The only time I ever used the FAST agc position, is when listening to AM signals, like SWBC.....esp on the 41 and 49m SWBC? bands...both of which can experience rapid, fast fades.?

What might make more sense is to use something like a 1 megohm pot and one cap. Then? (on paper) you should be able to tweak the agc from fast...to slow...and everything in-between.? It's just an RC time constant.?

?

What ever happened to ...'hang agc' ?? If you had a 90-200 msec? 'hang time', before the decay started, imo, it would sound a lot smoother, both on CW and also SSB.?

Somehow this discussion got off on measuring plate current, which makes no sense since there is already a means of checking cathode current using the two cathode resistors. In any case, my issue was definitely the shunt resistor, which had elevated to 9 ohms but physically looked OK! Replaced with three parallel 10 ohm 1/2 watt resistors and now get good cathode current readings.

The two cathode resistors in my unit had been changed to 10 ohm 2W and they measure very close to that and equal. Unfortunately, my brand-new tubes are not balanced, one has? .2 volts and the other .35 volts across their respective resistors or 20ma ans 35ma. I am going to check my stock for more 6JB6's and hopefully I can find a better match. How much of a difference is a problem? Also checked the screen resistors and they are closely matched so I don't think anyhing other than the tubes themselves would be causing the unbalance.

One problem I see is considerable power drop off on the higher bands. How much CW output should a good T4XC produce? I get 110W on 80 meters but down to about 60W or less on 40 and less even less on higher bands. I suspect the 12BY7 driver and I am going to look for one of them in my stock also. I have a 12.6mhz crystal but it apparently is not working as I get no drive on 160. Another thing to check.

Once I am confident that it is not a tube issue, I will do a complete alignment. I have a question about the RF and Mixer stage alignment procedure in section 5-8 of the manual. Item c-5 and c-6 refer to detuning by putting a screwdriver on the rotor. This seems like a weird way to do things. Has anyone come up with a better approach. Also, item c-5 says to tune the front injection trimmer, while item c-6 says to tune the rear injection trimmers - note plural. There are 4 trimmers, does that mean to tune the 3 rear trimmers? Perhaps someone has a better way of doing this?

Doug, WA3DSP

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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

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Sorry...
1 amp across a 1 ohm resistor will measure 1 volt.

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

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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

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

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Also here:



Jim W7RY

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

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They are non polarized and more reliable than an electrolytic.

See here:



73, Jim W7RY

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 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

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

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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

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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

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

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

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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

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

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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.

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

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