20-05-2015, 07:54 PM
(20-05-2015, 06:54 PM)boater sam Wrote: OK, been very careful getting the timebase speed switch assembly out, all 16 wires lengthened and re- attached to the pcb.
Power up and we are back where we started, 2 dots in the middle won't move.
I have checked R152,R137,R136,R138. They are certainly not high but it is impossible to isolate them without cutting them out.
TR82 has 13.6v on the base, but there is only 0.1v across R136 so it is not conducting. That transistor is an FRB749, new to me, ( which has a beta of 77 ) spec says BC109C but the other transistors which should be BC109C too, are BC239C. Swapped a BC239C into TR82 from TR72 and vise versa. No change.
I have checked all diodes on this pcb, all OK.
Beginning to loose patience with this 'scope, its just so difficult to work on. I have a DM63 with a focus problem that may be a better bet!
Sam, we need a more focused approach (pun intended!)
Please, let's just look at TR82, as we know that has a problem.
TR82 is a current source. It is required to make the X-amp work correctly. But if there is 0.1V across R136, then there is no current, and this is why the spots are on the middle of the screen. Let's establish why.
You have measured the resistors in-circuit, but I think you need to isolate them to measure them properly. That is just a case of snipping one lead-out on the PCB side and bending it up. Very easy to reconnect afterwards with a blob of solder.
Can you clarify one thing for me? When you say "TR82 has 13.6v on the base", what does that mean? Is it +13.6V with respect to 0V (which is what your sentence implies), or is it -13.6V w.r.t. 0V? Or, is it 13.6V w.r.t. the -14V rail? It matters
A better way of describing the base voltage would be to state the voltage across R138. So, what is it? Earlier it was 0.9V - has that changed?
I see from your parallel thread on UKVRRR that the voltage across R138 ought to be 5.6V, which is quite possible based on the resistor values. If so, the current delivered is 33mA. Right now, there is nothing, and the only way to get current flowing is to have a sensible voltage across R136. And that can only come from R152 and R137. Mostly R137.
Sorry to drone on, but we really need to grip this first. Based on earlier advice I gave, I don't understand why you've done most of what you've done - we just need to conclusively eliminate R152 and R137. Please report back once you've measured them (having snipped one end of each first).
Cheers,
Mark
