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Post by Deleted on Jul 28, 2015 10:37:14 GMT -5
Hi I am having again a problem with one neckboard. This neckboard is slightly modified, only some caps have been changed and some diodes have been removed and some resistors where replaced. See the schematics. Also in the G2 section two resistors where replaced with identical values new ones with some ferrite beats on one. Nothing shocking. I have these exact same modifications on another standard board and it performs super. This board did perform well a few hours after the changes and than suddenly when I started the 9500 after 30 seconds or so the neckboard failed. I put in another neckboard and everything was ok. What it does now is it goes form low brightness to high brightness and repeats. Sometimes it stays at constant brightness. There is no video and I can see the backtrace lines as if there is no G2? I careful checked the replaced resistors and everything in the G2 part and they where all ok and they where soldered well. I know I very carefully checked all replaced caps and checked all solder connections and they did perform well for a few hours. So they where ok. Also now I can find nothing wrong there. Who has any idea where to look. What could explain this? This board was new to me and not tested long.   |
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Post by gjaky on Jul 28, 2015 11:08:59 GMT -5
I am pretty sure you have a failing transistor there, first candidates are Q12, Q19 and Q26 to check.
Surprised to see that MP removes D10 and D12, while they indeed loading the output stages capacitively, also have an invaluable role in protecting the output transistors from failing when an arc strikes...
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Post by Deleted on Jul 28, 2015 14:56:07 GMT -5
I did a quick check on all transistors with diode meter and they all looked normal. Will check better tomorrow. But I am thinking that seeing the retrace lines would that not indicate that the G2 is gone? Can you explain why you think it is in the video path?
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Post by gjaky on Jul 28, 2015 16:50:40 GMT -5
Well you said you have checked already a few things on the VNB, and since the G2 circuit is not very complicated I'd think the problem is elsewhere.
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Post by Deleted on Jul 28, 2015 17:38:55 GMT -5
Well you said you have checked already a few things on the VNB, and since the G2 circuit is not very complicated I'd think the problem is elsewhere. I was thinking about replacing all thes G2 resistors but I checked them all so you are right. perhaps I have to reverse all modifications if I can not find a transistor that is broken. To bad this board had a very good bandwidth. When I combined it with the higher bandwidth Moome it became to sharp and to noisy so putting in the lower bandwidth Moome made it just right. |
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Post by mastertech on Jul 28, 2015 17:58:49 GMT -5
You have to check that spark gap and cap on the G2 line. Is that spark gap open or closed frame?
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Post by barclay66 on Jul 29, 2015 2:49:44 GMT -5
Hi,
In order to rule out that the problem is related to the G2 section, You could try measuring the voltage after R81 (10K). For that You will need a DMM that can withstand voltages up to 1000V. If the voltage that is measured (should be around 500-800V, depending on the G2 setting) doesn't change with the brightness pulsating, then the problem is somewhere else.
Like gjaky I don't think that it's the best idea to remove D10 and D12 and I don't see why it should do any performance improvement.
You might want to check C17. It's the only coupling capacitor on the VNB. Maybe Your new cap (is it 43µF? It's difficult to read on Your schematic) is defective or quadrupling the original value isn't doing good...
Regards,
barclay66
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Post by gjaky on Jul 29, 2015 5:46:44 GMT -5
Just as a side note.
Actually the only real performance improvement on that modified board is done by those two removed diodes, the rest are just small trims.
The CRT is seen by these amplifiers mostly as a capacitance around 10pf. The maximal slew-rate is limited by the output capacitance (load) and by the maximal current of the amplifiers. The maximal output current is inherent for the circuit and is hard to increase by "simple" modifications. The two diodes being reverse biased acting as two capacitors tied to the output, each of these diodes have a capacitance of 1,5pF, therefore they are increasing the capacitance on the output by 30%. So removing these diodes can indeed result in significant improvement in performance at the price of the lower reliability.
A good compromise here if there are multiple diodes used in series, eg. using 2-2 diodes of this same type will cut half the added capacitance by the diodes, and the protection function only slightly worsened.
