Peavey XR 886, 1997, mixer amp

Is that the model that has a fan mounted horizontally blowing down onto a skimpy heatsink ?

Graham

overheating.

on

in

the

aluminium

skimpy

Blowing up into an adequate , for fan blown, heatsink with a bank of 12 motorola 70483080 TO3 . As far as I can see the 2 uncut-down heatsink vanes that go across the fan do little functionally for cooling and probably disrupt the air flow.

The air has to come in horizontally under the base , up the one inch of fan axis and then have to abruptly divert horizontally again down the 2 (separated by these vanes) relatively small channels in the heatsink. I'll take a pic perhaps

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Perhaps the sensible approach to this is extend the rubber feet by half an inch as this is a known cure. There is no obvious place to put a secoind fan at the air outlet.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

So every XR 886 overheats?

an

fan

Not to the extent of tripping any of the thermal switches but enough to concern the owner into trying to improve airflow - admirable thinking for a non-techie. Just giving greater clearance to the airflow under an amp should not normally affect the amp temperature though. I would suggest that it is due to forcing the air to follow a tortuous path so a generic design flaw. I've heated up the fan motor, etc and there seems nothing wrong with this fan as well as no change of fan noise reported by the owner.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

What say if an amplifier was designed in such a way that the thermal protection was intended to operate at some given device temperature, say the safe working temp of the output transistors, and the cooling arrangments designed accordingly? Output transistors get hot, heatsink gets hot, thermal devices trip, amplifier shuts down or whatever it`s designed to do.

But, by increasing the airflow from the fan[1] or improving the cooling to the heatsink in some other way, results in the heatsink remaining cooler when the actual output devices themselves are still running at a high temperature. Bearing in mind that the temp sensors may not be directly monitoring the devices temp, but some other area on the heatsink, that might lead to the thermal protection failing to operate.

By changing the design of the cooling, you could be blowing cool air over the temp sensors and not the output devices. Result = a trip to silicon heaven.

I have seen Peavey mixer amps shut down in hot weather when placed in direct sunlight and driven hard, that`s the protection doing what it`s designed to do tho. Generally speaking, if a decent quality amplifier is overheating, it`s operator error, not a design fault. (IMO)

Ron(UK)

[1] This is hypothetical of course, I realise you havent changed the fan or it`s cooling path in this instance, but I think it`s something to be aware of. Maybe someone with experience in 'designing' power amplifiers could comment?

How do you define adequate ? I design my own heatsinks for amps you see and most others I come across are very inadequate, notably recent Peaveys.

Graham

A simple solution at least.

You CAN 'gang' 2 fans in series to increase the air pressure but I've never needed to do it.

Also, fitting a 'thicker' fan will increase the air pressure. You may find 12,

19 and 25mm versions of certain axial fans. The thicker they get, the more pressure they generate and that helps achieve good airflow. The CFM figures are largely just a guidline since there are with zero pressure gradient.

Graham

Would depend how they're operated, load impedance and the style of music (especially so). I have researched this area extensively and the variations are enormous.Stnadardised tests such as 1/8 or 1/3 power noise are especially useless.

Graham

Yes it will by reducing the back pressure.

Graham

That's what causes the high back pressure (and hence low air flow). For good cooling the airflow path should be in a striaght line with air vents in and out as large as possible (consistent with safety regs) and there are some other cute aspects which are even less well known too.

Graham

onto a

and most

For fan blown o/p devices the heatsink surely does not have to be much more that a metal matrix for holding the devices to. Does not have to be multifluted or with those chisseled multi-vanes etc (sorry don't know the generic term , end up curved ) or a great mass of metal for convection cooling

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

good

and out

other cute

Agreed , I'll take a photo later to show how tortuous this path is.

Even more dramatic was an Ashdown amp I had to repair recently for another fault. Supposedly fan assisted cooling but the fan "design" location was just circulating air inside the casing as no baffles etc to guide air in and out the single "vent" port. Owner much appreciated the amp running much cooler and being able to feel air coming out of the vent.

s.e.r added as design issue

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

I have done !

There's something worth adding though.

To optmise air flow you need a high 'static pressure'. There's a huge variation in this characteristic. The best examples I've found are NMB (formerly Boxer) and Panasonic's Panaflow fans. Be careful to order the high pressure version. They make versions with various motors speeds. In the case of NMB, the 'best' ones used to have 'B50' as part of the part number. I forget Panasonic's method of marking.

DO NOT use cheap 'off brand' fans for applications like this.

I see Farnell have NMB but only as a special US import. You didn't say what size it is but the popular 80mm size would be for example ....

Read the data sheet here

to see just how radically. the airflow varies with the Bxx model number.

Graham

The larger area the better.

That certainly helps.

Those increase surface area which is a GOOD THING.

It most certainly DOES NOT need to have great weight. But almost none of the colling in SR amp is classic convection anyway. What typically helps is maximum surface area of the fins/vanes to create the best opportunity to let the heat transfer to the air flow, but you can improve even that by 'slotting' the 'vanes' to create a slightly turbulent air flow.

Trust me I could write a lecture on the subject.

Graham

I recall many years ago that there was a 'manufacturers mod' for some power amp or other ( dont recall which) where you had to bend all the vanes of the heatsinks - which were like combs - away from the vertical in various different directions to improve the marginal cooling just that tiny bit to prevent the thermal protection tripping. Looked damn ugly but did the trick.

What works best Graham, a thick ally heatsink with many thick fins, or a thick ally plate with many convoluted thin sheet fins?

Ron

more

the

maximum

heat

Anyone know the generic name for this type of what I assume is chisel-action-cut vaned heatsink. Is it made using a shaper type machine with some sort of chisel or plane blade?

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

This is the amp in question

Yellow ring stands in for the 4 inch fan removed (position marked with Fs )along with metal sheet that encloses the heatsink. The view of this amp is upside down as the fan is at the base. So drawing air horizonatally theough a "designed" 5/8 inch space under the amp. Air then goes through the

1 in of fan vertically and then has to sharply divert horizontally to flow either way through the 3 ducts in each direction. The central duct contains no active ducts and as far as I can see only interferes with air flow as they are right up against the fan housing. Air then vents mainly at either end when the metal closure in place with not enough space to add a fan or 2 at those points and nowhere near the mains transformer and main air outlet vent . A lot of venting must go up and out the mixer pannel slider holes etc as well. The 2 TO3 mounted thermal switches are marked S and restrict the air flow through the uppermost duct in the pic, the leads and spades as much as the switch body

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

I think that the worst part of that desgn IIRC is that the board is mounted inverted with the sinks beneath, thus any heat which tried to escape by convection has to pass through the pcb, heating the other components. Am I correct in that assumption?

Ron