454 Fireworks -- almost fixed! Low horizontal gain


Sean Turner
 

Hey folks,

Quick question. I almost have "ol sparkey" fixed thanks to Walter from Sphere and the VintageTek museum. Many thanks to both for finding me the transistors I needed to fix the vertical amplifier. That is working great now!

However, there is a problem with the horizontal amplifier. The gain is too low, and the 1X gain adjustment doesn't have enough range (to express another way: markers from my 184 are too close together).
Observations:

(1) all the semiconductors in the horizontal amplifier seem fine.
(2) I probed the horizontal amplifier with the signal conditions specified in the manual and found no discrepancies.

Has anyone seen this? I thinking it could be a leaky cap or something now, but just figured someone else might have seen a similar issue in these scopes.

Thanks!

Sean


Harvey White
 

In the horizontal amplifier, check the values of the resistors going from emitter to emitter for each stage.  Check the value of the gain adjust and x10 pots as well.  Those affect the gain, all other things being equal.

 Harvey

On 5/22/2021 11:25 PM, Sean Turner wrote:
Hey folks,

Quick question. I almost have "ol sparkey" fixed thanks to Walter from Sphere and the VintageTek museum. Many thanks to both for finding me the transistors I needed to fix the vertical amplifier. That is working great now!

However, there is a problem with the horizontal amplifier. The gain is too low, and the 1X gain adjustment doesn't have enough range (to express another way: markers from my 184 are too close together).
Observations:j

(1) all the semiconductors in the horizontal amplifier seem fine.
(2) I probed the horizontal amplifier with the signal conditions specified in the manual and found no discrepancies.

Has anyone seen this? I thinking it could be a leaky cap or something now, but just figured someone else might have seen a similar issue in these scopes.

Thanks!

Sean





Sean Turner
 

Thanks Harvey, I will check those when I get back into it later today.

Sean

On Sat, May 22, 2021 at 09:50 PM, Harvey White wrote:


n the horizontal amplifier, check the values of the resistors going from
emitter to emitter for each stage.  Check the value of the gain adjust and
x10 pots as well.  Those affect the gain, all other things being equal.

Harvey


Mark Vincent
 

Sean,

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.

Mark


Sean Turner
 

Mark,

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.

Sean

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


Sean,

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.

Mark


Mark Vincent
 

Sean,

A thermal problem helps in narrowing the culprit(s) down. Test the 9V zener on the regulator board and the one by the 536 ohm resistor on the bottom board near the long shaft of the mode switch with a VTVM/VOM. If bad, use a BZX85B9V1. The second zener is part of the vertical circuit input circuit. Better to check and confirm than assume.

If you have freeze spray, start at the horizontal outputs and move back. A transistor could be temperature sensitive. I have seen this before while the transistor will check good. A brief release of the cooler is all that is needed. It is best to change both in a push-pull circuit to keep the balance to specs. This is for a power part or signal part.

If the cooler does not work on the parts, check the voltages while working right and when it has reduced sweep length. A drop in the B+ supplies will also do this. A muffin fan will help keep the area you want cool long enough to do testing. A piece of paper, envelope, etc. will help with air direction to isolate areas/parts. I have done this to isolate a thermal problem.

In my 453/4/A scopes, I replace more resistors to increase wattage and replace the electrolytics and most to all small (1mfd axial tantalums with film). I find most of the small tantalums will have at least several ohms or higher, e.g. 10 ohms or higher ESR. The maim filter cans I restuff with larger values, e.g. 200mfd to 270mfd UCY. The 10mfd output filter for the unregulated higher voltage, I put in a larger capacitance. Enough times the original electroyltics were already starting to go bad to being dried out to <1mfd when measured before restuffing. At near 50 years old, they are due to fail. This is my personal preference due to what I have seen and expericed. You do what you want with your piece. Doing the main filters takes the maximum amount of time and work in the areas to service in these series. I do not mind doing it because I know what is in there is now is new.

I know you will be thrilled when you get your scope working right. Let us know what you find is causing the problem(s).

Mark