Repairing Arcam P7 power amp

[Bob Latham]

I've owned my Arcam P7 amplifier since around 2003. In general I like
it as it serves for hi-fi and for home cinema. It does however seem
to have some issues. Not least, with age comes poor contact on the
speaker relays. Fortunately Farnell sell the same relay so I swap
'em. You can't get to the contacts without taking the relay off the
board so swap it whilst its out.

There is also a long term tendency to run too hot, I think all P7
owners have experienced this. I've seen P7s really hot and people
tell me their's run with the fan running all of the time. My fan has
never started even once, I do something about it before it gets to
that temperature.

Things running hot, worry me, reliability, power consumption and
cooking capacitors or even the risk of fire. So when mine start to
get noticeably warmer I open the amp up and turn down the quiescent
current. A year or two later it will start to creep back up and I
repeat the job.

What is causing this? Something must be changing but what?

Bob.

[Brian Gaff]

Is it a dc coupled design? I would suspect some change in either the value
of a resistor or a leaky capacitor somewhere changing bias conditions. They
really should have put in a sensor as a few micro of drift on a dc amp
input, is going to be amplified, Its acting a bit like a servo.
I've never been up for dc coupled amps without protection, since I had a
very nice sounding din based receiver branded as Memorex which was dc
coupled and it fried a speaker when a transistor went leaky in an input
stage. Bah humbug.
Brian
--
----- --
This newsgroup posting comes to you directly from...
The Sofa of Brian Gaff...
***@blueyonder.co.uk
Blind user, so no pictures please
Note this Signature is meaningless.!

Post by Bob Latham
I've owned my Arcam P7 amplifier since around 2003. In general I like
it as it serves for hi-fi and for home cinema. It does however seem
to have some issues. Not least, with age comes poor contact on the
speaker relays. Fortunately Farnell sell the same relay so I swap
'em. You can't get to the contacts without taking the relay off the
board so swap it whilst its out.
There is also a long term tendency to run too hot, I think all P7
owners have experienced this. I've seen P7s really hot and people
tell me their's run with the fan running all of the time. My fan has
never started even once, I do something about it before it gets to
that temperature.
Things running hot, worry me, reliability, power consumption and
cooking capacitors or even the risk of fire. So when mine start to
get noticeably warmer I open the amp up and turn down the quiescent
current. A year or two later it will start to creep back up and I
repeat the job.
What is causing this? Something must be changing but what?
Bob.
--
Bob Latham
Stourbridge, West Midlands

[Phil Allison]

Brian Gaff Bullshits Wildly Again:

------------------------------------

Post by Brian Gaff
Is it a dc coupled design?

** Be a rare beast if it were not.

Post by Brian Gaff
I would suspect some change in either the value
of a resistor or a leaky capacitor somewhere changing bias conditions.

** Or thermal deterioration inside the unique, 5 leg Sanken Darlington output devices

Post by Brian Gaff
really should have put in a sensor as a few micro of drift on a dc amp
input, is going to be amplified,

** No it isn't.

Direct coupled amps have unity gain at DC.

Post by Brian Gaff
I've never been up for dc coupled amps without protection, since I had a
very nice sounding din based receiver branded as Memorex which was dc
coupled and it fried a speaker when a transistor went leaky in an input
stage. Bah humbug.

** As Forest Gump said, "Shit Happens".

Crowbar circuits as fitted in the Quad 405 do a good job.

Relays are useless, contacts always develop distortion and they never work with major DC faults.


..... Phil

[Bob Latham]

Post by Phil Allison
Relays are useless, contacts always develop distortion and they
never work with major DC faults.

Interesting. Thanks Phil.

Cheers,

Bob.

[Trevor Wilson]

Post by Bob Latham
I've owned my Arcam P7 amplifier since around 2003. In general I like
it as it serves for hi-fi and for home cinema. It does however seem
to have some issues. Not least, with age comes poor contact on the
speaker relays. Fortunately Farnell sell the same relay so I swap
'em. You can't get to the contacts without taking the relay off the
board so swap it whilst its out.
There is also a long term tendency to run too hot, I think all P7
owners have experienced this. I've seen P7s really hot and people
tell me their's run with the fan running all of the time. My fan has
never started even once, I do something about it before it gets to
that temperature.
Things running hot, worry me, reliability, power consumption and
cooking capacitors or even the risk of fire. So when mine start to
get noticeably warmer I open the amp up and turn down the quiescent
current. A year or two later it will start to creep back up and I
repeat the job.
What is causing this? Something must be changing but what?
Bob.

