I have used the S8015L, S8020L, S8020LTP SCR's in the past.

Thanks. Before I buy diodes and SCR's, I will lift them from the board and test them. Hopefully I can identify if they are bad or not.

 

I have used the S8015L, S8020L, S8020LTP SCR's in the past.

Hi Max,

I think that I have found bad SCR's. First, I tested the three diodes by pulling one lead from the circuit board. With the multimeter in diode test mode, all three read OL when measuring between cathode-anode. Measuring anode-cathode (red-black) gives 0.527V for D31, 0.530V for D32 and 0.542V for D33.

For the SCRs, I did a resistance test between anode-cathode first with gate disconnected and second with gate touching anode to trigger it. Without the gate triggered, I should essentially read open circuit. With the gate on, I should see a low resistance. What I found is that without the gate triggered, I still read a high resistance regardless of lead polarity.

SCR1

R a-k (no trigger) 17.1M
R a-k (triggered) 49.2 ohms
R k-a (no trigger) 18.7M

SCR2 readings were similar.

Looks to me like I need new SCRs. F7260-ND from Digikey ( SCR 800V 20A TO220AB-L ) should fit the bill.

 

They are usually the cause!

 

They are usually the cause!

I replaced the two SCR's and it did nothing. I was really hoping that I had found the cause of the problem. Of course, I have to wonder what took out the SCRs, or did the failed SCRs take something else out.

 

Any further test generally involves a 'scope, check out where the firing signal is missing!

 

Any further test generally involves a 'scope, check out where the firing signal is missing!

I'm losing hope that I will be able to scope it out without having a schematic.

 

I'm losing hope that I will be able to scope it out without having a schematic.

I checked the gates on both SCRs with an oscilloscope and didn't see any firing signals.

 

Voltage on the speed potentiometer goes from 0V to 11.6V (center to one side. It is probably part of the 12v control circuit. Both the 7812 and 7912 are outputting the correct voltage. Also, the output relay on the right seems to work. When I hit the Run switch, the relay pins close as they should.

are you sure? I see 7915' and 7815'

 

are you sure? I see 7915' and 7815'

My mistake. They are 7815 and 7915 and as I mentioned, both are working.

 

There are three ICs with 14 pins on the PCB. I guess it's the opamps. Measure the voltages on their inputs and outputs and look for a faulty opamp. It's a pity that the manufacturer did not provided the circuit diagram

 

There are three ICs with 14 pins on the PCB. I guess it's the opamps. Measure the voltages on their inputs and outputs and look for a faulty opamp. It's a pity that the manufacturer did not provided the circuit diagram

Those three are L324 quad op amps. That means 12 different opamps to check when I don't even have a schematic to follow. It makes it near impossible to figure out what signal should appear where.

 

Those three are L324 quad op amps. That means 12 different opamps to check when I don't even have a schematic to follow. It makes it near impossible to figure out what signal should appear where.

It is not easy. But opamps are the most suspect components. You can look at their outputs and inputs with an oscilloscope.
If the voltage at the "+" input is higher than at the "-" input, the output must be close to +15V, and vice versa.
If it is worth repairing the controller, of course

 

It is not easy. But opamps are the most suspect components. You can look at their outputs and inputs with an oscilloscope.
If the voltage at the "+" input is higher than at the "-" input, the output must be close to +15V, and vice versa.
If it is worth repairing the controller, of course

I was just thinking that the reason they are there is to provide a changing output signal dependent on what else is going on in the circuit. I'll scope them out and report back.