Value of 1W, R67 near the footswitch socket, measures about 1200 ohm but
overheated colour bands look like 11 ohm ?

The polished stainless steel control panel has the legends only silk screen
printed on. No more durable than the white markings on valves/tubes. On/Off
and standby legends already worn off.
What is the recognised way of ameliorating this, remove pots etc, clean off
panel with very weak solvent and spray varnish over ?

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

N Cook wrote:
Value of 1W, R67 near the footswitch socket, measures about 1200 ohm but
overheated colour bands look like 11 ohm ?


Do you mean R97? 1.6k 2 watt?


Ron(UK)

Ron(UK) wrote in message
...
N Cook wrote:
Value of 1W, R67 near the footswitch socket, measures about 1200 ohm but
overheated colour bands look like 11 ohm ?


Do you mean R97? 1.6k 2 watt?


Ron(UK)

Yes, i was reading it upside down , goes to tip of footswitch plug, but I
would call it a 1W, when I replace it, it will be bigger.
Not the main problem which is drop in gain and distortion intermittently


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

The gain on both channels is too high in my reckoning.
Feeding only a 1mV RMS, 400 Hz, (600 ohm source) into a guitar amp input I
would not expect the output to be on the uncomfortable side of listening to
continuous sine at vol setting 3.
0.12V ac rms over 8 ohm speaker at "3" and 0.39V ac at "6".
At "3" setting 0.19V ac (measured as if it was sine RMS) on both splitter
driver outputs
No distortion at that sort of low level input.
What and where in the preamp to look for somethiong amiss?

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

N Cook wrote:
The gain on both channels is too high in my reckoning.
Feeding only a 1mV RMS, 400 Hz, (600 ohm source) into a guitar amp input I
would not expect the output to be on the uncomfortable side of listening to
continuous sine at vol setting 3.
0.12V ac rms over 8 ohm speaker at "3" and 0.39V ac at "6".
At "3" setting 0.19V ac (measured as if it was sine RMS) on both splitter
driver outputs
No distortion at that sort of low level input.
What and where in the preamp to look for somethiong amiss?

Have you tried it with a guitar?

Ron

Ron(UK) wrote in message
...
N Cook wrote:
The gain on both channels is too high in my reckoning.
Feeding only a 1mV RMS, 400 Hz, (600 ohm source) into a guitar amp input
I
would not expect the output to be on the uncomfortable side of listening
to
continuous sine at vol setting 3.
0.12V ac rms over 8 ohm speaker at "3" and 0.39V ac at "6".
At "3" setting 0.19V ac (measured as if it was sine RMS) on both
splitter
driver outputs
No distortion at that sort of low level input.
What and where in the preamp to look for somethiong amiss?

Have you tried it with a guitar?

Ron

Nope, I've always intended picking up a magnetic pick up from somewhere but
never have, let alone a guitar.
But all other amps i've fed this sig gen into then around 100mV is the norm
for just speaker output signal monitoring purposes, rather than dummy load
checking

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

"N Cook" wrote in message
...
The gain on both channels is too high in my reckoning.
Feeding only a 1mV RMS, 400 Hz, (600 ohm source) into a guitar amp input I
would not expect the output to be on the uncomfortable side of listening
to
continuous sine at vol setting 3.
0.12V ac rms over 8 ohm speaker at "3" and 0.39V ac at "6".
At "3" setting 0.19V ac (measured as if it was sine RMS) on both splitter
driver outputs
No distortion at that sort of low level input.
What and where in the preamp to look for somethiong amiss?

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/





Your measurements mean nothing in a guitar valve amp. They are designed to
distort. A Lot.

1. Put a sine wave at around 400 Hz into the power amp in jack, use a dummy
load, set the bias with a scope, measure the power output and observe the
waveform to make sure its not far off what it should be. Thats your power
amp working then.

2. Plug in a guitar and play it. If you can't hear anything amiss or don't
know what you should be listening for then don't bother trying to fix valve
guitar amps, or you'll be going round in circles measuring things, wondering
why there is so much distortion and the frequency response isn't very flat
at all.