**Hard to say. Though I hate those 5 pin Darlingtons with a passion
('cause they're Sanken devices and Sanken has a nasty habit of deleting
devices from their range, leaving no possibility of substitutes), they
usually exhibit pretty decent bias stability. I would suggest replacing
the bias pots with a good quality 9 ~ 12 turn preset. These can be
sourced from Farnell/Element14, RS Components, etc for less than 6
Bucks. I always replace bias pots with multi-turn things, if there is
any possibility of bias instability.

[Bob Latham]

I would suggest replacing the bias pots with a good quality 9 ~ 12
turn preset. These can be sourced from Farnell/Element14, RS
Components, etc for less than 6 Bucks. I always replace bias pots
with multi-turn things, if there is any possibility of bias
instability.

Ok, I get that, thanks for the suggestion.

The multi turn pots I'm familiar with have their 3 pins spread out
more than the tiny standard things. Are there any pin compatible MT
pots or is it a case of fitting one pin and wire patches for the
other two? Brand or type preferences?

Photo of my amp here showing pots near bottom of image.

Loading Image...


Centre modules was removed at the time while I was changing its relay.

Notice the second module has a square pot which is the one Arcam
repaired in 2015. they claimed they have never changed a pot which
suggests the board I got back wasn't the one I sent.

Thanks

Cheers,

Bob.


Cheers,

Bob.

[Trevor Wilson]

Post by Bob Latham

I would suggest replacing the bias pots with a good quality 9 ~ 12
turn preset. These can be sourced from Farnell/Element14, RS
Components, etc for less than 6 Bucks. I always replace bias pots
with multi-turn things, if there is any possibility of bias
instability.

Ok, I get that, thanks for the suggestion.
The multi turn pots I'm familiar with have their 3 pins spread out
more than the tiny standard things. Are there any pin compatible MT
pots or is it a case of fitting one pin and wire patches for the
other two? Brand or type preferences?
Photo of my amp here showing pots near bottom of image.
http://www.mightyoak.org.uk/IMG_0029.JPG
Centre modules was removed at the time while I was changing its relay.
Notice the second module has a square pot which is the one Arcam
repaired in 2015. they claimed they have never changed a pot which
suggests the board I got back wasn't the one I sent.

**
https://uk.farnell.com/w/c/passive-components/potentiometers-trimmers-accessories/trimmer-potentiometers?track-resistance=470ohm|500ohm&no-of-turns=5turns|9turns|10turns|11turns|12turns|13turns&potentiometer-mounting=through-hole

https://uk.rs-online.com/web/c/passive-components/variable-resistors/trimmer-resistors/?applied-dimensions=4294790962,4294723853,4294873306,4294870576,4293625343,4292049446,4294295627,4292049407,4292049449,4294878033,4294436201

RS probably stocks the one most suitable for your needs:

https://uk.rs-online.com/web/p/trimmer-resistors/7432312/

Check the correct value, as I can't be bothered looking at the schematic
for you.

[Jim Lesurf]

Things running hot, worry me, reliability, power consumption and cooking
capacitors or even the risk of fire. So when mine start to get
noticeably warmer I open the amp up and turn down the quiescent current.
A year or two later it will start to creep back up and I repeat the job.
What is causing this? Something must be changing but what?

Not sure and would need to investigate to tell. However my first guess is
that the wiper contact resistance is rising, and this tends in standard
'rubber zener' designs to allow the bias to rise. (Other types of
variations in the pot may also occur, but the wiper seems the most probable
guess given no other info.)

if you replace with a different type of pot, check the pot
inductance/capacitance as some amps might go unstable if the replacement
differs significantly from the chosen one for the design. One of those
things that is probably fine, but might not be.

Jim

[Bob Latham]

Post by Jim Lesurf
Not sure and would need to investigate to tell. However my first
guess is that the wiper contact resistance is rising, and this
tends in standard 'rubber zener' designs to allow the bias to rise.
(Other types of variations in the pot may also occur, but the wiper
seems the most probable guess given no other info.)

