OK - I have now replaced IC3 (14-pin NAND gate) and that has brought back the receiver to full normal volume, but I still have no TX output.?
I had to use a Dremel with a disc cutter to cut through the legs and then remove each pin one by one. A new socket has been soldered in and a new 7SN74ACT00N is in place.
On key down I can detect a constant carrier on my HF receiver at the frequency of interest, but it is solid (ie not keyed with the paddle) if that helps.?
Voltages are;
IC3
Pin 1 - key up 5V, key down 2.5V
Pin 2 - key up 5V, key down 2.5V
Pin 11 - key up 0V, key down 5V
Pin 12 - key up 5V, key down 0V
so I think that is working correctly.
The voltages on the centre pin of Q1, Q2 and Q3 are 0V on key up and 3.65V on key down.
I had previously replaced Q5 and Q6 (and Q1, Q2 and Q3) after the fault appeared, but I wonder if the faulty IC3 had in fact wrecked the replacements too. What would readers suggest I check/replace now? I have spare BS170s and MPS751s, but am loath to replace Q5 and Q6 yet again as the board can't take much more desoldering.
Steve G0KYA
?
