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7704A - MOD3089
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Can someone please tell me where I can find the details regarding MOD3089 for a 7704A? I was perusing one of the PDF scans of the service manual and notice there are several mods that seem to relate to the Guernsey versions of this scope, one of which is MOD3089 which changes R3468 to a 953-ohm resistor.
I have had problems getting the vertical positioning as close to correct as I can and one of the ways I did that was to place a 1k across R3468 (a 1.8k) which has helped that a bit. It makes the character height a bit smaller which keeps them both from being at the very top of the two graticule lines so I would be interested in knowing the details of that mod. Thanks, Barry - N4BUQ |
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Make that a 2k that I placed across R3468. That makes it pretty close to 953-ohms. I need to stop relying on my memory...
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Barry - N4BUQ Can someone please tell me where I can find the details regarding MOD3089 for a |
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Barry,
I wonder if some of the resistors on that board are out of tolerance. There are ones that should be raised in wattage. Ones to the E and C of the outputs should be 1/2W with a few others that 1/2W is advised. Some models of the boards have a 430 ohm 1/4 resistor that is usually hot to touch if not already discoloured. That one needs to be 1W. The 7704A board does not have this resistor. Mark |
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Hi Mark,
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The resistor in question is still in tolerance. The thing that made me start looking at that one is that other versions of the readout board have a pot for character height. Given that, I started experimenting with some lesser values and found that about 1k ohms gave me much better results. The tall characters are nice but when they're both bumping at the top of the graticule line, then I found it annoying and, without adjusting vertical centering to accommodate that, this seemed to be a good alternative. I just wonder what prompted the Guernsey folks to do that (along with several other tweaks in those mods). Thanks, Barry Barry, |
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On Sun, Dec 17, 2023 at 03:26 PM, n4buq wrote:
There was a mod on Beaverton built 7704A's which changed R3468 from 953ohm to 1.8kom. This is opposite to the Guernsey change. So either the Guernsey mod sheet is wrong or for some reason they changed it back what is was initially. This is what Mod Summary says about Beaverton Mod M24523 done at B180000: "To allow usage of the Readout circuit board in another instrument R3468 953ohm, (pn 321-0191-00), was replaced with a 1.8kohm, 1%,0.125W metal film resistor (pn 321-0641-00)." /H?kan |
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That very interesting. Mine's a B190000+ so that explains why I have the 1.8k. I still like the shorter characters and will likely either replace it with a 953-ohm or leave the 2k parallel resistor.
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Thanks so much for that tidbit of info. Barry - N4BUQ From: "zenith5106 via groups.io" <hahi@...>There was a mod on Beaverton built 7704A's which changed R3468 from |
zenith5106
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