Yes, seems reasonable, thank you.

Post by Jim Lesurf
if you replace with a different type of pot, check the pot
inductance/capacitance as some amps might go unstable if the
replacement differs significantly from the chosen one for the
design. One of those things that is probably fine, but might not be.

That is an unfortunate complication but you don't think that's
likely? Of course I don't know the values of inductance and
capacitance for the existing cheapo devices.

Same question as to Trevor.....

The multi turn pots I'm familiar with have their 3 pins spread out
more than the tiny standard things. Are there any pin compatible MT
pots or is it a case of fitting one pin and wire patches for the
other two? Brand or type preferences?

Photo of my amp here showing pots near bottom of image.

http://www.mightyoak.org.uk/IMG_0029.JPG

Thank Jim.

Cheers,

Bob.

[Jim Lesurf]

Post by Bob Latham

Post by Jim Lesurf
if you replace with a different type of pot, check the pot
inductance/capacitance as some amps might go unstable if the
replacement differs significantly from the chosen one for the
design. One of those things that is probably fine, but might not be.

That is an unfortunate complication but you don't think that's
likely? Of course I don't know the values of inductance and
capacitance for the existing cheapo devices.

It's impossible to be certain as it tends to depend on things that aren't
obvious from a diagram or even layout. You'll probably be OK. I mentioned
it as a just-in-case you find and try to use something that's fairly
different in construction to the original.

It is a reason to prefer pots of a similar size and construction if you can
get them - assuming they are reliable enough for like-for-like replacement
is acceptable.

Jim

[Trevor Wilson]

Post by Bob Latham

Post by Jim Lesurf
Not sure and would need to investigate to tell. However my first
guess is that the wiper contact resistance is rising, and this
tends in standard 'rubber zener' designs to allow the bias to rise.
(Other types of variations in the pot may also occur, but the wiper
seems the most probable guess given no other info.)

Yes, seems reasonable, thank you.

Post by Jim Lesurf
if you replace with a different type of pot, check the pot
inductance/capacitance as some amps might go unstable if the
replacement differs significantly from the chosen one for the
design. One of those things that is probably fine, but might not be.

That is an unfortunate complication but you don't think that's
likely? Of course I don't know the values of inductance and
capacitance for the existing cheapo devices.
Same question as to Trevor.....
The multi turn pots I'm familiar with have their 3 pins spread out
more than the tiny standard things. Are there any pin compatible MT
pots or is it a case of fitting one pin and wire patches for the
other two? Brand or type preferences?
Photo of my amp here showing pots near bottom of image.
http://www.mightyoak.org.uk/IMG_0029.JPG
Thank Jim.
Cheers,
Bob.

**This one will do just fine:
https://uk.rs-online.com/web/p/trimmer-resistors/7432312/

[Woody]

Post by Trevor Wilson

Post by Bob Latham

Post by Jim Lesurf
Not sure and would need to investigate to tell. However my first
guess is that the wiper contact resistance is rising, and this
tends in standard 'rubber zener' designs to allow the bias to rise.
(Other types of variations in the pot may also occur, but the wiper
seems the most probable guess given no other info.)

Yes, seems reasonable, thank you.

Post by Jim Lesurf
if you replace with a different type of pot, check the pot
inductance/capacitance as some amps might go unstable if the
replacement differs significantly from the chosen one for the
design. One of those things that is probably fine, but might not be.

That is an unfortunate complication but you don't think that's
likely? Of course I don't know the values of inductance and
capacitance for the existing cheapo devices.
Same question as to Trevor.....
The multi turn pots I'm familiar with have their 3 pins spread out
more than the tiny standard things. Are there any pin compatible MT
pots or is it a case of fitting one pin and wire patches for the
other two? Brand or type preferences?
Photo of my amp here showing pots near bottom of image.
http://www.mightyoak.org.uk/IMG_0029.JPG
Thank Jim.
Cheers,
Bob.

https://uk.rs-online.com/web/p/trimmer-resistors/7432312/

Er, I don't think so. The pots in the photo will have the centre leg at
the back as viewed. The trimmer above has its centre leg forward, so if
fitted the adjuster screw would be at the back making access impossible.
Another suggested would have put the adjuster screw at the side which
will present a similar access problem would it not?
The Farnell pot with the trimmer on top is probably the best option even
though it too will be difficult to adjust.

