- Thread starter julianf
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The large BT151 SCR, rated at 500v and 12 Amps, will drive the motor, the quad LM324 op-amps generate the variable-width trigger pulses, and there is a small BT169D SCR and a 2N6027 UJT - unijunction transistor - in there too.
The fault could be anywhere. You would need an oscilloscope to properly troubleshoot, and as @m_c said, there is no mains isolation so the circuit would be live. If your 'scope is powered by AC mains and grounded, you would have to be careful where you put the ground probe ( and your fingers ! ).
The motor should only draw 1 Amp and the large SCR does not look burnt. You might be able to get a low voltage reading ( DC or AC ) on a DVM between the Gate and the Cathode of the BT151 ( left-most and right-most pins when facing the metal mounting tab ), and this may vary when you turn the pot. This would show that the variable-width gate pulses do exist.
The fault could be anywhere. You would need an oscilloscope to properly troubleshoot, and as @m_c said, there is no mains isolation so the circuit would be live. If your 'scope is powered by AC mains and grounded, you would have to be careful where you put the ground probe ( and your fingers ! ).
The motor should only draw 1 Amp and the large SCR does not look burnt. You might be able to get a low voltage reading ( DC or AC ) on a DVM between the Gate and the Cathode of the BT151 ( left-most and right-most pins when facing the metal mounting tab ), and this may vary when you turn the pot. This would show that the variable-width gate pulses do exist.
julianf
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I pulled out my scope (422 tektronix) and the trace seems to have gone on holiday, out of neglect.
(ive probably not plugged it in in 10 years or more - lights up but no trace)
Anyway, i then borrowed a component tester off a friend. Theres not much active on the board, so i just desoldered the first part -
I dont know what im looking at, but its got "numbers" so i assumed it was ok, and re-soldered it -
I pulled the second component, and tested it -
Again, seems ok.
I pulled the third component (the BT151) and was unable to test it -
Ill order a replacement, and see what becomes of it.
(ive probably not plugged it in in 10 years or more - lights up but no trace)
Anyway, i then borrowed a component tester off a friend. Theres not much active on the board, so i just desoldered the first part -
I dont know what im looking at, but its got "numbers" so i assumed it was ok, and re-soldered it -
I pulled the second component, and tested it -
Again, seems ok.
I pulled the third component (the BT151) and was unable to test it -
Ill order a replacement, and see what becomes of it.
julianf
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This will do as a replacement, right?
It looks like this is the datasheet -
Im not familiar with these, but, so far as i can tell, the spec is identical on the 800v unit as to the 500v unit that i have broken?
Obviously they are pennies at rapid, but with a MOQ and postage, hence looking on ebay.
It looks like this is the datasheet -
Im not familiar with these, but, so far as i can tell, the spec is identical on the 800v unit as to the 500v unit that i have broken?
Obviously they are pennies at rapid, but with a MOQ and postage, hence looking on ebay.
As the final high-power component in the speed controller, directly driving the motor, the state of this thyristor would explain why it doesn't work!
Yes, an -800v rated BT151 is a perfectly good replacement for a BT151-500.
julianf
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Sadly, the BT151 has not resolved it.
I braved my loft for the LM324N, and that did not do it either (the one i took out tested bad on the tester)
I then found a few shifty dry joints (which may have been half the issue with it failing in the first place) and reflowed the whole board, but still nothing.
At this point, im going to give up and spend the £50 on a new board.
(i know theres 3rd party options for less cash, but this is the original board, and doesnt come with the stack of warnings about turning on on low power)
I braved my loft for the LM324N, and that did not do it either (the one i took out tested bad on the tester)
I then found a few shifty dry joints (which may have been half the issue with it failing in the first place) and reflowed the whole board, but still nothing.
At this point, im going to give up and spend the £50 on a new board.
(i know theres 3rd party options for less cash, but this is the original board, and doesnt come with the stack of warnings about turning on on low power)
julianf
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To conclude -
The new OEM pcb turned up today, i fitted it, and the unit now works.
Its always irritating, spending £50 for the sake of a 80p component, but, without a scope, the time spent was outweighing the money, so, £50, and it now works.
The unit is one of these -
www.axminstertools.com
Its re-badged chinease, same as a lot of axminster tools, but their parts support is really pretty good.
The new OEM pcb turned up today, i fitted it, and the unit now works.
Its always irritating, spending £50 for the sake of a 80p component, but, without a scope, the time spent was outweighing the money, so, £50, and it now works.
The unit is one of these -
Axminster Professional AP635DS Drum Sander - 230V
This twin drum finishing sander has a compact footprint and is designed for finishing boards, panels and frames. The sanding drums are aluminium, machined and dynamically balanced for smooth running. The drums are powered by a 1.5kW 230V. Being a twin...
www.axminstertools.com
Its re-badged chinease, same as a lot of axminster tools, but their parts support is really pretty good.
julianf
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I need to revisit this.
I have a different motor to run from the pcb, and now it runs way too fast.
The pcb has a couple of trimmers on it, but, of course, no documentation.
There are two trimmers on the pcb -
(note, the photo shows the scrap pcb, but the one in the machine is exactly the same)
And here is the back -
To attempt to make things easy for someone with more skill than me, ive drawn some lines on the photo below -
Im making the assumption that the small black trimmers are standard footprints, and the wiper is the leg by itself.
Now, to me, it looks like trimmers L and H set the voltages that the user accessible potentiometer (the white, yellow, and black wires) divide?
But not understanding the board, im just guessing really.
Can someone who knows better than me confirm? You might assume that H and L on the trimmers stand for "High" and "Low" but, really, i think i may be risking things if i make assumptions like that.
Thank you.
I have a different motor to run from the pcb, and now it runs way too fast.
The pcb has a couple of trimmers on it, but, of course, no documentation.
There are two trimmers on the pcb -
(note, the photo shows the scrap pcb, but the one in the machine is exactly the same)
And here is the back -
To attempt to make things easy for someone with more skill than me, ive drawn some lines on the photo below -
Im making the assumption that the small black trimmers are standard footprints, and the wiper is the leg by itself.
Now, to me, it looks like trimmers L and H set the voltages that the user accessible potentiometer (the white, yellow, and black wires) divide?
But not understanding the board, im just guessing really.
Can someone who knows better than me confirm? You might assume that H and L on the trimmers stand for "High" and "Low" but, really, i think i may be risking things if i make assumptions like that.
Thank you.
Yes, I see the circuit that way too. One trimmer sets how high the positive end of the user pot can reach, and the other one determines how close to zero volts the low setting can go.
If you make a note of where the trimmers are set now, and then turn the "High" one left hand down a bit, the new motor may slow to the speed that you want.
julianf
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I was tempted to just "have a go" but, in another life, I used to deal with a specific bit of equipment that required a scope to set the trimmers on, and I know from this prior experience that some trimmers you just can't get back in the same place if you mess with them.
I don't know this circuit well enough to know if that was the case so wanted to be sure before just having at it!!
