[Al_The_Man]

Anyone have a copy of the schematic for the positioner DC drive board for the BP II (non-CNC), this is the SCR drive common to both XY axis.
The board is marked Bridgeport Controls part #1920000.
I would appreciate a copy if anyone has.
Al.

[magnadave]

Al, Did you ever find the schematic? I just pulled one out with a couple of burnt resisters. Hard to tell what size they are.

[Al_The_Man]

I believe I have it somewhere, I would need to look it out. I will post it If I find it today
Al.

[Al_The_Man]

This is all I have.
Al.

[magnadave]

Thanks Al, I needed the complete manual as well. Only problem is that the drive schematics don't call out the resistor values. I need to replace R3 and R48. You wouldn't happen to have a close up picture of the drive. I could get the values from the colour codes. Attached is my drive unit.
Attachment 302338

[Al_The_Man]

I don't have the drive anymore unfortunately.
Al.

[magnadave]

Thanks anyway. At least I have the manual.

[inertialabs]

Hey Dave I have one that isn't burnt up I will try to get over to the welding shop tomorrow and get you a picture of it

[inertialabs]

Al
Thanks for the manual that is way better than the one i just bought for $25 on EBay
Sorry I didn't see that sooner

[john-100]

after looking at the circuit diagram on page 68 of the manual posted by Al in post 4

R3 will be the same value as R4

R3 could of been damaged by a short on the +15V output or zenner diode D9 being faulty - D9 is the same as D4


R48 and C12 form a snubber across the 115V supply

I'd replace both R48 and C12 as I expect it was C12 failing that burnt up the resistor

if C12 is about 100nf then expect R48 to be about 40 ohms

the capacitor needs to be a "self healing" metallized polypropylene type


John

[inertialabs]

Any idea of what this thing is ?

[inertialabs]

The cabinet had a couple spares in it

[john-100]

in the first photo the device at the top of the heatsink will be 1 of the 3 diodes D3 , D4 , D5 on the circuit diagram

the two spares in the 2nd photo are the thyristors D1 and D2 on the circuit diagram

the 5 devices form a controlled bridge rectifier

if the thyristors are triggered at the start of each half cycle

you get full DC power just as you would expect with 5 diodes

by delaying when the thyristors are triggered you reduce the average power to the motor
( zero when you don't trigger them !)



John

[magnadave]

Those are the SCR's (silicone control rectifier). They control the current/voltage output.

[magnadave]

John would I be correct in saying that the snubber you talk about is for controlling electrical noise coming in from the 115v supply? Since this is a manual machine and not CNC I don't really need to have a precise feed rate. Do you think the SCR drive can be run without this circuit?

[inertialabs]

If the thyristors are triggered at the start of each half cycle

you get full DC power just as you would expect with 5 diodes

by delaying when the thyristors are triggered you reduce the average power to the motor
( zero when you don't trigger them !)


So It sounds like if the Thyristors are not triggering and this could be the cause of the motors barley turning ?

Is there a good way to test for this ?

[john-100]

yes the thyristors or SCR's to give them their alternative name are the devices that control the power to the motor

the circuit will work without the snubber but you could find you have the SCR's triggered by noise / fast rising transient voltages on the mains supply

how you test the control will depend on what test equipment you have
an oscilloscope would make it easy but I don't expect you have one available
a multimeter is the minimum

to test the control on the work bench you could use a 60 to 100W GLS mains bulb (old type with a tungsten filament) as a dummy load instead of the motor

the speed control should control the lamps brightness like a normal dimmer

if it has a pronounced flicker the you need to test the SCR's to check both are working
or possibly the current limit protection is operating


John

[Jonp]

Those are the SCR's, I've replaced at least six or eight of them in our drive in the last ten years or so

[magnadave]

Thanks for the info John. Very helpful. I do have access to an oscilloscope and will test the drive.

[Al_The_Man]

What i ended up doing on one drive before I got the print was to use a KB DC drive and replace the SCR's and Diodes with higher power ones and mounting outboard on a heat sink.
Al.