Cambridge A60
 

I've been fixing a Cambridge A60 - turned out to be nothing more than both
speaker fuses blown. But thought I might as well check the quiescent
current while I'm at it. And the method given in the manual is weird -
replace the PS DC fuses with 15 ohm resistors and measure the voltage
across them.

Not having any high watt 15 ohm resistors to hand is there a different way?

--
*Toilet stolen from police station. Cops have nothing to go on.

Dave Plowman London SW
To e-mail, change noise into sound.

Cambridge A60
 

On Sat, 11 Jun 2011 18:20:33 +0100, "Dave Plowman (News)"
wrote:

I've been fixing a Cambridge A60 - turned out to be nothing more than both
speaker fuses blown. But thought I might as well check the quiescent
current while I'm at it. And the method given in the manual is weird -
replace the PS DC fuses with 15 ohm resistors and measure the voltage
across them.

Not having any high watt 15 ohm resistors to hand is there a different way?

Take the fuse out and just put in an ammeter of suitable range. I
think that if it is a modern digital type, its internal resistance
will be low enough. Ohms law will translate volts across 15 ohms into
current.

d

Cambridge A60
 

"Dave Plowman (News)" wrote in message
...
I've been fixing a Cambridge A60 - turned out to be nothing more than both
speaker fuses blown. But thought I might as well check the quiescent
current while I'm at it. And the method given in the manual is weird -
replace the PS DC fuses with 15 ohm resistors and measure the voltage
across them.

Not having any high watt 15 ohm resistors to hand is there a different
way?


I don't know that those resistors have to have a lot of power rating. If
you are adjusting quiescent current, you're only talking a few dozen
milliamps. That doesn't drop a lot of voltage and therefore, the power
being dissipated is nominal.

Cambridge A60
 

In article ,
Arny Krueger wrote:

"Dave Plowman (News)" wrote in message
...
I've been fixing a Cambridge A60 - turned out to be nothing more than
both speaker fuses blown. But thought I might as well check the
quiescent current while I'm at it. And the method given in the manual
is weird - replace the PS DC fuses with 15 ohm resistors and measure
the voltage across them.

Not having any high watt 15 ohm resistors to hand is there a different
way?


I don't know that those resistors have to have a lot of power rating.
If you are adjusting quiescent current, you're only talking a few dozen
milliamps. That doesn't drop a lot of voltage and therefore, the power
being dissipated is nominal.

They say to use 5 watt ones.

--
*A backward poet writes inverse.*

Dave Plowman London SW
To e-mail, change noise into sound.

Cambridge A60
 

"Dave Plowman (News)"

I've been fixing a Cambridge A60 - turned out to be nothing more than both
speaker fuses blown. But thought I might as well check the quiescent
current while I'm at it. And the method given in the manual is weird -
replace the PS DC fuses with 15 ohm resistors and measure the voltage
across them.

Not having any high watt 15 ohm resistors to hand is there a different
way?


** Measure the DC voltage across the emitter resistors - easy done with
any DMM.

Then use Ohm's law.


..... Phil

Cambridge A60
 

In article , Don Pearce
wrote:
On Sat, 11 Jun 2011 18:20:33 +0100, "Dave Plowman (News)"
wrote:

I've been fixing a Cambridge A60 - turned out to be nothing more than
both speaker fuses blown. But thought I might as well check the
quiescent current while I'm at it. And the method given in the manual
is weird - replace the PS DC fuses with 15 ohm resistors and measure
the voltage across them.

Not having any high watt 15 ohm resistors to hand is there a different
way?

Take the fuse out and just put in an ammeter of suitable range. I think
that if it is a modern digital type, its internal resistance will be low
enough. Ohms law will translate volts across 15 ohms into current.

If you do this I recommend also having a shunt capacitor across the place
(fuseholder) where you connect the ammeter. (i.e. shunting the ammeter at
HF.)

