MXR Limiter Problem

[davebungo]

I am trying to repair my MXR Limiter (1980 UK Mains Powered model).  I have no circuit diagram. There is a large 2 (ish) watt carbon resistor on the live input which has gone high resistance (~1K8) and as a result de-soldered itself from the PCB.  The trouble is, the heat has obliterated the colour codes so I have no idea what the original value was.  The pedal worked fine before the resistor failed, so I'm pretty sure there is no underlying fault with the input Transformer (I could be wrong).  Any clues would be gratefully received.
P.S. Let me know if this is an appropriate question for this forum i.e. is this forum only for home build stuff.  Thanks.

[EdJ]

Is that the red box limiter?
If so i think i have the board laying around somewhere.Let me know and i will look it up for you.
Greetings,Ed

[davebungo]

Is that the red box limiter?
If so i think i have the board laying around somewhere.Let me know and i will look it up for you.
Greetings,Ed

That's the one! but its the UK/Euro 220-240 Volt model.  The input resistor goes on the live input and straight into the mains transformer.  I guess it must be some form of inrush limiting resistor or something along those lines.  The manual specifies a current consumption of 12mA but I tried out a 1K resistor in place and measured 62V AC across the resistor = 62mA.  Could it be that there is a primary fault or a fault in the rectifier circuit on the secondary?

[Paul Perry (Frostwave)]

Could it be that there is a primary fault or a fault in the rectifier circuit on the secondary?

It's certainly possible, I've seen more than one 'miniature' 230v transformer with shorted turns lately (possibly due to mains surges & spikes).
maybe try isolating the power supply & driving it off an alternative supply, if you have a suitable one already, to see if the main circuit is OK.

[davebungo]

Could it be that there is a primary fault or a fault in the rectifier circuit on the secondary?

It's certainly possible, I've seen more than one 'miniature' 230v transformer with shorted turns lately (possibly due to mains surges & spikes).
maybe try isolating the power supply & driving it off an alternative supply, if you have a suitable one already, to see if the main circuit is OK.

Is there an easy way to check for shorted turns in a transformer?  Do they generally occur in the primary winding?  Thanks.

[davebungo]

Turns out that the resistor on the live input to the transformer had gone low impedance.  The design (if that is what you call it) looks pretty suspect to me.  Turns out that with 120VAC on the input to the transformer, you get a nice 18V on the bridge rectifier output which is more than enough to feed the 15V reg.  In other words, they used a US transformer and bodged on a resistor to drop the UK supply down.  This means that the resistor on the live input must drop HALF of the 220/240V input.  How silly is this?  Why didn't they just fit an appropriate transformer?  (I can guess why not).  Anyway, replaced the resistor (8K2 5 watt) and things are working again.  Thanks for the replies.