Last view of the boards shown below... http://72.52.250.47/images/R4B-5.jpg
Paul, W9AC ----- Original Message ----- From: Paul Christensen To: drakelist@zerobeat.net Sent: Saturday, March 26, 2011 5:27 PM Subject: [Drakelist] R-4B Modifications I just completed my R-4B modifications. For those not previously tuned in, I was interested in adding the Sherwood R-4C power supply and audio amp mod to the R-4B. The stock high-voltage dividers, power transformer, and audio output tube were all contributing far too much heat for my comfort level although I'm sure thousands of R-4Bs have been running just fine. A side benefit to the heat reduction is less change of temperature to the PTO as the bulk of generated heat exists very close to the PTO. I could have just as easily installed a small fan but this seemed like a more efficient alternative. When I started, I thought this would be a "slam dunk" project. It wasn't -- far from it. The changes needed result in extensive circuit changes to the R-4B. The Sherwood PS board requires a low voltage AC source but neither the 6V nor 12V filament supplies will work for several reasons, one of which is that neither will produce enough voltage to feed the Sherwood board's CT full-wave rectifier and 7812 regulator. The 7812 requires at least 3V of in/out differential and then one must add 1 volt for "brown-out" conditions. A bridge rectifier wouldn't be enough without a "kludgey" doubler added. I explored several alternatives, including finding another off-the shelf transformer but nothing would fit within the space allowed on the chassis. Finally, I ended up replacing the R-4B power transformer with a NOS R-4C transformer. Oh, wait a minute -- the R-4B uses a mix of 6V and 12V tubes. The R-4C only uses 6v tubes from a single filament secondary. Luckily, I was able to find all 6V equivalents for five 12V tubes. For example, the 12BA6 is now a 6BA6, a 12AX7 is now a 6AX7. I could have left the 12AX7 as Garey, K4OAH pointed out to me, the 12AX7 is designed with a center-tapped filament. So, a change at the tube socket world have worked but I did find a 6AX7 and left the socket alone. http://72.52.250.47/images/R4B-1.jpg This worked well since the secondary winding needed for the Sherwood board was now there and the R-4B would now have only 6V tubes of the exact same performance parameters as their 12v counter-parts. The R-4C transformer is not a direct replacement. Mounting centers are slightly different -- enough that I had to drill two new chassis holes. With the transformer installed, it was time to install the Sherwood PS Board and new audio amp: http://72.52.250.47/images/R4B-3.jpg It took several iterations to get these boards to fit in a way that would minimize wiring and stay out of harms way of B+, the headphone jack, and other existing hardware. The next photo shows changes to the transformer wiring: http://72.52.250.47/images/R4B-4.jpg Note in the photos that with all this work being done, I felt it a good idea to change the main power supply filter cap . I replaced it with a direct replacement from HayseedHamfest.com -- and all paper caps were changed to metalized Polypropylene "yellow jackets." Wiring isn't final and several solder connections need TLC. Also, anti-vox is not yet re-connected to the headphone jack, but that's the last item on the list. Another potentially troublesome issue goes back to the voltage dividers for low-voltage biasing/switching of three circuits in the R-4B. By replacing the heat-generating voltage-dividers, a common 7812 regulator now feeds these circuits but care must be taken to ensure ample isolation. Each circuit comes right back to the 7812 -- no daisy-chains and generous use of 100 ohm series isolation resistors with film bypass caps were used on these lines. So far, so good. Hard to believe, but after this much wiring change, it all worked the first time. I was prepared to spend a full day troubleshooting. Instead, I was listening to QSOs in five minutes! That's a first for me and is probably the result of taking about six weeks in time bits to methodically go through each and every connection -- then check and cross-check against the schematic. The R-4B now runs incredibly cool now -- the same as a Sherwood modified R-4C which isn't too surprising. Audio from the Sherwood AMP-4 board is strong without hiss or other artifacts. Low-end response was extended by changing C187 by a factor of 10X. This cap is just ahead of the AF volume control. In its stock form with 0.1 uF looking into approx. 2.5K (depending on the wiper arm position), the - 3dB point calculates to 650 Hz. This number is confirmed by measurement and is the primary reason why the R-4B sounds a bit thin on the low end. Was it all worth it? For me, I suppose it was. I detest cabinet heat. However, had I known all the complicating issues and extensive re-wiring needed, I would not have done this since the thought was the Sherwood mod would have been "plug 'n play." This said, I do believe this is the 'right" way of distributing low voltages in receivers. In fairness to Drake, compact ICs like the 7812 and LM383 were not yet developed in the 1960s. On the other hand, in the absence of the 7812, perhaps a 12V Zener and pass transistor (e.g., 2N3055) could have been used to better regulate the R-4B's low voltage circuits from a dedicated transformer secondary like that implemented in the R-4C. Paul, W9AC ------------------------------------------------------------------------------ _______________________________________________ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
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