8 replies to this topic

[kevvywevvywoo]

I have another interesting one here. MA5000VZ. The fans run flat-out fast at switch-on when cold. Channel 1 does not work and has the IOC bright and ODEP on.  I've checked the thermal trip in the transformer. There's no diagnostic lights on internally. Output modules test ok. ODEP -/+ voltages are fine, temp IC sensor volts is fine.  

Ch2 also has a small fault, though i don't think its related. The soft-start precharge doesn't work. The amp powers up and when the soft-start time has run out the main relay comes in, dimming the lights and sometimes blowing the breaker because the capacitors are empty.

Any clues?!

[kevvywevvywoo]

Ok, sorted the soft start issue. It was C282 leaky.
I've also sorted the fan issue, someone had cut the ODEP lines where they meet the control board.

However I still have the Ch2 in IOC protect. Measuring the +/- dc bus gives uneven readings..39V on one and 48V on the other. I've fitted a new ;'main' back card and that fixes it..so the fault's on there somewhere.  

[joust]

IOC is a bit tricky. Verify the output termination board, where the flat output cables lead. Perhaps it has broken cable. Also, verify the U750, the ISENCE detector.
I believe I responded to a recent post for a similar symptom.

Alain

[kevvywevvywoo]

Hello Alain, yes the previous amplifier had the same symptoms, but this is a different amplifier and it isn't the flat ribbon..it was the first thing I checked!!

This was a difficult one. I discovered that the +ODEP voltage was perfect at 12.6V...however I spotted today that the voltage was positive with respect to 0V...it should be negative -12.6V !! So we had a hard odep fault on Ch1.

After 3 long days of testing, swapping chips, testing for leaky capacitors etc., I finally found the fault. The Thick Film resistor network RN101 had an out-of-tolerance section (pins 3-5). I found out by exchanging the network for the one in Ch2 and the fault transferred over.

I now just need to make & fit a new ribbon cable to replace the one damaged by a previous engineer.

[joust]

Great! This one had turned into a b*! I must be lucky: I've still to find a faulty network! In all my Crown repairs no resistor network has fallen.
Biggest fault on MA5Ks are the solder connections (base leads) of the output devices. Every few years they have to be inspected for cracks and such. Though, most of the repairs I've seen were related to ouput device faults. Also make sure that output devices are well 'thermal pasted', heatsink and device!

Regards,

Alain

[kevvywevvywoo]

Quote

In all my Crown repairs no resistor network has fallen.

I agree, me neither. I was so glad to find it.

On your last point, I have spent today resoldering the output devices on 2 MA5000 amps. Brittle, crystallised solder and loose connections on the end transistors causing the pcb to become loose.  Sometimes this causes the transistors to fail.

[Wrighty]

I've had two Macro-Techs in, with faulty resistor networks... Both were a right pig to find!

[joust]

On later MA5K, Crown added to the end transistors, a long mounting screw, that passes through the boards and are fixed with a small nut, thus securing the boards and transistors together. The 'flapping' or looseness (sic?) of the boards usually stressed the solder joints until they break.

[kevvywevvywoo]

Ah, never seen any with long pins. Any amps I've seen with this problem are usually rather old by then.  I always check the joints even if the fault is not ostensibly on the output modules.