Re: Puzzling problem with the AGC in my R4-C
Hi Bill,
?
There are multiple issues to consider.? I always felt the Fast AGC decay speed was useless, and installing an extra C value (or R/C pair) across the Fast setting
is a good idea.? I would keep the 1 meg + 0.01 in the Fast position.? I still wouldn’t use that Fast decay on SSB, but maybe it is useful in rare instances on CW.? This really fast decay speed may go back to early Drake QSK operation. ?This is from memory
in the days of the Drake 2A and 2B so it could well be inaccurate. ?
??
The other problem is the dual-diode product detector in the late R-4As, all R-4Bs, all R-4Cs and early TR-7s is terrible.? The instantaneous IF level during
AGC attack exceeds the BFO injection, causing awful (from my perspective) AGC attack distortion.? The faster the decay the more often the AGC attack distortion occurs. ?A simple dual-diode product detector (which is a mixer) should have at least 5X the LO
injection level as the IF level to avoid distortion.? This isn’t even close to being the case in those stock Drake product detectors.
?
Another issue is the stock dual-diode product detector allows detected audio to leak back into the IF, modulating the AGC voltage.? This may explain why even
during a steady-state test with a signal generator it also shows distortion changing with decay speed selection.? A doubly balanced Gilbert Cell product detector doesn’t have either of these problem.?
?
My first R-4C replacement product detector used an MC1496 Gilbert Cell chip that had a very high parts count.? Later I changed to an Texas Instrument TL-442
Gilbert Cell chip, but when that went out of production the NE602 or SA602 was on the market with only a modest parts count. ?Attached is a PDF with the schematic of the 602 Gilbert Cell product detector.? Note: in later TR-7s Drake changed to an MC1496 product
detector.? Déjà Vu.?? ?
?
I always wondered why Drake engineers were “tone deaf”, seemingly unaware of AGC attack distortion.? The original R-4 and early R-4As had a tube product detector
which did not have this problem.? I also had to fix the AGC and product detector distortion in my 75S-3C, so Drake didn’t have a monopoly on distortion.
?
73, Rob, NC0B
?
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From: [email protected] [mailto:[email protected]]
On Behalf Of Bill Leonard N0CU
Sent: Sunday, March 12, 2023 9:51 PM
To: [email protected]
Subject: Re: [DRAKE-RADIO] Puzzling problem with the AGC in my R4-C
?
Rob,
Thanks for the info.
I have a couple of questions:
-Is the distortion I am experiencing on the M & F AGC settings normal? If not, have you seen this problem before?
-The only mod I have found is the addition of a 1 Meg resistor in series with a 0.01 uF capacitor that was recommended by VE3EFJ in his article "4.0 R4B and R4C Mods and Tech". It was added on the AGC board to improve AGC transients. I haven't tried removing
it. Should I?
--
Bill N0CU
|
Re: Puzzling problem with the AGC in my R4-C
Rob,
Thanks for the info.
I have a couple of questions:
-Is the distortion I am experiencing on the M & F AGC settings normal? If not, have you seen this problem before?
-The only mod I have found is the addition of a 1 Meg resistor in series with a 0.01 uF capacitor that was recommended by VE3EFJ in his article "4.0 R4B and R4C Mods and Tech". It was added on the AGC board to improve AGC transients. I haven't tried removing it. Should I?
--
Bill N0CU
|
Re: Puzzling problem with the AGC in my R4-C
Be sure the Drake AGC circuit hasn’t been modified.? Having worked on many R-4Cs over a 40 year time period it wasn’t unusual to have radios come into the lab
with AGC issues.?? Also in that time period I never had to replace a standard Drake AGC capacitor.
?
Attached is a PDF that includes component listings for the stock AGC and the two additional speed I typically added.
?
Rob, NC0B
?
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From: [email protected] [mailto:[email protected]]
On Behalf Of Bill Leonard N0CU
Sent: Sunday, March 12, 2023 7:02 PM
To: [email protected]
Subject: Re: [DRAKE-RADIO] Puzzling problem with the AGC in my R4-C
?
Gary,
The S meter is behaving as you described with a steady signal.
The only issue that has me puzzled is how the audio distortion increases as the AGC time constant is reduced. I haven't seen this with any other receiver I have owned, analog or digital. I can see how distortion due to clipping could occur if the AGC risetime
isn't fast enough to keep the signal from clipping, but my understanding is that the R4-C AGC switch only varies the decay time, and I wouldn't expect that to cause clipping. So, if the radio is behaving as it should, is this a characteristic of the Drake
AGC design? If so, it isn't a problem because I don't operate CW, however, I would simply like to know if there is a problem with my R4-C that I need to track down.