[Trevor Wilson]

Post by Woody

Post by Trevor Wilson

Post by Bob Latham

Post by Jim Lesurf
Not sure and would need to investigate to tell. However my first
guess is that the wiper contact resistance is rising, and this
tends in standard 'rubber zener' designs to allow the bias to rise.
(Other types of variations in the pot may also occur, but the wiper
seems the most probable guess given no other info.)

Yes, seems reasonable, thank you.

Post by Jim Lesurf
if you replace with a different type of pot, check the pot
inductance/capacitance as some amps might go unstable if the
replacement differs significantly from the chosen one for the
design. One of those things that is probably fine, but might not be.

That is an unfortunate complication but you don't think that's
likely? Of course I don't know the values of inductance and
capacitance for the existing cheapo devices.
Same question as to Trevor.....
The multi turn pots I'm familiar with have their 3 pins spread out
more than the tiny standard things. Are there any pin compatible MT
pots or is it a case of fitting one pin and wire patches for the
other two? Brand or type preferences?
Photo of my amp here showing pots near bottom of image.
http://www.mightyoak.org.uk/IMG_0029.JPG
Thank Jim.
Cheers,
Bob.

https://uk.rs-online.com/web/p/trimmer-resistors/7432312/

Er, I don't think so. The pots in the photo will have the centre leg at
the back as viewed. The trimmer above has its centre leg forward, so if
fitted the adjuster screw would be at the back making access impossible.

**Which will take an extra 60 seconds to rectify.

Post by Woody
Another suggested would have put the adjuster screw at the side which
will present a similar access problem would it not?

**Which is why I ignored that particular variant.

Post by Woody
The Farnell pot with the trimmer on top is probably the best option even
though it too will be difficult to adjust.

**Which is why I suggested the one I did.

[Bob Latham]

Post by Trevor Wilson

Post by Woody

Post by Trevor Wilson

Post by Bob Latham
Photo of my amp here showing pots near bottom of image.
http://www.mightyoak.org.uk/IMG_0029.JPG

https://uk.rs-online.com/web/p/trimmer-resistors/7432312/

Er, I don't think so. The pots in the photo will have the centre
leg at the back as viewed. The trimmer above has its centre leg
forward, so if fitted the adjuster screw would be at the back
making access impossible.

**Which will take an extra 60 seconds to rectify.

I take it you mean bending the legs so that the adjustment is on top
and the pot sits below the top edge of the PCB?

Post by Trevor Wilson

Post by Woody
The Farnell pot with the trimmer on top is probably the best
option even though it too will be difficult to adjust.

Yes, except unfortunately according to the circuit diagram the pot is
only 100 ohms and that series does seem to go that low. However, on
the same the same theme...

https://uk.farnell.com/bourns/3296p-1-101lf/trimmer-25-turn-100r/dp/9353020?st=100ohm%20multi%20turn%20pots

Looks OK to me unless experts here can spot an issue.

Is anyone else a bit suspicious of this stated 100 ohms? Seems very
low to me. I know the obvious solution is to open the amp and measure
an existing one but it is such a faff to get inside the amp, not
because of the amp itself but because of how/where it is installed.


Thanks everyone for your help.


Bob.

[Bob Latham]

Post by Trevor Wilson
**Which will take an extra 60 seconds to rectify.

I take it you mean bending the legs so that the adjustment is on top
and the pot sits below the top edge of the PCB?

Post by Trevor Wilson

Post by Woody
The Farnell pot with the trimmer on top is probably the best
option even though it too will be difficult to adjust.

https://uk.farnell.com/bourns/3296p-1-101lf/trimmer-25-turn-100r/dp/9353020?st=100ohm%20multi%20turn%20pots

Found a better photo of original pot.

Loading Image...

I'm hoping that bending the centre leg of the new pot backwards
backwards to go into the hole behind the pot will end up with the
adjustment screw being clockwise to increase Iq as it is with the old
pots otherwise a bit more leg bending needed.

