TR7 2nd IF/Audio board variations and S-Meter Calibration

This page details my investigations of variations of the TR7 2nd IF/Audio board, especially the S-Meter circuit. I will not cover the product detector section as this has been adequately covered elsewhere on the internet. This adventure began with my attempts to calibrate the S-Meter circuit, and led me to some known variations in this area of the board. I also found other variations that had not been previously documented.

There seem to be three general types of design changes applied to the TR7 over time:

These changes may or may not have been made in production or at the Drake repair center, depending on the vintage of the board and whether the change was considered critical in nature. I have found all three so far in the two TR7s that I have been working with. These units both have the Version 1 2nd IF/Audio board. One unit is serial number 32xx, the other 34xx. My first exposure to these changes was a mailing list discussion HERE, between Carel PA0CMU and Garey K4OAH. That was only the beginning.

Below are the variations I found on the two boards that I had at my disposal.

Component Designation
 Board 32xx
 Board 34xx
CR1101 AA119
R1112 22K
Connected to supply side of RFC1104 (+10) by factory modification.
Connected to supply side of RFC1104 (+10) by factory modification.
R1127 1K
Connected to +10R through RFC1101 by factory modification.
Connected to +10R through RFC1101 by factory modification.
R1187 10K
1.5K added in series with anode of CR1119 by factory modification.
1.5K added in series with anode of CR1119 by factory modification.
R1188 180 Ohm
Not present
Not present
R1120 8.2K
Missing - possibly removed to provide more IF gain

R1187 and R1188 were added to correct loud pops in the speaker when switching between CW and AM modes. The value of R1187 does not seem to be critical. 10K will cause the rig to "wake up" somewhat slower when switching to CW mode than 1.5K. The change to 1.5K may have been a service change to resolve customer complaints about slow switching to CW mode. The positions of R1187 and CR1119 as factory installed on the Version 1 board are reversed from what is shown on the schematic, with the resistor on the anode side instead of the cathode side of CR1119. This is shown in the Version 1 service manual photo. The R1112 connection was changed to correct a pulsing of the T/R relay when switching to CW mode, although in my opinion it should be connected to the circuit side of RFC1104 instead of the supply side for proper supply decoupling. The R1127 connection to +10R appears to have been changed in production to correct a PCB layout error. These changes should be made to any old Version 1 boards that do not have them.

R1112 Relocation by Factory Modification

PCB Trace Cut and Added Jumper for R1127 Correction by Factory Modification

An alternate version of the R1127 mod found in K8AC's article on TR-7 Key Clicks
R1127 has been relocated to the bottom of the board.

The S-Meter Circuit

Version 1 S-Meter circuit
Version 1 S-Meter circuit

Version 2 S-Meter circuit
Version 2 S-Meter circuit

Documented variations in the S-Meter circuit between Version 1 and Version 2 include R1101 being replaced by another pair of 1N4148s in series, CR1113 and CR1114; CR1101 is removed and replaced with a direct connection. Carel PA0CMU reports a unit with CR1101 replaced with a 4.7K resistor. R1111 has been changed from 2.2K to 3.3K. Confused yet?

OK, back to the S-Meter calibration. As I noted above, all this started with an inability to properly adjust the S-Meter sensitivity. The TR-7 service manual states that a "properly aligned" receiver should read S-9 with a signal level of approximately 30 uV on all bands (page 3-3). Furthermore, the document Service Group TR7 Alignment Procedures  posted on the Drakelist web site states that S-9 will typically be -73 to -77 dBm (30 To 50 uV). I was seeing more like 100+ uV using either alignment procedure.

I am usually very reluctant to "modify" a circuit to achieve proper operation that should have been there in the first place, but both boards that I had on hand exhibited similar behavior and I was relatively confident that there was nothing "wrong" with either board. I modified the Version 1 2nd IF/Audio boards to reflect the S-Meter circuit changes that Drake made in later revisions as follows:

    R1101 - Replaced by 2x 1N4148
    R1111 - Changed to 3.3 K

I also replaced CR1101 with a 1N6263 Schottky diode. This is the only non-Drake change that I made. In later revisions, CR1101 is eliminated entirely, but I like the idea of having some isolation between the wattmeter circuit and R1105/Q1102. Of the two boards I had on hand, one had a Germanium diode and the other used Silicon. The schematic and parts list specify a Germanium part, so I used a 1N6263 Schottky as a compromise; similar voltage drop to Germanium, better switching characteristics than Silicon.

After the changes were made, I was able to get much closer to the desired 50 uV for S-9 figure, which seems to be the de-facto standard for "modern" rigs. Final results using a combination of the two alignment methods noted above are S-9 = 42/59 uV (AM/SSB) for radio SN34xx, and 40 uV for radio SN32xx (I did not check both AM and SSB on this sample). Close enough ... for now.

Update July 26 2016

I was never totally happy with the results of my S-Meter experiments so I have reverted to pretty much the original configuration in the main rig. My current practice is to set S9 = 30 uV with the IF Gain adjustment (R1136) after the other S-Meter/AGC adjustments are completed. The IF Gain is only vaguely mentioned in the service manual and conventional wisdom seems to have been to just set it to max. I have found that tends to make the RX too "hot". The sensitivity is not improved by setting it to max. That just makes the radio SEEM more sensitive because there is more system gain. It also makes the S Meter very generous. I prefer to have the S-Meter reasonably close to a standard and use a preamp on the high bands if I want more gain so that weak signals develop some AGC action.

Here is the adjustment procedure that I currently use. This is the service manual alignment procedure from page 3-1, with my modifications highlighted:

The S-meter adjustments are located on the 2nd IF/Audio Board (Section 2-11). These adjustments must be made in the AM mode (to prevent errors due to BFO leakage), and must be made in the order outlined below. Failure to do so will result in improper S-Meter and AGC operation. (Only the pedestal adjustment is critical for proper ACG operation). Most adjustments should be made with the shield cover in place; you will need a long, narrow straight-blade screwdriver to reach the adjustments near the bottom of the board. The screwdriver shaft should be insulated with heat shrink tubing or electrical tape to prevent accidental shorts. A small flashlight will prove useful to sight on the adjustments through the provided holes in the shield cover. NOTE: The +10 volt regulator adjustment must be properly set before any adjustments are performed, or improper AGC action may result.

Standard disclaimers apply; this is what worked for me, it may not work for you. Your mileage may vary. I'm not responsible if you damage your radio. User experiences, comments, and suggestions are always welcome and may be added to this web page; flames will be ignored.

Acknowledgments: I would like to thank Carel PA0CMU and Garey K4OAH for their previous work uncovering revisions to the 2nd IF/Audio board, Thom K3HRN for his tireless work on the Drakelist web site and email reflector, and Floyd K8AC for the definitive article on resolving key clicks in the TR-7.

Please join us in the Yahoo DrakeRadio group and the Drakelist reflector.

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Updated 8/4/2016