Otherwise you may get amplifier instability due to the increased rail lead
inductance, etc. No idea if this *will* happen with the A60. But it can
occur for some designs/layouts. Use 0.1 microF or more. FWIW I always used
1 microF.

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

Cambridge A60
 

On Sun, 12 Jun 2011 09:40:53 +0100, Jim Lesurf
wrote:

In article , Don Pearce
wrote:
On Sat, 11 Jun 2011 18:20:33 +0100, "Dave Plowman (News)"
wrote:

I've been fixing a Cambridge A60 - turned out to be nothing more than
both speaker fuses blown. But thought I might as well check the
quiescent current while I'm at it. And the method given in the manual
is weird - replace the PS DC fuses with 15 ohm resistors and measure
the voltage across them.

Not having any high watt 15 ohm resistors to hand is there a different
way?

Take the fuse out and just put in an ammeter of suitable range. I think
that if it is a modern digital type, its internal resistance will be low
enough. Ohms law will translate volts across 15 ohms into current.

If you do this I recommend also having a shunt capacitor across the place
(fuseholder) where you connect the ammeter. (i.e. shunting the ammeter at
HF.)

Otherwise you may get amplifier instability due to the increased rail lead
inductance, etc. No idea if this *will* happen with the A60. But it can
occur for some designs/layouts. Use 0.1 microF or more. FWIW I always used
1 microF.

Slainte,

Jim

Shouldn't be a problem from what I remember of the Cambridge design,
but it never hurts to be safe. Somewhere in a junk box I still have a
modified fuse holder for the panel mounted bayonet fixture. It has a
couple of fat leads with banana plugs on the end for a meter for
current measurement.

One thing that could be a gotcha, though, is an autoscaling meter.
This could just induce oscillation in combination with the amp. A
fixed scale meter would be best.

d

Cambridge A60
 

"Dave Plowman (News)" wrote in message
...
In article ,
Arny Krueger wrote:

"Dave Plowman (News)" wrote in message
...
I've been fixing a Cambridge A60 - turned out to be nothing more than
both speaker fuses blown. But thought I might as well check the
quiescent current while I'm at it. And the method given in the manual
is weird - replace the PS DC fuses with 15 ohm resistors and measure
the voltage across them.

Not having any high watt 15 ohm resistors to hand is there a different
way?


I don't know that those resistors have to have a lot of power rating.
If you are adjusting quiescent current, you're only talking a few dozen
milliamps. That doesn't drop a lot of voltage and therefore, the power
being dissipated is nominal.

They say to use 5 watt ones.


Not exactly high power in my book. My high power resistors are rated at
250-300 watts.

Cambridge A60
 

On Sun, 12 Jun 2011 07:11:48 -0400, "Arny Krueger"
wrote:


"Dave Plowman (News)" wrote in message
...
In article ,
Arny Krueger wrote:

"Dave Plowman (News)" wrote in message
...
I've been fixing a Cambridge A60 - turned out to be nothing more than
both speaker fuses blown. But thought I might as well check the
quiescent current while I'm at it. And the method given in the manual
is weird - replace the PS DC fuses with 15 ohm resistors and measure
the voltage across them.

Not having any high watt 15 ohm resistors to hand is there a different
way?


I don't know that those resistors have to have a lot of power rating.
If you are adjusting quiescent current, you're only talking a few dozen
milliamps. That doesn't drop a lot of voltage and therefore, the power
being dissipated is nominal.

They say to use 5 watt ones.


Not exactly high power in my book. My high power resistors are rated at
250-300 watts.


The 5 watt rationale is obvious. Still physically small and wire ended
yet big enough not to go pop if the current is a bit higher than
expected on firing up.

