On 10/18/2020 7:32 PM, David Farber wrote:
On 10/18/2020 6:38 PM, Phil Allison wrote:
Â* David Farber wrote:

====================

By the way, I noticed a potential problem with checking the input
waveforms to the gates of Q800 and Q802. If I put one scope probe across
one transisor, from G to E, that should be OK however if I want to see
both waveforms together while powering up the unit and then connect a
second probe to the scope on the other transistor, the common ground of
the probe will short out Q802. Should I just float the ground on one of
the probes? Will that work?



** Both signals come from the same transformer - out of phase.

You can monitor the outputs of the 3525 IC or the primary side of the
tranny without the iso in use.

Plus check continuity of secondary windings and their connections to
the IGBTS.

....Â* Phil


Thanks for following up with the details Phil.

--
David Farber
Los Osos, CA

Note: The tests I performed below were without the control board
connected. The control board is responsible for connecting the earth
ground to the audio ground. The schematic of the control board is he
https://cdn.shopify.com/s/files/1/04...feb10.pdf?2161

The schematic of the main board is he
https://cdn.shopify.com/s/files/1/04...feb10.pdf?2161

I received the correct parts and installed them. I checked the waveform
at the primary side of T2. 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. I checked the signal across Q802 from drain to source and it
was a very clean square wave and measured approximately 400 Vpp. 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