P
Phil Allison
Guest
Hi,
had one of these powered mixers on the bench this week - but without a clear fault report! The unit belongs to a church and was brought in by a regular customer, who did not use it personally.
The main problem proved to be similar, intermittent faults in both power amps - with very low and distorted (ie half sine wave) output when one or both were in fault condition. Both amps could work for periods of a hour or more between episodes of the fault condition.
The amp PCB did not respond to impacts or wiggling any multi-pin connectors and there are no rail fuses. So there was no option but to remove the heatsink and unbolt all 16 flat-pak devices to release the PCB.
This revealed a surprising situation - the solder on each leg of every TO3P device was visibly cracked, no need for a magnifying lens to see them either. Some devices almost fell of the PCB when it was released. Strangely, no other solder joints on the PCB showed any problems at all.
The unit is maybe 15 years old and looked well treated, so WTF was going on ?
The best hypothesis I have is the heatsink is the main culprit - it is a single piece of extrusion, about 20cm square, with raised, flat surfaces on opposite sides for two rows of TO3Ps bolted into threaded holes. The PCB itself is secured by 4 bolts fitted into stand-offs in a square pattern.
So, whenever the aluminium gets hot it expands and this pulls on all the flat-paks, tensioning the legs of each device against the solder joints. Do this thousands of times and fatigue cracks appear.
Not really sure if the solder was Pb free or not ( there is no marking on the unit ) but they all did have that dull look of the evil Pb fee stuff.
Anyone here seen the same thing before ?
BTW
The power amps use a really odd circuit topology called "EEEngine" by Yamaha - with six power Darlington devices, two of them operating as self oscillating, switching current sources.
..... Phil
had one of these powered mixers on the bench this week - but without a clear fault report! The unit belongs to a church and was brought in by a regular customer, who did not use it personally.
The main problem proved to be similar, intermittent faults in both power amps - with very low and distorted (ie half sine wave) output when one or both were in fault condition. Both amps could work for periods of a hour or more between episodes of the fault condition.
The amp PCB did not respond to impacts or wiggling any multi-pin connectors and there are no rail fuses. So there was no option but to remove the heatsink and unbolt all 16 flat-pak devices to release the PCB.
This revealed a surprising situation - the solder on each leg of every TO3P device was visibly cracked, no need for a magnifying lens to see them either. Some devices almost fell of the PCB when it was released. Strangely, no other solder joints on the PCB showed any problems at all.
The unit is maybe 15 years old and looked well treated, so WTF was going on ?
The best hypothesis I have is the heatsink is the main culprit - it is a single piece of extrusion, about 20cm square, with raised, flat surfaces on opposite sides for two rows of TO3Ps bolted into threaded holes. The PCB itself is secured by 4 bolts fitted into stand-offs in a square pattern.
So, whenever the aluminium gets hot it expands and this pulls on all the flat-paks, tensioning the legs of each device against the solder joints. Do this thousands of times and fatigue cracks appear.
Not really sure if the solder was Pb free or not ( there is no marking on the unit ) but they all did have that dull look of the evil Pb fee stuff.
Anyone here seen the same thing before ?
BTW
The power amps use a really odd circuit topology called "EEEngine" by Yamaha - with six power Darlington devices, two of them operating as self oscillating, switching current sources.
..... Phil