I have a KB motor controller that runs at high speed with no adjustment.
I'd like to try and fix.
I checked the pot and I think it is supposed to be around 5k.
I measured 6.8k or something out of circuit (it has push on crimp lugs) I disconnected to measure it was linear from 0 to 6.8K
I looked up the part numbers of what I believe to be two SCRs (A69258 proprietary Teccor) I'm told the little fuse replacement is a S8025LTP.
When I futz around with my volt meter the resistance from what I believe is cathode to anode it measures 500k when I touch the + probe to the anode and the the gate whilst the - probe on the cathode I measure a short.
If I then lift the probe off the gate whilst leaving it on the anode I measure 100k.
So I kind of sorta think they are good.
So I drug out my handy dandy scope in a can that connects to my laptop.
I measured the gate voltage and it posted it below.
Which surprised me I was expecting a few volts DC which would control the turn on point of the SCRs??????
There are three more TO-220 devices they look to be diodes which is confusing (a Teccor A68063 and and two more A68064s)
I was expecting the two SCRs to rectify a DC voltage and four diodes in a bridge configuration with the motor connected across the bridge????
But only three diodes does not a bridge make?????
I don't want to reverse engineer it.
I just need to understand enough about the design to figure out what is wrong/why it won't adjust.
Is there someone out there that can tell me enough about what is going on that I can figure out what is and is not working?

The thing is not connected to the motor as the motor is on a giant plate thingy that a horse stands on for a massage therapy action out in the barn.
Thanks in advance for any help suggestions.

 

If it is the SCR bridge version, you need the two SCR's and the two Diodes for the bridge controller Digikey has them, or at least the equivalents.

 

What are you using as a reference point for the scope measurements?
Are you using a 10:1 probe?

 

Thanks Max
But this has three diodes?
And the SCRs appear to be good (at least with a voltmeter whatever that is worth)
Why so much voltage on the gate of the SCRs?

 

What are you using as a reference point for the scope measurements?
Are you using a 10:1 probe?

I'm attached to chassis ground the black alligator lead on the green AC input line and DC output.
10X scope probe

 

Thanks Max
But this has three diodes?
And the SCRs appear to be good (at least with a voltmeter whatever that is worth)
Why so much voltage on the gate of the SCRs?

I doubt whether the scope traces will really tell you anything, using ground as a reference on SCR gate measurements. To display the gate waveform, you should be making a differential measurement using two scope channels with the probes connected to the cathode and gate of an SCR and the two probe grounds connected to chassis ground.
The symptoms suggest that you probably have one or both SCRs shorted, anode to cathode. With the voltmeter, measure the AC voltage between each SCR's anode and cathode with the power on.

 

Thanks Max
But this has three diodes?
And the SCRs appear to be good (at least with a voltmeter whatever that is worth)
Why so much voltage on the gate of the SCRs?

The bridge consists of two rectifiers and two SCR's, the third rectifier is reverse biased on the bridge output.
I believe I have a schematic somewhere.
Carful using the 'scope GND as these drives are not isolated from the mains supply.

 

Schematic

 

Schematic

Hey Max
Mucho thanks for the schematic.
"Carful using the 'scope GND as these drives are not isolated from the mains supply."
Yeah I was poking around in the back without a probe condom and got a few sparks already.

I have to go fix a welder and a pressure washer.
I'll get back to this tonight.

 

Hi folks
I got wrapped around the axel fixing other stuff and have just got back to this.
Once I got the schematic from Max.
I gave up on making measurements from chassis GND and referenced the GND of the schematic (F-).
Things make a little more sense.....well sort of kinda maybe
The output to the dc motor does vary a little bit.
The plots below are from F- to the black with the goes to the motor A+.
The AC on the wave form is really confusing seems you have full wave rectifying when I tun the pot in one direction and half wave if I turn the pot in the other direction (the middle is even more confusing).

I need to figure out what Keith is trying to tell me above.
(As mentioned above I reckon measurements should be made from F-)
So I'm still contemplating the two scope probe advice.

"The symptoms suggest that you probably have one or both SCRs shorted, anode to cathode. With the voltmeter, measure the AC voltage between each SCR's anode and cathode with the power on."
But can understand what to do with the last part
so SCR1 A to K is 64.5vac SCR2 is 66vac. (SCR2 I measured from L2 to A+ I could not reach the device)
I'd say they are not a short but I remain cornfuzed.
Any more suggestions?

 

"The symptoms suggest that you probably have one or both SCRs shorted, anode to cathode.

Generally if one of the components of a full wave bridge is shorted, results is a dead short to the incoming power.

 

Generally if one of the components of a full wave bridge is shorted, results is a dead short to the incoming power.

Before I knew anything about this (still don't know very much)
I would have guessed the one or more of the SCRs are bad for the exact reason no adjustment.
I don't understand how they have sixty volts across them if they are shorted (as measured with AC voltmeter).
Maybe it is because I have no load connected.... said another way uh uh I dunno know.


If the schematic is correct and I measure with the O'scope I have different signals on the cathode and anode of both SCRS.
Seems if they were shorted I would measure the exact same signal if I were measuring a short.
In the measurements below A+ would be the cathodes of both SCRs as they are tied together.
and L1 would be connected to SCR1's anode and L2 would be SCR2's anode.

I don't have plots but I have what appears to be the exact same signal on the cathode as the gate.
I need to probe both simultaneously to make sure.

 

Measuring the SCR voltages with a meter is not going to tell you about the trigger pulses. Also, recall a breakdown failure mode for SCRs where they turn on without triggering. I think.
For testing under load you can substitute a regular 120 volt filament type light bulb between A+ and A- to put some load on the circuit.