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


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

Join TekScopes@groups.io to automatically receive all group messages.