Re: Epiphone valve standard not working

On Oct 7, 12:32 am, Phil Kett <phil_k...@xxxxxxxxxxx> wrote:
Richard Kaulfuss wrote:
Phil Kett <pk...@xxxxxxxxx> wrote in
news:985nc5trdrt1n19qkqf488mhh9ac27mrso@xxxxxxx:

On Tue, 6 Oct 2009 13:36:36 +0000 (UTC), Richard Kaulfuss
<m...@xxxxxxxxxxx> wrote:

Phil Kett <phil_k...@xxxxxxxxxxx> wrote in
news:_sSdnYMVca8_j1bXnZ2dnUVZ_vGdnZ2d@xxxxxxxxxxxxx:

Phil Kett wrote:

Hi,

I've just turned on my amp and it's not working (was working fine
the last time it was turned on!).

The light in the power switch is coming on so there's power but no
output.

Looking in the back the valves aren't glowing as I'd expect them to
be. I suspect there might be a fuse blown somewhere but never
having worked on a valve amp before (though I'm well aware of the
dangers) I thought I'd ask here to see if anyone has any advice
before digging into the chassis!
Bad form to reply to my own post I know but I thought I'd post an
update.

I pulled the chassis out last night, there are three fuses on the
circuit board and they are all fine.

At this point I'm a bit lost - I just don't know enough about tube
amps to diagnose the fault. If what I've read is accurate though,
I'd guess that it's the heater circuit that's not working. As the
fuses are ok does that mean it's likely to be the transformer that's
died?
Possible, but unlikely. A more probable cause is a cold or dry solder
joint (or even a break) at the start of the heater chain, for example
at the point where the wires from the transformer connect to the PCB,
if the valve bases are PCB mounted. Be wary of any charge stored in
the reservoir capacitors after the amp is switched off - it can take
several minutes to reduce to a safe level when the valves are not
conducting.
Well, I've just checked the output from the transformer and got 0.1V -
not good.

I have to agree :-(

Checked continuity on those wires and there's no resistance.
Looks like that coil on the power transformer has fused.

By 'no resistance', do you mean open-circuit? If you probe gently where
the wires come out of the bobbin, you might be lucky enough to find a
break there.

By no resistance I mean that my meter is showing 0 ohms, or at least
close enough to set the buzzer of the continuity meter off. I wouldn't
have expected that from a transformer winding.



There is a brown 'disk' (I suspect a capacitor) in series with these
outputs (where they connect to the circuit board) that looks like it's
slightly burnt. The only markings on it are 'CHANGE' - it's not shown
on the circuit diagram I have.

It's probably a thermistor to limit the inrush current. A rather unusual
measure IME, and possibly indicates that the transformer was marginally
specified. If the transformer voltage had been normal, I might have
suspected that as the culprit.

Looking at the circuit diagram there isn't a fuse on that part of the
circuit - is that normal? I would have thought that'd mean that if
anything went wrong the first thing that's going to take the brunt of it
would be the transformer...



I guess the next question is whether or not it's worth trying to
source a replacement transformer for it.

There's no harm in looking. If you can find out the spec for the HT
winding (I'm assuming there aren't any others), you can probably source a
beefier transformer for less than the cost of an OEM replacement.

There are a total of three windings on the transformer - there's the HT
and a 12V supply for the DSP - looking closer at the diagram I can see
that the heater circuit needs 6.3v, I'm wondering if I could perhaps
find another transformer (or even a wall wart as a test) to see if the
rest of it still works? There's probably enough room on the chassis to
mount another transformer for the heater circuit as a 'kludge'



Thanks for your advice and help!

Phil

If it is indeed the transformer, you should not have that much of a
problem finding a replacement.

There are several firms that provide "upgrade" kits for the Epi which
include a new speaker and a transformer.

In fact, a quick google revealed that the transformer dying seems to
be a common problem, and various better-quality replacement units can
be had.
.

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