1176 rev D power supply low, due to low mains voltage?

[ibvee]

Got an 1176 rev D here, which had some trouble. Recapped the thing, sounds much better now.

Also recapped the power supply, after which the DC voltage went up from 24V to 27V, after that I changed the lamps to the original ones, in there were 756 because the original 1819 weren't bright enough. However they seemed to draw too much current, now with the original 1819 lamps the voltage has increased to 29V.  Without the lamps the voltage measures 30,5V which it should be, as there's a 30V zener to regulate it. However, with the lamps in, it doesn't come above 29V, at which voltage the zener doesn't do anything. Just to be sure I also replaced the rectifier diodes, but this didn't do anything.

Now looking at the schematic, the transformer is rated for 120V or (in this case) 240V. The mains voltage here where I live is 220V. The secondary of the transformer measures (under load) 28V, which makes it more or less a 1:8 transformer. Which should measure 28*sqrt(2)-1,4 = 38V after the diode bridge. However, (due to the lamps) the voltage there is now only 32V.
Could it be that the design requires those extra 20/8 = 2,5V from the transformer, which it gets with the 240V mains voltage. in order to get the proper 30V volts? Or am I overlooking something?

Any thoughts?

Thank you!

 

[PRR]

Factory-made or home-made?

Did you replace the 2,000uFd power caps?

This design IS very fussy. It can't take much raw-supply variation with only 10% headroom above Zener voltage.

First because it is obvious: move the hot-leg of the lamp-string from the positive rail to the lighter-loaded negative rail. That might un-sag the pos rail enough to regulate, without hurting the neg rail.

Then I suppose the obvious next step is a 230V:12V 0.5A (6VA) transformer wired as a 220V:231V auto-transformer, put in front of the 1176 power plug.

 

[ibvee]

Thank you PRR.

This is a factory made unit. Has been modified along the way as it appears. The input transformer has dissapeared. Probably should find another in order to change it back to the original stage again.

Yes, the 2,000uFd are indeed replaced.

thank you for confirming my thoughts, and the suggestions. I'll get back to work.