3. If you are going to be fixing valve amps you need a stock of valves.
Best bet is to change the whole lot to a set you know works, then go through
the amp fixing any faults. Then go back and fit the originals, changing any
that are bad.



Gareth.

N Cook wrote:
Value of 1W, R67 near the footswitch socket, measures about 1200 ohm but
overheated colour bands look like 11 ohm ?

The polished stainless steel control panel has the legends only silk screen
printed on. No more durable than the white markings on valves/tubes. On/Off
and standby legends already worn off.
What is the recognised way of ameliorating this, remove pots etc, clean off
panel with very weak solvent and spray varnish over ?

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/





There is a repository of guitar amp schematics on the web, including
most of the vintage Fender models, including their many revisions.
Google would probably pull it up, or I might have bookmarked it. I'll
check....

jak

jakdedert wrote:
N Cook wrote:
Value of 1W, R67 near the footswitch socket, measures about 1200 ohm but
overheated colour bands look like 11 ohm ?

The polished stainless steel control panel has the legends only silk
screen
printed on. No more durable than the white markings on valves/tubes.
On/Off
and standby legends already worn off.
What is the recognised way of ameliorating this, remove pots etc,
clean off
panel with very weak solvent and spray varnish over ?

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/





There is a repository of guitar amp schematics on the web, including
most of the vintage Fender models, including their many revisions.
Google would probably pull it up, or I might have bookmarked it. I'll
check....


This is a good site for diags

http://blueguitar.org/schems.htm

Ron(UK)

Ron(UK) wrote:
jakdedert wrote:
N Cook wrote:
Value of 1W, R67 near the footswitch socket, measures about 1200 ohm but
overheated colour bands look like 11 ohm ?

The polished stainless steel control panel has the legends only silk
screen
printed on. No more durable than the white markings on valves/tubes.
On/Off
and standby legends already worn off.
What is the recognised way of ameliorating this, remove pots etc,
clean off
panel with very weak solvent and spray varnish over ?

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/





There is a repository of guitar amp schematics on the web, including
most of the vintage Fender models, including their many revisions.
Google would probably pull it up, or I might have bookmarked it. I'll
check....


This is a good site for diags

http://blueguitar.org/schems.htm

Ron(UK)

Indeed it is. I don't think it was the one to which I
referred...possibly even better!

jak

Ron(UK) wrote in message
...
jakdedert wrote:
N Cook wrote:
Value of 1W, R67 near the footswitch socket, measures about 1200 ohm
but
overheated colour bands look like 11 ohm ?

The polished stainless steel control panel has the legends only silk
screen
printed on. No more durable than the white markings on valves/tubes.
On/Off
and standby legends already worn off.
What is the recognised way of ameliorating this, remove pots etc,
clean off
panel with very weak solvent and spray varnish over ?

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/





There is a repository of guitar amp schematics on the web, including
most of the vintage Fender models, including their many revisions.
Google would probably pull it up, or I might have bookmarked it. I'll
check....


This is a good site for diags

http://blueguitar.org/schems.htm

Ron(UK)

Schema for this one , or near enough , was on that site, ac voltages agree
reasonably well throughout from input to output. I assume that with the test
4mV in and gain controls at 50 percent they mean using a dummy load or
serious ear defenders required.

After all that it looks as though it is the 8 ohm / 4 ohm (auto) diverter
switch in the unused Switchcraft 1/4 inch external speaker socket. It looks
like a silicone rubber loop around the tip contact is the way around that,
after cleaning. The internal speaker tip contact could do with one also.
Its only 5 years old

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

N Cook wrote in message
...
Value of 1W, R67 near the footswitch socket, measures about 1200 ohm but
overheated colour bands look like 11 ohm ?

The polished stainless steel control panel has the legends only silk
screen
printed on. No more durable than the white markings on valves/tubes.
On/Off
and standby legends already worn off.
What is the recognised way of ameliorating this, remove pots etc, clean
off
panel with very weak solvent and spray varnish over ?

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/







I tried lacquering a test spot but it did not like the metal, would wear
away as quickly as the Lettraset type silkscreen printing. I did add a few
white Lettraset legends totally worn off
A sheet of celluloid over the lettering and under the bush nuts and taped
,hidden, along the sides looks quite presentable.


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/