From: [email protected] [mailto:[email protected]] On Behalf Of Doug Crompton WA3DSP
Sent: Tuesday, March 21, 2023 11:28 PM
To: [email protected]
Subject: Re: [DRAKE-RADIO] R4C power supply question

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Rob,

Yes I completely rebuilt the supply adding a 7812 regulator board which are dirt cheap on Amazon and elsewhere. I also added an LM380 audio board. again, dirt cheap, and everything runs from regulated 12V. I use 3 diodes in series to lower the low voltage to the 7812 to about 14.5-15 volts and the 7812 loafs along.

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Reply:? I never used your diode idea, and at least the way I used a copper strap from the 7812 to the chassis, 17 volts into the regulator was a non-issue.? I used the LM-383T which puts out more power than the LM-380, but 100mW produces plenty of audio with a typical high efficiency speaker. I also used the LM-383T in my SE-3 AM sync detector, but running on 18 volts instead of 12 volts.

My R4C never had the C201 as far as I remember although I ripped quite a bit off the power supply board. There is no heat there anymore. I do have some very low level hum that is coming from somewhere ahead of the volume pot. It is absolutely clean with the pot disconnected. It is very low level, but being a purist, I was looking for possible fixes.

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Reply:? My engineer carefully specified how my LM-383T audio amp board had to have a single ground return. Otherwise there would be a ground-loop hum problem.? That is likely what you are observing.?

I wonder if others have measured filament voltage. I used several digital meters and they all agreed that the filaments were running at about 7.0-7.1 volts. This is with a 121 volt line. I figured I needed about .3 ohms to lower it so I wound about 6 feet of #30 enameled wire on a high value 2W resistor which dropped it about .5 volts or so.

Reply:? Yes the typical filament voltage is slightly higher than +10% above nominal 6.3 volts on most of the hundreds of R-4Cs that came through my shop.? I recently was dealing with an R-4C that took 5 minutes to warm up, and the filament voltage was 6.1 volts.? I never came up with a solution to the slow warmup.? I never found tube life a problem with the elevated filament voltage.?

I was also wondering when the LM380 first came out. The earliest data I can find is an app note dated 1972, so it had to be early in the R4C production, My R4C is SN 28387 which is very close to the end of production in 1976 or later so it certainly could have had a much better power supply and audio amp at that late date.? Think of the space that would have been saved as most of the powers supply and audio boards would have gone away, I guess by 1976 they were working on the TR7 design which used the newer chips.

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Reply:? Drake is notorious for using big dropping resistors in products.? Look at the L-4B power supply and all the heat in those large resistors.? The TR-7 wasn’t any better with all the heat in regulators sinked to the chassis near the PTO.? Also why put the 100 watt PA right by the PTO??? Why put the R-4C power transformer right behind the PTO?? The earlier Drake R-4, R-4A and R-4B receivers had the power transformer oriented 90 degrees from where it is in the R-4C.? In the R-4C case, the transformer magnetic field couples into the PTO slug-tuned inductor causing hum sidebands on the PTO output. If you operate the PTO outside the radio it has no line-related sidebands.? If you remove the R-4C power transformer, punch two new holes in the chassis, and remount the transformer 90 degrees from normal (as it is in earlier models), the line-related hum sidebands on the PTO signal are drastically lower.? Please see attached PDF from my 2012 Drake Dayton Forum presentation.

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COMMENTS:? Consider the parts count in the TR-7 DR-7 board with all those TTL chips.? The parts count on that board, and likely many others, could have been drastically reduced, decreasing assembly time and cost.? The change in the R-4C from 6HS6 mixer tubes to 6EJ7s, and the subsequent circuit change for LO injection, was a terrible step backwards.? Any RFI noise on the AC line passed right into the grid of the 3^rd mixer.? That didn’t happen with the 6HS6 circuit. ?Drake did well for decades in spite of design errors and the lack of production updates that would have saved money and made the products better. I wonder if the culture at Drake had been more proactive whether the TR-8 would? have had a chance in the market?? David Assaf W5XU is the TR-8 guru, but I don’t know whether a schematic even exists for the TR-8. ?Was it designed well?? I? have no idea. ?????

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73, Rob, NC0B

Doug, WA3DSP