--
Bill N0CU
|
Sorry...
1 amp across a 1 ohm resistor will measure 1 volt.
73, Jim W7RY
On 3/12/2023 9:28 PM, Jim W7RY via
groups.io wrote:
Here is what I would do.
Install a 1 Ohm 1% 1 Watt resistor in series with the plate supply
at the bottom of the plate choke.
Then simply measure the current across it with a milli-volt meter,
(aka your quality digital meter).
1 amp of plate current would be one volt across the resistor. 100
mA would be 100 mV. So essentially 200 mV would be 200 mA. etc...
etc....
Easy peasy.
Lets not make rocket surgery out of this?
73, Jim W7RY
On 3/12/2023 2:05 PM, Doug Crompton
WA3DSP wrote:
Yes, now that I have gone through this, testing of the shunt
resistor on an unknown T4 should be one of the first things you
do even before you apply power. It should definitely be replaced
with a higher wattage rating. It seems Drake engineers were very
good about not underrating parts. I wonder why they selected a
1/2W that carries 1/2W or possibly more as it heats. It is a
recipe for failure.
I measure my shunt. It is about 9 ohms. I will be replacing with
three 10 ohm 1/2W 5% in parallel. That should fix the problem.
Then I can check the 15 ohm resistor values and measure the
voltage to set the bias correctly and see what the resulting
meter reading is.
I would expect, if the shunt was entirely open, it could cause
meter damage. Perhaps back to back diodes across the meter or at
the shunt would be in order. My guess is there are a lot of T4's
out there not reading cathode current correctly, thus the bias
is being incorrectly set, but since the current is always going
to read higher than actual there would not be tube damage only
signal distortion.
?Doug, WA3DSP
--
Thanks and 73, Jim W7RY
--
Thanks and 73, Jim W7RY
|
Here is what I would do.
Install a 1 Ohm 1% 1 Watt resistor in series with the plate supply
at the bottom of the plate choke.
Then simply measure the current across it with a milli-volt meter,
(aka your quality digital meter).
1 amp of plate current would be one volt across the resistor. 100 mA
would be 100 mV. So essentially 200 mV would be 200 mA. etc...
etc....
Easy peasy.
Lets not make rocket surgery out of this?
73, Jim W7RY
On 3/12/2023 2:05 PM, Doug Crompton
WA3DSP wrote:
Yes, now that I have gone through this, testing of the shunt
resistor on an unknown T4 should be one of the first things you do
even before you apply power. It should definitely be replaced with
a higher wattage rating. It seems Drake engineers were very good
about not underrating parts. I wonder why they selected a 1/2W
that carries 1/2W or possibly more as it heats. It is a recipe for
failure.
I measure my shunt. It is about 9 ohms. I will be replacing with
three 10 ohm 1/2W 5% in parallel. That should fix the problem.
Then I can check the 15 ohm resistor values and measure the
voltage to set the bias correctly and see what the resulting meter
reading is.
I would expect, if the shunt was entirely open, it could cause
meter damage. Perhaps back to back diodes across the meter or at
the shunt would be in order. My guess is there are a lot of T4's
out there not reading cathode current correctly, thus the bias is
being incorrectly set, but since the current is always going to
read higher than actual there would not be tube damage only signal
distortion.
?Doug, WA3DSP
--
Thanks and 73, Jim W7RY
|
Re: Puzzling problem with the AGC in my R4-C
Also here:
Jim W7RY
On 3/12/2023 9:05 PM, Jim W7RY wrote:
They are non polarized and more reliable than an electrolytic.
See here:
73, Jim W7RY
On 3/12/2023 7:36 PM, Bill Leonard
N0CU wrote:
Thanks Jim.
I'll get them on order from Mouser.
I am curious what performance feature about these capacitors
requires their use in the AGC circuit.
--
Bill N0CU
--
Thanks and 73, Jim W7RY
--
Thanks and 73, Jim W7RY
|
Re: Puzzling problem with the AGC in my R4-C
They are non polarized and more reliable than an electrolytic.
See here:
73, Jim W7RY
On 3/12/2023 7:36 PM, Bill Leonard N0CU
wrote:
Thanks Jim.
I'll get them on order from Mouser.
I am curious what performance feature about these capacitors
requires their use in the AGC circuit.