Clear in this photo is "100" stamped on the top which I think
confirms what the diagram says.

Bob.

[Trevor Wilson]

Post by Bob Latham

Post by Trevor Wilson
**Which will take an extra 60 seconds to rectify.

I take it you mean bending the legs so that the adjustment is on top
and the pot sits below the top edge of the PCB?

**Correct. You may even be able to locate a pot with the correct
orientation for your needs. I don't want to spend the time looking for
you though.

Post by Bob Latham

Post by Trevor Wilson

Post by Woody
The Farnell pot with the trimmer on top is probably the best
option even though it too will be difficult to adjust.

https://uk.farnell.com/bourns/3296p-1-101lf/trimmer-25-turn-100r/dp/9353020?st=100ohm%20multi%20turn%20pots
Found a better photo of original pot.
HTTP://www.mightyoak.org.uk/BiasPot100.jpg
I'm hoping that bending the centre leg of the new pot backwards
backwards to go into the hole behind the pot will end up with the
adjustment screw being clockwise to increase Iq as it is with the old
pots otherwise a bit more leg bending needed.

**Who cares if it's clockwise or anti-clockwise? It's irrelevant.

Post by Bob Latham
Clear in this photo is "100" stamped on the top which I think
confirms what the diagram says.

**There you go. 100 Ohms it is.

[Phil Allison]

Trevor Wilson wrote:

--------------------

Post by Trevor Wilson
**There you go. 100 Ohms it is.

** Says so on the schem too:


Loading Image...

The topology is bias trim setting critical.

If the crummy 100ohm carbon trimmer rises in value by 10 ohms, the bias voltage increases by about 50mV - which appears across a 0.2ohm resistance.

Equates to 250mA increase in idle current.

With an overall DC supply of 110V, this adds almost 30W of extra heat in each channel. Times 7, so the fan will be running hard.

If on my bench, I would first determine the needed resistance for each channel, and replace the trim pots with a metal film resistors or combination to get the same value.



..... Phil

[Trevor Wilson]

Post by Phil Allison
--------------------

Post by Trevor Wilson
**There you go. 100 Ohms it is.

https://4.bp.blogspot.com/-WWvAbzf8Du4/Vlq3mHC7CgI/AAAAAAAANCE/iyd43SH_9mU/s1600/1.jpg
The topology is bias trim setting critical.
If the crummy 100ohm carbon trimmer rises in value by 10 ohms, the bias voltage increases by about 50mV - which appears across a 0.2ohm resistance.
Equates to 250mA increase in idle current.
With an overall DC supply of 110V, this adds almost 30W of extra heat in each channel. Times 7, so the fan will be running hard.
If on my bench, I would first determine the needed resistance for each channel, and replace the trim pots with a metal film resistors or combination to get the same value.

**Way the best idea. It is certainly an unusually low value pot.

[Bob Latham]

Post by Trevor Wilson
**Way the best idea.

OK, thanks.

Post by Trevor Wilson
It is certainly an unusually low value pot.

I thought that.

Cheers,

Bob.

[tony sayer]

In article <***@sick-of-spam.invalid>, Bob Latham <***@sick-
of-spam.invalid> scribeth thus

Post by Bob Latham

Post by Trevor Wilson
**Way the best idea.

OK, thanks.

Post by Trevor Wilson
It is certainly an unusually low value pot.

I thought that.
Cheers,
Bob.

Did you ever phone Arcam and ask them what they did or do to cure it or
have they got a modification like the QUAD 303 had two versions of bias
arrangements?..

[Phil Allison]

tony sayer wrote:

------------------

Post by tony sayer
Did you ever phone Arcam and ask them what they did or do to cure it or
have they got a modification like the QUAD 303 had two versions of bias
arrangements?..

** The Quad 303 has no issue with biasing since, due to the use of output triples, bias is independent of output device temperature.

The two bias setting methods seen are due to making running improvements over a very long production run.

OTOH - the Arcam uses a topology that depends on exotic Sanken 5 lead transistors plus a 5 cent trim pot remaining stable - or it overheats.




..... Phil

[tony sayer]

Post by Phil Allison
------------------

Post by tony sayer
Did you ever phone Arcam and ask them what they did or do to cure it or
have they got a modification like the QUAD 303 had two versions of bias
arrangements?..

