Fishman Lunchbox whoops Loudbox Hexfets

[oc disorder]

Has anyone worked on these...?
Was blowing mains fuses... appears the hexfets not tight enough on heatsink
end result = shorted out supply.
One is a IRF640N (N-ch 200v 18A) the other is a IRF9540N (P-ch 100v 23A) .
As I have no schematic (the "box" is practically brand new) I was wondering
if these are the correct components 'cause as a complimentory pair they seem to have different specs.... Also seem to be soldered top and bottom
of doublesided board have to chop them then clean up the debris. grrr
Expensive but Vifa tweeta..and real ply in the construction from China.....
Any links to schematic? (dubiously optimistic)
Thanks

[Mars Amp Repair]

Nevah hoid of dah ting!
I would expect if someone replaced the Pwr transistors & the amp has non-original numbers in it that they would have tightened them sufficiently. Did they seem loose?

How many watts is this amp supposed to deliver. I would think the weakest of the 2 power transistors capable of handling 18Amps would be more than enough, so the higher rated one would have no problem at all. the mismatch is probably no issue as both xistors are most likely way over rated.

Be certain there is enough silicone heat sink grease after you replace the xistors. glen

[Enzo]

I would not be surprised that the two transistors are not "matched." It is not critical.

Did you contact Fishman and ask for schematics?

[oc disorder]

Thanks for replying.
Firstly I have approached the local agent for the service manual as in the
past the some manufactures get sh***y if you bypass their "zoneing" arrangements
but so far no luck.

Both the fets were the original factory ones I would have thought either a pair of
9640N/640N or 9540N/540N would have been used.It was relatively easy to
tighten 1/4 of a turn without overdoing it hence my misgivings regarding
coupling to the heatsink.

Both were short-circuited and mounted on a strip of silicon impregnated rubber
to the heat sink.I thought of using mica insulators and heat sink compound
as I read a post elsewhere where someone conducted some comparative
thermal tests which concluded mica + paste superior to "silly pads".
Is this the case ?

The majority of the board consists of S.M.D. components apart from regulators,
bridge rectifier and an LM3886 for the tweeter also mounted on the silicon rubber strip.

Its a 100 watt bi-amped amp and from the owners manual - tweeter amp 10w rms
continuous and woofer amp 90w rms continuous which feeds a 8" 4ohm 125w speaker.

Well, got exact replacements and fitted them tested it with my old 70's yamaha
accoustic with a retrofitted fishman piezo pu and sounded great !

But it runs very hot contemplated frying up some bacon and eggs on it !

It appears that the quiescent current is too high and I noticed there is a removable
jumper in order to place an ammeter in series with the source of the IRF9540N.

Measuring between these two points I got approx 1.1mA without input signal.

Meanwhile I'm waiting hopefully for the schematic ......

[Enzo]

Firstly I have approached the local agent for the service manual as in the
past the some manufactures get sh***y if you bypass their "zoneing" arrangements
but so far no luck.

Never spend times thinking of reasons NOT to try something. The FIRST place I look for schematics is the manufacturer. The worst they can do is tell you "no," or just ignore you. Stamps for email are pretty cheap where I live.

The transistors need to make good thermal contact, so if they were flat against the heatsink, they were tight enough. I bet I could tighten most transistor screws a little if I tried.

Both were short-circuited and mounted on a strip of silicon impregnated rubber
to the heat sink.I thought of using mica insulators and heat sink compound
as I read a post elsewhere where someone conducted some comparative
thermal tests which concluded mica + paste superior to "silly pads".
Is this the case ?

The Sil-Pads work well enough, I seriously doubt the thing blew up for lack of mica. Mica and white goop might be "better" than sil-pads, but that is not the issue. The issue is will the sil-pads do the job that is needed. After all, liquid nitrogen is a better chiller than my refrigerator, yet that inefficient refrigerator manages to chill my beer anyway.

Point is, that the transistors need X amount of heat removed. If the sil-pad does that, then it doesn't matter that some other product could have removed more heat.

A sil-pad comes out of the box ready to slap in place, then mount the transistor and no clean up. A mica washer means you have to grease the heatsink, slap the mica in place, grease the mica, THEN mount the transistor, and hope you didn't smear white grease all over. This adds lots to the labor, and have you priced heat goop lately? And I don't know about you, but whenever I work around the stuff, EVERY TIME I get it all over the place.

A manufacturer has to consider cost and ease of application.

As to running hot, does it get hot just sitting there idling? Or only if you play through it? Is there anaything else on the heat sink besides the MOSFETs?

[oc disorder]

Thanks for the reply Enzo (do you ever sleep?)
I have asked Fishman via their webpage email for details no response as yet.
I guess it could have just been component failure (fets).
The "heatsink" is actually a u shaped (maybee 1/16" Al) pcb housing which
attaches to the back panel with 5 nuts.
It gets hot to touch just idleing .
I realise mosfets run hot but thought after fitting new ones I hadn't really determined what had caused the problem in the first place.
There are also regulators, a bridge rectifier and a LM 3886 on the same heatsink.
I had before tested it in series with a 100w light bulb to make sure there were no faults remaining and now have it powered up normally and intend to leave it running for a few hours with the occasional burst of guitar abuse !

The fet Operating Junction and temperature range from the data sheet is -55 to + 175 °C so maybe I'm being over anxious.

I realise manufactures don't purposefully build things to break down although
the accountants seem to run R & D these days.

One other minor thing is here in Aust we have a healthy 240 volt mains.
Most of modern equipment I see these days is made for the European 220v.
This amp is 230 so there is slightly more stress than it expects although
minor.

From what I read mains voltages differ across the US so I guess you've
encountered that before.

I'll post a follow up once the real world testing phase is over.

I agree the sil-pads are much faster and cleaner to use ..

Some times jobs seem too easy so I just have to over complicate the issue.
Hence my logon name !

Thanks for your valued input.

[Enzo]

I am guessing the 3886 is for the tweeter?

Hey, a heatsink is a heatsink, even if it is just a lid from a tin can.

HAving Vregs on the heatsink is certainly going to warm it up. That 1ma you measured is not going to heat up much. I bet if you lay a finger on the Vregs, and then on the MOSFETs, you will find the Vregs a lot warmer. I'd bet my lunch miney.

And you realize, I hope, that American equipment must be turned upside down for proper operation in Australia.

Watch closely and see how many posts I make during the daylight hours. Darn few. That is when I go home. We work musician hours here - I open the shop at 6:30PM and close at dawn.

[oc disorder]

All's well that ends well

Well got a reply from fishman the Hexfets were the correct ones and my 1.1ma mis-reading should have been 11ma.They aim for about 15ma idle.
Played it loudly and checked the heatsink hot but within reason and at idle
only warm.
Must have just been component failure...

Of course it sounded better upside down ......

"We work musician hours here - I open the shop at 6:30PM and close at dawn. "

Here in OZ we would need liquid nitrogen to chill the beer 'cause it wouldn't
be in the fridge for long enough if we were open all night for muso's....

Not to mention actually getting anything done....!

Once again thanks for your valued input Enzo...

[Enzo]

They cut off beer sales at 2AM here. I have to think to go get some before 2, or I don't get any. Usually nothing in the fridge but OJ - the drink, not the football felon.