Re: R4C power supply question
¿ªÔÆÌåÓýHi Paul, Comments inserted below. Ro ? From: [email protected] [mailto:[email protected]]
On Behalf Of Paul Christensen
Sent: Wednesday, March 22, 2023 11:27 AM To: [email protected] Subject: Re: [DRAKE-RADIO] R4C power supply question ? Rob, ? I don¡¯t recall if you and I reached the same but independent conclusions at the same time, but I discovered the power transformer issue back in 2011 when my R-4C and R-4B underwent thorough upgrades.? However, the transformer fix only affected the R-4C. ? Reply:? We must have arrived at the same conclusion and solution independently.? First I tried some mu metal shielding to no avail, and my R-4C was likely modified sometime in 2011 before my 2012 Hamvention presentation. ?The hum sidebands, solder braid and grounding solder lug improvement were in my 2010 Drake Forum presentation with attribution where appropriate.? My original R-4, when I was in high school, needed a better ground than provided by the PTO spring .? As a 16 or 17 ?year old I demonstrated the problem to Drake at the factory and was blown off as a ¡°snot nosed kid¡±.? Hi Hi? ??They replaced my PTO saying the newer production PTOs had harder ball bearings that provided a better ground.? Drake never put the flexible grounding braid on the PTO into production until late TR-7s.? ? ? ? Although the root cause is different, and as your slides show, it has the classic characteristics of so-called ¡°hum modulation,¡± essentially FM-like sidebands at 60 Hz intervals.? I first noticed it on my R-4C when listening to the crystal calibrator up at 500 kHz on the top end of the PTO.? As the PTO is adjusted toward the 500 kHz end, the ferrite core moves closer to the power transformer.? That coupled with the wrong transformer orientation creates this effect in the R-4C, sometimes even after separately grounding the PTO case. ? Reply:? No question about that. ? I think the reason this had gone unnoticed for decades is because the CW portion of the bands are at the other extreme of the PTO ferrite core position.? If the reverse were true (i.e., CW portion at the high end of the bands), no doubt some folks would have picked up on this limitation much sooner.? Hum modulation effects are much more apparent when listening in CW mode than when listening to SSB transmissions although when bad enough it makes voice sound ¡°fuzzy.¡± ? Reply:? I agree, but I found the effect on ¡°fuzzy¡± SSB unacceptable, too.? I wasn¡¯t doing much CW in those days.?? ?? ? After the transformer mod, 60 Hz sidebands are diminished to the point of inaudibility.? It would probably be wrong for everyone to start modifying their R-4C receivers.? ? Reply:? Yes likely too much trouble, but if an R-4C owner is a nitpicker like you and I the difference is very significant. ? As a rule, I get deeply perturbed by the presence of any hum or buzz in receiver audio.? Shortly after working on the Drake receivers, I moved on to Collins 75S-3A and 75S-3C C receivers and eliminated hum in those receivers but it took many hours of work.? ? Reply:? I eventually owned an S Line and spent a few years ¡°fixing¡± some of its issues.? PDFs attached.? ? On my bench at the moment is a Hallicrafters SR-400A ¡°Cyclone III¡± transceiver that will soon get the same treatment. ? Comment:? I briefly owned a Squires Sanders SS-1R and modified it to cover 160 meters.? Unfortunately I sold it before I started testing receivers.? I have always wondered how well it would? have performed from a noise floor and dynamic range standpoint.? See link to the receiver.? http://www.w1vd.com/SS-1R.html ? 73, Rob, NC0B ? ? Paul, W9AC ? ? tom: [email protected]
<[email protected]> On Behalf Of Rob Sherwood ? Hi Doug, ? Please see comments inserted below. ? Rob, NC0B ? From:
[email protected] [mailto:[email protected]]
On Behalf Of Doug Crompton WA3DSP ? Rob, ? 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.
? 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.? 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.?
? 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. ? 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 3rd 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. ????? ? 73, Rob, NC0B |
Rob Sherwood