20-11-2015, 04:53 PM
(19-11-2015, 07:09 PM)pwdrive Wrote: One of the IF transformers not resonating at the correct frequency might also cause an increase gain (apparent) due to capacitive shunting due to the meter probes and leads, a 10 pf loading would shift the IF transformer primary down by about 12 khz according to my calcs, beyond the half power point possibly, the only proper way is by circuit alignment checks.
Lawrence.
Given I'm satisfied the output and power circuits are now performing O.K., it seems clear it's the Mixer stage that now needs some attention. I did spend a fair bit of time on the H.T. and output part of the circuit and this finally involved replacing one of the large H.T. cans.
So, today, here is what I found so far. The aerial does seem to be connected O.K. The switches could be a problem area but seem fairly robust (certainly by today's standards). They are ganged and I did trace the aerial lead to the switch.
The first thing now that has my attention is the resistor to the Oscillator Control Grid. I ought to point out, though, that access to the mixer tube base is a nightmare in itself. In fact, I can't even see directly the solder point to the grid. I have to use a lamp to even stand any chance of seeing anything and am totally baffled as to why the manufacturers boxed all of this in!!
Assuming this is indeed the control grid resistor it measures about 53 K. The schematic gives I think 48 K. I have a strong feeling this must be the Osc C.G. resistor as it connects between chassis and grid at the same point as the cathode. That is, with schematics you often find where there is a resistor connection to a pin, it only shows on the schematic as chassis if it shares a ground with the pin itself. There are no resistors in the schematic at 53 K. I think I'll buy a new battery for my meter today and double check this figure. Possibly though I may need to start by changing this resistor but I'll make sure I'm right first.
I can't say I'm all that impressed by the way this set was built. I'm sure it will sound great and has many good points in the design but I dislike boxing in tube bases. I may even bypass sets in future that don't grant at least some access to the bases.
The actual anode I tested for voltage does, of course, connect to the I.F. Transformer.