And of course if you are expecting a dissipation of about a milliwatt,
5W is an extremely high power resistor.

d

Cambridge A60
 

"Don Pearce" wrote in message
...
On Sun, 12 Jun 2011 07:11:48 -0400, "Arny Krueger"
wrote:


"Dave Plowman (News)" wrote in message
...
In article ,
Arny Krueger wrote:

"Dave Plowman (News)" wrote in message
...
I've been fixing a Cambridge A60 - turned out to be nothing more than
both speaker fuses blown. But thought I might as well check the
quiescent current while I'm at it. And the method given in the manual
is weird - replace the PS DC fuses with 15 ohm resistors and measure
the voltage across them.

Not having any high watt 15 ohm resistors to hand is there a
different
way?


I don't know that those resistors have to have a lot of power rating.
If you are adjusting quiescent current, you're only talking a few dozen
milliamps. That doesn't drop a lot of voltage and therefore, the power
being dissipated is nominal.

They say to use 5 watt ones.

Not exactly high power in my book. My high power resistors are rated at
250-300 watts.

The 5 watt rationale is obvious. Still physically small and wire ended
yet big enough not to go pop if the current is a bit higher than
expected on firing up.

I suspect that the 5 watt rationale is based on making the part finger-tip
safe. Or it makes for a durable part with heavy leads.

If someone makes a mistake with a bias pot, both they and the equipment are
probably better off with the low value part - it will act like a fuse.

And of course if you are expecting a dissipation of about a milliwatt,
5W is an extremely high power resistor.

If you do the math, you can easily see that 5 watts is vast overkill.

Cambridge A60
 

Dave Plowman (News) wrote:
I've been fixing a Cambridge A60 - turned out to be nothing more than
both speaker fuses blown. But thought I might as well check the
quiescent current while I'm at it. And the method given in the manual
is weird - replace the PS DC fuses with 15 ohm resistors and measure
the voltage across them.

Not having any high watt 15 ohm resistors to hand is there a
different way?

**Use 1/2 Watt resistors. The Pdiss will be minimal. Better still, use 1 Ohm
resistors and make appropriate calculations to compensate.


--
Trevor Wilson
www.rageaudio.com.au

Cambridge A60
 

In article ,
Trevor Wilson wrote:
Dave Plowman (News) wrote:
I've been fixing a Cambridge A60 - turned out to be nothing more than
both speaker fuses blown. But thought I might as well check the
quiescent current while I'm at it. And the method given in the manual
is weird - replace the PS DC fuses with 15 ohm resistors and measure
the voltage across them.

Not having any high watt 15 ohm resistors to hand is there a
different way?

**Use 1/2 Watt resistors. The Pdiss will be minimal. Better still, use 1
Ohm resistors and make appropriate calculations to compensate.

I'll post the instructions later just in case there's something there that
gives the reason for it using 5 watt resistors.

--
*A fool and his money are soon partying *

Dave Plowman London SW
To e-mail, change noise into sound.

Cambridge A60
 

In article ,
Dave Plowman (News) wrote:
I'll post the instructions later just in case there's something there
that gives the reason for it using 5 watt resistors.

Here they are :-


1. Remove power supply fuses(3.15A).

2. Replace fuse with 15ohm wirewound resistors (about 5 watt).

3. Connect a DVM across the resistor in the positive fuseholders place.

4. Turn both IQ presets (RV2, 102) fully anticlockwise.

5. Switch on and observe meter reading. It should be in the region of 0.5
to 0.6V (500 to 600mV).

6. Turn up RV2 until the reading increases by about 0.2V.

7. Now turn up RV102 until the reading increases by a further 0.2V. The
meter reading should now be about 1.0V.

8. Leave the amplifier to warm up for a few minutes during which time the
reading will rise to approximately 1.4 to 1.6V IT SHOULD NOT BE
ALLOWED TO RISE ABOVE 2.0V

9. The amplifier can now be fully checked providing no loads are
connected. Aural monitoring can be done using headphones.

