Peavey PV2000 slave amp

[N_Cook]

Ok last summer, stored in an outbuilding over winter , got down to -15 deg C
southern England last December.
Now one channel shows protect LED on . What does DOT (TM) accronym? stand
for ? and anyone any inkling what would be most ssceptible to
cold/condensation? Nothing seems amiss cold testing on that channel, not
powered up , monitored , yet. I have a schematic for it, a lot of socketed
op amps in that area, so changing them and the sockets would be a good
start methinks.

 

[William Sommerwerck]

What does DOT stand for?

dead output transistors

<grin>

 

[N_Cook]

dead output transistors

<grin>

I now see it is DDT (TM) for speaker protection , there was no DC on the
output of either channel so presumably false protect cct and the crowbar
triac is ok. Still can't find what DDT stands for though

 

[William Sommerwerck]

A quick Googling shows that DDT stands for "distortion detection technique",
a system that prevents (or reduces) clipping.

http://www.peavey.com/media/pdf/manuals/80300112.pdf

Why should I have had to do this for you?

 

[Meat Plow]

Ok last summer, stored in an outbuilding over winter , got down to -15
deg C southern England last December.
Now one channel shows protect LED on . What does DOT (TM) accronym?
stand for ? and anyone any inkling what would be most ssceptible to
cold/condensation? Nothing seems amiss cold testing on that channel, not
powered up , monitored , yet. I have a schematic for it, a lot of
socketed op amps in that area, so changing them and the sockets would
be a good start methinks.

Reseat the socketed devices. Temperature isn't a factor, humidity more
likely the cause of slight corrosion of the pinned devices.

 

[N_Cook]

Reseat the socketed devices. Temperature isn't a factor, humidity more
likely the cause of slight corrosion of the pinned devices.

Looks as though a TO92 has decided to take in condensation, go ohmic and
move the bias a bit negative.
TO92 failures the same, with a load of Crest audio slave amps left over
winter ,almost outdoors , by someone else.

 

[N_Cook]

At least these amps have separate LV DC supplies and the biasing part can be
probed out of casing , powered from +/-15V bench ps without +/-92V around

TO92 60V, 0.6A , 9V on one end of the 47K base resistor and 8mV on the
other. Checks out DVM diode test and no obvious DVM resistance between pins.
Now removed can see ,under x30, slight build up of corrossion crud where the
legs enter the plastic, marginally more than the layer of corrossion over
the rest of the legs.
I will let the owner see the result under pocket magnifier , that and bill
might change his storage arrangements.

 

[Meat Plow]

At least these amps have separate LV DC supplies and the biasing part
can be probed out of casing , powered from +/-15V bench ps without
+/-92V around

TO92 60V, 0.6A , 9V on one end of the 47K base resistor and 8mV on the
other. Checks out DVM diode test and no obvious DVM resistance between
pins. Now removed can see ,under x30, slight build up of corrossion crud
where the legs enter the plastic, marginally more than the layer of
corrossion over the rest of the legs.
I will let the owner see the result under pocket magnifier , that and
bill might change his storage arrangements.

Some people just don't realize that electronics of this type really need
to be stored in a climate controlled storage unit.

 

[N_Cook]

Some people just don't realize that electronics of this type really need
to be stored in a climate controlled storage unit.

The other owner , with the Crest audio amps thought covering with a plastic
sheet , include a bag of silical gel, and leave in a Dutch barn was
sufficient. He never even recharged the silica gel - having no knowledge of
that process.

I wonder why TO92 and not TO220 or TO3 fail in these circumstances. I
suppose the leads corrode and swell and then like concrete spalling due to
rusting of rebar, the plastic splits enough for capilliary water condensate
to migrate up into the die. Something to do with small diameter leads for
TO92?
I will try to remember to split open this TO92 and see if it cleaves along
the corrossion plane.
Different maker to the Crest TO92 so not a batch problem.

 

[Meat Plow]

The other owner , with the Crest audio amps thought covering with a
plastic sheet , include a bag of silical gel, and leave in a Dutch barn
was sufficient. He never even recharged the silica gel - having no
knowledge of that process.

I wonder why TO92 and not TO220 or TO3 fail in these circumstances. I
suppose the leads corrode and swell and then like concrete spalling due
to rusting of rebar, the plastic splits enough for capilliary water
condensate to migrate up into the die. Something to do with small
diameter leads for TO92?
I will try to remember to split open this TO92 and see if it cleaves
along the corrossion plane.
Different maker to the Crest TO92 so not a batch problem.

I think the corrosion infiltrated the die. Like ice crystals in concrete
as you said. Repeated heat thaw cycles exacerbate the issue. Furthermore,
not having a fresh/sufficient desiccant furthers the process. Why the
TO92 suffers more would only be guessing. May be a batch issue. I know
there are plenty that don't suffer from extreme conditions. But they may
be saved from frequent use.

 

[N_Cook]

I think the corrosion infiltrated the die. Like ice crystals in concrete
as you said. Repeated heat thaw cycles exacerbate the issue. Furthermore,
not having a fresh/sufficient desiccant furthers the process. Why the
TO92 suffers more would only be guessing. May be a batch issue. I know
there are plenty that don't suffer from extreme conditions. But they may
be saved from frequent use.

Different manuafacturer and series of TO92 transistors rather than different
amp maker , so less likely a batch problem. In TO92, TO126 and TO220 the
encapsulation all goes down to the die IIRC. Perhaps corrossion on small
diameter leads has a better forcing/splitting effect into the plastic, then
capilliary. Continue the bad environment long enough and the other classes
of encapsulation would start failing also is my guess.
Further back in thread 80mV not 8mV on the base

 

[N_Cook]

Assuming it is B-E junction failure going ohmic. It will be low current
transistors showing the effect first, it may be inside other higher current
ones but not manifesting as a problem yet. For fault finding I suppose one
route is to check TO92 first and of those the lightest current ones first eg
highest base resistor values first , if no other knowledge.
A trouble with these sort of biasing circuit areas is an almost total lack
of grounded nodes so everything is floating up or down together.

 

[N_Cook]

looks as though the precipitating problem was failing 15V zener.
For some odd reason there are LV +/- 15V LV supplies and +/-15 supplies from
the +/-82V rails linked together via diodes at the pa.
The LV +15V supply is from 7815 but the -15V is from 1W zener. The -92V
supplied 15V zener stable but the LV one starts at -15.6V (diode offset from
the other one of 15.2V presumably) and drops through 15.8V . When the
difference between nominal +/-15V rails is about 1V it goes outside the
capture range of the half U100 dual op amp that feeds back correction into
the pa bias area (why there is no bias adjust on these amps)