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Tektronix 564 WW resistor for -100V supply

Brenda
 

HI everyone! I thought I would take my 564 out and see what I possibly do to get the storage section of the CRT working again. I fired up the scope and only to find that absolutely nothing worked. I measured all the low voltage supplies and they are all severely off. The -100V was reading -77, and with that, decided to check everything in the -100V circuit. All of the resistors were still in tolerance except for R617 which is an 80K WW 1W resistor and it was completely open. The brand of the resistor is Reon. I am wondering if any 80K resistor would work in it's place as long as the wattage is greater than 1W and is within the 1% tolerance spec? Or does it have to be a WW resistor?

Brenda

Trevor
 

The choice of wire-wound was probably for temperature stability of the -100 volt power supply. Since R617  forms a voltage divider with pot R616 and R618, both also specified as wire-wound, the circuit designer wanted resistors with tracking temperature coefficients. The 1% spec is superfluous. You could use any resistor type, but you would compromise temperature stability if the three resistances don't track. If you use a carbon-composition resistor as a replacement, say 82K, consider changing R618 to carbon-composition as well. Then see how the -100 volt supply behaves as the scope heats up.

An RN65 78.7K metal film resistor is probably a better choice, however.

-Trevor

On Thursday, May 23, 2019, 10:22:08 PM EDT, Brenda via Groups.Io <brendda75=yahoo.com@groups.io> wrote:

HI everyone! I thought I would take my 564 out and see what I possibly do to get the storage section of the CRT working again.  I fired up the scope and only to find that absolutely nothing worked. I measured all the low voltage supplies and they are all severely off.  The -100V was reading -77, and with that, decided to check everything in the -100V circuit.  All of the resistors were still in tolerance except for R617 which is an 80K WW 1W resistor and it was completely open.  The brand of the resistor is Reon. I am wondering if any 80K resistor would work in it's place as long as the wattage is greater than 1W and is within the 1% tolerance spec? Or does it have to be a WW resistor?

Brenda

Dave Hills
 

I would use a low Tempco metal film, say 15 to 50ppm. The only reason I can think of using WW in that location is that carbon comp and WW were the primary choices when that scope was designed, and carbon would have caused the supply output to drift all over the place as the instrument warmed up. The 80k value may be difficult to find, but a modern standard value of 80.6k or 78.7k will work fine as would a series combination of 49.9k and 30.1k. You might consider replacing R618, 10k with the same type so as to match the Tempco and minimize the warm up drift.

Dave

On Thu, May 23, 2019 at 07:22 PM, Brenda wrote:


HI everyone! I thought I would take my 564 out and see what I possibly do to
get the storage section of the CRT working again. I fired up the scope and
only to find that absolutely nothing worked. I measured all the low voltage
supplies and they are all severely off. The -100V was reading -77, and with
that, decided to check everything in the -100V circuit. All of the resistors
were still in tolerance except for R617 which is an 80K WW 1W resistor and it
was completely open. The brand of the resistor is Reon. I am wondering if any
80K resistor would work in it's place as long as the wattage is greater than
1W and is within the 1% tolerance spec? Or does it have to be a WW resistor?

Brenda