Hi Dennis:
Yes, we are on the same page. The B and A I have are identical in the
detector stages. The collector and Base bias network is as you
describe. Dirt simple.
Tom suggested I try to put the GP replacement into one of the less
critcal stages and use the original 3394 in the detector. I did that
and it all works just fine. The 3393 that I had on hand is a bit higher
in Hfe so your diagnosis is right on.
Just a curious situation in such a simple circuit. It is all so much fun!
Trying to leave my original one-owner R4A un-modified but will be adding
a full-wave AM detector to the B.
All is right with the world.
Thanks All
Curt
On 2/27/2013 12:30 PM, Dennis Monticelli wrote:
Curt,
I only have the schematic for my R4-B but that should be close enough.
The feedback connection I was seeing includes the C179 on the base
which would represent the dominant pole in the collector-base neg
feedback path that appears to exist on the schematic. But in looking
more closely at the S2 wafer switch contact arrangement, it now
appears that a fixed bias current is applied to the junction of R116,
R117, and R118 when in AM mode. If so then that junction becomes
biased to a voltage that represents the power supply for Q5 where R117
is the collector load and R118 is the base current bias. The ratio of
these R's is 100. This means a beta greater than 100 would
theoretically saturate Q5. The 2N3394 carries a spec of beta that is
55 min and 110 max (the 3394 is a factory selection from the wider
range of beta that comes straight out of fabrication) so it looks like
Drake was flirting with the edge of disaster in that design. They may
have even done a manual beta selection among their transistor stock
because there were other places in the receiver that could accept the
higher beta 3394's that were culled. Your replacement transistor
probably has a beta >100 as most modern types do. My recommendation
is to replace the 2.2M with a higher value or maybe better yet put one
of those tiny PCB trim pots in series (500K) and adjust it so that the
collector voltage sits at a comfortable bias point.
Dennis AE6C
On Wed, Feb 27, 2013 at 7:48 AM, Curt Nixon <cptc...@flash.net
<mailto:cptc...@flash.net>> wrote:
Hi Dennis:
Yep...the DC values do not suggest saturation. Is the feedback
you refer to the base bypass cap? I thought that to be a part of
the detector (the charge cap). I guess I dont see anything else
on my schematic that looks to be feedback. Straight voltage
dividing for bias and collector, just the diode in series with the
base source.
Pretty sure I have tried all variants of the pinouts and checked
the devices on the Huntron to be sure.
I'm really betting on the Gain-Bandwidth issue now. Old
devices/vs new ones.
Curt
On 2/26/2013 9:31 PM, Dennis Monticelli wrote:
The saturation you are seeing should not be happening based upon
simple DC specs. Either the pinout is not correct as has been
suggested or perhaps the new transistor is oscillating due to a
higher gain-bandwidth product. The circuit does use a feedback
connection.
Dennis AE6C
On Tue, Feb 26, 2013 at 12:06 PM, Tom Holmes <thol...@woh.rr.com
<mailto:thol...@woh.rr.com>> wrote:
:-).
I wonder if there is an asterisk on the schematic next to
some bias resistor
that says hand chosen.
Oh well. Have fun!
Tom Holmes, N8ZM
Tipp City, OH
EM79
> -----Original Message-----
> From: drakelist-boun...@zerobeat.net
<mailto:drakelist-boun...@zerobeat.net>
[mailto:drakelist-boun...@zerobeat.net
<mailto:drakelist-boun...@zerobeat.net>]
> On Behalf Of Curt Nixon
> Sent: Tuesday, February 26, 2013 12:30 PM
> To: drakelist@zerobeat.net <mailto:drakelist@zerobeat.net>
> Subject: Re: [Drakelist] R4A-B Detector Amp Q5
>
> Hi Tom:
>
> The collector goes to hard saturation value. Less than
.2V. I did the
test using a
> Huntron on a few of the swaps just to be sure.
>
> I also carefully watched the base voltage established thru
the detector
diode. It
> stays right around .5-.6V. Even repalced the diode to see
if that might
be the
> case but same result.
>
> At this point, I guess I am going to try to find a genuine
3394 and call
it a day. It
> all works fine when I put in a working device from another
> R4 so suspect something particular about the device.
Transistors were a
lot less
> controlled in 1968 so it may not be close. Dont have a
curve tracer and
not going
> to remove it again to do an Hfe test on it the hard way.
