When I want to start the bias adjustment of my new build K-4040, LED 12 0.5V is already on, although the other LED’s (LD-3 is burning faint) , are off.
Adjusting VR1 (dipswitch 1 in down (on) position) does not lighten up any of the following leds.
Any suggestion were to start checking? All DC voltages seem to be okay.
When I’m adjusting any of the VR’s, the LD’s will go on from LD-12, to LD-11 to LD-10, so moving in opposite direction.
What also worries met is that LD12 is already on, in the first step of checking voltages ( So, when you need to check the 6.3V ac heater voltage (no valves insterted) LD-12 is already burning…LM39314 faulty?
It is normal for the right led to be on when no dipswitch is pressed.
Once a dipswitch is pressed, it should turn off and the dot can be adjusted starting from the left hand side.
Probably, an assembly problem in the readout secton. Check negative bias at all tubes.
Many thanks for your reply. Unfortunately the manual does not mention that LD-12 should be on, so that is a little confusing. Voltage V+ on Pin 3 should be approx 12V?
I checked the mainboard various times for construction failures in read oud section, but I can’t see anything suspicious.
After rebuilding unit for third time yesterday, reading still goes from right to left…
To be on safe side I will buy a new LM3914 and see what happens after inserting new IC.
No need to buy a new IC.
Some K4040 kit batches contain trimmers which work ‘inverted’.
Turning all trimmers to the opposite extreme will solve the problem.
To make sure, as indicated before: remove tubes and measure bias. Turn trimmers so that negative voltage is maximal (most negative) before inserting tubes.