--
Bill N0CU
--
Thanks and 73, Jim W7RY
|
Re: TR7 Service Kit - Extender Boards Needed
Thanks Gary, I did notice that those switches are a bit "dirty".? I haven't put the rig on the bench yet. just getting prepared with everything I might need to give it a thorough restore to full operation.? I'll definitely be cleaning the switches and controls.? John
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Re: Puzzling problem with the AGC in my R4-C
Gary,
The S meter is behaving as you described with a steady signal.
The only issue that has me puzzled is how the audio distortion increases as the AGC time constant is reduced. I haven't seen this with any other receiver I have owned, analog or digital. I can see how distortion due to clipping could occur if the AGC risetime isn't fast enough to keep the signal from clipping, but my understanding is that the R4-C AGC switch only varies the decay time, and I wouldn't expect that to cause clipping. So, if the radio is behaving as it should, is this a characteristic of the Drake AGC design? If so, it isn't a problem because I don't operate CW, however, I would simply like to know if there is a problem with my R4-C that I need to track down.
--
Bill N0CU
|
Re: Puzzling problem with the AGC in my R4-C
? S=Slow AGC ?response time M=Medium AGC ?response time F=Fast AGC ?response time
Fast is only really suitable for use in CW and RTTY. SSB will get quite distorted in the Fast AGC position and less so in the Medium position but still not zero for strong signals. Slow AGC is always best for SSB.
From what you describe, your radio is behaving as it should. The S meter reads, meaning that the AGC is working. The change in S meter response to a pulsed signal is expected because the S mater is tracking what it can insofar as the AGC is holding things back due to the time constant.
Try injecting a steady signal into the radio and you should see that, after the slow time constant is passed, the S meter will read the same in the Slow position as it does in the Fast position.
Those old capacitors you describe are paper caps, which is why they do not show a polarization marking. You can replace them with Mylar caps. ?Most Mylar caps you buy from reputable sources are rated to 630 volts and this presents no problem in terms of function or lifetime.
Gary
W0DVN
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On Mar 12, 2023, at 9:10 PM, Jim W7RY via groups.io <jimw7ry@...> wrote:
?
I recommend these in the proper values:
Please don't order some chinese junk from amazon or ebay. Use these
from Mouser or Digikey.
73, Jim W7RY
On 3/12/2023 7:01 PM, Bill Leonard N0CU
wrote:
They are the two large caps on the AGC board. They are the size of
small electrolytic capacitors, but they are not polarized, so I
don't know what type they are:
C73: 0.22 uF
C74: 0.047 uF
--
Bill N0CU
--
Thanks and 73, Jim W7RY
|
Re: Puzzling problem with the AGC in my R4-C
Thanks Jim.
I'll get them on order from Mouser.
I am curious what performance feature about these capacitors requires their use in the AGC circuit.
--
Bill N0CU
|
Re: Puzzling problem with the AGC in my R4-C
I recommend these in the proper values:
Please don't order some chinese junk from amazon or ebay. Use these
from Mouser or Digikey.
73, Jim W7RY
On 3/12/2023 7:01 PM, Bill Leonard N0CU
wrote:
They are the two large caps on the AGC board. They are the size of
small electrolytic capacitors, but they are not polarized, so I
don't know what type they are:
C73: 0.22 uF
C74: 0.047 uF
--
Bill N0CU
--
Thanks and 73, Jim W7RY
|
Re: Puzzling problem with the AGC in my R4-C
They are the two large caps on the AGC board. They are the size of small electrolytic capacitors, but they are not polarized, so I don't know what type they are:
C73: 0.22 uF
C74: 0.047 uF
--
Bill N0CU
|
Re: Puzzling problem with the AGC in my R4-C
Any capacitor (especially an old one, even more so where it's in a
very hot environment all its life) can easily go bad.
What are the values of the capacitors?
73, Jim W7RY
On 3/12/2023 3:13 PM, Bill Leonard N0CU
wrote:
The receiver
seems to work fine when the AGC switch is set to S. However, in
the M position there is noticeable distortion on both noise and
signals. In the F position, the distortion becomes significant.
The distortion sounds like clipping, but none is seen on the
audio output with a scope. The S meter shows no difference with
the three settings. I am using the Sherwood audio amp and power
supply. I set up an HP sig gen to provide a pulsed RF signal.
What I see is the AGC pulse in response to that input signal
increases in voltage by x3 in the M position and by x7 in the F
position compared to the S position. The only changes to the AGC
circuit that I see when the delay is changed to the M & F
positions are the time constants (two RC networks). The
resistors in those networks check ok and to the limited extent
that I can check the two capacitors, they seem ok. What types of
capacitors are they and do they have a history of going bad? Any
suggestions would be appreciated.
