Tektronix 50 ohm attenuators
John Rehwinkel <spam@...>
Hi All,This is simply a 3900 ohm, 1/8 watt, 5% tolerance resistor.
I expect a quarter watt unit would do fine. You can find
these at Radio Shack. Or I can send you some. I have
plenty (I bought out a Radio Shack when it was going out of
307-1020-00 ATTENUATOR,FXD:50 OHM,2X
You can actually make these -- though for compensated high
frequency use, you generally have to parallel the resistors
with small capacitors.
There are two basic arrangements for resistive matched-impedance
attenuators, "tee" style with two series resistors and one shunt,
and "pi" style with one series resistor and two shunts. Here is
an attempt at ASCII diagrams:
in o--/\/\/\---+---/\/\/\--o out
series | series
in o--+---/\/\/\---+--o out
| series |
< shunt < shunt
= (gnd) = (gnd)
Note that these are unbalanced (signal referenced to ground)
attenuators -- there are also balanced varieties. When I'm
building these myself, I tend to use the tee variety, as it
seems a little easier to wire up.
Here are the resistor values for the listed attenuators (since
the input and output impedances are the same (50 ohms), the
attenuators are symmetrical, that is both the series resistors
in the tee attenuator are the same and both the shunt resistors
in the pi attenuator are also the same.
series (2) shunt
2X 8.58 141.
5X 19.1 55.9
10X 26.0 35.1
shunt (2) series
2X 291. 17.7
5X 131. 44.7
10X 96.2 71.1
Resistor values are in ohms, and I've rounded them to
3 places. Note that most of these aren't standard
values, so you'll have to obtain precision resistors
(a good idea any to maintain calibration) or build
them up out of combinations of standard values (this
doesn't work well for high-frequency use).
(currently repairing a 7844 and a 570)
I'd be careful of using a Radio Shack resistor in that spot.
Depending on where it's being used, a 1/8-watt CARBON COMPOSITION
resistor might be required as the 1/4-watt (or an 1/8-watt) CARBON
FILM resistor may have too much inductance for the application. You
can still get carbon comp resistors at Mouser Electronics
(http://www.mouser.com ), but they have only the 1/4-watt size, not
1/8-watt. I looked through my parts and did find a 1/4-watt 3.9K
carbon comp resistor. Measure the hole spacing in the PCB and see it
it'll fit. You'll need about 5/16" (7mm-8mm) between centers. My
part (used) has remaining lead length of about 5/16", so check to
make sure your holes aren't TOO far apart or you'll have to add a
little lead to the part.
I checked through the 50-ohm thin film parts that I have (all new)
and didn't find what you needed, which surprised me. 7A24 attenuator
parts must be different than those for the 7A19. I have 307-1018,
1034, 1035, 1036, 1037, 1038 and 1040-00 plus the mounting studs/pins
(they're staked onto the board 214-1797-00) and the hold-down leaf
springs (344-0248-00). E-mail me if you need some pieces or the 1/4-
watt resistor (I can mail the resistor for $0.34 or you can have a
nasty minimum order and shipping charges from Mouser).