Tranny arrived today. I found some spanners big enough to get the old one
out. Fitted the replacement, along with an inrush thermistor from Farnell,
and two new level pots (both the originals went round and round ... ).
Gingerly powered it via the variac and Lo! Up it came !

Which was a relief, because I'm not sure that it looks like one that I would
want to be trying to repair an output stage on, particularly if it was the
lower board that had failed ...

Bucketfuls of watts poured out of both channels, and the fan cut in just
when expected, so I heaved a sigh of relief, screwed the cover back on, and
wrote the ticket out. Then gave myself a hernia getting it off the bench ...
:-)

Arfa

Arfa Daily wrote:

Tranny arrived today. I found some spanners big enough to get the old one
out. Fitted the replacement, along with an inrush thermistor from Farnell,
and two new level pots (both the originals went round and round ... ).
Gingerly powered it via the variac and Lo! Up it came !


** Hope you also tried a few on-off cycles to check out the new PTC.

Did you find one close to the original at Farnell ?


Bucketfuls of watts poured out of both channels, and the fan cut in just
when expected, so I heaved a sigh of relief, screwed the cover back on, and
wrote the ticket out. Then gave myself a hernia getting it off the bench ...
:-)

** What are you plans for disposing of it?

Nearby skip ?


..... Phil

Phil Allison wrote:


** What are you plans for disposing of it?

Nearby skip ?


** I'm asking about the old transformer, of course.



.... Phil

On 21/08/2015 01:58, Arfa Daily wrote:
Tranny arrived today. I found some spanners big enough to get the old
one out. Fitted the replacement, along with an inrush thermistor from
Farnell, and two new level pots (both the originals went round and round
... ). Gingerly powered it via the variac and Lo! Up it came !

Which was a relief, because I'm not sure that it looks like one that I
would want to be trying to repair an output stage on, particularly if it
was the lower board that had failed ...

Bucketfuls of watts poured out of both channels, and the fan cut in just
when expected, so I heaved a sigh of relief, screwed the cover back on,
and wrote the ticket out. Then gave myself a hernia getting it off the
bench ... :-)

Arfa

Butchery autopsy report on the Tx?
Bifilar wound hotspot at the most insulation-stressed point? blown
thermal fuse? failed PbF coilwire/tail joint?

"Phil Allison" wrote in message
...
Arfa Daily wrote:

Tranny arrived today. I found some spanners big enough to get the old one
out. Fitted the replacement, along with an inrush thermistor from
Farnell,
and two new level pots (both the originals went round and round ... ).
Gingerly powered it via the variac and Lo! Up it came !


** Hope you also tried a few on-off cycles to check out the new PTC.

Did you find one close to the original at Farnell ?


Yeah, I powered it on and off a number of times, and it was all ok. I found
some fibreglass mat sleeve in a box of junk to enclose the new thermistor.
As to the suitability of the thermistor, I found one rated at 20 amps at
Farnell. Slightly smaller diameter - but still big - and a bit thicker.
Couldn't find the original value though. The replacement was an ohm or so
lower, but I figured that some limiting has gotta be better than none, and
as soon as the relay drops in - which is actually quite quick - it's shorted
out of circuit anyway.



Bucketfuls of watts poured out of both channels, and the fan cut in just
when expected, so I heaved a sigh of relief, screwed the cover back on,
and
wrote the ticket out. Then gave myself a hernia getting it off the bench
...
:-)

** What are you plans for disposing of it?

Nearby skip ?

Maybe. Or maybe just return it to the shop and let them show it to the owner
/ chuck it in their industrial-rated bin. Or I might hang on to it to
autopsy later, but I'm not sure that I can actually be bothered to spare the
time. These days, I never seem to have the time to do all the things that I
really *want* to do, let alone the stuff that's just for interest's sake.
Perhaps it should just be a case of 'It's bolloxed, the replacement worked,
the repair put another meal on the table, move on ... ' :-)

Arfa


.... Phil

On 8/21/2015 3:22 AM, N_Cook wrote:
On 21/08/2015 01:58, Arfa Daily wrote:
Tranny arrived today. I found some spanners big enough to get the old
one out. Fitted the replacement, along with an inrush thermistor from
Farnell, and two new level pots (both the originals went round and round
... ). Gingerly powered it via the variac and Lo! Up it came !

Which was a relief, because I'm not sure that it looks like one that I
would want to be trying to repair an output stage on, particularly if it
was the lower board that had failed ...

Bucketfuls of watts poured out of both channels, and the fan cut in just
when expected, so I heaved a sigh of relief, screwed the cover back on,
and wrote the ticket out. Then gave myself a hernia getting it off the
bench ... :-)

Arfa

Butchery autopsy report on the Tx?
Bifilar wound hotspot at the most insulation-stressed point? blown
thermal fuse? failed PbF coilwire/tail joint?

