Paul Barker wrote:I believe the absolutely safe way to sort this out for the 813 is for me to obtain one of the transformers you have and use the HAMEG to set the voltage to 10v. Then you duplicate.
So where do I get this transformer?
The transformer is a Vossloh-Schwabe made in Germany, Lite Line 105/12. I guess you will not find it on the local market. Besides, I think that we can work to approximate voltages with any given unit. Probably the best solution will be to work on the output transformer as well.
You actually helped a lot by pointing out that the Fluke 116 does not cut the mustard. The guy in the video uses a Fluke 87... that unit costs 4-5 times more.
I did some more thinking and thinkering and found some correlation. The voltage increase you have seen is proportionate to the reduction in ripple, i.e. the increase of DC RMS voltage at the "input" where we add the capacitor. My voltage increase is a little higher since I added a 330uF cap (I had those at hand). My increase in DC voltage is 310/208=1.49 (just measured with and without cap).
The nominal voltage was 11.5 (thus across 2x 0.15 ohm I should get 10V RMS across the heater). Voltage across the heater is slightly above 22V, and dividing by the factor 2.29 this is very near 10V (as expected). Voltage drop across the 0.15 ohm resistor shows a current draw of almost 5A (again calculated with the 2.29 factor), this sligthly lower draw corresponds to the slightly lower voltage than 10V.
On the tweaked unit voltage across the heater gets about 31V which is about 13.5 V (!) if the 2.29 factor is correct. If 13.5V, current draw is probably above 5A. Voltage drop across 0.63 ohm resistors should be at least 3.15V, meaning that the voltage at the output of the transformer should be at least 16.65 (probably more, if current is proportional it could be 6.5A, and the voltage drop up to 4.26V, meaning 17.76V!). 11.5V x 1.49 = 17.135, which we can assume as a reasonable approximation!
I would need to burn 7V, and this means more than 1 ohm resistance. Now it seems much easier to tweak the output transformer! The result I am after should be around 22-23V under load (measured with my DMM, based on the assumption that with a square wave average and RMS are the same, and since I am measuring average P-P, the correction factor 2.29 gets us the "correct reading"...
Now I need to try to tweak my output transformer... if possible (should be fairly easy if toroid).
Have you got some "normal" garden variety DMM that measures average AC values? You could check the math by comparing your HAMEG with the garden variety average unit...
I was also considering to go and buy some less sophisticated units to tweak if mine do not have easily tweakable output transformers... anyway, the video you linked to is enormously helpful (but the price of the Fluke 87 is very high).