Re: Just got a 475, nothing showing on display

Michael W. Lynch

On Wed, Jul 29, 2020 at 04:47 AM, donald collie wrote:

Is the fse on the main PCB at the back left blown? If so, it is almost
certain that the Tantalum capacitor associated with this circuit is
shorted......................Donald Collie
Check F1318 as Donald mentioned. Is it open? You should have 15V Unregulated at that fuse. If F1318 is open, check C1318, which is a tantalum cap in the primary of the HV transformer, this can short and stop the HV circuit from working. Check the power transistor Q1318, this is a common failure point as well on the same circuit.

+5: 4.99v, 48.5 ohm to ground, 0mv ripple voltage<
-8: -7.96v, 40.8 ohm, 0mv ripple<
+15: 14.98v, 68 ohm, 4mv ripple<
+50: 50.00v, 2.78k ohm, 36v* ripple<
+50 unregulated: 67v, OL, 31.9v* ripple<
+105/160: 139.4v, OL, 13.8v* ripple<
+110: 111.1v, OL, 20.9v* ripple<
Your Low Voltage power supplies seem to be working, the voltages are very close to ideal. Worry about ripple after later. You really need a scope to accurately measure ripple.

Concentrate on getting a spot or a trace. Shotgun replacement of parts which are not known or obviously bad is not the best way to proceed, replace only OBVIOUSLY burned or defective parts at this time.

Clean that beam finder switch with DE-OXIT or other cleaning method and exercise the switch, could be as simple as a dirty switch.

But it sounds like the same problem I have with a 465. I have a dead HV Multiplier<
Regarding the HV Multiplier that Stephen mentioned, this is also a very common failure which will open F1318. On most 465/475 Series scopes there is an isolation jumper on the ground or common side of the HV Multiplier, this can be a simple wire jumper or wire with a plain white ceramic insulator. If present on your scope, this jumper will be located under that HV shield of the bottom board near to and in front of the two nylon nuts that secure the HV Multiplier to the circuit board. This jumper is often not clearly identified in the manual. If you have this jumper, carefully unsolder and lift one end. This will take the HV multiplier out of circuit and allow the HV Transformer to generate the -3kV required for the CRT to partially function . Once the Multiplier is out of circuit, you should be able to observe some sort of visual reaction from the CRT, such as a short trace, a dot or a faint glow (depending on other failures).

WARNING: Keep in mind that you may now have potentials in excess of 3kV present IF you do this step and the HV Circuit comes back to life. Extreme care should be used when working on any component located under that HV Shield as High voltages may remain, even after the power is switched off.

Make sure that you have the correct version of the Service Manual for your scope. Study the Theory of Operation, Schematics and the Circuit descriptions. Do not attempt to proceed without the proper manual, either paper or digital.

A last recommendation (from someone who has been there recently): Focus on one problem at a time, do not try to diagnose/fix everything at once. Systematic diagnosis and repair yields the best results, As I stated previously, "wholesale" or "shotgun" replacement of components is not the most effective way to fix the problem. One lesson that I learned, REGARDLESS of the brand or type of equipment is that you must have the correct power supply voltages at all test points of the supply before proceeding to repair other faults. On this point, you seem to be on track.

Good Luck!

Michael Lynch
Dardanelle, AR

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