I once had a Pb coilwire/tail joint fail from over heat on a two HP
motor. Kinda ticked at myself, I disassembled all the windings figuring
I'd rewind it, never did. The joint was 8-#18 wires connected to
a #2 or #3 cable. (Military motor, 28V)
I didn't know the term "coilwire/tail joint fail" at the time. :-)
It powered my gokart, took a few years until it got tired of those
250 amp to 300 amp currents from a standing start.
Mikek

Mikek

---
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On Fri, 21 Aug 2015 01:58:25 +0100, Arfa Daily wrote:

Tranny arrived today. I found some spanners big enough to get the old
one out. Fitted the replacement, along with an inrush thermistor from
Farnell, and two new level pots (both the originals went round and round
... ). Gingerly powered it via the variac and Lo! Up it came !

What's the spec of this tranny, Arfur?

" ** Hope you also tried a few on-off cycles to check out the new PTC. "

Umm, shouldn't that be NTC ?

Plus heat = plus ohms in a PTC and an NTC is the opposite.

Or has my brain been screwed up all these years ?

(shaddap)

Or is this new math where positive is negative, kinda like the government's estimates of unemployment and inflation ?

In a TV (telly for y'all on the otheer side), the NTC degaussing thermistor was in series going to the (must obviously be full wave) rectifiers from the transformer. The ddegaussing coild was across the NTC thermistor. (in the old tube/valve sets)

Later, they did switch to PTC thermistors and fed the coil right off the mains. That was a PTC.

This is not the only place I noticed this error.

It might be kinda importat because if you put a PTC in series with the primary of the transformer, as it heats up it is going to raise in resistancve and give the transforme and get even hotter and give the transforme less power and get even hotter to the point where it is hardly conducting at all.

Bottom line, Ppositiv Temperature Coefficient = hotter is more resistonce or less ? It really does make a difference.

"Butchery autopsy report on the Tx? "

Good idea. Also you end up ith some magnet wre for the juk drawer. Comes in handy sometimes.

"Bifilar wound hotspot at the most insulation-stressed point? "

Plausible.

"blown
thermal fuse? failed PbF coilwire/tail joint? "

Not if it was pulling excessive current and frying the thermistor.

"Cursitor Doom" wrote in message
...
On Fri, 21 Aug 2015 01:58:25 +0100, Arfa Daily wrote:

Tranny arrived today. I found some spanners big enough to get the old
one out. Fitted the replacement, along with an inrush thermistor from
Farnell, and two new level pots (both the originals went round and round
... ). Gingerly powered it via the variac and Lo! Up it came !

What's the spec of this tranny, Arfur?


Single primary at 0 - 240v

One multi-tap secondary 120v - 80v - 40v - 0 - 40v - 80v - 120v

and one single centre tapped secondary 13v - 0 - 13v

It doesn't say on it what the VA rating is, but it's a lot. This thing is 14
cm diameter by 12 cm tall and almost too heavy to pick up with one hand ...

And there's two of them in the amp - one for each channel :-)

Arfa

wrote:



" ** Hope you also tried a few on-off cycles to check out the new PTC. "

Umm, shouldn't that be NTC ?



** The device used in the smaller XLS602 model is a PTC - about 27mm diameter and blue in colour. Measures 6 ohms at room temp rising sharply to over 10kohms when hot. It is rated to stand 240VAC after tripping.

A normal inrush surge is too brief to heat it to the trip temp ( about 120C ) while a relay bridges the device well before the amp un-mutes, so it does not have to carry the amp's running current.

If there is a major fault somewhere at switch on, the relay never operates and the PTC trips limiting AC current to under 25mA.

I'm not sure if Arfa's model use a PTC or an NTC, but he seems to have used an NTC in his repair.


.... Phil

Arfa Daily wrote:


What's the spec of this tranny, Arfur?


Single primary at 0 - 240v


** As seen from the outside, internally there are almost certainly twin 120V primaries, wired in series as was the case with the XLS602 tranny.


One multi-tap secondary 120v - 80v - 40v - 0 - 40v - 80v - 120v


** So it's a class H, multi-rail amp.


It doesn't say on it what the VA rating is, but it's a lot. This thing is 14
cm diameter by 12 cm tall and almost too heavy to pick up with one hand ...


** The tranny in the XLS602 was 16cm dia x 8cm high.

Nearly the same in volume and weight.

FYI

The VA rating and regulation percentage of commercial mains transformers closely follows their primary resistance, not by any simple formula but ones with the same resistance have similar VA ratings.

Egs, for 240V rated transformers:

50VA = 60 ohms & 13%
160VA = 12 ohm & 8%
300VA = 4.7 ohms & 4%
800VA = 1.4 ohms & 4%
1000VA = 1.1 ohms & 4%



.... Phil

"Phil Allison" wrote in message
...
Arfa Daily wrote:


What's the spec of this tranny, Arfur?


