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250W amp - Mark Bass , Little Mark 250, of 2008, Italy

Mains fuse blew in the middle of normal use. No mention of PbF anywhere but it is. Unlikely the problem though. Seems bad thermal design as audio-out vaned heatsink has cooling air through it but the non-vaned SMPS one does not have, as that intended? air will go through the path of least resistance over the uncluttered preamp . I can see a baffle going in here to direct air over this plain block heatsink. Anyone experience of these little amps? Both SMPS powerFETs s/c all round. Each has a discoloured patch on the mounting-plate where the die overlies. No burning/blow-holes of these powerFET encapsulations or 10R SMD gate droppers or anything else found suspect from cold testing/close inspection, nor suspect looking PbF. The SMPS supervisor IC ident is ground off and a schematic for a similar amp has no ident. I could find details of any 8 pin 0.1 inch pitch package with push/pull drive in 1991 DATA linear colated listing- any suggestions for this sort of search? p1 supply low side p3 probably Enable p4 Vcc p5 and p7 hi/lo outputs p6 mid connection to both output powerFET Does not seem to be any thermal shutdown for the SMPS section

Reply to
N_Cook
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I've got one of those here I gave up on. Life's too short and doesn't pay enough for this kind of palaver.

Good luck!

Gareth.

Reply to
Gareth Magennis

but

Anyone

amp

So what was the failure on your one?

Reply to
N_Cook

Both Power amp and Power supply. Couldn't get a replacement for a blown PSU Mosfet, (Farnell had a one year waiting list) so subbed in the highest spec one I could find, but it still didn't power up, (amp disconnected) so I gave up.

If I come across something like this where schematics and parts are not available, I am not about to spend 6 difficult hours on it, and only be able to charge 3, so I now cut my losses and refuse. This, together with muting all TV adverts the past 5 years, has improved my life immeasurably.

Gareth.

Reply to
Gareth Magennis

From digeykey filter search , these are 8 pin SMPS ICs , now to see if dual drive

LTC1041

NJM2103

TDA4605

FAN7680

SG6203

Reply to
N_Cook

On Tue, 15 May 2012 16:12:10 +0100, "N_Cook" put finger to keyboard and composed:

Could we see it?

- Franc Zabkar

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Reply to
Franc Zabkar

different powerFET pair but overlay device numbers/types etc correcpond in the SMPS area otherwise

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Reply to
N_Cook

Will jury-rig with a couple of IRF740 in there at 70 percent mains to check if the supervisor is working. Can "totem pole" types be used as dual drive? A TDA8130 would go in there pinning-wise, with 6 of the 8 pins rewired, no idea if correct operational parameters though. Before trying a pair of 20 amp/600V powerFETs. Is there any point in roughing the surface of a flat shiney aluminium block heatsink or painting black for better heat transfer, not really space to add vanes to the block

Reply to
N_Cook

Different heatsink types and different layout to the pics shown in that pdf

Reply to
N_Cook

The SG6203 in that list is totem-pole but the source/sink split is not available at the output unlike the TDA8130, so none suitable from that sub-set

Reply to
N_Cook

On Wed, 16 May 2012 08:27:42 +0100, "N_Cook" put finger to keyboard and composed:

If I use the following search parameters ...

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... I get several promising leads, but all of them have the Vcc and Ground pins interchanged.

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... and lots more.

- Franc Zabkar

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Reply to
Franc Zabkar

in

I tried googling "switch mode" "8 pin" "totem pole" "switch mode" "8 pin" "hi output" "low output" and similar and found nothing suitable

Reply to
N_Cook

"Franc Zabkar"

** The Italian schem may well contain a drawing error re pins 1 and cos the IR2151 certainly looks the part.

Also, the amplifier output stage is NOT well protected from a short - those 6.2 V zeners are not gonna save any of the IRFP mosfets, which BTW need to be matched sets in order to share current.

PLUS, once the power amp fails short from rail to rail - the PSU has got only one option.

It blows up.

Wot a dago piece of shit.

... Phil

Reply to
Phil Allison

in

I checked the pcb traces and pins 1 and 4 are swapped over on that schematic , the 270K dropper agrees but ground and supply are swapped so pinning agreement with those IR... ones

Reply to
N_Cook

Would you trust amp repair to a company where they pull knobs away with carpenter's pliers? I assume that is the reason for the ground off ident, to lock into their repair shop

Reply to
N_Cook

"N_Cook"

** Told ya ....

.... Phil

Reply to
Phil Allison

With 2x IRF740 in there and 60 percent mains there is high current draw, some smps type oscillator start up noise and mains draw current drops right back with 180V over the main bridge rect , switch off and that 180V takes a long time dropping to 150V with some low level audible osc type noise . No led or fan tries turning , perhaps needs more than 60 percent for this or something else wrong, will have to try monitoring SMPS output before venturing higher

Reply to
N_Cook

On Wed, 16 May 2012 12:34:31 +0100, "N_Cook" put finger to keyboard and composed:

If you measure the Vcc of the IC, then that should narrow down the suitable candidates. Vcc is determined by an internal zener, not by the external components. It doesn't sound like this IC is the problem, though.

- Franc Zabkar

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Reply to
Franc Zabkar

schematic

When I get back to it I will try a 72V zener over one of the Brown-out? zener chain of 220/200V zeners, while srill trying with 60 percent mains (2 x200V caps across the main DC - reminder to myself to find a workaday formula for "Vz" of zeners at 1/10 to 1/10000 of rated current )

Reply to
N_Cook

On Thu, 17 May 2012 08:13:30 +0100, "N_Cook" put finger to keyboard and composed:

I would think that 420VDC would be required to "turn on" the series connected zener pair. A 240VAC source would produce 350VDC, so ISTM that these zeners would be sensing an overvoltage rather than a brownout. That said, the overvoltage would have to be an extremely large one (+20%).

- Franc Zabkar

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Reply to
Franc Zabkar

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