Post by Phil Allison"Arfa Daily"
"Phil Allison"
Post by Phil AllisonPost by Phil Allison** Ever see one of these ?
An evil mix of EL84s, ECC83s and SMD on the same PCB.
Oh, and a DSP chip hiding on the back of a tiny PCB that holds 3 jack
sockets.
But none of these were the problem.
The amp would run OK for about 10 mins and then power itself down -
all the lights go out and the sound fades away.
Turning the power off and waiting a few minutes restored operation,
but not for long.
Who has come across this one?
** All the clues are there.
The amp powers itself off, valve heaters and the dial light goes out and
will not restart by itself.
A few minutes with the AC off does the trick.
Sounds like it needs a good squib of WD40 to me, Phil ... :-)
** LOL !!
But seriously, it's gotta be something to do with either that connector,
SK4 that they've put in the way, or the PTC, F1 that follows it ??
** Give that man a Kewpie doll.................
? Maybe faulty, maybe hotting more than normal up for some reason ? Maybe
had the wrong pilot lamp fitted, although I would have expected the two 10
ohm Rs to have taken care of that. Otherwise, apart from a heater in a
valve going short to a cathode maybe, about the only other thing is the
hum balance pot, although I'm not sure in what way practically that it
could fail, to place an excess load on the PTC ...
** For some whacky reason, the Chinese included a PTC ( aka Polyswitch )
in series with the 6.3 volt AC heater supply - adding to their bad
judgement, they picked one with too low a current rating.
The heater draw in the Cub 12R is a tad over 3 amps ( = 20 watts) and is
the largest load on the AC tranny.
Seems these Polyswitch devices deteriorate with age, if run hot all the
time - until they trip and go high resistance. With no heater supply, the
dial lamp goes out and the amp goes slowly silent and the same scenario
can be repeated indefinitely.
From reading the Laney user's forum, the fault is endemic with the model
and you need to fit a 6 amp rated PTC to fix it for sure.
Or just leave it out.
... Phil
Interestingly, on the schematic copy that I have, it shows F1 as a linear
thermistor which has the designation "PTC 6.0" by the side of it, which
would imply that it is specced as a 6 amp device. However, a little further
up the schematic in the AC feeds to the opamp supply rectifiers, there are
two more devices, this time shown as fuses with the component references F2
and F3, and designated as "PolySW0.3" which would imply to me that these are
300mA opening-current devices, and that Laney believe them to be a different
thing altogether from a PTC ?? Although polyswitches are of course PTCs,
they are designed to be very fast. On the other hand, there are positive
temperature coefficient thermistors that are similarly 'slow' to NTC ones.
There are plenty of characteristic graphs on the web showing 'slow' linear
PTC thermistor action, and others showing fast PTC polyswitch action.
Although that said, I don't really know why Laney would have designed one
into there anyway. An NTC thermistor to mitigate heater surge when they are
cold, maybe, but a PTC - linear characteristic or polyswitch
characteristic - seems altogether 'wrong' in that position, and I agree with
you that it's probably best just bypassed ...
Arfa