Hi All, I sent my R-4C (S/N 26903, May 1977) to Rob Sherwood for alignment and installation of the 5 basic modifications on his website (filter cap kit, AMP-4, MIX-4, PD-4, R-4C power supply board). Rob found that my R-4C had issues with the OEM stock first mixer and last IF tubes. Attached are parts of the conversation, which has a few morsels of wisdom in it regarding the R-4C specifically, and receiver testing in general.
Best, Hugh WA2LXB __________________________________ WA2LXB: Regarding the first mixer tube and the last IF tube, those are the kind of failures that I would not be able to diagnose at my shack due to lack of experience and lack of test gear. For my education, would you mind briefly explaining how you diagnosed the two failed tubes? I would assume that it was a several step process, starting with the most generic observations and then proceeding to instrumented testing...again starting generically and then moving to the details. Rob's REPLY: Your radio was pretty basic from that standpoint. There are certain levels in dBm or microvolts that should make the S meter read certain values. If the preselector peak is appropriately audible, but the S meter reading is low, then total gain is suspect. In your case when I wiggled the first mixer tube, it made a lot of noise. Replacing the tube brought the gain up to nominal. The other issue was random crackling in the speaker, and at times moving the S meter off of S1. Since the MIX-4 (Sherwood mod) had eliminated the tube before the last IF tube, I decided to replace the 6BA6. This got rid of the random crackling. One can rarely detect a problem with an antenna connected since band noise is way above receiver noise. I never use a tube tester (for the R-4C), as with all Drake products, the tester is worthless. Drake uses very high grid resistor values, and any kind of grid leakage will seriously affect performance. Substitution is always needed, and hand selection, too. NOS tubes are one's best bet, and for the 6BA6 and 6BE6 I use the mil version 5749 and 5750. To test any radio requires as a minimum a 100 MHz scope and a decent signal generator. Of course any DVM is going to be adequate for most service issues. I don't use C & R boxes, but I do have the best spectrum analyzers and signal generators beyond what would be generally needed for service. All are HP and Tektronix. Testing 100 dB radios really pushes the state-of-the-art. When there are less obvious issues with the C-Line, I use a spectrum analyzer and an FET probe to follow the signal from antenna input clear to the input to the product detector. Sometimes that is the only way to troubleshoot. Message: 1 Date: Sat, 9 Jul 2016 15:10:13 -0400 From: "Mark" <mar...@cox.net> To: <firstname.lastname@example.org> Subject: Re: [Drakelist] Drakelist Digest, Vol 57, Issue 3 Hi Dan, I reached the conclusion based on two factors. The serial number of this R-4C is 28188, which according to Ron's website (www.wb4hfn.com) put its date of manufacture in November of 1977. The GE tubes, all the same, are OEM marked, with the date code 7743, made the 43rd week of 1977. All the other tubes are Sylvania, but not these mixers! 73 de NO8J -----Original Message----- From: Drakelist [mailto:drakelist-boun...@zerobeat.net] On Behalf Of drakelist-requ...@zerobeat.net Sent: Saturday, July 09, 2016 12:00 PM To: email@example.com Subject: Drakelist Digest, Vol 57, Issue 3 Today's Topics: 1. 6EJ7 Mixers - Meshed Plates - Metal PTO Gears (Mark) 2. Re: 6EJ7 Mixers - Meshed Plates - Metal PTO Gears (Dan Martin) ---------------------------------------------------------------------- Message: 1 Date: Sat, 9 Jul 2016 08:49:42 -0400 From: "Mark" <mar...@cox.net> To: <firstname.lastname@example.org> Subject: [Drakelist] 6EJ7 Mixers - Meshed Plates - Metal PTO Gears I picked up an R-4C in the flea market this year at Dayton. It was a later one, made sometime in 1977. I noticed that the mixer tubes, 6EJ7's are original and GE with date codes 1977 and were made in Great Britain. The plates on these 6ej7's are mesh, not solid. I've always thought that Drake used Sylvania tubes and that their 6EJ7's are quieter than most - less crackling noise. Did Drake try these mesh plate tubes to address the noise issue and are they any better? Finally, the PTO has metal gears on it, not molded nylon. Was that a design change? 73 NO8J Message: 2 Date: Sat, 9 Jul 2016 11:21:50 -0400 From: Dan Martin <pit...@comcast.net> To: Mark <mar...@cox.net> Cc: email@example.com Subject: Re: [Drakelist] 6EJ7 Mixers - Meshed Plates - Metal PTO Gears Message-ID: <f7ee056a-5291-4551-8c9d-581bbae44...@comcast.net> Content-Type: text/plain; charset="utf-8" Like you, I've always associated Sylvania with factory Drakes. Sure could have been exceptions, I guess. Curious how you know your GE mixers to be "original"? I understand metal gears were used in the later/last runs of 4-Line gear. My 23XXX and 25XXX 4C and 4XC, though well into the production life of the 4-Line, still have nylon gears. This link explains some of the changes and differences in the 4C over its lifetime and may prove interesting. http://www.zerobeat.net/drakelist/drakemod/drmod37.html Dan WB4GRA Sent from my iPhone _______________________________________________ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist