On 11/9/2020 8:06 PM, Phil Allison wrote:
David Farber wrote:
==================
As I powered it up slowly with the Variac,
there was no gradual change in the duty cycle. It was either flat line
or full on.
** Very bad.
Check the 22uF on pin 8 of the PWM IC.
This cap enables the Soft Start circuitry.
https://www.ti.com/lit/ds/symlink/uc...ct%252FUC3525A
Using a Variac slowly may be an issue, but nothing will happen until the +/- 15V rails appear, then it should ramp up the PWM in about 1 second.
.... Phil
I hadn't realized that the ramp up occurred within 1 second. I powered
it on without the Variac and sure enough, the waveform made a gradual
change within that 1 second timeframe. Everything seems to powering up
OK. Checking the audio will be next.
I'm still curious about the power supply question I asked in my previous
post (see below) regarding how the two bridge diode circuits seem to be
adding voltage in a series circuit.
============================================
At this point I could reassemble everything and see if it actually will
perform properly. But before I do that, I would like to figure out the
operation of the power supply circuit that is rectified by diodes
D805-D808. It's easy to see that when pin 20 of T3 goes positive, D805
conducts and becomes the V+ line. From that point, it feeds the load and
then I imagine it ends up going through ground via the terminal marked
HI RAIL on T3 and also through the other ground circuit which is in
series with the array of capacitors, C811-812, C849-850, and C34, C36,
and C51. Apparently there is also a connection to the bridge circuit,
D809-D812. Is there some voltage adding going on there with the
secondary transformer windings interacting with the two bridge circuits?
This is not a configuration that I have come across before. At the very
least I'd like to know where ground is for the V+ supply coming from the
cathodes of D805 and D806. I am measuring 90 volts individually across
capacitors C850 C852.
Thanks for your reply.
--
David Farber
Los Osos, CA