>
> It is just a curiosity now. I taught solid-state design
and theory in
Navy and later in
> college and thought I had seen most issues. ;)
>
> Curt
> KU8L
>
> On 2/26/2013 12:02 PM, Tom Holmes wrote:
> > Well, it was worth a shot. Since I don't have the circuit
in front of
> > me I can't make a more educated guess.
> >
> > Since you caught the lead issue, I'll assume that you
also did the
> > diode test on the replacement parts. I have seen a few
cases where the
> > NTE doc's are wrong about the leads though. When the
collector voltage
> > goes to near zero, is it .2 V or .6 V? The first case is
a saturated
> > transistor; the second is a diode junction, which would
suggest the
pinout info is
> wrong.
> >
> > When I get back from some errands, I'll look in my NTE
book to see if
> > I can find any other clues.
> >
> > Happy hunting!
> >
> > Tom Holmes, N8ZM
> > Tipp City, OH
> > EM79
> >
> >
> >> -----Original Message-----
> >> From: drakelist-boun...@zerobeat.net
<mailto:drakelist-boun...@zerobeat.net>
> > [mailto:drakelist-boun...@zerobeat.net
<mailto:drakelist-boun...@zerobeat.net>]
> >> On Behalf Of Curt Nixon
> >> Sent: Tuesday, February 26, 2013 11:43 AM
> >> To: drakelist@zerobeat.net <mailto:drakelist@zerobeat.net>
> >> Subject: Re: [Drakelist] R4A-B Detector Amp Q5
> >>
> >> Hi Tom:
> >>
> >> Yes...there are the lead arrangement issues but I
accounted for them.
> >> The typical EBC Vs ECB issue. Easy in this case because
they used
> >> the triangular hole pattern instead of the inline
pattern on both the
> >> R4A and
> > B version
> >> modules.
> >>
> >> Curt
> >>
> >> On 2/26/2013 10:49 AM, Tom Holmes wrote:
> >>> HI Curt..
> >>>
> >>> It almost sounds like there is a different lead
arrangement on the
> >>> 3393. Any well designed circuit of that era would have
had to
> >>> tolerate the typical high variability of Hfe to avoid
tedious hand
> >>> picking of parts, although that may have been done in
this case.
> >>>
> >>> Tom Holmes, N8ZM
> >>> Tipp City, OH
> >>> EM79
> >>>
> >>>
> >>>> -----Original Message-----
> >>>> From: drakelist-boun...@zerobeat.net
<mailto:drakelist-boun...@zerobeat.net>
> >>> [mailto:drakelist-boun...@zerobeat.net
<mailto:drakelist-boun...@zerobeat.net>]
> >>>> On Behalf Of Curt Nixon
> >>>> Sent: Tuesday, February 26, 2013 10:40 AM
> >>>> To: Drake Forum
> >>>> Subject: [Drakelist] R4A-B Detector Amp Q5
> >>>>
> >>>> GM All:
> >>>>
> >>>> Has anyone else had trouble getting a general purpose
sub working
> >>>> in the
> >>> Q5 AM
> >>>> detector amp position?
> >>>>
> >>>> I tried several close NTE GP subs and also a 3393
which is same
> >>>> parameters
> >>> ex
> >>>> Hfe which is slightly different.
> >>>>
> >>>> The transistor comes on but with the grounded emitter,
pulls the
> >>>> collector
> >>> voltage
> >>>> to near zero. As soon as I put in a "real" orignal
> >>>> 3394 from a R4A, it works as it should--good fidelity
and collector
> >>> voltage at about
> >>>> 5V from the supply rail of 10V.
> >>>>
> >>>> Is this design so sensitive to Hfe as to be marginal
or need to be
> >>>> hand
> >>> selected?
> >>>> Certainly the 3393 is well within the spec range of
the 3394.
> >>>>
> >>>> Thanks
> >>>>
> >>>> Curt
> >>>> KU8L
> >>>>
> >>>> _______________________________________________
> >>>> Drakelist mailing list
> >>>> Drakelist@zerobeat.net <mailto:Drakelist@zerobeat.net>
> >>>> http://mailman.zerobeat.net/mailman/listinfo/drakelist
> >>>
> >>> _______________________________________________
> >>> Drakelist mailing list
> >>> Drakelist@zerobeat.net <mailto:Drakelist@zerobeat.net>
> >>> http://mailman.zerobeat.net/mailman/listinfo/drakelist
> >>>
> >> _______________________________________________
> >> Drakelist mailing list
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