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 <[email protected]> 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 <[email protected]> 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: [email protected] >> [mailto:[email protected]] >> > On Behalf Of Curt Nixon >> > Sent: Tuesday, February 26, 2013 12:30 PM >> > To: [email protected] >> > 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: [email protected] >> > > [mailto:[email protected]] >> > >> On Behalf Of Curt Nixon >> > >> Sent: Tuesday, February 26, 2013 11:43 AM >> > >> To: [email protected] >> > >> 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: [email protected] >> > >>> [mailto:[email protected]] >> > >>>> 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 >> > >>>> [email protected] >> > >>>> http://mailman.zerobeat.net/mailman/listinfo/drakelist >> > >>> >> > >>> _______________________________________________ >> > >>> Drakelist mailing list >> > >>> [email protected] >> > >>> http://mailman.zerobeat.net/mailman/listinfo/drakelist >> > >>> >> > >> _______________________________________________ >> > >> Drakelist mailing list >> > >> [email protected] >> > >> http://mailman.zerobeat.net/mailman/listinfo/drakelist >> > > >> > > >> > > _______________________________________________ >> > > Drakelist mailing list >> > > [email protected] >> > > http://mailman.zerobeat.net/mailman/listinfo/drakelist >> > > >> > >> > _______________________________________________ >> > Drakelist mailing list >> > [email protected] >> > http://mailman.zerobeat.net/mailman/listinfo/drakelist >> >> >> >> _______________________________________________ >> Drakelist mailing list >> [email protected] >> http://mailman.zerobeat.net/mailman/listinfo/drakelist >> > > > > _______________________________________________ > Drakelist mailing > [email protected]http://mailman.zerobeat.net/mailman/listinfo/drakelist > > > _______________________________________________ > Drakelist mailing list > [email protected] > http://mailman.zerobeat.net/mailman/listinfo/drakelist > >
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