--
Bill N0CU
--
Thanks and 73, Jim W7RY
|
Re: TR7 Service Kit - Extender Boards Needed
Clean the push buttons for “Fixed XMIT” before you go in to extensive troubleshooting. Best to clean the “FIXED RCVR ?button and the “AUX Program” rotary switch as well. I think this will fix your issue.
Gary?
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On Mar 12, 2023, at 6:15 PM, John B <wb9fhl@...> wrote:
?I have a TR7 that I picked up at a local hamfest, it came with the matching power supply.? I've gone through the power supply, cleaned it up and tested it.? All is good there.? The TR7 receives but on transmit it does not lock on frequency, it just counts up. I would like to obtain a set of the extender boards (part of the Drake TR7 Service Kit) but it appears that there are none to be found anywhere.? If someone has a set they would like to sell or let me borrow for my repair effort I'd much appreciate it.? I would be happy to pay the round-trip postage.? ? John
|
TR7 Service Kit - Extender Boards Needed
I have a TR7 that I picked up at a local hamfest, it came with the matching power supply.? I've gone through the power supply, cleaned it up and tested it.? All is good there.? The TR7 receives but on transmit it does not lock on frequency, it just counts up. I would like to obtain a set of the extender boards (part of the Drake TR7 Service Kit) but it appears that there are none to be found anywhere.? If someone has a set they would like to sell or let me borrow for my repair effort I'd much appreciate it.? I would be happy to pay the round-trip postage.? ? John
|
Puzzling problem with the AGC in my R4-C
The receiver seems to work fine when the AGC switch is set to S. However, in the M position there is noticeable distortion on both noise and signals. In the F position, the distortion becomes significant. The distortion sounds like clipping, but none is seen on the audio output with a scope. The S meter shows no difference with the three settings. I am using the Sherwood audio amp and power supply. I set up an HP sig gen to provide a pulsed RF signal. What I see is the AGC pulse in response to that input signal increases in voltage by x3 in the M position and by x7 in the F position compared to the S position. The only changes to the AGC circuit that I see when the delay is changed to the M & F positions are the time constants (two RC networks). The resistors in those networks check ok and to the limited extent that I can check the two capacitors, they seem ok. What types of capacitors are they and do they have a history of going bad? Any suggestions would be appreciated.
-- Bill N0CU
|
Yes, now that I have gone through this, testing of the shunt resistor on an unknown T4 should be one of the first things you do even before you apply power. It should definitely be replaced with a higher wattage rating. It seems Drake engineers were very good about not underrating parts. I wonder why they selected a 1/2W that carries 1/2W or possibly more as it heats. It is a recipe for failure.
I measure my shunt. It is about 9 ohms. I will be replacing with three 10 ohm 1/2W 5% in parallel. That should fix the problem. Then I can check the 15 ohm resistor values and measure the voltage to set the bias correctly and see what the resulting meter reading is.
I would expect, if the shunt was entirely open, it could cause meter damage. Perhaps back to back diodes across the meter or at the shunt would be in order. My guess is there are a lot of T4's out there not reading cathode current correctly, thus the bias is being incorrectly set, but since the current is always going to read higher than actual there would not be tube damage only signal distortion.
?Doug, WA3DSP
|
Re: Cinch Jones connector identification
On Sun, Mar 12, 2023 at 11:54 AM, Jim Shorney wrote:
The RV75 works just fine without it
Thanks, Jim. Yes, I am aware the RV75 will work without the RIT cable but curiosity has the best of me and it appears to be an easy task. Rick K8EZB
|
Re: Cinch Jones connector identification
Jones plug nomenclature on the web suggests P-3312-CCT. However, you only need this if you actually want to use RIT. The RV75 works just fine without it an honestly I have never missed it because I rarely use RIT anyway. 73 -Jim NU0C On Sun, 12 Mar 2023 11:37:35 -0700 "Rick Boswell" <frboswell@...> wrote: I recently acquired an RV75 to use in my 7-line station. The RV75 requires a cable from the RIT SWITCH phono connector on the rear panel to the ACCESSORIES connector on the rear panel of the TR7. This cable and CJ "adapter plug" wre not included in my purchase. Looking for the correct CJ part number for a plug compatible with the TR7 ACCESSORIES" connector in order to make the cable/adapter.
Thanks,
Rick
K8EZB
|
Rob Sherwood
Doug Crompton WA3DSP
Jim Shorney