Metrix type 920 RF generatorOK could not resist ! I emptied my box of BANANIA where I have been keeping all my salvaged power resistors for the past 30 years now, in search of a suitable dropper resistor for the power supply. Manual says it's 10K 10% and 2 Watts. The one in place is 8.2K and more like 10 Watts, way too big.
Spent an hour sorting through all these resistors, eventually got lucky and found not one but two 10K ones, with a more reasonable wattage to boot. 7 Watts, half the size of the existing one, and still 3+ times more than what Metrix meant to use... so should do it.
One is a ceramic bodied / sand packed one, with radial leads, and reads 9,75K which is 2,5%, so plenty accurate enough, meets the required 10% easily.
However I chose to use the other one, as was much better : much older package, looks more period / fitting, has axial leads which is what we need, and Fluke DMM measured it between 9,99 and 10.00K ! good enough for me !
![Grin ;D](https://www.eevblog.com/forum/Smileys/default/grin.gif)
So I soldered that in place, and measured voltages. Previously we got 157 Volts instead of 135 indicated on the schematic. Now I get 145 V ! Half way there, still 10 more Volts to go... hmmm... what gives.
Anyway, it was enough of a change that IF that was causing my problems, it would now show SOME change, at least some... change. But it did not.. at all.
Modulation signal still runs at 360HZ bang on, no change at all. Did not make any effort to even slightly towards the expected 400Hz.
As for modulation factor, same thing. Still stuck at 50% instead of the required 30%.
So.... it was good fixing the power supply, needed doing, but it's not what's causing me problems.
So not sure there is much point trying to increase the value of the dropper resistor above the specified 10K. Voltage levels on schematics, I find, are really only a mere guide line... plus they are measured usually with a low-ish impedance analog meter not with a VTVM, so me using a DMM is bound to read a bit higher than what they measured with an analog meter...though it should not be enough to justify a 10 volts difference... that's a bit much.
Anyway, it does not worry me more than that, I would happily leave it at that...
So back to the other most likely cause : a resistor somewhere that drifted, or some bad cap...
But that's not the end of the world eh...I am just nit picking here. Most important was fixing the power supply.