On Wed, 19 Mar 2014 17:44:01 +0000, RJH wrote:

On 18/03/2014 23:32, Phil Allison wrote:
"RJH"

My power amplifier has just stopped working. It's a Rose English made 70W
stereo amplifier, about 20 years old. Here it is with the top off:

http://flic.kr/p/mbRV1M

When I switched it on there was a noticeable 'phutt', but no noise at the
speakers. Now nothing - no mechanical or electrical noise, and the power
LED doesn't come on. Also, when I disconnected the speaker cables, one had
sheared off at the banana plug. So a short is a possibility.

I've checked the 4 internal fuses and the mains 5A fuse for continuity.
All fine.

Any other diagnostics I could do, given limited understanding of such
things and a basic multimeter?


** Check for continuity through the AC plug active and neutral pins -
resistance reading should be about 10 ohms.


Out of interest, what's the significance and proper application of that
test? I tried it on an amp that I know to work (a NAD 3020) and it reads
20 ohms. On a CD player and DAB tuner - both 100 ohms.

Generally the higher the power rating of the unit, the less will be
the resistance. Those you quote follow the pattern pretty nicely.

d

"RJH"
Phil Allison wrote:
"RJH"

** Check for continuity through the AC plug active and neutral pins -
resistance reading should be about 10 ohms.


Out of interest, what's the significance and proper application of that
test? I tried it on an amp that I know to work (a NAD 3020) and it reads
20 ohms. On a CD player and DAB tuner - both 100 ohms.


** The resistance of the primary winding of a mains transformer is
characteristic of its VA (or power) rating. Small transformers have high
values and large ones low values.

I hunched the toroidal tranny in your amp was about 200VA rating - which
corresponds to 8 to 12 ohms primary resistance.

20 ohms corresponds to a 120VA tranny, 100 ohms to a 35VA one.

These figures are for 240VAC rated transformers, 120VAC ones have about 1/4
the resistance value for the same VA.




..... Phil

"RJH"

I've checked the 4 internal fuses and the mains 5A fuse for continuity.
All fine.

Any other diagnostics I could do, given limited understanding of such
things and a basic multimeter?


** Check for continuity through the AC plug active and neutral pins -
resistance reading should be about 10 ohms.

Check the fuse in the plug too.


Thanks - no continuity across one of the pins.

I didn't see a fuse holder built into the 3 pin mains socket at the back
of the amp. One of these:

http://www.maplin.co.uk/p/1a-time-de...-10-pack-gl59p

has blown. I'll get some tomorrow and give it a go.


** A 1A slow fuse is still a bit small for your amp - it will fail at the
moment of switch on quite regularly.

Note how the internal one is 5A.

I would go for a 2A slow fuse.



..... Phil

On 19/03/2014 23:35, Phil Allison wrote:
"RJH"
Phil Allison wrote:
"RJH"

** Check for continuity through the AC plug active and neutral pins -
resistance reading should be about 10 ohms.


Out of interest, what's the significance and proper application of that
test? I tried it on an amp that I know to work (a NAD 3020) and it reads
20 ohms. On a CD player and DAB tuner - both 100 ohms.


** The resistance of the primary winding of a mains transformer is
characteristic of its VA (or power) rating. Small transformers have high
values and large ones low values.

I hunched the toroidal tranny in your amp was about 200VA rating - which
corresponds to 8 to 12 ohms primary resistance.

20 ohms corresponds to a 120VA tranny, 100 ohms to a 35VA one.

These figures are for 240VAC rated transformers, 120VAC ones have about 1/4
the resistance value for the same VA.


Ah, thanks very much, good to know. New fuse (1.6A) and the amp's
working - measures 7 ohms.


--
Cheers, Rob

On 18/03/2014 20:07, RJH wrote:
My power amplifier has just stopped working. It's a Rose English made
70W stereo amplifier, about 20 years old. Here it is with the top off:

http://flic.kr/p/mbRV1M

When I switched it on there was a noticeable 'phutt', but no noise at
the speakers. Now nothing - no mechanical or electrical noise, and the
power LED doesn't come on. Also, when I disconnected the speaker cables,
one had sheared off at the banana plug. So a short is a possibility.

I've checked the 4 internal fuses and the mains 5A fuse for continuity.
All fine.

Any other diagnostics I could do, given limited understanding of such
things and a basic multimeter? A likely cause?




All working now, drama over :-)

Thanks for the guidance.

--
Cheers, Rob

"RJH"
Phil Allison wrote:


** The resistance of the primary winding of a mains transformer is
characteristic of its VA (or power) rating. Small transformers have high
values and large ones low values.

I hunched the toroidal tranny in your amp was about 200VA rating -
which
corresponds to 8 to 12 ohms primary resistance.

20 ohms corresponds to a 120VA tranny, 100 ohms to a 35VA one.

These figures are for 240VAC rated transformers, 120VAC ones have about
1/4
the resistance value for the same VA.


Ah, thanks very much, good to know. New fuse (1.6A) and the amp's
working - measures 7 ohms.


** Ah - a 7ohm primary toroidal is normally rated at 225VA for 240V
operation.

The inrush surge will be around 40 amps peak, single cycle, worst case.

That is an awfully large surge for a 1.6A fuse to tolerate.

Better have a few spares on hand ....



..... Phil