CROWN 5000VZ

jbl_man wrote: Whats the situation at the output? Put the meter across the speaker terminals ,whats the DC readings on each channel?

Thanks a lot for your help so far!

the best tool is the service manual, measure the +/-15V, odep and temp ref. voltages on the test headers on the rear control board. If these are amiss in any way it will give you a better pointer as to what's wrong.
The transformer thermostats can cause the fans to run flat out as can any ODEP voltage that's not in spec, since the speed control is influenced by the ODEP circuit.
Dont worry about small amounts of DC on the outputs, it'll just be a stray voltage because the protect circuit removed the power from the output modules. By switching off the drive to the low-side transistors, the faulty channel(s) lose their ground reference and can't loop any current through the output terminals.

You're right regarding the ODEP voltages. The +ODEP should be a negative reading and the -ODEP should be a positive reading, both around the 12V mark depending on the temperature of the heatsinks. In a working amp the ODEP adjustments are rarely very far from the centre of the pot.

Under normal conditions the amp gets hotter or takes more current and the ODEP voltage starts to move towards zero. During a hard fault the polarity of the odep can swap over so the +ODEP goes positive and vice versa.
There a re one or 2 typos in the service manual for example the output of the temperature sensing is quoted in mV but it's actually in V.

Next thing then is to check the heatsink temperature measurements, also on the test plug, around 2.8v each. If these are wrong then thats why the ODEPs are wrong.
There are another couple of lv rails to test, I think the diagram refers to them as 'bias supply' or similar, around 2.5V each. They are generated in various ways depending on the board revision, sometimes off the display board (!) and sometimes off the back board.

Obviously with having both channels faulty it's probably an issue common to both sides but it might be the same fault but on each channel, its not unheard of.

Triple check the tracks are all ok, each output transistor bank is monitored for current via sense lines, you can see these radiating out from the transistors where they measure the volt drop across the emitter resistors to control the SOAR via ODEP. Only 1 high reading will put the ODEP out of range on that channel. They're very thin tracks and can corrode, I read earlier you've already seen corrosion on there. Check the thru-hole vias.
I would buzz the tracks through from start to the actual pcb header. I've had tracks fail underneath the header itself. Take into account the low value of the emitter resistors which may make an open track look ok, it's a good idea to lift the little connecting wires that go from the emitter resistors to the pcb so you dont affect the results.

PS Make sure the rear ribbon cables are intact especially regarding corrosion and tearing. The output voltage of the amp is measured just behind the speaker terminals on a little board accessed behind the bottom half-cover and if the output is not sensed properly here it will cause all sorts of fault symptoms. Easily buzzed through from the terminals to the modules. Dont forget to clean the tinned brass pillars in between the output pcbs, they can corrode like mad.



Edited by kedwardsleisure - 14 May 2020 at 9:27am

the odep voltages are normally around 12V, the 2.5-3v readings are the temp sensors. When checking/adjusting the ODEPs you simply have to refer to the temp readings because it influences where the ODEP voltages sit.

If one of the temps is off the scale then you need to find out why. Dont just null it out on the pot and hope the amp won't notice. The heatsink temps should be approx the same at a cold switch on.

Edited by kedwardsleisure - 19 May 2020 at 12:45pm