** The Quad 303 has no issue with biasing since, due to the use of output
triples, bias is independent of output device temperature.
The two bias setting methods seen are due to making running improvements over a
very long production run.

Yes was told that by Mike Albinson he just said hack the board around
and use the bipolar transistor arrangement over the diodes which i did
with a multi turn pre-set still working well:)

Post by Phil Allison
OTOH - the Arcam uses a topology that depends on exotic Sanken 5 lead
transistors plus a 5 cent trim pot remaining stable - or it overheats.

Sanken?, Used to use those in the Audiolab amps but just conventional
devices..

Post by Phil Allison
..... Phil

[Phil Allison]

tony sayer wrote:

------------------

Post by tony sayer

Post by Phil Allison
** The Quad 303 has no issue with biasing since, due to the use of
output triples, bias is independent of output device temperature.
The two bias setting methods seen are due to making running
improvements over a very long production run.

Yes was told that by Mike Albinson he just said hack the board around
and use the bipolar transistor arrangement over the diodes which i did
with a multi turn pre-set still working well:)

** Yep - that'll work.

Post by tony sayer

Post by Phil Allison
OTOH - the Arcam uses a topology that depends on exotic Sanken
5 lead transistors plus a 5 cent trim pot remaining stable
- or it overheats.

Sanken?, Used to use those in the Audiolab amps but just conventional
devices..

** The Sanken's 5 leg beasties ( SAP numbers) have built in 0.2 ohm emitter resistors and a number of diodes on the same chip as the power transistor.

Connections to the resistor and diodes account for the two extra legs.

The diodes are there for biasing an output pair, so the idle current tracks the actual temp of the chip. Adjustment is via single resistor or trim pot.

Sanken failed to warn designers that the trim pot used must be high quality - like a cermet multi-turn type.

The 0.2ohm resistor was a mistake, removed in recent versions of the same devices. A burst of high current could cause failure and it had a non zero tempco - unlike the usual wire wound types used for such a job.



.... Phil

[Bob Latham]

Post by tony sayer
Did you ever phone Arcam and ask them what they did or do to cure
it or have they got a modification like the QUAD 303 had two
versions of bias arrangements?..

In 2015 I did ask them if changing the pots was a good idea and they
said they had never changed one.

I tried to speak to them last year about another matter and they
refused any conversation saying everything has to go via the dealer
now. This of course means They've lost me as a customer.

Cheers,

Bob.

[tony sayer]

In article <***@sick-of-spam.invalid>, Bob Latham <***@sick-
of-spam.invalid> scribeth thus

Post by Bob Latham

Post by tony sayer
Did you ever phone Arcam and ask them what they did or do to cure
it or have they got a modification like the QUAD 303 had two
versions of bias arrangements?..

In 2015 I did ask them if changing the pots was a good idea and they
said they had never changed one.
I tried to speak to them last year about another matter and they
refused any conversation saying everything has to go via the dealer
now. This of course means They've lost me as a customer.

Shame I'd be pissed of by that approach too!..

Post by Bob Latham
Cheers,
Bob.

[Bob Latham]

Post by Phil Allison
If on my bench, I would first determine the needed resistance for
each channel, and replace the trim pots with a metal film resistors
or combination to get the same value.

Strewth Phil that is a radical idea.

Can I read into that that you think the need for bias adjustment goes
away completely if the bias resistor is stable and it is that
resistor that is causing my issue?

Thanks Phil,

From time to time you can be an absolute star.


Cheers,

Bob.

[Bob Latham]

Post by Phil Allison
--------------------

Post by Trevor Wilson
**There you go. 100 Ohms it is.

https://4.bp.blogspot.com/-WWvAbzf8Du4/Vlq3mHC7CgI/AAAAAAAANCE/iyd43SH_9mU/s1600/1.jpg
The topology is bias trim setting critical.
If the crummy 100ohm carbon trimmer rises in value by 10 ohms, the
bias voltage increases by about 50mV - which appears across a
0.2ohm resistance.
Equates to 250mA increase in idle current.
With an overall DC supply of 110V, this adds almost 30W of extra
heat in each channel. Times 7, so the fan will be running hard.
If on my bench, I would first determine the needed resistance for
each channel, and replace the trim pots with a metal film resistors
or combination to get the same value.

