JohnnySH

Hi Alain

Thank you for your reply,

I have really ended up replacing the same part, only not from crown but a UK supplier

Are you saying the new upgraded design uses a different type of body thermister,

If so, can this type of thermister be used on my macro-tech 5000i ?

and would it be beneficial to change to this type ?

also how does it mount to the heatsink

do you have a part number or supplier ?

does the new thermister have the same characteristics as the old part

anymore info would be very much appreciated as I would like to make my two amplifiers as reliable as possible

Thank you for your feed back

Kind Regards

John

Hi

Hiss is tied up with the Hi Frequency side of the amplification, most people complain about hums, earth loops, DC Hum etc the low frequency side.

So this is quite a rare condition, if no input source is connected to the amplifier and this hiss still existed, then there is an internal fault with the amplifier and thus would need to go to a service center where by an oscilloscope could be used to see exactly what is going on with the amplification circuitry and what is causing this loud hiss.

I would have a guess and say this fault lies on the front end amplification of the faulty channel and could be a faulty OPAMP.

Simply take your faulty amplifier to a good repair center, it will be much quicker any easier than messing around crown support.

Good luck

Hi

No I did not try a 20k resistor,

I first cut the two old thermistors legs from the circuit board and did a resistance check with a analogue ohm meter

I always prefer using this type of meter on the lowest possible reading because in draws more current through the part when taking a measurement, thus if part if faulty, it's more likely to show, instead of using a DVM.

The good thermister read about 22k and bad thermister read about 1k,

with the parts connected to the circuit board I was getting lower reading, so I knew to get true readings I had to isolate the two thermisters from the circuit board.

From my two reading 20k and 1k i knew there was problems

I then started to investigate thermisters and managed to find a similar device and check the spec of the device and viewed a chart of temperature vs resistance for this part.

At 1k on the chart the temperature would have been very high can't quite remember exactly, so I knew this part had failed and was fooling the amplifier in to thinking the heatsink was very very hot. I also knew that both thermisters should have been 20k in value and thus were not.

the 4 new parts I ordered were from RS components, RS Number 697-4581, Part Number NJ28PA0203F Made In France By AVX

http://uk.rs-online....7633D4E4F4E4526

I did have to sleeve the two legs as the original parts had sleeve coated legs and these new parts did not. apart from that they look identical in every way.

A little bit cheaper than what UK Crown distributor wanted to charge me ! and I got the parts next day ! not in 3-4 weeks time.

Now when I turn on my amplifier CH1, CH2, Ready Light turn on, NO fans spin until you start working the amplifier

One Fixed amplifier, not bad for under a week from failure and at a cost of £2

Hope this helps others,

Regards

John


PS also I was a little shocked that there was so little help, support and input from the tech guys on this forum, though I would like to say thank you to dakos. I'm only too pleased to be a professional electronics engineer and able to repair this type of equipment myself, 1) I know the job has been carried out correctly, 2) It saves me a small fortune.

Please to say I have repaired my much love amplifier,

My diagnoses was spot on, it was the 20k thermister, on the output heatsink, ch2 thermister had gone low to about 1k!

Crown UK distributor want to charge me about £16 and I would have to wait for 3 weeks to get the parts from the usa to the uk.

I did some research and managed to get the exact same part from a uk electronics supplier for £2 each

I have ordered 4 so I can change all thermisters in my 2 crown macro-tech 5000i amplifiers.

I am truly amazed that this grade of amplifier uses such a cheap thermister that can fail as has in my case and cause such problems,

Even though I have replaced all thermisters with brand new items, I now have little faith in them, I know the could fail again,

Crown really does over inflate the replacement cost of there parts,

anyway any one else with this problem, you now know how to resolve.

regards john

Dear Dakos

Thank you for you informative reply,

I'm still waiting to speak with Crown on Monday,

1) to try to get a circuit diagram and

2) to find out a little more about the thermal sensors and thus what values they should be?

When you say the outboard thermal sensors, do you mean the ones on the heatsink ? if so, the values you suggest are different to mine ?

The working CH1 from cold is about 6.2k and the bad CH2 is around 1k thus seems to have gone low in value ?

Please forgive my ignorance but what are the SMPS thermistors, and where can they be located.

I am also a little amazed that such an expensive, high quality well built and constructed amplifier can fail over one poorly designed thermal sensor ?

The amplifier was built in 2009, so not very old and it may still be warranted.

I appreciate your feedback

Thanks again