Re: Space Duel HV advice

"Christopher X. Candreva" wrote:

> I've got a wave comming out of the 555, and on the collector of Q904.
> Instead of being amplified at Q905 (the big 2N6557) however, it becomes very
> small.
>
> The schematic gives some voltages at certain nodes of the circuit, I assume
> these are for troubleshooting purposes. It shows the collector of Q905 as
> 24.8v. I see -25v. It shows 5.9/5.4 at the base/emitter of q900 (the BU409),
> I have 26v at both (same as collector). The tail end of ZD902 should have
> 29.4v, I again show -25.

Ahhh. Collector of Q905 is definitely wrong. If you look at the schematic you
will see that the circuit of Q904 and Q905 are set up to basically invert the
signal coming from the 555 (polarity of the signal doesn't matter, it just
needs to go high and low at 15KHz). When Q905 turns on it will conduct current
from the collector to the emitter. The emitter is hard wired to to -27V. The
schems show the volage as being positive. Why... If you look at the schem,
there is only one way that the collector can be a positive voltage... through
the drive coil and via R917. The purpose of the coil is to induce of voltage
onto the base of Q906 and turn it On/Off while isolating it from the rest
of the circuit. That is a bit beyond what we need to deal with tho. The
problem is with the collector voltage. If you measure with a volt meter
you should have +27V on the top side (schematic) of R917, if you don't, then
you have a cold/cracked solder joint between the +27V input pin and that
resistor. Being that R917 is only a 3.3 Ohm resistor it should maybe have
about +25V there. If you don't (voltage is much lower), the resistor is
bad (open). You may also have an open driv coil but I have never seen one
go bad. It usually reads about .7 Ohms so you will have very little voltage
drop across it. Check the resistor, Im 99% sure it is open.

report back.... :-)

>
> I do see the correct -19.2v at the collector of Q904.
>
> I replaced Q901, 902, and 903. I swapped the 2N6557 with one in another
> monitor -- it was good. The BU409 and BU207 test OK.
>
> I'm really open to suggestions at this point.
>
> -Chris
>
> ==========================================================
> Chris Candreva -- chris@westnet.com -- (914) 967-7816
> WestNet Internet Services of Westchester
> http://www.westnet.com/
Received on Tue Oct 19 00:44:15 1999