10. If all is now okay switch off and replace the 15ohm resistors with
3.15A fast blow-fuses.

--
*A person who smiles in the face of adversity probably has a scapegoat *

Dave Plowman London SW
To e-mail, change noise into sound.

Cambridge A60
 

In article , Dave Plowman (News)
wrote:


2. Replace fuse with 15ohm wirewound resistors (about 5 watt).

That is interesting. I was wondering if they'd specified 5 Watt because
they wanted physically large wirewound resistors to get some inductance as
well, for some reason.

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

Cambridge A60
 

In article ,
Jim Lesurf wrote:
In article , Dave Plowman (News)
wrote:


2. Replace fuse with 15ohm wirewound resistors (about 5 watt).

That is interesting. I was wondering if they'd specified 5 Watt because
they wanted physically large wirewound resistors to get some inductance
as well, for some reason.

Dunno - I'd never seen this method used hence bringing it up. If I'd
suitable 5 watt resistors lying around I'd not have bothered. ;-)

--
*My designated driver drove me to drink

Dave Plowman London SW
To e-mail, change noise into sound.

Cambridge A60
 

"Jim Lesurf is a know nothing ****** "

2. Replace fuse with 15ohm wirewound resistors (about 5 watt).

That is interesting. I was wondering if they'd specified 5 Watt because
they wanted physically large wirewound resistors to get some inductance as
well, for some reason.


** How ****ing pig ignorant & stupid.

FFS - break a 5W WW resistor open take a ****ing look it it.

IDIOT !!!!




.... Phil

Cambridge A60
 

In article , Dave Plowman (News)
wrote:
In article , Jim Lesurf
wrote:
In article , Dave Plowman (News)
wrote:


2. Replace fuse with 15ohm wirewound resistors (about 5 watt).

That is interesting. I was wondering if they'd specified 5 Watt
because they wanted physically large wirewound resistors to get some
inductance as well, for some reason.

Dunno - I'd never seen this method used hence bringing it up. If I'd
suitable 5 watt resistors lying around I'd not have bothered. ;-)

I can see a point of having a bigger resistance since it makes the current
easier to monitor. And will drop the rail if a lot of current is drawn. But
having in the past both measured the inductance of resistors and taken them
apart I know that some wirewound types are far from being a plain
resistance. Not all were non-inductively wound. And some had a fair bit of
shunt capacitance. Who knows, maybe it is the shunt capacitance they are
hoping for. :-)

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

Cambridge A60
 

On 15/06/2011 15:44, Phil Allison wrote:
"Jim Lesurf is a know nothing ******"

2. Replace fuse with 15ohm wirewound resistors (about 5 watt).

That is interesting. I was wondering if they'd specified 5 Watt because
they wanted physically large wirewound resistors to get some inductance as
well, for some reason.


** How ****ing pig ignorant& stupid.

FFS - break a 5W WW resistor open take a ****ing look it it.

IDIOT !!!!


As usual Phil cuts to the chase.

Cambridge A60
 

"atriage"
Phil Allison wrote:
"Jim Lesurf is a know nothing ******"

2. Replace fuse with 15ohm wirewound resistors (about 5 watt).

That is interesting. I was wondering if they'd specified 5 Watt because
they wanted physically large wirewound resistors to get some inductance
as
well, for some reason.

** How ****ing pig ignorant& stupid.

FFS - break a 5W WW resistor open take a ****ing look it it.

IDIOT !!!!

As usual Phil cuts to the chase.


** If you open a 5W, WW resistor - there is a ceramic tube inside with a
short coil of resistance wire wound on it.

The tube is about 3.5 mm in diameter, 20mm long and there might be 12 turns
on it.

Using an on-line calculator gives 70 nH of inductance.

Using the formula XL = 2 . pi. F. L - F comes out as ** 17MHz ** for a
reactance of 15 ohms.

Do the same calc for a 15 ohm film resistor of 2W or 3W rating and the
answer is much the same.

Bonkers.



..... Phil