Repair ZP120

Show first post

407 replies


Be sure to purchase at least two of these thermistors because the first will likely blow again as soon a you apply power. I suggest that you track down the root cause before applying power again. This is not a hard as it seems, just follow the current that blew up the original thermistor. This will lead you to a really sick component. Be sure to replace everything in that current path.

Also, I am leery about running up the power voltage as a strategy to troubleshoot switch mode power supplies. When the voltage is much lower than expected, the switch mode stuff will really try hard to maintain the specified output. This could imply much more current than average use cases.

If you have lab power supplies available, separate the ZP120 into modules, power them up and test them separately. The lab supply will have current limit capabilities to protect itself and the device under test from unreasonable current levels.
Userlevel 2
Upon closer examination I see that a current limiting in-rush thermistor has over heated and cracked. Its a SCk054 (schematic TH16001) which I'll have to replace to progress further.
Userlevel 2
Lacking a schematic to do proper trouble shooting I slowly applied power to force evidence of a fault without hopefully doing damage. One of the varisters eventually exploded. I desoldered its remains. Although I had measured it as open it was evidently damaged and shorted out under power. I reassembled the unit but still don't have power so I'll look for another faulty component on the power board. At least now I can apply full power without any shorting occurring.

A flash of light usually indicates something has died -- in a hurry -- usually letting out its smoke. Of course, everyone knows that smoke is required for proper operation and after the smoke leaks out, operation ceases. Further, the flash of light (former) device is usually the victim of another issue.

Some units will blow fuses for no good reason other than the fuse was probably poorly specified and fatigues over time, eventually failing. I don't have enough data to know if ZP120 is one of these units or not.

I can't recommend the strategy of inserting a fuse eliminator as an expedient. Unless I know that a given (dead) unit is subject to nuisance fuse failures, I'll track down the root cause of the failure before applying power to a unit. Powering a (dead) unit prior to repairing the root cause typically expands the failure, fuse or not. Also, even if they appear to be OK, replace any components in the current path of the root failure because these components were probably stressed during the original failure and you will likely be faced with another similar failure as these stressed components fail in the future.

Inspect the board using a magnifying glass. There is almost always some sort of visual physical clue indicating which component gave out its smoke. Of course, with our modern, ever shrinking components, these physical clues can be small too. Sometimes the only clue is a small black dot on the non visible side of a surface mount component.
Userlevel 2
I bought a used Z120 with an unidentified power issue. It was dead.

The 5A fuse is open. No other components show visible damage. When I shorted out the fuse to see if the amp would power up there was a momentary flash of light near the centre of the power board. I say momentary because I quickly opened up the fuse again.

It appears to me as if something arced or there is a short somewhere downstream of the fuse. Everything I measure with an ohm meter seems to react ok. The varistors near the large filtering caps are not shorted. Before I try to replicate the action of shorting the fuse again to see what is arcing I thought I'd see if anyone had any ideas as to the fault.

Also, has anyone created a schematic of the power supply?
Userlevel 2
This guide might help although it is for the ZP100.
Userlevel 2
I too am keen to work out where the fuse is. I tried here and here but not much joy. Anyone got better info? B.