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Old 8th Jun 2019, 2:04 pm   #83
WaveyDipole
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Join Date: Mar 2013
Location: Leicester, UK
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Default Re: Hameg HM307-3 Cleaning and Repair

Quote:
Originally Posted by OldTechFan96 View Post
Thanks for that, Wavey Dipole. I think I understand. Does it remove the faint line shown in the attached image?
Yes, that's a fine example of what happens when the return sweep is not blanked. You get a visible trace of the electron beam being swept back from right to left, but since this happens much faster than it takes to sweep the signal trace from left to right, it appears fainter.

Quote:
Originally Posted by OldTechFan96 View Post
What do you think about the BF440? With it having the same voltage on each junction I think it may be faulty. I have checked the resistors around the transistors and they are good.
After replacing C352 you now have a 33v difference between the -900v and -867v rails (-909-(-874) = 33) which is as it should be. There should also be a 100v difference between the -900v and the -1kv rail. Since you have -909v on the -900v rail, it may be simply that the -1kv rail is at -1009v which is not a major problem. However, the zener should still be maintaining a 100v difference between those two rails. If you measure 100v across C350 then all is well and good. If you are seeing less than that, then this might affect the maximum achievable brightness and there are only two components that are likely to affect this - C350 and the zener itself. Disconnect the capacitor. If the voltage across the zener now rises to 100v replace the capacitor, otherwise replace the zener.

Regarding the BF440, I am not entirely sure, but the reading does look odd. If you have a 2N2907, 2N3906 or 2N4404 to hand then you could try one of these. Even a BC327 might do at a push. It has a lower transition frequency, but it should be fine for a test. Just be careful about the pinout of any substitute. For example, the 2N4404 is EBC vs CBE.

Since we are dealing with pulsed signals it is difficult to understand what is going on at the output using a multimeter alone. The output pulse has an amplitude of +33v relative to the -900v point, but since the switching is so rapid, a multimeter would just average this out, with the result being affected by the duty cycle of the pulse. The voltage drop across the transistor alone would be fairly insignificant (approximately 0.6v) while it is turned on, however, since the blanked state duration is but a fraction of the unblanked state, I would expect the measured reading to "favour" the -900v unblanked state. The fact that the measurement seems to be actually at the blanked state voltage may suggest that T352 is hard on, which may be either because it is leaky, or D351 is faulty or both. Your strange test result does seem to suggest that all may not be well with the transistor, but I would check both transistor and diode anyway.

Last edited by WaveyDipole; 8th Jun 2019 at 2:14 pm.
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