[Amps] LK550 (etc. ) QSK board schematic question (schematic section copy sent by e-mail upon request)

[wb0gaz at yahoo.com]

 (I can supply schematic section cited by e-mail upon request to wb0gaz at yahoo.com, as it is a photo of the manual page I am referencing.)

Next step in my continuing work on recovery of a LK550 with a history many prior adverse experiences.

(some acknowledgements of prior topics I've raised during this project): Thanks Curt/KU8L re clarification on TS590 integration with LK550 - now have it set for relay enabled switching and connected to the amplifier's RELAY input (with QSK board present.) The wording in the TS590 manual around switching an amplifier wasn't very clearly worded, but it did eventually sink in what was happening, and it is working fine for the needed use. From previous advice here, I've de-powered the ALO/Meter board (it is now only serving as a passive mounting for the two meter scaling and protection components), and while I have repaired the step-start board, I cannot re-install it due to lack of correct mating connector - it is bypassed by prior owner. None of these issues (TS590 interconnect, ALO/Meter board faults, nor step-start board) are of concern today.

The QSK board is not the original item from Amp Supply - that board was damaged beyond repair by too many previous butchery attempts (attempted butchery of electronics evidently cannot be prosecuted in Colorado). For this reason, I'm not starting from a "clean" broken amplifier, but rather one with prior ill-fated modifications/repairs, and a cloned QSK board. I cloned the QSK board (creating PCB artwork from the schematic in the manual, roughly following the physical dimensions, mounting holes, and component placement on the original board, but adding silkscreen nomenclature, a connector for closed-box test point measurements, and taking advantage of double-sided plated-thru board fabrication.) Copies of the cloned board were made from an online PCB manufacturer, they seem to be of good quality.

>From what I can ascertain, the cloned QSK board is doing most of it's job correctly, however, K3 does not appear to actuate (K3 would route the amplifier's driver/input SO-239 port to the input (ATI) filter board when amplifier is enabled and radio is transmitting.) I've been able verify the radio is controlling the cloned board's K1 (amplifier enabled whenever STBT/XMIT switch on amplifier is in XMIT setting), K2 (bias cut-off control relay and cascade control to K3), and K4 (main vac relay for amplifier output T/R switching), also the associated circuitry for driving K4 is causing the STBY/XMIT switch on the amplifier to illuminate whenever the radio is in transmit mode (no RF, just using it to verify the radio can key the amplifier). The notable problem remaining is at K3 (small relay for amplifier input T/R switching) - it not appear to be getting energized. This is probably a board layout error inhibiting K3 operation, but there appears to be a schematic error (see below) of concern.

Half of K2 (pins 4, 6, 8) appear to create a ground-on-transmit condition (passed via CR10) to energize K3 (the RF input/drive path switching relay) to allow the transceiver's transmit RF (from the back panel SO239) to reach the ATI (relay selected input band matching) board. Symptom right now - it appears that K3 never gets it's "lower" relay coil connection pulled to ground via CR10 and K2 (4, 6, 8) - both sides of it's coil sit at 12V regardless of TX or RX state, as long as the amplifier is enabled (which delivers 12V to power-up the QSK board).

When I cloned the board artwork from the schematic (with hints from the original board only about rough component placement and board dimensions), I didn't catch a likely error in the schematic: pin 4 (common of the section of DPDT to control K3 coil via CR10) goes to ground. Normally open (de-energized position) pin 6 is left open. Pin 8 (energized position) goes to board holes/contacts 8, 12, 13, and the cathode of CR10 (anode of CR10 goes on to K3's coil "lower" end of winding, so that pulling that end of K3's coil to ground would cause K3 to energize.) Here's the hitch - in the schematic, pins 8, 12 and 13 all show going to ground (as the schematic shows board contacts 8=ground tied to 12/13, which causes those to be grounded. If the board were following this correctly, CR10 cathode would be grounded, leaving K3 forever energized. Operationally, the board is not ever energizing K3 coil, so on top of a schematic error, I appear to have incorrect handling of K3 coil, preventing it from finding ground during transmit.

As the original QSK board is mangled beyond recognition, I cannot yet determine what the correct wiring around this part of K2 should be. It seems from the schematic that CR10's cathode would be forever grounded, however, the board is not actually doing this, so not only is there possibly a schematic error, I may have misrouted (on the PCB artwork) the handling of CR10 cathode.

Anyone care to take a look at the QSK board schematic and/or board itself and let me know whawt should be going on around this part of K2?


Thanks!

Dave