R4C Sherwood audio mod questions
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I've been looking over documentation I've assembled on the Sherwood audio stage modification.? I have diagrams from the April 1979 HR article, from MODS.dk and from the files section here.? Since none of these except for the HR article are dated, I'm unsure which is the latest improvement.? Hopefully someone can point me in the right direction.? Apparently after the HR article, there were changes made as follows but I couldn't find an explanation for the changes: 1. The .22 cap and 1500 resistor on the input were eliminated, 2. The 200 uF cap on pin 2 was changed to 400 uF, the 390 ohm resistor was changed to 270 ohms.? Which version of the circuit is the best one to use?
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73, Floyd - K8AC |
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On Jan 2, 2025, at 8:33?AM, Floyd - K8AC via groups.io <floydsense@...> wrote:
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开云体育Hi Floyd, ? My most recent post of the AMP-4 information was yesterday at 1:37 PM MST. ? For everyone’s convenience, attached to this email are PDFs for the following: AMP-4, FC-4, MIX-4, PD-4, RPS-4 and the FPS-4 and alignment tips document. If there is a document repository location associated with this email reflector, would someone please deposit these 6 documents there. ? Rob, NC0B ? From: [email protected] [mailto:[email protected]]
On Behalf Of Floyd - K8AC via groups.io
Sent: Thursday, January 02, 2025 9:24 AM To: [email protected] Subject: Re: [DRAKE-RADIO] R4C Sherwood audio mod questions ? Hi Rob.? Where did you post that stuff?? I've looked here in files and photos, at your website and on Mods.dk with no luck. ? 73, Floyd |
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On Sat, Jan 4, 2025 at 05:19 AM, Floyd - K8AC wrote:
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>The text specifies the inverting input capacitor as 200 uF while the schematic shows it as 330 uF.
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The computed -3 dB high-pass turnover frequency is 123 Hz for 330 uF, and 204 Hz for 200 uF.? Pick either one based on personal roll-off preference.
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Paul, W9AC? |
开云体育Hi Floyd, ? Thanks for pointing that out.? I’ll look at the board in my R-4C and see which is correct. It likely doesn’t make much difference with the inverting input impedance. ? Rob, NC0B ? From: [email protected] [mailto:[email protected]]
On Behalf Of Floyd - K8AC via groups.io
Sent: Saturday, January 04, 2025 6:19 AM To: [email protected] Subject: Re: [DRAKE-RADIO] R4C Sherwood audio mod questions ? Rob - maybe it makes no difference at all, but there is a discrepancy in the component values for the AMP-4.? The text specifies the inverting input capacitor as 200 uF while the schematic shows it as 330 uF. ? 73 & Happy New Year, Floyd - K8AC |
开云体育I stand corrected.? Rob, NC0B ? From: [email protected] [mailto:[email protected]]
On Behalf Of Paul Christensen via groups.io
Sent: Saturday, January 04, 2025 6:52 AM To: [email protected] Subject: Re: [DRAKE-RADIO] R4C Sherwood audio mod questions ? On Sat, Jan 4, 2025 at 05:19 AM, Floyd - K8AC wrote: ? >The text specifies the inverting input capacitor as 200 uF while the schematic shows it as 330 uF. ? The computed -3 dB high-pass turnover frequency is 123 Hz for 330 uF, and 204 Hz for 200 uF.? Pick either one based on personal roll-off preference. ? Paul, W9AC? |
开云体育Hi Paul, ? I was reviewing my 2012 Hamvention Drake Forum presentation and came across a picture of the AMP-4 in my R-4C.? The inverting input has a 220 uF capacitor installed.? There may well be some AMP-4s with 330 uF capacitors for the inverting input. ? The bulk of the attached Drake presentation deals with drastically reducing the line-related sidebands on the R-4C PTO.? I know you and I found this solution independently about the same time, likely in 2011.? R-4C owners may find this attached document interesting.? ? It is tedious to rotate the R-4C power transformer 90 degrees, but it really makes a difference.? A hole punch makes the job mechanically easy even if there is a good bit of work removing and reinstalling the wires. ? We will likely never know why Drake mounted the transformer properly in the R-4, R-4A and R-4B, but mounted it incorrectly in the R-4C. ? 73, Rob, NC0B ? From: [email protected] [mailto:[email protected]]
On Behalf Of Paul Christensen via groups.io
Sent: Saturday, January 04, 2025 6:52 AM To: [email protected] Subject: Re: [DRAKE-RADIO] R4C Sherwood audio mod questions ? On Sat, Jan 4, 2025 at 05:19 AM, Floyd - K8AC wrote: ? >The text specifies the inverting input capacitor as 200 uF while the schematic shows it as 330 uF. ? The computed -3 dB high-pass turnover frequency is 123 Hz for 330 uF, and 204 Hz for 200 uF.? Pick either one based on personal roll-off preference. ? Paul, W9AC? |
开云体育&驳迟;”It is tedious to rotate the R-4C power transformer 90 degrees, but it really makes a difference.”? ? For the curious, the magnetic field created within the transformer couples to the PTO which is located directly in front of the transformer.? The field produced is much stronger in one orientation. ? ? As the R-4C dial is turned from 0 to 500 kHz, the PTO’s ferrite slug moves closer to the transformer which results in even greater field coupling.? Audio from the crystal calibrator at 0 is cleaner than it is at 500 where a “fuzzy” sounding carrier is heard.? That fuzziness is created by the 60 cycle sidebands shown in Rob’s PowerPoint slides. ? Paul, W9AC ? |
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Sounds as if one of you has actually rotated the transformer 90 degrees.? I just took a look at that area of the receiver and decided that task ranks right up there with removing your own appendix.? I wonder if it wouldn't be possible to shield the PTO can with mumetal?? I have no idea what the material is like to work with, but it seems to be readily available.? And thanks for the comments on the capacitor value.? I'll finish getting my version of the AMP-4 board installed today and hopefully I'll have audio output.
