2021 ~ Fine Wine and Seasoned Women get better with age -- so do the Drake Twins
The Drake A Line: R4A and T4X
June 13th ~ The Mystery is Still Mysterious!
Yesterday the R4A was working for 4 hours and then suddenly the problem reappeared. This is awful --- so I turned the radio downside upside and with an insulated stick poked something (I don't know exactly what) and it has been working ever since.
When the shoemaker was in there with his soldering iron -- do we have a bad solder joint that turns intermittent? I did reheat many joints that looked pretty bad; but may have missed one or two. So It is now working but I cannot definitively say this was the problem and here was the solution.
I also had some reports that even though the R4A and the T4X are slaved to one VFO that there may be a difference in frequencies. I am using a cable kit made for connecting the two units.
Now one report I am discounting and it sort of "pissed me off"! The Drake dial gradations are 1 kHz on the tuning knob and you could come visually close to reading 0.5 kHz; but anything else is a crap shoot.
So this ham yesterday on 40M was complaining that I was off frequency by 100 Hz. What a lid! Then he says he was not familiar with Drake equipment and that I should recalibrate the display so it was on (his) frequency.
Someone needs to put a straight pin in his coax and may his antenna get struck by lightning!
Out of not so good things, good things do happen and the R4A/T4X sojourn is no exception. I had to learn a lot more about the R4A than I had originally anticipated.
An early 1960's design -- it is clever and innovative. Drake had an excellent engineering staff -- and the follow on TR-7 is a benchmark of engineering excellence.
I am surprised at the stability of the Drake PTO. Yes you heard me utter those words, But I have been thinking about how to retrofit a Si5351 in there somewhere.
Hope to use this pair next weekend for a back yard field day. How cool would that be?
The Problem may have been found = C107.
I had it running for 5 hours with the AGC fully functional and it did not drop out. There does not seem to be good reason why C107 -- But disconnecting it made it work. If it would short upon failure then there would be no AGC voltage and it should not work with the AGC OFF. But if it were open it should work as I simply lifted one end of C107 and it works. I will replace C107 tomorrow. For good measure I did replace C108.
This saga may continue; but for now it has worked for 5 hours with no hiccups. I did retouch a lot of the solder joints that had been touched by the "shoemaker". So maybe that was a part of the solution as well.
More Mysterious Stuff ~ 6/11/2021
Spoiler Alert --it is not C108!
So after fiddling all day yesterday there was no music at the end of the day. Had the garage door open as it was a pleasant day, yet I had no pleasant results. I got so frustrated that I hauled out my Fender Strat and played a few bars wherein one of the neighbor ladies who was walking by stopped and applauded and said you are a man of many talents. That only made things worse! Dumb me, why didn't I start playing the guitar in High School (alternative chick magnet)? Moreover, troubleshooting talents were definitely not working today.
When all else fails read the manual. So I pawed through the pile of papers I printed off and read about the AVC system
Q4 shown as a 2N3858; but in other Drake schematics is a 2N3877, is the AVC workhorse. The other key pieces are V5 (part of the IF Amp) and C162 (220PF). Signals are sampled from the Plate of V5 through the 220 PF and sent to the base of Q4. That all looks pretty standard. Yet another connection to V5 is through R23 (330K) from the Grid circuit now connects to the Collector of Q4. This also is TP2.
Supposedly you should see in normal operation about -1.35 volts at TP2 with the RF Gain at wide open. With the RF gain minimum, TP2 jumps to -25 volts. With the AGC OFF you see about -1.5 volts and -25 Volts. When the AGC is working you see -3.5 Volts and - 22 Volts for the Fast and -4.3 Volts and -22 Volts for the Slow. These I think should all be the same -- so maybe a clue. When the Slow Fast does not work you see -20 Volts at TP2 (cutoff) no matter where the RF gain control is situated.
With the AGC OFF or when the AGC Fast/Slow is working you do see signals on the Base of Q4 coming from V5. When the AGC is not working no signals to the base. Ostensibly this is because upstream stages are at cut-off so nothing on the Plate of V5.
Hold it guys -- how can you have a minus voltage on the Collector of Q4 --this is an NPN transistor. But wait there is a -75 VDC being pumped into the Emitter through R127 which is a 33K resistor. So could this be a case that even though there is a Minus Voltage on the Collector there is also a more Negative voltage on the Emitter so that the Collector is more Positive (relative) to the Emitter? Drake always did strange things and so this will require a bit more measurement and evaluation on my part.
When the AGC is OFF essentially R82 is shorted out. But a voltage divider consisting of R123 (22K) connected to R111 (10K) in series with R110 (220) and R87 (1K) where the other end is either connected to R86 (12K) or Ground via the Mute RCA Connector. Essentially R123 is connected at one end to -75 volts and the other end to a string of resistors. At minimum gain the RF gain pot removes a portion of the voltage divider and so the Negative Voltage at TP2 rises cutting off the receiver. R86 plays a role in discharging C108 or C109 as a part of the Fast/Slow AGC.
The official Drake explanation is as follows:
The AVC Amplifier Q4 is biased past cut-off with no signal input to the receiver. The base of Q4 is coupled to the plate of V5 (2nd IF) through Capacitors C162 and C161 [Note C161 on my schematic is for the AM Detector and so when you head off to AM, C161 provides for an AM Time Constant as the SLOW/FAST AGC is not in the loop.]
