EB5AGV's TR-4C repair notes

Keep them glowing!

Here you have some notes I wrote about the TR-4C transceiver repair.


A new puppy arrived yesterday to my shack. It is a TR-4C, with AC-4/MS-4. I got it in Spain, so I saved lots of the usual shipping charges. The price was also right. I got a shock when I discovered that it HAS the 34-PNB installed :-)!. It has also the two optional 10m crystals. So I think it has all the possible internal accesories, am I right?.

The TR-4C was very dirty internally, but cleaned up very well and chassis is in an 8.5+ condition. I got also a manual copy (in spanish) and some spare tubes, INCLUDING a final set. The front panel is about an 8, with a couple of small scratches. The knobs are all in very good shape. Only thing I don't like of this unit is that cabinet was repainted in an slightly bright black. I'll probably repaint it in the future. The AC-4/MS-4 is in very good shape also.

About the installed PA tubes, there is one curious thing: the tubes are NOT really 6JB6A, although they are marked as 6JB6A... A mistery?. Well, the tubes are another sweep tube (marked in the glass) with an small 'pinout changer' fiber glass PCB in the base... I suspect some tube manufacturer discovered a treasure in those old sweep tube boxes he had some years just getting dust... Perhaps is time to find some of these unmodified tubes, develop an small PCB and use them for our lovely Drakes!. BTW the tube structure looks exactly the same as a 6JB6A so it should be only a matter of pinout.

Well, I'll continue working on my unit (now it is disassembled in lots of parts, as I enjoy cleaning the rigs the most I can) and will let you know what I find.


The 'converted' 6JB6A tubes are marked in the glass as 6JM6. The manufacturer and origin are 'National Electronics, Made in USA'. I'll report the output power and such when I check it.


Some time ago I sent a message to the List about a TR-4C I had got, which had some PA 'converted' tubes instead of the classic 6JB6A. These tubes were originally 6JM6 (duodecar 12 pin base) but have been 'converted' to 6JB6A (novar 9 pin base) by means of an small fiberglass PCB circuit. Well, I've checked my trusty RC-25 RCA manual and also the converted tubes, to find that the electrical ratings are almost the same and the pin connections to convert them are as follows:

6JB6A - 6JM6
1 - 3
2 - 5
3 - 2
4 - 12
5 - 1
6 - 5
7 - 3
8 - 4
9 - Not connected

So it is possible to use 6JM6 tubes instead of 6JB6A!. I have no idea, though, if these tubes (6JM6) are easier to find than 6JB6A but, at least, it is another possibility we have to maintain our nice Drake gear working :-)!.

By the way, power output with these tubes is similar to what I get with 'real' 6JB6A.


I have done my first alignment of a TR-4 transceiver, exactly a TR-4C. It has been a rewarding experience :-)!. I've gone through all the alignment steps in the manual, and have found interesting things:

-The S-meter was a bit low on readings. Now it reads fine.

-IF gain was way out of alignment, so I have now LOTS more gain, specially noticeable in higher bands. I was a bit discouraged with the performance on 15 and 10, but, after the alignment, it is very near the performance of my R-4C.

-All the RF and MIXER stages were a bit out of adjustment, although performance has not apparently improved.

-The 9.0 MHz oscillator was also bad aligned and this generated different (and a bit distorted) recovered audio for USB and LSB. It is very easy to align (only C130 is involved) and it is a welcomed performance change!.

-PA tubes NEEDED neutralization. Now I can adjust them in 10 meters without esporadic oscillations and the minimum plate current means really maximum power output ;-)

Summing up, now I have a very capable transceiver, with very good transmitted audio and good recovered audio (I prefer the sound of the R-4C). And all of this in only about three hours of work. I highly recommend to perform the alignment, if you suspect your TR-4 is not working fine.

Only thing still doesn't work is the relative power indicator. When I press the LOAD button, I don't get any reading, so I do the PA adjustments with my trusty Bird 43 wattmeter. I have checked the switch and it works, so I suspect of the rectifier diodes... Is there any common failure I should know about?.


In my last posting, I commented a problem I still had in my recently aligned TR-4C:

Only thing still doesn't work is the relative power indicator. When I press the LOAD button, I don't get any reading, so I do the PA adjustments with my trusty Bird 43 wattmeter. I have checked the switch and it works, so I suspect of the rectifier diodes... Is there any common failure I should know about?.

BINGO!!!. Now I have it working fine :-) :-)!.

As I suspected, one of the diodes in the power detector circuit was bad; it had developed a short. The culprit was D9. This diode in my TR-4C was a 1N270 and, as usual (Murphy Law #13), I had none around... But in the TR-4C schematic I have, it is a 1N4148, so I found a couple of them in my diode box and replaced D9 and D11 (also a 1N270) with 1N4148s. It has not been an easy task, as the small PCB (the same which has the plug-in relay) is not easily removed: you should unsolder some 'big' wires in a very tighly packaged zone. But, anyway, I'm very happy with the results!. Now I get a nice relative power reading, without looking at the external wattmeter. Yahoo!!!.

Hope this helps to someone else, as I got some messages from other Drake users with similar troubles. By the way, you can CHECK the diodes removing only the plug-in relay. But if you need to change any of them, you need to suffer as I did ;-).

Best regards from a happy camper!.


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