| Electronics > Repair |
| Quick repair project - Marconi 2024 |
| << < (3/15) > >> |
| mianchen:
It's very impressive work! Well done. |
| comprotudo:
Hello, I have also a 2024, witch a "little" problem. When switch it on and activate the control knob to tune the frequency, level etc. he does not work, even I can see the under line, the values does not change. After waiting a little I switch the generator on/off and everything work fine. Does anyone have the same problem? Any clue about that?? Regards Rodrigo |
| grumpydoc:
Maybe the rotary encoder is unreliable. One of the menus has an option to do a keyboard and encoder test, try that to make sure the encoder is working correctly. |
| comprotudo:
I have checked that. When it runs ok I'm able to check the rotary encoder. When it fails at beginning, the rotary check fail. Probably the generator makes a initial check and some times he can't "see" the encoder. I need some one can confirm this fact. Rodrigo --- Quote from: grumpydoc on May 02, 2013, 04:29:23 pm ---Maybe the rotary encoder is unreliable. One of the menus has an option to do a keyboard and encoder test, try that to make sure the encoder is working correctly. --- End quote --- |
| grumpydoc:
To continue an old thread.......... The Marconi had been behaving itself nicely for quite some time but it looks like it wants to attract some attention to itself. I was using it for some bandwidth/frequency response checks on a 'scope I've just repaired and tried to dial up a 20MHz output only to be greeted with and no output either - rats! :-BROKE Some fiddling later I noticed that it's only lost output from 18.75MHz through to 37.5MHz, everything else is fine, so checking the service manual I noticed this in the description of the error OK, that looks promising in terms of narrowing it down, lets have a look at the schematic. The band pass filters seem to use steering diodes to switch themselves in and out of the signal path. Click on the image above for the full circuit diagram of this section. So it looks like SW9 is the switching voltage point - that comes from here I had to reduce that a bit so it's slightly hard to read - again you can click on the image for the full sized one. Basically two 74HC138's are used to form a one of 16 decoder and then three LM324's are used as comparators/level shifters to drive the filter sections. One thing which does seem odd is that SW10 has two comparators driving it (IC604a and IC606b) as does SW1 (IC604c & d) but the others just have one comparator. Absolutely no idea why they did that I'm really hoping that one of the 74HC138's outputs has died - or the comparator. If not I'm stuck, the only other active components are the steering diodes (which I suppose it could be). After that I can't see any of the inductors or caps in the filter section suddenly keeling over. It's a bit late to open up the RF section and start looking so that will have to wait until tomorrow but I'll report back. In the meantime - thoughts anyone? |
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