replace it.




I know the larger uF caps are prone to dry out and deteriorate, but I
tried shunting a few of them with alligator leads and known-good units.
It didn't seem to make much difference.

Shunting the caps is not a particularly effective troubleshooting
technique because most of these caps are not open, but leaky. Shunting a
good one across a leaky one will have minimal noticeable impact on
performance unless one lead of the original cap is disconnected first.

Come to think of it, you might as well replace all the paper and
electrolytic caps in this radio. Most of them are probably going to be
bad anyway.


What are my options? Is there a common failure mode for the BFO in this
set? Short of removing parts and testing them out-of-circuit what are my
options? Does anyone have a functional BC-348 Q who could give me
nominal voltage readings around the pins of the VT-233 that I could
compare with mine?


I don't have a BC-348 presently, but this site has manuals freely
available for download that will tell you practically everything you
need to know about this radio in almost nauseating detail:

http://www.jamminpower.com/main/bc348.jsp

The files there are huge, but these are the clearest BC-348 manual scans
I have ever seen...Well worth the download.


Side note: The crystal filter also appears not to work. Basically, it
behaves like an open...nothing gets past it, unless bypassed by the
switch. Do these particular crystals tend to deteriorate and die with
age? What do you replace it with, a crystal ground to the IF freq?

You will probably have to get one from a junker BC-348 chassis. IIRC,
the IF frequency of this radio is 910KC, which is somewhat unique.


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Vintage radio schematics, Binary newsgroup archives, TV Test pattern DVD
and other great radio-related stuff is just one click away at:


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BC-348-Q Repairs - Looking for advice
 

IIRC, the IF frequency of this radio is 910KC, which is somewhat unique.


"...somewhat unique."
.....a little bit pregnant.
.....almost alive.

GRRRRRR.......

Either it is, or it isn't.

.....unusual.
.....somewhat rare.
.....not typical.
.....specific to this unit.
.....hard to find.

Otherwise, the advice is directly on point.


Peter Wieck
Wyncote, PA

BC-348-Q Repairs - Looking for advice
 

On Wed, 14 Jun 2006 10:54:52 GMT, Peter Wieck wrote:

IIRC, the IF frequency of this radio is 910KC, which is somewhat unique.

"...somewhat unique."
....a little bit pregnant.
....almost alive.

GRRRRRR.......

Either it is, or it isn't.

However, "almost" doesn't mean _a_bit_; indeed, if there were only two of something, then each could be _almost_ unique.

Otherwise, the advice is directly on point.

Yep.

BC-348-Q Repairs - Looking for advice
 

H. P. Friedrichs wrote:
I recently acquired a BC-348 Q in fairly decent shape and have set about
getting it into useable condition.

First, I found a dead tube (open filament). Since some of the others
tested weak on my tester, I collected a full complement of NOS tubes
and repopulated the set. No go.

Next, I found a wiring error made by somebody who had attempted the 28v
to 6 volt filament conversion. The error left both sides of one of the
tube's filaments hooked to A+, i.e. no way it could light up. This was
easily corrected.

Once I fixed this, I could hear stations...albeit faintly. While poking
around the 1st RF section, I found an open (literally cracked open)
plate resistor. With that replaced, the radio generates enough volume to
drive a small speaker to comfortable levels.

I cannot seem to get the BFO working, however. The resistors around the
BFO seem to check out ok. The condition of the caps are unknown at this
time, though I have not run across bad silver micas or the pF dogbones.

I know the larger uF caps are prone to dry out and deteriorate, but I
tried shunting a few of them with alligator leads and known-good units.
It didn't seem to make much difference.

What are my options? Is there a common failure mode for the BFO in this
set? Short of removing parts and testing them out-of-circuit what are my
options? Does anyone have a functional BC-348 Q who could give me
nominal voltage readings around the pins of the VT-233 that I could
compare with mine?

I would like to help. I have a 348Q in quite good working shape, but I
would not be able to get into it to take measurments for about 10 days
due to family upheaval. You can download a manual from the BAMA site,
which should give voltage and resistance norms.

Side note: The crystal filter also appears not to work. Basically, it
behaves like an open...nothing gets past it, unless bypassed by the
switch. Do these particular crystals tend to deteriorate and die with
age? What do you replace it with, a crystal ground to the IF freq?

I'd be interested in corresponding with somebody who has been successful
in restoring one of these radios. My email address is

hpf atsign gainbroadband dotcom

Thanks,
Pete
AC7ZL

BC-348-Q Repairs - Looking for advice
 

Peter Wieck wrote:
IIRC, the IF frequency of this radio is 910KC, which is somewhat unique.



