Q1 Transistor on EME-232 Drive

[tjohnson]

Hi All,


I had a rather strange experience with my PCW8512 2nd drive - the double density model.  It is an EME-232 model.  I was replacing the belt on the drive, put it back in and basically Q1 transistor was smoking big time.  It is completely burnt out - it took a shortwhile for me to notice the smoke and the place stinks now  


I thought maybe I put the connectors on wrong and had somehow put too much voltage on but all connectors look ok.  From the tracks this looks like it is used for the motor that spins the disk so thought maybe I had fitted the belt incorrectly causing it to jam and for the motor to draw too much current and overload the transistor.  Either way the transistor is completely smoked out and unreadable.  Does anyone (Bryce perhaps) know the part number of this transistor so I can at least attempt to solder in a replacement and see if the drive has life again, otherwise it is one for the bin


The board is a z70313 and looks virtually identical to this board used in the EME-156 http://www.cpcwiki.eu/index.php/File:EME-156_Bottom.jpg.  which is a z70312.  The Q1 transistor is in the top left corner.


Not having much luck tbh, the Parados ROM Craigsbar sent me I managed to wreck by inserting it the wrong way round now I'm starting small fires replacing drive belts!


Cheers Trevor

[Bryce]

The part No. of Q1 varies on these boards, and yes, it switches the motor on and off. If it's a small retangular transistor, then it's probably a B793-S which was used on many of these boards. Can you post a photo of the part and check if any markings on the part have survived?
It probably burnt out due to a wrong connector connection or more likely, a wire being trapped when you put the drive back together.

Bryce.

[tjohnson]

Thanks Bryce it is a small rectangular transistor.  It really is in a bad way, when i unsoldered it a leg feel off.  Don't think any markings remain but will check again later when i can.  Not sure if anything else is damaged the pcb took a real beating.

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[tjohnson]

This is all that is left of the transistor.  The only letter I can make out on it is a 4 but I think that is likely to be part of the product code or some other marking rather than the main code which is probably below it.

I had a look on the other drive which is a different type and it has a B793-R the best I can find about this is that it could be this on the data sheet attached.  The pin that has burnt off is the emitter which is connected to the motor.

finding a replacement could be interesting -  I think this might work http://uk.farnell.com/fairchild-semiconductor/bc636ta/transistor-bipol-pnp-45v-to-226aa/dp/2453810

[Bryce]

You could try the B793 out of the other drive first? It certainly shorted, so you need to find out what caused the failure before putting a new part in.

Bryce.

[tjohnson]

You could try the B793 out of the other drive first? It certainly shorted, so you need to find out what caused the failure before putting a new part in.

Bryce.

I really don't want to risk another drive for this, that part doesn't seem readily available anymore so don't want to burn out another component.  Im looking to buy a substitute instead and thought from the specs the bc636 looked close.  They are cheap and readily available so will probably give that a go.

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[tjohnson]

I guess i should start with the motor, its a simple two wire affair.

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[Bryce]

I really don't want to risk another drive for this, that part doesn't seem readily available anymore so don't want to burn out another component.  Im looking to buy a substitute instead and thought from the specs the bc636 looked close.  They are cheap and readily available so will probably give that a go.

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I have a few spare B793 transistors here. I can send you one for the price of postage. Send me a PM with your address.

Bryce.

[tjohnson]

I have a few spare B793 transistors here. I can send you one for the price of postage. Send me a PM with your address.

Bryce.

Thanks Bryce, messaged.  I tested the motor on a bench power supply at 12v it is working fine and drawing about 20ma.

The PCB is a mess, the emitter track has lifted slightly, see attached.

[Bryce]

That shouldn't be an issue. You can run a wire from the next pad on that track.

Bryce.

[CraigsBar]

Here is what's on my EME-232

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[tjohnson]

Thanks Craigsbar that is interesting as the board on my 232 is slightly different and only has a single transistor mounted on a different orientation to your board, is it the same part number on the pcb as mine? .  I checked the voltages coming out the power cable to the drive and they read 12v and 5v all fine.  Bit at a loss to understand why this has happened.

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[Bryce]

When you're testing the new transistor, only connect the base and collector and make a temporary wire bridge for the emmiter that you can quickly disconnect. If the transistor starts getting hot you can disconnect immediately.

Bryce.

[gerald]

Thanks Craigsbar that is interesting as the board on my 232 is slightly different and only has a single transistor mounted on a different orientation to your board, is it the same part number on the pcb as mine?

Craigsbar picture show the ME56PB41 version of the EME-156/232

[tjohnson]

Craigsbar picture show the ME56PB41 version of the EME-156/232

N11008 shown on the ME56PB41 quick search indicates it is some sort of fuse, added to protect the transistor maybe from this sort of thing??

[Bryce]

Yes, some versions of the 3in drive have a component fuse on the 12V supply.

Bryce.

[tjohnson]

So I picked up a bc636 transistor while I wait for the right one to wing its way over here (thanks Bryce) and have soldered that in.  The motor now spins, reads disks and no magic smoke appeared.  I've got an IR thermometer and that didn't register any overheating on the transistor.  However, the motor never stops spinning even when drive A stops.  This is tested on a PCW.  As far as I can work out the BC636 should be a suitable replacement, I found various links that indicated it as suitable other links that indicated an NTE383 as suitable to replace the 793 and the BC636 suitable to replace that.  I know little about electronics so struggle to understand the datasheets.  Clearly something is working to a certain extent as the drive now functions but I don't think it should continuously spin all the time.  Any thoughts?

[tjohnson]

Yes, some versions of the 3in drive have a component fuse on the 12V supply.

Bryce.

I'm wondering if the original transistor was an npn instead of pnp, do you know?

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[tjohnson]

Ok things are not so rosy but think I'm getting somewhere.  I did some more testing today and the drive motor is permanently enabled, then I noticed a very hot smell and then puff more magic smoke appeared before I could switch off the machine, however this time I think it has come from Q2 which is now scorched and likely toast.   So anyway I have found that the PCW service manual appears to have a circuit diagram (see attached) and Q2 is a digital transistor and used take the 5v signal from pin 8 and to supply Q1 base and switch on the Q1 transistor so I think Q1 should have been a NPN which is what the circuit diagram suggests.  So now I have a drive that works because the motor is always enabled because the PNP I've soldered in but ultimately with 2 blown transistors.


From the PCW circuit diagram it looks like I need a UN2213 (Q2) and 2SD1198 (Q1) or suitable equivilents.  The UN2213 is easy to come by, the 2SD1198 less so.


Atleast I'm learning something as I'm smoking stuff

[Bryce]

Then don't install the PNP transistor that I sent you!

Bryce.

[tjohnson]

Then don't install the PNP transistor that I sent you!

Bryce.

I've now managed to fix the drive.  Replaced the blown SMT with another DTC144EKA (NPN) purchased from RS and I replaced the main power transistor with a BD681 darlington (NPN) which is much bigger than the old one, the main reasons for getting this was that it was readily available in Maplins and had the correct leg configuration, B/C/E instead of the more usual C/B/E.  It now works again with the motor switching on and off and no smoke so far.  Still no real idea why the transistor went pop in the first place, it feels like it got shorted to ground somehow and I wonder if when I screwed the pcb board back down I manage to break through the emitter track by fitting the screw with the anti rattle washer on the wrong fixing which then caused the short - you can see slight scoring in the pcb picture.

[Bryce]

Most likely, as well as pinched wires, shorting tracks through the screws is the next most common way of messing up a PCB
Glad you got it sorted. Keep the B593 I sent you for something else.

Bryce.