Hello. Y will try to relate my experience with an OS-5100RA with the same crt drive board.
The first symptom was F741 burning aparently without reason at intervals, at the begin
of weeks, after days, and finally hours. Simultaneously and sporadically with this fault,
the crt filament no turn on, in spite of having a good oscillation level on collector
of Q746 (2SD613F). Dont try to measuring the filamet voltage with a tester, because
if the transistor is working, you will have 1.47KV with respect to ground. After verifying
the crt´s socket, inclusive changing it, I noticed that diconecting the crt´s socket,
and measuring the conduction of the filament winding from the H-V Module, it measured
open. So that taked out the H-V Module, and after cuting the black plastic cover in
the line of the two points that feed the filament, I found that the two wires are soldered
to a printed circuit at the top of the module and was VERY bad soldered.
To removing the pink sealant used as insulation, I used a hot air blower at
aproximately 180° centigrades, (356 fahrenheit) and a screwdriver. To isolating this again I used hot glue and a layer of PTFE of 0.5mm thicknes, (0.019685 inch) covering the cutted parts.
As the voltage at this point is 1.47KV, you can use maylar instead of PTFE.
After this, the filament always turned on with Q746 working ,but keeped burning
F741 sometimes. After some days F741 blowed and Q746 shorted. After checking
the H-V Module´s conection to the colector of Q746 have found the same fail as the
filament. After applying the same procedure as with the filament, the conections
was fixed. BUT after trying several transistors to replacing the 2SD613F without success, found the datasheet where says that the "F" means hfe from 160 to 320. After comparing with other of my oscilloscopes, an Tek 7623, noticed that the transistors used in the HV tranformers in oscilloscopes must work in the lineal region,and therefore the base bias and coupling are low. This means that on the colector you must have a sinusoidal waveform. In my case, Y found an old good quality TIP31 (Motorola) that worked OK. About the schematics Y have found that is very similar to LG Goldstar OS-3020, in particular the file "LG Goldstar OS-3020 Schematics PT2.pdf" page 5. I hope this information can help you.
Hello. I found two photos that I took when I did the first repair of the H-V Module.
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