Re: 454 Fireworks -- almost fixed! Low horizontal gain

Sean Turner


Thanks for the write up. I feel fairly sure that whatever is causing this is definitely temp related. The horizontal gain is fine when the scope is first fired up, and then a few minutes later it goes low. Linearity is perfect; it's just the gain is low.

I will start looking at the various passives noted in this thread. I do have a lot of new 1 and 2W Allen Bradley resistors on hand (from the estate of the late Stan Griffiths, no less), otherwise I will shop from a reputable parts house.


On Sun, May 23, 2021 at 11:33 AM, Mark Vincent wrote:


Harvey is right about resistors. Some I have found that should be raised in
wattage are: R757, R980, R984, R993, R1014 and R1024 to 2W, R1063, R1063,
R1074 and R1076 to 1/2W. These will affect gain as the originals are smaller
in wattage than what I put in mine. It is likely they are out of tolerance.
The two emitter resistors to Q1034 and Q1044 that go to the 75V supply can be
bad. If any look brown in the body, change them. The 13,300 ohm can be done by
a 10,000 ohm 1W in series with a 3300 1/2W. The 64,900 ohm resistors can be
replaced with a pair of 130,000 ohm 1/2 or 1W in parallel. The additional 100
ohms is within the 1% of the original. The resistors I used are 1%. Some of
the original resistor physical sizes can be different in the same stage/piece.
I have had these two emitter resistors be bad due to the size being too small.
Mount the resistors off the board to make sure they have air flow around the
whole body. I will usually get the 1W and larger resistors from China off
ebay. superiorbuy2014 is a seller that has these resistors. There are others.
Mouser also has plenty to choose from. The physical size of the resistor I
always look at before getting. A 3,68 x 8,72mm resistor is not 2W. The
substrate will tolerate getting hot. The element will change too much in
resistance negating the good to excellent temperature coefficient and can
overheat the board and any other parts in contact or very close to the body of
the resistor. The 1% Chinese resistors have a temp. coefficient of 100 ppm/C.
I have done enough through the years with these types to say I will use them.
Calculating the drift and using low drift/1% resistors made me decide to stick
with what I found There are times where the drift will be the most important
and has made a device better in stability, noise, etc. better. Before others
say I am wrong, I am going by what I found out and have seen in person for
some years.

If you want to extend the life of the trigger lamp, change the 82 ohm 1/2W
resistor, R793, by the TO-5 transistor to 120 ohm 1W. That will greatly
increase the life of the lamp and still be easily visible when it is flashing
or on constantly when a signal is applied. Here is where tight temperature
coefficient is not necessary. A bleeder is another example where drift is not
a concern.

You may already have done this in the vertical circuit, R318 and R385 being
raised to 2W and the pot adjusted for 15V at the base of the outputs. You can
also use 2N3866A as the outputs. The A version is heavier duty, higher ft, and
is supposed to have a higher voltage capacity.

I like the nickname you gave the scope. That nickname would fit after what it
did initially.


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