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

[N_Cook]

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

 

[N_Cook]

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.

So what was the failure on your one?

 

[N_Cook]

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

LTC1041

NJM2103

TDA4605

FAN7680

SG6203

 

[Franc Zabkar]

a schematic for a similar amp ...

Could we see it?

- Franc Zabkar

 

[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

 

[N_Cook]

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

 

[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

 

[Franc Zabkar]

different powerFET pair but overlay device numbers/types etc correcpond in
the SMPS area otherwise
http://www.eserviceinfo.com/download.php?fileid=51892

If I use the following search parameters ...

http://www.google.com/search?q=vb+vs+en+vcc+rt+com+filetype:pdf

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

http://www.datasheetcatalog.org/datasheet/irf/ir2151.pdf
http://www.irf.com/product-info/datasheets/data/ir2153z.pdf
http://www.irf.com/product-info/datasheets/data/irs27951s.pdf
http://www.dianyuan.com/bbs/u/83/3675811258537068.pdf
http://bbs.dianyuan.com/bbs/u/70/2970761224726220.pdf

.... and lots more.

- Franc Zabkar

 

[N_Cook]

If I use the following search parameters ...

http://www.google.com/search?q=vb+vs+en+vcc+rt+com+filetype:pdf

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

http://www.datasheetcatalog.org/datasheet/irf/ir2151.pdf
http://www.irf.com/product-info/datasheets/data/ir2153z.pdf
http://www.irf.com/product-info/datasheets/data/irs27951s.pdf
http://www.dianyuan.com/bbs/u/83/3675811258537068.pdf
http://bbs.dianyuan.com/bbs/u/70/2970761224726220.pdf

... and lots more.

- Franc Zabkar

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

 

[Phil Allison]

"Franc Zabkar"

If I use the following search parameters ...

http://www.google.com/search?q=vb+vs+en+vcc+rt+com+filetype:pdf

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

http://www.datasheetcatalog.org/datasheet/irf/ir2151.pdf
http://www.irf.com/product-info/datasheets/data/ir2153z.pdf
http://www.irf.com/product-info/datasheets/data/irs27951s.pdf
http://www.dianyuan.com/bbs/u/83/3675811258537068.pdf
http://bbs.dianyuan.com/bbs/u/70/2970761224726220.pdf

** 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

 

[N_Cook]

If I use the following search parameters ...

http://www.google.com/search?q=vb+vs+en+vcc+rt+com+filetype:pdf

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

http://www.datasheetcatalog.org/datasheet/irf/ir2151.pdf
http://www.irf.com/product-info/datasheets/data/ir2153z.pdf
http://www.irf.com/product-info/datasheets/data/irs27951s.pdf
http://www.dianyuan.com/bbs/u/83/3675811258537068.pdf
http://bbs.dianyuan.com/bbs/u/70/2970761224726220.pdf

... and lots more.

- Franc Zabkar

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

 

[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

 

[Phil Allison]

"N_Cook"

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

** Told ya ....



..... Phil

 

[N_Cook]

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

 

[Franc Zabkar]

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

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

 

[N_Cook]

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

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 )

 

[Franc Zabkar]

When I get back to it I will try a 72V zener over one of the Brown-out?
zener chain of 220/200V zeners ...

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

 

[N_Cook]

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

That 47K must put the current into the uA regime
I doubt the Ebers-Moll sort of stuff applies in this sort of illegitimate
area of operation.
Tried a 6.2V 1W zener
Vz 6.22V (initially then rising) @ 78mA
96 percent of Vz @ 7.4mA
95% @ 810uA
92% @ 102uA
77% @ 9.6uA
66% @ 2.6uA (4.12V)

 

[N_Cook]

I'd forgotten to at least try the PA and its ok up to +/-25V dc on a bench
PS , 63V rated caps, so less than the 80V of that schematic.
The PA is certainly different to that schematic, pair of those 5 pin SAP
devices with intimate thermally connected sensing diodes

 

[N_Cook]

470K not 47K

I thought startup was a bit quiet, somewhere along the way I've managed to
short G-S on the high side IRF740. The other one changes state on DVM
"diode" test. Shorted turns on the switch mode transformer ? Change C68,
perhaps punch through ?