Crest CPX 1500 Power Amp - Noisy Fan

[TomCarlos]

Friends...

I picked up a Crest CPX 1500 Power Amp. The unit is working fine with the exception of the VERY noisy fan. The amp has a 2 way switch that changes the fan speed based on heat. When turning the unit on and this means the low speed, the fan is noisy. And I say noisy because comparing the Crest to my Peavey CS800 or QSC GX3 power amps, the Crest fan is much louder.

I don't know if this is normal for this amp. Before I open the case and go exploring, I thought I would post a query here.

And for anyone needing the schematic for the CPX1500, see attached file.

Tom

[Leo_Gnardo]

I picked up a Crest CPX 1500 Power Amp. The unit is working fine with the exception of the VERY noisy fan.

It was meant to amplify very noisy music. But seriously folks... I find sometimes fans with blades loaded with caked-on dust make a racket & behave much quieter after a cleanup. If the fan's buried inside, and loaded with dust, probably the heat sinks also are filthy and your prize will benefit from a good clean up. Other than that, a new fan may be in order. In many cases the noise factor is included in the specs & you may be able to select a nice quiet one that way.

[Jazz P Bass]

On certain fans a label can be lifted to access the bearing so that you can get at it to apply a drop of oil.

[TomCarlos]

Thanks Leo and John...

I opened the case and found some gunk on the fan and minor dirt inside the case. I cleaned the fan and was able to add a drop of oil under the label. Thanks for the tip. The fan isn't as noisy as before. But for a few bucks, I might try a new one.

SuperRed CHA8024EBN-K
DC 24Vm
.24A

[Leo_Gnardo]

for a few bucks, I might try a new one.

Back in the good ole days, Rotron in nearby Woodstock NY was the major fan builder, their best known model the "muffin fan." Now we get all our fans from you know where*, the old Rotron factories stand empty.

* except for Papst, made in Germany, built like a brick outhouse. Costly but they do last practically forever.

[TomCarlos]

Yup... everything is made ashore these days.

I found a replacement fan, from Jameco.com (Part # 136161). The specs are:

24vdc
38.74 CFM
150ma
27.8 dba

Heck... for $10, why not try it!

[Enzo]

Check the dimensions.

[TomCarlos]

Friends.... A follow up to this thread...

I replaced the stock fan with the unit from Jameco (see #6). At start up, the replacement fan is definitely more quiet. But I have noticed something about this amp (and it did the same thing with the stock fan).

You turn the amp on, the fan is running, amp sounds fine, all is good. At the 6 minute mark (give or take), the fan kicks into high gear - and this is with NO input signal. I checked the output - clean, no oscillation on either channel. I let the amp cool down, then repeated the test by removing the top cover of the amp. Again, the same thing happens at around 6 minutes.

The heat sinks are arranged in a V shape. The fan comes stock with a cardboard cover that directs the air flow across the back of each heat sink. (See photo). At the 6 minute mark, the heat sink (where they meet up, at the front end of the amp) does feel a skosh warm, but I would not describe it as hot. You can leave you hand on the heat sink and you won't get burned!

I have run this amp for a couple hours at a moderate output level- sounds just fine.

So I am wondering, is it normal for a fan (in a Crest amp or any other amp) to kick into a higher gear and stay there, even with no input signal?

[Leo_Gnardo]

At least the temp sensor system seems to be responding, even if it seems to be "jumping the gun" a bit. Does the fan speed ever return to slow mode? Whether it does or not, seems you're better off with the new fan. Unless your application requires an amp that runs nearly silently*, I'd say leave 'er alone, you've done everything you can.

*Story time. Late 70's Clair Brothers was hired to do PA & monitors for Paul Simon tours. They used "S4" racks, each had four custom-modified Phase Linear 700 amps, probably a dozen or more racks, each with a couple of 8 inch Rotron "Tarzan" fans for cooling, larger versions of the famed muffin fan. Comes a quiet portion of the show, Paul hears the amp rack fans whizzing. Then Clair had to keep him satisfied, right? They installed fan speed switches so as to not interrupt the sensitive musician. Plus assign their system techs something else to do: flip those switches on cue. Keep 'em happy, collect the paycheck!

[TomCarlos]

Thanks Leo....

The fan stays in the fast mode once it kicks into high gear. Compared to other amps (and I have a fairly new QSC), I would say the fan noise is reasonable.

I just thought it was odd that this speed change seems to be "time" based. And I say that because of testing the amp with the top cover on and off.

[Leo_Gnardo]

I just thought it was odd that this speed change seems to be "time" based. And I say that because of testing the amp with the top cover on and off.

It must be heat based. If you were able to put the amp in a much warmer or colder ambient, I'll bet the 6 minute delay would change accordingly.

[TomCarlos]

You are absolutely right Leo.

I found a big a$$ house fan and pointed it to the heat sinks. I had the amp running for 30 minutes and the amp fan remained in the slow mode. At "T" plus 6 minutes after I removed the house fan, the amp fan kicked into gear.

So this tells me a couple things... the design might have been improved by separating the heat sinks from one another and better usage of a cooling fan. It also tells me that the power devices are pulling plenty of idle current!

Tom

[Leo_Gnardo]

You are absolutely right Leo.

Good I can take off this dratted dunce cap now. Ha! I thought you might try firing up your Crest inside the local butcher's meat locker. Or haul it out to the desert.

[Jazz P Bass]

If you really want to know, measure the heatsink temp.
60 degrees C (140F) is where the high speed is supposed to turn on.

Rather than being 'time' based, I would expect that that is the length of time that it takes to heat up the circuit at idle.

Maybe check the bias current of both amps.

crest_cpx-900_1500_thermal management.pdf

[g1]

I'd be a bit surprised if this is normal. I'd think the fan should be able to cool the unit back down to low speed when idling with no signal. Otherwise why even have 2 speed operation.
Agree with what Jazz said about checking the bias. If that's ok I'd check that both the thermistors are acting the same as far as resistance at various temperatures. Maybe one is wonky or has been replaced with a different spec. part. I'm assuming they are mounted one on each heatsink?

edit: I deleted my comment about the hysteresis adjust as that is for thermal shutdown, not fan speed.