- 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...
Its re-badged chinease, same as a lot of axminster tools, but their parts support is really pretty good.