As the amplitude of the signal on the plate of V5 increases a point is reached at which collector current will flow in Q4, during a portion of each cycle resulting in an amplified negative voltage across R83 (2.2M).
When the SSB/CW switch (AGC Positions) is in the FAST AGC Position the voltage across R83 will charge through Capacitor C108 ( 0.001). Capacitor C108 tends to discharge through R83 during the portion of the cycle which Q4 is cut-off. The time constant for Fast is 25 milliseconds.
In the Slow AGC position, the circuit functions the same as for Fast AVC but Capacitor C109 is added to the charging circuit which now makes the time constant 750 milliseconds.
The Grids of V1, V4, V5 and V9 are connected to the AVC voltage. CCW rotation of the RF Gain control increases the negative bias on the AVC controlled tubes thus limiting their maximum gain. Isolation of the mute line from ground swings the bias to - 30 volts cutting off the AVC Controlled tubes. The Minimum bias of the AVC Controlled tubes is set by R87 (1K) RF Sensitivity control.
Given the above explanation I am at a loss as to why the AGC Slow/Fast will suddenly go bonkers. All I know is that with time (and heat) the AGC Fast/Slow control goes to -20 V and the radio stops working. But put the AVC to OFF and it works. So the issue is limited to everything and anything in the AGC Loop.
Noteworthy C108 is in play for either Fast or Slow AGC -- if that were to open or short then R83 would be impacted. Seems like a place to start looking.
Updated Mystery Sojourn 6/10/2021
The good news I have been in contact with a Drake Guru and he has been most helpful. He supplied me a schematic for my version. (I found out there are at least 5 versions of R4 R4A) and they are all different. He also suggested some troubleshooting tests.
I did follow those tests but nothing has shown up. But the Good News I now know that Q6 is the 50 kHz BFO and Q5 is the AM detector.
I removed Q4 which it turns out was a 2N2222 but not the 12AV6 nor the 2N3877 or 2N3858 as shown on the schematic versions. I replaced the 2N2222 with a good one and when it works that transistor works OK.
A friend suggested that when the circuit dies because of heat, that I hit it with the cold spray to identify which is the heat sensitive component -- I expended half a can and did not identify anything.
But it definitely is a heat issue given that the garage was cold this morning and it worked for about 45 minutes before crapping out. That is when I applied the cold spray. Nada!
I removed the 2N2222 and find that the rig will work in any of the AGC positions and that is a clue --all paths lead through Q4. True when moved through those positions it is simply seeing that the AGC is OFF but that also says that something in the switching is not the main issue -- it is the transistor and the circuits around it.
I say this because when it stops working the TP2 reading is -20 VDC which essentially cuts off several tubes. When it is working the AGC off is - 1.6 volts but the Fast / Slow are around - 3 VDC. So even when it is working there is something amiss as I was led to believe that the TP2 voltage should all be about that same level.
Against the advice of KB1GMX I am planning on completely rebuilding Circuit Board 11 as the parts were cheap -- about $10 max for all new parts. Then if the problem persists I know it is an issue elsewhere.
Enter the mystery part of this project!
So Ok I have the Twins operating and have made contacts; but there is always the head scratching mystery. I think I purposefully find these really dirt cheap rigs because others are smarter than me and see there likely will be issues and they steer clear. In one of the videos I mention something about the AGC.
So let me elaborate the problem. Turn on the R4A from a cold start and all works OK and then after about a hour you find that when the AGC control is on Fast or Slow that the R4A mutes and nothing is heard; BUT with the AGC Off --the R4A works normally. Now how is that for a mystery? Let it cool down and the cycle repeats itself.
But that is not the full and complete mystery. So let us start with some background stuff.
- The R4A I have is S/N 5XXXR somewhere between an R4A and R4B in the design
- The R4A uses a 12AV6 for the AGC and the R4B uses a 2N3877. Mine has the 2N3877
- My AGC Circuit Board looks like the R4B board.(It is Circuit Board #11)
- When using with the T4X there is a Mute Cable Connection and the Mute input is connected to part of the AGC. When not using the T4X you must install a shorting plug.
- Inserting the shorting plug and having no connection to the T4X does not resolve the problem
- The issue appears to be focused on V5, Q4 and the Mute connection. But as I found out with a Ten Tec Rig sometimes the problem is entirely somewhere else.
- The "duh" factor it must be heat related.
- I hate it when prior owners are "shoemaker's" trying to perform delicate electronics work using improper tools. Burnt wiring, odd hanging components and spliced wiring that has been welded into the circuit. Shoemaker was a derisive term I learned from my grandfather who being a craftsman would often described such poor workmanship in that manner.
So I need to devise some sort of cogent plan to track, trace and identify likely suspects and then fix the problem.
The Drake Twins -- having fun with the past!
This gear was acquired dirt cheap and still has plenty of miles left on the chassis. Minimum work was required to get this far. Will be doing some on the air testing and that may uncover some problems not seen so far. But I am excited with the possibilities.
Documented 1st QSO's with the A Line!