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repairing a 184

bill K7WXW
 

I have a 184 that partially works: the 10MHz signals along the 10HMz, 100MHz, and 200MHz, and 500MHz. The problems are in the 1us and 5us chain. Among other things, there is some poorly done rework in this part of the circuit, the ferrite beads shown on the schematic (input and output side of the delay line) are gone, there are some questionable transistor and part substitutions.

But this is my first 184, so I don't know what a "good" one looks like. Perhaps someone that has worked on one can tell me:

What is the purpose of the delay lines on the 0.5us and 1.0us marker lines?

The ferrite beads on the input and the delay line seem to be in other gear, too, and listed as 0.6uH. In the schematic, they look like they are wound as a transformer with the secondary tied to ground at both ends. Anyone know a substitute part and how they were wound?

The delay line looks like it has been mucked with, too. Is there a better way of testing it than putting a pulse into it and measuring the delay to see if it matches?

There is a lot of 10MHz signal appearing on the trigger output, especially when the the marker is set to 0.1us, and some marker edges appearing in the trigger output at other mark rates. I noticed that the 12V rail in the two buffer amps at the trigger and marker outputs is tied together through a resistor and that the voltage is changing with the marker. That shouldn't be happening, should it?

I am trying to get the unit back to its schematic form and address the crosstalk issues. Any help with these questions is much appreciated. 73 Bill

Tom Gardner
 

On 08/12/19 20:57, bill K7WXW wrote:
I have a 184 that partially works: the 10MHz signals along the 10HMz, 100MHz, and 200MHz, and 500MHz. The problems are in the 1us and 5us chain. Among other things, there is some poorly done rework in this part of the circuit, the ferrite beads shown on the schematic (input and output side of the delay line) are gone, there are some questionable transistor and part substitutions.

But this is my first 184, so I don't know what a "good" one looks like. Perhaps someone that has worked on one can tell me:

What is the purpose of the delay lines on the 0.5us and 1.0us marker lines?
The manual states, p 3-2 col1, "The resultant output markers from the emitter of A123 are delayed by approx 76us by delay line L123 to allow stacking with other selected time markers.".

You can select more than one time marker simultaneously, e.g. if you press the 1us and 10us buttons simultaneously then every tenth marker is of a higher amplitude.

bill K7WXW
 

On Sun, Dec 8, 2019 at 01:14 PM, Tom Gardner wrote:


The manual states, p 3-2 col1, "The resultant output markers from the emitter
of A123 are delayed by approx 76us by delay line L123 to allow stacking with
other selected time markers.".

You can select more than one time marker simultaneously, e.g. if you press the
1us and 10us buttons simultaneously then every tenth marker is of a higher
amplitude.
Thanks Tom. I read that section at least three times and missed the last sentence every single time. Going to read the whole thing again and see if I missed answers to other questions. If not, hoping others here will have some ideas.

Eric
 

Bill, I have a working 184 if you need me to take some high rez pictures for you let me know I am pretty sure mine has not been tampered with.

bill K7WXW
 

Eric - that would be great, thank you. The problem? The delay line and the connections of interest are under the countdown board, the one with 23 individual connections. :\ So it's a project: you have to remove at least the three power wires (A, B, and C) and undo six mounting screws to lift the board and take photos. No worries if that's more work than you want to do!

On my unit, someone modified the delay line, removed the chokes, drilled holes in the countdown PCB and pulled the delay line inputs and outputs through the holes and soldered them on top. Why? No idea. I guess they were trying to fix some other problem but the rework is a mess. Without trying to describe it, what I found underneath the countdown board would partially explain why I was seeing the 0.1us edges as crosstalk on the 0.5us and 1ns output lines.

If I can figure out how the chokes/transformers were constructed, and the delay lines were arranged, I will undo the modification, clean up the mess, and replace the components damaged by the rework.