I recently opened my Crest VS900 and noticed that the big, vertical mounted 330ohms/10W resistor marked as R92 gets extremely hot, so hot that you can already smell it after a minute the amplifier was switched on, let alone touching it. 
You can also see burn marks on the fan connector that's directly next to it.
What could be causing it? The fan (115VAC Sunon) was already lubricated and cleaned some time ago.
My VS is an original euro version and we have around 225-228V in the socket.
(The mentioned resistor is just under the TR1 triac)
You can also see burn marks on the fan connector that's directly next to it.
What could be causing it? The fan (115VAC Sunon) was already lubricated and cleaned some time ago.
My VS is an original euro version and we have around 225-228V in the socket.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
(The mentioned resistor is just under the TR1 triac)
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As I calculate the 330W resistor will dissipate about 7.3W, if it running from 115V AC, and series connected with the fan.
Maybe You can change the resistor to 470ohm/10W. It will reduce the fan speed somewhat, the dissipation of the resistor will be about same (7.4W).
If I would do the design of this amplifier, I would use 20W resistor....
Sajti
I measured that the fan runs at 68,3V now. The VS900 seems to have a 3 step fan control, it's always on and at 55*C it speeds up and at 75% it should go full throttle.
Crest did a trick to somehow cool it - they let the fan suck the air inside the amplifier, alongside the radiator and it comes out on the sides of the amp.
Crest did a trick to somehow cool it - they let the fan suck the air inside the amplifier, alongside the radiator and it comes out on the sides of the amp.
An externally hosted image should be here but it was not working when we last tested it.
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If You want to keep everything same, than change the resistor to 330ohms 20-25W. Multicomp has 330ohms 25W resistor, available from Farnell. And it's not expensive.
Sajti
I'm concerned whether it will fit or not. The 10W is already packed quite tight. Is the 25W also a cement resistor or wirewound?
Wirewound will most prbably not fit, unless I cut the mounting flaps and let it hang on the wires...
I think that I could just disconnect the fan, I'm using the Crest at home and since it's an Class H amp it should stay cool.
Wirewound will most prbably not fit, unless I cut the mounting flaps and let it hang on the wires...
I think that I could just disconnect the fan, I'm using the Crest at home and since it's an Class H amp it should stay cool.
Yes. If You don't use the fan, it will turn off itself, in few minutes with closed box.
Sajti
I'm not agree. It will reduce the temperature of the resistor. The ventillation should be OK, as the resistor is close to the heatsink.
Sajti
Thank you for all replies!
@JonSnell Electronic,
I thoroughly cleaned and lubricated the fan, it runs smooth.
The R92 and the main heatsink don't rely on each other, they need the blower to keep things cool but again - even with the fan on and the lid closed that resistor gets hot enough to burn the fan connector and start to smell.
@PQatPIT,
What 220Vac fans did you use?
@JonSnell Electronic,
I thoroughly cleaned and lubricated the fan, it runs smooth.
The R92 and the main heatsink don't rely on each other, they need the blower to keep things cool but again - even with the fan on and the lid closed that resistor gets hot enough to burn the fan connector and start to smell.
@PQatPIT,
What 220Vac fans did you use?
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Be careful with the fan replacement. 220V fan means less air delivered to the heatsink. It may save Your resistor, but it can results early thermal shut down.
Sajti
I am fully aware of that, the amp is used in a home environment so for now it only needs a gentle breeze
The resistor issue has the highest priority for now, then comes the fan replacement/speed tweakin.
I thought about using two 10W resistors in parallell as that would give more surface to dissipate heat...
The resistor issue has the highest priority for now, then comes the fan replacement/speed tweakin.
I thought about using two 10W resistors in parallell as that would give more surface to dissipate heat...
I am fully aware of that, the amp is used in a home environment so for now it only needs a gentle breeze
The resistor issue has the highest priority for now, then comes the fan replacement/speed tweakin.
I thought about using two 10W resistors in parallell as that would give more surface to dissipate heat...
If You have enough space to do that. Be carefull with fixing the 2nd resistor, because it's working on mains.
Sajti
The 230V euro model seems to have a separate 115V winding on the transformer but I will be careful, promised!
I think this is not really separate winding, just a half of the primary windings.
Sajti
Not sure anymore but I'd guess they were Fulltech UF12A23-BTH.
https://www.google.fi/search?q=UF12...ved=0CAcQ_AUoAWoVChMIzJfsxa3YxgIVZ_ByCh2c2grw
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