This is a cute little QRP rig that covers 40m, 15m, 10m and 6m. They’re hard to find and they’re even harder to find intact and not-broken.
(Ie, so I don’t forget!)
The TX power control and ALC loop are split across the filter board and the IF board and feed into the RF board.
The forward / reverse power on the filter board are both fed via D6 -> VR1 -> D10 (reverse) and D7 -> VR2 -> D9 (forward) into IC1 (2/4), which acts as a positive/buffer amplifier and generates a positive control voltage representing how much to negatively influence the ALC / TX power control loop.
The TX power meter is tapped off of D7, through R40 / D14 and eventually into the positive side of IC1 (1/4). That acts as a positive gain amplifier, out through VR5 -> D16 into the RFM (pin 1) on J4. The RFM connector also is fed the S meter signal from the RXM (pin 3) on J3.
The ALC signal is generated from two signals fed into a differential amplifier IC 1 (3/4):
Ok, so the PCL line is fed from the IF board, which is generated from the PCV (1) and MIN (2) pins on J10, and those go to the RF power pot on the front panel. The MIN pin connects to the ground side of the pot and controls the “minimum” voltage the pot is provided above ground. The PCV signal feeds into a voltage divider (R139 / R136), through some diode logic to make sure things go a certain way (D51/D54), and out to PCL. The other diodes (D47 -> D57) and the transistors (Q37, Q38, Q39, Q40) control when to provide which minimum and maximum PCL control voltages to the filter board to feed the TX power control / ALC loop.
The G2 signal (TX gain) is generated from the PCL signal, through R141 to 8V (via R141, 100K), D55 and Q40. There’s some transistor switching and voltage dividing going on. If SSB/FMB is high then Q40 will be turned on, lowering the G2 voltage feed on the 8V side; I think this lets the PCL signal assert more of G2.
G2 feeds into the RF board via G2 (pin 2) on J18. It feeds a positive control voltage to both the HF path (Q26) and 50MHz path (Q29).
Finally, ALC is fed into one of the inputs into Q42 on the IF board. That’s the transmit IF (TIF) final stage amplifier and as an active ALC voltage is negative, it will reduce the gain of that final IF stage before the TIF signal is handed to the RF board for modulation.
Note there’s no ALC voltage / control loop feeding into the RF board. The RF board is only looking at the G2 voltage (TX gain) derived from the above logic and RF power control (PCL) generated from the front panel.
You absolutely should verify the finals current by desoldering the two links on the finals board (A and B, which are unlabeled, sigh) and measure the current between them. I destroyed one of my driver transistors because the circuit didn’t actually calibrate down to 0mA bias current.
The A and B connectors aren’t labelled on the board or drawn on the service manual. It’s similar to the TS-660 finals board though (and that service manual does have pictures) - the “A” jumper is the one closest to VR1, and the “B” jumper is closest to VR2.
The 50MHz TX bandpass filter (Section 3, TX calibration) instructions aren’t as clear as they should be. The 50 MHz TX BPF is actually T37 -> T42 (ie, T37, T38, T39, T40, T41, T42) and TC3/TC2 for max power.
The instructions just say TC3/2, then T37 / 42, and they don’t make it clear it’s the whole range between T37 -> T42.
If your radio (like mine) was very broken, the alignment instructions may just not be enough to get the radio into good working order. After a lot of reverse engineering I finally figured out what’s going on.
If there are issues in the HF RX input / bandpass filter path, then you’re going to have problems calibrating the TX side.
Note that the calibration instructions actually have you check the RX path including the 50MHz and HF RX filters, but before the IF mixer. The cathode on D3 is connected to D15 and D16 cathodes, and those three diodes are switch/pin diodes switching in the “right” RX path into the 8.83MHz trap before the IF mixer pair (T20 and Q5/Q6.)
This means:
Instead, I did something a bit sneakier:
Then to test a few things:
That verifies that you’re getting a nice flat -40dB passthrough and you’re not seeing anything odd going on (eg D2/D4 are shorted, which lead to my -10dB behaviour.)
Then:
That verifies that there’s nothing weird on the input side (dirty attenuation relay, broken T1, something wrong with T2/T3 trap, etc, etc.)
Then:
If you have a problem at this point then it is likely on the filter board, the cables between the filter board to the RF board, or the SO239 connector and cabling on the back of the unit. My TS-670 had corroded relays because they weren’t the sealed units. Sigh.
Once that’s done, you’ve eliminated anything weird going on early in the receive path which could cause RX attenuation, and you can work on the RX BPF calibration as per the manual.
On my NanoVNA with DC block / 20dB attenuators on both input and output, each bandpass filter shows up at around -16 to -20dB. Yes, because of the RX amplifier stages involved, you’re going to get a decent amount of gain showing up and if you didn’t have the 20dB attenuator on the input into the NanoVNA you’d quickly overload the NanoVNA receiver and maybe even damage it.
I’ve found that C61 and R31 on the control board are inverted on the PCB - C61 goes to the junction of L28/R32, then the ground of C61 goes to R31, that then goes to ground
I am referring to R43,R44,R45,R46 before the Q5/Q6 mixing transistors!
(TBD)
(TBD)
The service and user manuals should be used together to figure out what’s going on.