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73, Floyd - K8AC |
开云体育Hi Floyd, ? Yes I rotated my transformer 90 degrees, and my technician? made the change to two other R-4Cs over time.? It takes less than two hours on the assumption you have a chassis punch, a hand drill and bits.? I am assuming Paul did the same thing.? It doesn’t take all that much time to disconnect and re-connect the transformer wires. ? I did procure a sheet of mu-metal and inserted it between the transformer and the PTO. Grounded or not, it didn’t do anything.? It is conceivable I didn’t do it the correct way.? Maybe if one had a sheet metal brake, a mu-metal shield could surround the PTO can.? That wasn’t an option.? ? ? Who was asleep at the switch at Drake deciding to make a change in the transformer mounting and not realizing it was a mistake? ? I often wonder if engineers? at various companies actually use the radios they manufacture and sell.? There are so many examples of design decisions, mechanical or electrical, that make no sense.? ? Rob, NC0B ? From: [email protected] [mailto:[email protected]]
On Behalf Of Floyd - K8AC via groups.io
Sent: Sunday, January 05, 2025 5:57 AM To: [email protected] Subject: Re: [DRAKE-RADIO] R4C Sherwood audio mod questions ? Sounds as if one of you has actually rotated the transformer 90 degrees.? I just took a look at that area of the receiver and decided that task ranks right up there with removing your own appendix.? I wonder if it wouldn't be possible to shield the PTO can with mumetal?? I have no idea what the material is like to work with, but it seems to be readily available.? And thanks for the comments on the capacitor value.? I'll finish getting my version of the AMP-4 board installed today and hopefully I'll have audio output. ? 73, Floyd - K8AC |
开云体育>Yes I rotated my transformer 90 degrees, and my technician made the change to two other R-4Cs over time.? It takes less than two hours on the assumption you have a chassis punch, a hand drill and bits.? I am assuming Paul did the same thing.? ? I did the same although I don’t recall if it was possible to remove the transformer bolts, rotate the transformer, then drill new chassis holes. The wire lead length and existing routing/dressing may have precluded doing it that way. ? Mu metal was tried by forming a cut sheet around the transformer, then soldering the seam.? No luck – and it looked awful.? ? Photos attached showing the power transformer after rotation. ? Paul, W9AC |
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Well, I let my hole punches get away from me decades ago and I'd guess that they'd cost more than I'd like to spend these days.? Great work by you guys who figured out the source of the 60 Hz modulation.? I read some info at a mumetal manufacturer's site and had the idea of just building a can to fit over the existing PTO can.? They noted that the effectiveness could be greatly increased by adding a second layer with an insulating material in between (teflon sheet?).? Their description of the material said it was very malleable and ductile so maybe the work could be done by hand with a vise and steel ruler.??
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73, Floyd - K8AC |
开云体育Doing this kind of work is when a real spectrum analyzer with low RBW really pays off. ?IMHO, to get a good look at close-in 60 Hz sidebands requires an RBW of no more than 10 Hz, with 1 Hz offering excellent screen resolution. That rules out the toy analyzers like the TinySA.? ?I think Rob’s analyzers are all 1 Hz RBW.? I use an HP 8560E and Anritsu MS2721B. ? Paul, W9AC ? ? From: [email protected] <[email protected]> On Behalf Of Floyd - K8AC via groups.io
Sent: Sunday, January 5, 2025 1:31 PM To: [email protected] Subject: Re: [DRAKE-RADIO] R4C Sherwood audio mod questions ? Well, I let my hole punches get away from me decades ago and I'd guess that they'd cost more than I'd like to spend these days.? Great work by you guys who figured out the source of the 60 Hz modulation.? I read some info at a mumetal manufacturer's site and had the idea of just building a can to fit over the existing PTO can.? They noted that the effectiveness could be greatly increased by adding a second layer with an insulating material in between (teflon sheet?).? Their description of the material said it was very malleable and ductile so maybe the work could be done by hand with a vise and steel ruler.?? ? 73, Floyd - K8AC |
Floyd - K8AC
Rob Sherwood
Paul Christensen
