Wells Gardner K7500 - Turns on then off

[jconstanza]

The subject pretty much says it. The monitor turns on for about a split second then shuts off. I have replaced all the caps, & U101 (UC3842). I tested the HOT Q704 with my meter, it didn't test open (no long beep). Is their a possibility the HOT is bad or getting weak, is that even possible?

I am an amateur, any help I would greatly appreciate!
Thanx!

[grantspain]

http://www.wellsgardner.com/pdf/repair/K7500.pdf

[Kevin Mullins]

I tested the HOT Q704 with my meter, it didn't test open (no long beep).

Just for clarification ..... a long beep would indicate a SHORT or continuity ..... not open.

[sarge]

Try isolating your power supply from the monitor using a 60W light bulb. Do this by unsoldering R728 and R100(jumper) at C121. This isolates the power supply. Hook up the bulb between the pad where the resistors were hooked and ground. If the light bulb lights, you know it's in the secondary. Measure your B+. Should be about 118vdc. If it does not light, then you have a problem in the power supply of your monitor.

[jconstanza]

Well I sort of did that bulb test, I actually just lifted the one leg on R100. Used the frame as ground and the leg as positive. It lit up. Is that a bad way? I also have another 7400 that just ticks, I have replaced caps, HOT, and U701 on both and still got nothing.

Thanx!

[jconstanza]

I'm sorry I got the positive voltage from the leg off of L100

[sarge]

What did your B+ measure?

Are you the one also posting these problems in startech's forum?

[jconstanza]

I'm not getting anything for my B+ (TP 202). Is that where I should check it?
Yes.

[SirPeale]

If it's going into shutdown, the B+ should be zero, right?

K7500s are finicky beasts.  Try turning down the B+ pot JUST A HAIR.  And I do mean just a tiny itty bitty little bit.

[sarge]

I mean measure it with your power supply isolated and only feeding the light bulb.  If your bulb lit up you had to get some voltage. 

[jconstanza]

I am measuring the voltage off of the leg from L100 and I am getting 120vdc in which I can adjust. Their is some ticking right before it lights up, not much maybe about five or so rapid ticks.

Again, is this the proper way to isolate the power supply?

[sarge]

If you disconnected the circuit like I described in the above post then you could measure off of L100.  And since you are getting 120V and the light bulb lights your power supply is operating.  Although it shouldn't have to tick before it lights up.  The only thing I have seen cause this on a 7500 power supply is C117.  Are you sure you changed that one?  Its a 22@63 according to the schematic but I usually step the voltage rating up to a 100.