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Name of 3-pin plug for Hayes Smartmodem 2400?

B

bwann

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Oct 16, 2023
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Does this type of power plug have a name? The rubber boot on mine is starting to crumble and crack. I've been trying to find a replacement 3-pin plug in case I need it, but I'm not even sure where to start.
 

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Hi,

Which PINs are +8.5V and which is GND?


Did you ever find a part number for the connector?


.
 
Look at the picture, 8.5 VAC. No Ground, No +8.5.
Take a 9V transformer and you are ok.
Most of modems of that era used alternate current
for power supply.
 
noppa said:
Look at the picture, 8.5 VAC. No Ground, No +8.5.
Take a 9V transformer and you are ok.
Most of modems of that era used alternate current
for power supply.
Click to expand...

Hi,

Some of the modems show half the a.c. voltage with a center tap ... that's why I need to know what the PIN voltages are ... and which PIN is center tap.



.
 
The user manual (in the specification section) states 14V AC.

This implies a C.T. transformer.

If you don't have the transformer unit itself, have a look inside the unit and trace out the internal power wiring from the input connector, through the rectifier diodes to the smoothing capacitors.

The internal power supply may produce 1 or (more likely) 2 voltage rails.

Dave
 
daver2 said:
The user manual (in the specification section) states 14V AC.

This implies a C.T. transformer.

If you don't have the transformer unit itself, have a look inside the unit and trace out the internal power wiring from the input connector, through the rectifier diodes to the smoothing capacitors.

The internal power supply may produce 1 or (more likely) 2 voltage rails.

Dave
Click to expand...
Hi,

I'm not an electronics engineer to start trying to trace things on the circuit board:

1782163521911.png



.
 
wperko said:
I'm not an electronics engineer to start trying to trace things on the circuit board:
Click to expand...

Why not just ask ChatGPT?

daver2 said:
The user manual (in the specification section) states 14V AC.
Click to expand...

FWIW, there’s a schematic for a SmartModem 1200 on Bitsavers:


That basically agrees with that (it says 13.5v)… and you’re right, the manual for the 2400 on archive.org says 14v, but the fact that the case has “8.5” printed on it is a problematic difference…

Okay, so looking at pictures on eBay, yes, there are two (at least) different versions. One with “8.5” on the back comes with a power brick *that has molded into it* that it’s for use with modems that say that. The manual on Bitsavers says it’s for a “V-Series” 2400, and there is indeed one of those on eBay with a 14v transformer.

Anyway. Setting that aside, if you extrapolate from the 1200 schematic it looks like one of the three pins is chassis ground. So I would *guess* you could figure out which two pins were the AC pins by testing continuity between the shield on the RS232 connector and each of the pins in the power socket. The one that chirps is the one you do *not* hook up to your ~9v transformer.
 
I know I posted pictures here somewhere on the forum of the inside of a smartmodem power brick and while I can't seem to find them at the moment IIRC it's a normal step-down AC transformer and the third pin is bonded earth ground.
 
The Smartmodem 1200 schematic has regulators for +10V and -10V which would need a few volts more than that as an input, so that may be where the 13.5V requirement comes from.

RS232 mark and space voltages have a wide range so if you provided 8.5VAC you would get an acceptable +/-7V or so at these regulators.

I measure 7.6V for the RS232 signals on my Smartmodem 2400 which uses an 8.5VAC power supply..
 
Last edited:
I never did find the name of the connector, but looks like ya'll worked out the pinouts. On all the three-pins I have the pins paired together left/right are the AC and the lower with the angled indent is the ground/com. The whole Smartmodem/Optima series has a wide variety of input voltages that's maddening

  • Smartmodem 1200 - 13.5 VAC three pin
  • Smartmodem 2400 - 8.5 VAC three-pin
  • Smartmodem V-series 2400 - 14.0 VAC three-pin, extra notch
  • Smartmodem 9600 - 14.0 VAC three-pin, extra notch
  • Smartmodem Optima 144 - 9 VAC barrel
  • Smartmodem Optima 288 - 9 VAC barrel
 
bwann said:
I never did find the name of the connector, but looks like ya'll worked out the pinouts. On all the three-pins I have the pins paired together left/right are the AC and the lower with the angled indent is the ground/com. The whole Smartmodem/Optima series has a wide variety of input voltages that's maddening

  • Smartmodem 1200 - 13.5 VAC three pin
  • Smartmodem 2400 - 8.5 VAC three-pin
  • Smartmodem V-series 2400 - 14.0 VAC three-pin, extra notch
  • Smartmodem 9600 - 14.0 VAC three-pin, extra notch
  • Smartmodem Optima 144 - 9 VAC barrel
  • Smartmodem Optima 288 - 9 VAC barrel
Click to expand...
Just to complete your set, the Smartmodem 300 is also 13.5VAC, 3 pin.
 
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