开云体育

I have been working on a 7844 dual beam scope that has a beam intensity problem. The scope was working, then the power supply quit with a rapid tick. Ok I had another 7844 that has a different problem but a good power supply. Swapped the power supply, now both beams are overpowered, even at minimum intensity, power supply voltages are all in tolerance. The intensity controls do increase intensity but can't reduce the intensity below what was close to maximum before. Focus controls can adjust the focus but not enough. Readout intensity is normal.

Thoughts of where to start checking would be appreciated!

Mark
W7HPW

Mark

The d-c restorer diodes are bad, replace all of them with 1N4937. Also replace the 22meg 1/4W carbons with Vishay VR37 series, 594-5053DM22M00F. Get 10 of these to replace the 8 in the supply or get enough for both supplies and any other supplies you have. When these go too high, the brightness is high when cold then dims after 15-20 minutes without touching the intensity control. In mine, I recapped the scope and replaced some resistors with higher wattage.

I did put heatsinks on three of the four on each horiz. board. This is because two transistors are close and the heatsinks would not fit on both.

In mine, I did put in BUX48A as replacements for the TO-3s in the power supply. The beta was 1 for three and 2 for the fourth (two scopes). Both were hard to start and would tick several times before starting. After the replacements, instant start without any tick.

Mark

Mark,

A correction. There are (4) 22meg resistors in each scope, not 8.

Mark

Mark,

Thanks for the tip, but I am unsure where the d-c restorer diodes are, but I think you mean CR2352-55 and CR2342-45 and R2356 and R2346, all on the HV board A24?


?? Mark

Mark,

You are correct about the parts and board number. Also R2326 and R2336.

On the A21 and A22 boards, the 9100 ohm 1/4W and 20k pot are known to go bad due to being run beyond the rating of the parts. I put in a 56,000 ohm 1W in place of the 9100 ohm and used a 3/4W 20,000 ohm 10 turn pot to replace the original 1/4W. Pots in this area are likely open. That will also cause the intensity to go way off. The ones I removed from mine and other 7000 series felt wrong when you turned the adjustment. Some were open. The lower current for the bias circuit and higher resistance does not affect the bias or intensity. You will use short solid wire from the control to the board. The 'bottom' wire from the pot can be soldered to the pad which also helps secure it. The ones I used and had in stock are the blue body ones with the small adjustment on one end. Mouser carries these. The fine adjustment allows for the bias to be set easy. I did increase wattage of the 8200 ohm and 750 ohm to 1W on the A21 and A22 boards. I also put heatsinks on Q1150 and Q2050.

Mark