Single primary at 0 - 240v


** As seen from the outside, internally there are almost certainly twin
120V primaries, wired in series as was the case with the XLS602 tranny.


One multi-tap secondary 120v - 80v - 40v - 0 - 40v - 80v - 120v


** So it's a class H, multi-rail amp.


It doesn't say on it what the VA rating is, but it's a lot. This thing is
14
cm diameter by 12 cm tall and almost too heavy to pick up with one hand
...


** The tranny in the XLS602 was 16cm dia x 8cm high.

Nearly the same in volume and weight.

FYI

The VA rating and regulation percentage of commercial mains transformers
closely follows their primary resistance, not by any simple formula but
ones with the same resistance have similar VA ratings.

Egs, for 240V rated transformers:

50VA = 60 ohms & 13%
160VA = 12 ohm & 8%
300VA = 4.7 ohms & 4%
800VA = 1.4 ohms & 4%
1000VA = 1.1 ohms & 4%



... Phil


That's an interesting observation, Phil. I will keep that noted, as it's
potentially a good 'reverse lookup' for determining if a tranny has shorted
turns on its primary, if you know its VA rating

Arfa

Arfa Daily wrote:


The VA rating and regulation percentage of commercial mains transformers
closely follows their primary resistance, not by any simple formula but
ones with the same resistance have similar VA ratings.

Egs, for 240V rated transformers:

50VA = 60 ohms & 13%
160VA = 12 ohm & 8%
300VA = 4.7 ohms & 4%
800VA = 1.4 ohms & 4%
1000VA = 1.1 ohms & 4%



That's an interesting observation, Phil. I will keep that noted, as it's
potentially a good 'reverse lookup' for determining if a tranny has shorted
turns on its primary, if you know its VA rating


** That is a big "if" - cos with transformers fitted to commercial products it is not published.

I keep an index book with primary ohm values for commonly seen gear, also the mains rms current draw at idle, with standby off and on - if it exists.

Egs:

Marshall JTM60; 6.5ohms (250VA) 0.16A, 0.44A

Marshall VS100; 10.2ohms (175VA) .17A

Phase Linear 400; 1.9ohms (600VA) .17A

Roland Cube 60; 12ohms (160VA) .1A



..... Phil

"Phil Allison" wrote in message
...
Arfa Daily wrote:


The VA rating and regulation percentage of commercial mains
transformers
closely follows their primary resistance, not by any simple formula but
ones with the same resistance have similar VA ratings.

Egs, for 240V rated transformers:

50VA = 60 ohms & 13%
160VA = 12 ohm & 8%
300VA = 4.7 ohms & 4%
800VA = 1.4 ohms & 4%
1000VA = 1.1 ohms & 4%



That's an interesting observation, Phil. I will keep that noted, as it's
potentially a good 'reverse lookup' for determining if a tranny has
shorted
turns on its primary, if you know its VA rating


** That is a big "if" - cos with transformers fitted to commercial
products it is not published.

I keep an index book with primary ohm values for commonly seen gear, also
the mains rms current draw at idle, with standby off and on - if it
exists.

Egs:

Marshall JTM60; 6.5ohms (250VA) 0.16A, 0.44A

Marshall VS100; 10.2ohms (175VA) .17A

Phase Linear 400; 1.9ohms (600VA) .17A

Roland Cube 60; 12ohms (160VA) .1A



.... Phil


I used to do all sorts of stuff like that to make life easier for the
future, but somehow, I just don't seem to have the motivation any more. I
guess it's just part and parcel of getting older. I seem to really lack
enthusiasm the last couple of years. Every now and then I get one that
provides some satisfaction. Like the Studio Due Shark 250 moving head last
week. It was really slow to do its power on reset. It found the pan endstop
microswitch ok, then reversed direction as it should, to index the opposite
end of the pan range. But it moved REALLY slowly doing this. And instead of
stopping just short of the mechanical end point, it ran into it, leaving the
pan stepper motor clunk-clunk-clunking. And there the reset process stopped
for ever. I opened it up and took a look at the output from the pan position
encoder. It's made up of a slotted wheel running in two slotted optos side
by side, presumably to generate Gray code or some such to determine which
direction it's going. The output from both optos was poor, never getting
close to zero. I thought it was unlikely that both of them would be faulty,
so I took my air line, and blasted out the gaps, and the slots in the
encoder wheel. When retried, reset proceeded at normal speed, and fully
completed. When I checked the output from the optos again, this time it was
a full zero to 5 volts. A good example of what a problem 'dust in the works'
can be.

That fix gave me a good degree of satisfaction, but the week went downhill
from there ... :-\

Arfa