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Post by barclay66 on Jul 30, 2015 5:56:29 GMT -5
I see it is a ESD suppressor. What does it protect? The electronics that make the heater voltage on another board? Hi, It is quite probable that these suppressors are there for protecting the LVPS. Especially if the corresponding tube experiences a heater <-> cathode short which can happen in rare occasions. I think that there are lots of fault sensing and/or protection circuits in a Marquee and that a great deal of simplification could be done to the boards (ever heard about "Muntzing"?). But I wouldn't do that. After all, the reason for modding boards should be to increase performance without sacrifying reliability... Regards, |
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Post by Deleted on Jul 30, 2015 6:09:58 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. Thanks for explaining the diodes. Also another question. Does anyone have an idea why C67 and C66 are replaced with smd versions of the same value? The resistors that where replaced where original carbon so I guess the metalfilm resistors are a bit less noisy. Making the capacitance of C17 bigger is also somewhat a mysterie to me. Low frequencies are less attenuated? |
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Post by Deleted on Jul 30, 2015 6:11:17 GMT -5
I see it is a ESD suppressor. What does it protect? The electronics that make the heater voltage on another board? Hi, It is quite probable that these suppressors are there for protecting the LVPS. Especially if the corresponding tube experiences a heater <-> cathode short which can happen in rare occasions. I think that there are lots of fault sensing and/or protection circuits in a Marquee and that a great deal of simplification could be done to the boards (ever heard about "Muntzing"?). But I wouldn't do that. After all, the reason for modding boards should be to increase performance without sacrifying reliability... Regards, Hi Barclay, these mods where done by MP. |
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Post by barclay66 on Jul 30, 2015 6:22:47 GMT -5
Also another question. Does anyone have an idea why C67 and C66 are replaced with smd versions of the same value? The resistors that where replaced where original carbon so I guess the metalfilm resistors are a bit less noisy. SMD parts show less stray inductance than their leaded counterparts. Maybe this has a positive effect. Making the capacitance of C17 bigger is also somewhat a mysterie to me. Low frequencies are less attenuated? Well, this isn't clear to me either. Maybe it creates more "punch" on the picture as the transfer level of the signal (DC block and AC pass) is increased in general. Regards, barclay66 |
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Post by Deleted on Jul 30, 2015 6:34:16 GMT -5
I see it is a ESD suppressor. What does it protect? The electronics that make the heater voltage on another board? Hi, It is quite probable that these suppressors are there for protecting the LVPS. Especially if the corresponding tube experiences a heater <-> cathode short which can happen in rare occasions. I think that there are lots of fault sensing and/or protection circuits in a Marquee and that a great deal of simplification could be done to the boards (ever heard about "Muntzing"?). But I wouldn't do that. After all, the reason for modding boards should be to increase performance without sacrifying reliability... Regards, Ok I still do not get what exactly happened yesterday as when these protecting diodes are broken they should pull the whole heater line down and as I can see all three heaters should fail. I see only one connector from the psu. Also I measured all these caps that replaced the diodes but no one measures a short. I think it might have been some bad connection after all. |
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Post by mastertech on Jul 30, 2015 9:12:07 GMT -5
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? You have me a little confused here. D5 and D7 are circled in the attached picture. What are you saying was modified into a cap?  |
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Post by Deleted on Jul 30, 2015 9:49:29 GMT -5
D5 and D7 where! Two caps.
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Post by barclay66 on Jul 30, 2015 10:37:50 GMT -5
Yes,
Mike put in capacitors where the diodes had been. That seems to be a standard procedure on boards he's working on. I too have some MP modded neck boards and there the inductors (FB1 & FB2) were replaced with different ones and the diodes D5 & D7 were replaced with SMD capacitors...
Regards,
barclay66
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Post by Admin on Jul 30, 2015 10:47:48 GMT -5
Ahhhh, so he is decoupling the heater lines for noise but in turn taking away the protection. Why not piggy back the caps on the TVS's. And that's if, and I mean "IF" there is any noise on the heater lines and "IF" you could even see it in the image.
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Post by gjaky on Jul 30, 2015 10:59:03 GMT -5
Ahhhh, so he is decoupling the heater lines for noise but in turn taking away the protection. Why not piggy back the caps on the TVS's. And that's if, and I mean "IF" there is any noise on the heater lines and "IF" you could even see it in the image. Nah, the prescence of the TVS would alter the picture quality...  |
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Post by gjaky on Jul 30, 2015 11:05:13 GMT -5
Yes, Mike put in capacitors where the diodes had been. That seems to be a standard procedure on boards he's working on. I too have some MP modded neck boards and there the inductors (FB1 & FB2) were replaced with different ones and the diodes D5 & D7 were replaced with SMD capacitors... Regards, barclay66 Are D10, D12 also missing on your board? |
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Post by barclay66 on Jul 30, 2015 13:54:25 GMT -5
Are D10, D12 also missing on your board? Nope. |
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Post by Deleted on Jul 30, 2015 15:05:09 GMT -5
Ok I put the smd caps on C67 C66 and C59 again. Put a ferite beat on R76 and changed the FB1 and FB2 for unknown FB. Did that last thing on two boards so only the TVS are original. My first impression was that the noise was substantial lower. After a movie I still think that. No idea what part does it.
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Post by gjaky on Jul 31, 2015 5:29:03 GMT -5
Ok I put the smd caps on C67 C66 and C59 again. Put a ferite beat on R76 and changed the FB1 and FB2 for unknown FB. Did that last thing on two boards so only the TVS are original. My first impression was that the noise was substantial lower. After a movie I still think that. No idea what part does it. As I said the cathode act as a 10pF capacitor, that is tied between the filament (~ground) and the cathode itself. If one end of a capacitor is being excited the signal goes through it, so if there is noise on the filament line that can appear on the cathode side through the 10pF capacitance. |
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Post by mastertech on Jul 31, 2015 8:46:36 GMT -5
As I said the cathode act as a 10pF capacitor, that is tied between the filament (~ground) and the cathode itself. If one end of a capacitor is being excited the signal goes through it, so if there is noise on the filament line that can appear on the cathode side through the 10pF capacitance. Wow, that fishing hole is pretty deep.  |
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