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Post by Admin on Jul 29, 2015 9:15:44 GMT -5
When editing schematics and posting them for clearer view you are better off keeping them in their pdf format. This way they can be zoomed in and no clarity is lost. Note page 2 where "test test test" has been added. 00-260338-01P test.pdf (78.1 KB) |
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Post by Deleted on Jul 29, 2015 17:08:25 GMT -5
HE Guys! Good news. I first replaced the resistors in the G2 section and rechecked but nothing changed. I checked the surge protector there with the ohm meter it had some resistance far in the mega Ohms. How do you check these surge protectors? But than I soldered original caps back in the end section also the bp elco (C17). Also resoldered the resistors. After that tried again and yes it was ok again. I guess I had made a bad solder connection around the resistors or C17. Although I have the old caps in the end section again I see no difference in noise or bandwidth. Also the G2 section is original again with little difference. I know doing those mods reduced noise but I do not know which parts do cause that the most. However just when this neckboard worked again the other broke down. Yes I am so lucky  That other board had no light at all and there was not even glow in the heater. I measured a normal conducting path to the heater pins so yes the surge protector must have failed. Again I do not know how to check them but after replacing that board worked well again. On my modified board the surge protectors for the heater where not original. In the picture they are the smd brown part that is close to the black part. Does anyone know what kind of surge protector this is? Would this one make less noise?  |
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Post by Deleted on Jul 29, 2015 17:19:32 GMT -5
There is a schematic with modifications in pdf
Not drawn are changed D5, D7 and FB1 and FB2
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Post by barclay66 on Jul 30, 2015 1:59:53 GMT -5
On my modified board the surge protectors for the heater where not original. In the picture they are the smd brown part that is close to the black part. Does anyone know what kind of surge protector this is? Would this one make less noise? Hi, This looks more like a ceramic capacitor. You could try measuring it if you have a capacitance meter... Regards, barclay66 |
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Post by barclay66 on Jul 30, 2015 2:09:46 GMT -5
I checked the surge protector there with the ohm meter it had some resistance far in the mega Ohms. How do you check these surge protectors? Hi, These aren't surge protectors but spark gaps (therefore their designation SG1 ... SG3). They are designed for prevention of arcing as they will allow for a controlled path for the energy to discharge. The spark will stay inside and will (hopefully) prevent other parts from being damaged. Therefore their trip voltage is shown in the schematic. There's no practical way of testing these as they normally will be the same as an open connection. The megaohms that you measured were the resistors R81 (10K) and R33 (22M) which form one resistor (22.01M) in parallel... Regards, barclay66 |
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Post by Deleted on Jul 30, 2015 5:20:45 GMT -5
On my modified board the surge protectors for the heater where not original. In the picture they are the smd brown part that is close to the black part. Does anyone know what kind of surge protector this is? Would this one make less noise? Hi, This looks more like a ceramic capacitor. You could try measuring it if you have a capacitance meter... Regards, barclay66 Ok I tried and you are right it is a 1uF capacitance that was placed there. The schematic says the part is called D5 or D67 (smbj43c) it was modified into a capacitor. Now this capacitor had failed making it a short and making the heater voltage 0. I can imagine that capacitor being there to filter noise from the heater voltage but why would the original schematic put D5/D7 there? I think D5/D7 is a surge protector diode? Wait a moment it is a zener diode at 43V. It should stabilise the voltage I assume? I see it is a ESD suppressor. What does it protect? The electronics that make the heater voltage on another board? |
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Post by Deleted on Jul 30, 2015 5:22:16 GMT -5
I checked the surge protector there with the ohm meter it had some resistance far in the mega Ohms. How do you check these surge protectors? Hi, These aren't surge protectors but spark gaps (therefore their designation SG1 ... SG3). They are designed for prevention of arcing as they will allow for a controlled path for the energy to discharge. The spark will stay inside and will (hopefully) prevent other parts from being damaged. Therefore their trip voltage is shown in the schematic. There's no practical way of testing these as they normally will be the same as an open connection. The megaohms that you measured were the resistors R81 (10K) and R33 (22M) which form one resistor (22.01M) in parallel... Regards, barclay66 Ok thanks the SG is a controlled spark gap. Makes sense. |
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That other board had no light at all and there was not even glow in the heater. I measured a normal conducting path to the heater pins so yes the surge protector must have failed. Again I do not know how to check them but after replacing that board worked well again. 