In the end I chickened out of going for a static resistor. I'm sure
you're right Phil but too brave for me.

Photo of half done.
Loading Image...

All 7 done now. I didn't even need to remove the modules, did them in
situ.

The wiper centre pin has to be bent backwards underneath as the
orientation is opposite which means the pots cannot lie flat on the
board, did the best I could.

I set the quiescent to 15mV.

As Phil correctly says, the transistors have a 0.22 ohm emitter
resistor but the voltage is measured across both the NPN and PNP in
series so I think the current through each pair is

0.015/0.44 = 34mA.

Of course with two parallel output devices the current for a channel
is ~68mA which with 110 volt rails = 7.5 watts per channel or 52.5
watts just sitting there.

Time will tell if the quiescent is now more long term stable, fingers
crossed.

Thanks everyone.

Cheers,

Bob.

[Bob Latham]

Post by Trevor Wilson

Post by Bob Latham
I'm hoping that bending the centre leg of the new pot backwards
backwards to go into the hole behind the pot will end up with the
adjustment screw being clockwise to increase Iq as it is with the
old pots otherwise a bit more leg bending needed.

**Who cares if it's clockwise or anti-clockwise?

Me.

Post by Trevor Wilson
It's irrelevant.

Electrically and to you yes but not to me. As it happens, I think it
will come out right just by luck.

Cheers,

Bob.

[Bob Latham]

Post by Jim Lesurf
Not sure and would need to investigate to tell. However my first
guess is that the wiper contact resistance is rising, and this
tends in standard 'rubber zener' designs to allow the bias to rise.
(Other types of variations in the pot may also occur, but the wiper
seems the most probable guess given no other info.)

Would you expect a multi-turn pot like this one:

https://uk.farnell.com/bourns/3296p-1-101lf/trimmer-25-turn-100r/dp/9353020?st=100ohm%20multi%20turn%20pots

to be less inclined to have increasing "wiper contact resistance" ?

Thanks

Bob.

[Jim Lesurf]

Post by Bob Latham

Post by Jim Lesurf
Not sure and would need to investigate to tell. However my first guess
is that the wiper contact resistance is rising, and this tends in
standard 'rubber zener' designs to allow the bias to rise. (Other
types of variations in the pot may also occur, but the wiper seems the
most probable guess given no other info.)

https://uk.farnell.com/bourns/3296p-1-101lf/trimmer-25-turn-100r/dp/9353020?st=100ohm%20multi%20turn%20pots
to be less inclined to have increasing "wiper contact resistance" ?

Afraid I can't tell from the data given, etc. Someone who has used them may
know.

I'd tend to use an open 'skeleton' pot with multiple wipers so you can see
what the actual construction and materials used. But these can be more
fiddly to adjust precisely.

Chances are, it would be OK, but I can't say for sure.

Jim

[Phil Allison]

Jim Lesurf wrote:

-----------------

Post by Jim Lesurf

Post by Bob Latham
https://uk.farnell.com/bourns/3296p-1-101lf/trimmer-25-turn-100r/dp/9353020?st=100ohm%20multi%20turn%20pots
to be less inclined to have increasing "wiper contact resistance" ?

Afraid I can't tell from the data given, etc. Someone who has used them may
know.

** Multi turn trimmers are nearly all "Cermet" types.

The resistance elements are waaay more stable than carbon film that drift high.

Contact effect seme to be no problem either.

Only thing better is a fixed resistor.


..... Phil

[Bob Latham]

Post by Phil Allison
-----------------

Post by Jim Lesurf

Post by Bob Latham
https://uk.farnell.com/bourns/3296p-1-101lf/trimmer-25-turn-100r/dp/9353020?st=100ohm%20multi%20turn%20pots
to be less inclined to have increasing "wiper contact resistance" ?

Afraid I can't tell from the data given, etc. Someone who has used them may
know.

** Multi turn trimmers are nearly all "Cermet" types.
The resistance elements are waaay more stable than carbon film that drift high.
Contact effect seme to be no problem either.
Only thing better is a fixed resistor.

Brilliant

Thanks Phil.


Bob.