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Old 7th Jun 2019, 12:05 pm   #78
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
Z352 had -900V on each side. Checked C352 (1uf 25V tantalum) and its resistance varied between 10KΩ and 20KΩ. Replaced it with a ceramic capacitor and the voltage on the cathode of Z352 was closer to what it should be.
Sounds like someone replaced it at some point with an unsuitable part. There is bound to be 33v across the zener diode and therefore also across the parallel capacitor, which is why the circuit diagram indicates that a capacitor rated for a minimum of 40v should be used. Tantalum capacitors are very particular about excessive voltage being applied to them and tend to fail short.

Quote:
Originally Posted by OldTechFan96 View Post
The voltages on T354 were higher than expected. Measured 140V rail and it was a bit high at 148V. The output of the rectifier was also high at 198V. I noticed that C305 was leaking electrolytic on the PCB. It was removed and the board cleaned up with IPA. R304 is good at 1200Ω. This is one of the hot resistors mentioned earlier in the thread.

I replaced it with a 47uf 450V radial capacitor I had around. I probably will replace it in the future with a 250V axial capacitor.

This only brought the 140V rail back to 146V, still a bit too high. I noticed in my notes that in the past the rail measured 133V. I wonder is something attached to the 140V rail is drawing less current? 140V goes to the astigmatism control and the Y final amp.

Attached are some diagrams with some more voltage measurements.
Those two rails are unregulated so the voltage will vary depending on whether there is a load connected or not, and what the mains input supply is doing at the time. AFAIK, mains voltage in the UK is 230v +10%/-6% so the voltage on the transformer secondary can be expected to vary by a similar amount. As a result the rectified DC will also vary, so some variance is to be expected. You have cleaned up the leaking electrolyte and replaced the capacitor so if C306 and C307 are OK then it sounds like you are good to go for now.

With regards to R304 getting hot, the fault in the Y-preamp may have been a contributing factor, stressing the final stage and putting excessive demand on the supply. That might have been sufficient to make R304 run a bit hot and drag the voltage down. However, faulty smoothing capacitors can also cause such problems. With repairs having been made to the Y-amp and preamp, it one might expect it to run a bit cooler now. Of course, with a 4W resistor some heat under running conditions is only to be expected, but if it is still running excessively hot, you might also want to make sure that C307 is not electrically leaky.

With regards to the voltage reading at the emitter of T354, this will vary somewhat with the input voltage, but will also very much depend on the position of the 100k preset R369. The diagram shows circa +85v, which is approximately what might be expected when correctly adjusted, but 100k provides quite a wide range of input to the base of T354 so the voltage at the emitter (i.e. to pin 8 of the CRT) will also vary considerably as the preset is adjusted. I imagine that the preset is positioned close to one end of its possible travel? May also be worth checking that R370 is not gone high. For now it might be sufficient to simply adjust the preset to obtain approximately +85 volts at the emitter, or just leave it as it is. Once you have a running trace and can display a square wave, you will then be able to adjust this control visually for an evenly spaced trace.

It does sound like you are making good progress. Has the brightness of the trace improved now that you have replaced that tantalum capacitor and now that the voltage across Z352 is closer to what it should be?

Last edited by WaveyDipole; 7th Jun 2019 at 12:12 pm.
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