"...somewhat unique."
....a little bit pregnant.
....almost alive.

GRRRRRR.......

Actually a 910 khz IF makes a great deal of sense in a rig with an XTAL
filter, low frequency coverage and no broadcast band. It gives a
considerable improvement in image rejection on the higher bands. The
XTAL takes care of the selectivity lost by the higher IF.


Either it is, or it isn't.

....unusual.
....somewhat rare.
....not typical.
....specific to this unit.
....hard to find.

Otherwise, the advice is directly on point.


Peter Wieck
Wyncote, PA

BC-348-Q Repairs - Looking for advice
 

Thanks for all the thoughtful replies...it's one of the reasons that
this newsgroup is such an outstanding resource.

73
Pete
AC7ZL

Scott W. Harvey wrote:

H. P. Friedrichs wrote:

I cannot seem to get the BFO working, however. The resistors around
the BFO seem to check out ok. The condition of the caps are unknown at
this time, though I have not run across bad silver micas or the pF
dogbones.


Congratulations on getting this far....The BC-348 is a good, classic
military radio and quite a good listener when fully restored.

You need to examine every resistor in the set. Some of them can and will
drift upward in value quite substantially as they age under the best
conditions, and most of these radios were used under conditions that are
far from the best. If any resistor measures more than about 10% off the
marked value, I would replace it.




I know the larger uF caps are prone to dry out and deteriorate, but I
tried shunting a few of them with alligator leads and known-good
units. It didn't seem to make much difference.


Shunting the caps is not a particularly effective troubleshooting
technique because most of these caps are not open, but leaky. Shunting a
good one across a leaky one will have minimal noticeable impact on
performance unless one lead of the original cap is disconnected first.

Come to think of it, you might as well replace all the paper and
electrolytic caps in this radio. Most of them are probably going to be
bad anyway.


What are my options? Is there a common failure mode for the BFO in
this set? Short of removing parts and testing them out-of-circuit what
are my options? Does anyone have a functional BC-348 Q who could give
me nominal voltage readings around the pins of the VT-233 that I could
compare with mine?


I don't have a BC-348 presently, but this site has manuals freely
available for download that will tell you practically everything you
need to know about this radio in almost nauseating detail:

http://www.jamminpower.com/main/bc348.jsp

The files there are huge, but these are the clearest BC-348 manual scans
I have ever seen...Well worth the download.


Side note: The crystal filter also appears not to work. Basically, it
behaves like an open...nothing gets past it, unless bypassed by the
switch. Do these particular crystals tend to deteriorate and die with
age? What do you replace it with, a crystal ground to the IF freq?


You will probably have to get one from a junker BC-348 chassis. IIRC,
the IF frequency of this radio is 910KC, which is somewhat unique.

BC-348-Q Repairs - Looking for advice
 

H. P. Friedrichs wrote:


I cannot seem to get the BFO working, however. The resistors around the
BFO seem to check out ok. The condition of the caps are unknown at this
time, though I have not run across bad silver micas or the pF dogbones.

How did you determine that the BFO is not working? I'm thinking that
it might be working and just off frequency due to bad caps.

73, Roger

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Remember the USS Liberty (AGTR-5)
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BC-348-Q Repairs - Looking for advice
 

Roger,

I presumed it (the BFO) was not working because

a) There is no audible evidence of function, that is, it is not
producing beat notes against CW

b) I tried probing in that area with a scope and saw no evidence of
oscillation

c) I injected a sine wave from an external signal generator into that
circuit, and could then make out CW notes.

You might be right, it might be working but at the wrong frequency. On
the other hand, item (b) suggests it's not working at all.

73
Pete
AC7ZL


Roger D Johnson wrote:

H. P. Friedrichs wrote:


I cannot seem to get the BFO working, however. The resistors around
the BFO seem to check out ok. The condition of the caps are unknown at
this time, though I have not run across bad silver micas or the pF
dogbones.


How did you determine that the BFO is not working? I'm thinking that
it might be working and just off frequency due to bad caps.

73, Roger

BC-348-Q Repairs - Looking for advice
 

H. P. Friedrichs wrote:
I presumed it (the BFO) was not working because

a) There is no audible evidence of function, that is, it is not
producing beat notes against CW

b) I tried probing in that area with a scope and saw no evidence of
oscillation

c) I injected a sine wave from an external signal generator into that
circuit, and could then make out CW notes.

You might be right, it might be working but at the wrong frequency. On
the other hand, item (b) suggests it's not working at all.

What are the plate and cathode voltages on the BFO tube? I am thinking you
might have a bad resistor.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."