Cambridge audio A5 blowing fuses help

Status
Not open for further replies.
Jump to Latest
#41
greatsushi said:
So I can check the voltages without the two fuses in?
Click to expand...
with the fuses removed you can check the AC voltage of the transformer when there is no load. I thought that was posted some while back.
With the fuses in place you can check the DC voltage on the big capacitors.
If your DMM can do it, you can also check the AC voltage across the capacitors.

Have you built the Mains Bulb Tester?
That comes first.
You plug that into the wall socket and check the bulb is unlit.
You plug in a mains kettle to the MBT outlet socket. That should turn on the bulb full bright.
You plug in a small low power appliance and the bulb should either stay unlit, or glow very dimmly. A clear glass envolope helps in observing the very dim glow.


Now your MBT is ready to use.
 
Last edited:
#42
Ok thanks.

The discolored resistor is (yellow, purple black, brown, brown)
The green resistor is (yellow, purple, black, silver, brown)

I guess that the 1st brown on the discolored resistor should be silver? But it has been burnt so badly it's turned brown?
 
#43
AndrewT said:
with the fuses removed you can check the AC voltage of the transformer when there is no load. I thought that was posted some while back.
With the fuses in place you can check the DC voltage on the big capacitors.
If your DMM can do it, you can also check the AC voltage across the capacitors.
Click to expand...

It was posted I think, but I was still a bit unsure. By no load do you mean with no current flowing?

And checking the DC voltage on the big capacitors, do I do this by checking each one individually or measure the voltage across all of them?
 
#44
greatsushi said:
Ok thanks.

The discolored resistor is (yellow, purple black, brown, brown)
The green resistor is (yellow, purple, black, silver, brown)

I guess that the 1st brown on the discolored resistor should be silver? But it has been burnt so badly it's turned brown?
Click to expand...
have you looked up the colour codes.
0 to 9 are Blk, Brn, R, O, Y, Grn, Blu, V, G, W
Brn at the end is 1%, Gold is 5%
Second from the end is the multiplier, silver = /100, gold = /10, the rest follow the number codes.
Some have a tempco stripe. These need a bit more care to decipher
http://www.resistorguide.com/resistor-color-code/
 
#46
Right-O.

The resistor that isnt burnt = yellow, violet, black, silver, brown = 0.7 Ohms 1%.

I'll have to check on the others to see what the stripes are as I can't see them with the picture.

The gap between bands.. do you read the bands in order moving towards the biggest gap between 3rd and 4th band.
 
#47
if there is a gold stripe at the end then you start deciphering from the other end.
If it's brn at one end, then the other end is the start. If it's brown at both ends you have to try deciphering both ways and find which is impossible.
E24 has 24 values and many two colour combinations are not available, that immediately excludes counting from that end.

You omitted the yellow stripe. 4r7 for Y (4) V (7) Blk(0) S(/100) Brn(1%)

I was expecting 0r47 for an emitter resistor. 4r7 is too high. Ah, the output Zobel is 4r7
The emitter resistors are 0r27 5% will be R V S G
 
Last edited:
#48
Hi everyone

I've sorted myself a mains bulb tester and I've taken off the sap15's.

Turned the amp on and the bulb flashes but goes dim which I guess is the inrush current?

Red LED's illuminate, nothing burning or crackling so I can tell. So I started taking measurement with my DMM

AC voltage from one side to the black speaker terminal ~ 33.6 V AC
AC voltage across big capacitors = 0V
DC voltage " " = 44.25V
AC voltage across red and yellow wire from the transformer ~ 67.7V AC

To me that seems ok? Those measurements are with all the other boards disconnected so just the transformer and the power pcb connected.

Thanks everyone for your help so far
 
#50
What's the part designator numbers on those resistors ?

This looks promising for starters! Check all of the small TO-92 packaged transistors.. make sure none of them are getting warm. The ones I usually suspect when the SAPs have failed are the VI limiting transistors Q207,Q208,Q215,Q216. They often short from trying (and failing) to limit the fault current.
 
#51
Also, you cant measure current across something... so not sure what you are doing there. If you are measuring across resistors you should be in volts/millivolts mode. DONT try to measure across anything in current mode - you will create a short!
 
#53
I see the discoloured resistors you metion, they are indeed 4K7 and I would say they are R231 and R245. It seems a bit odd that they are discoloured as my calculations suggest theres roughly 43v dropped across them but theyre only dissipating around 350mW. Are they very hot ?
 
#57
Hi
You have to use the diode function on the voltmeter.
I repair loads of this amp boards, they are fitted to the A300/A500/A4/A5 and P500
Find the faulty channel (check Sap15n/p for shorts/open circuit legs E and S)
If there is nothing else damaged/burned replace trimpot RV202 (100 Ohm) and capacitor C220 (10uf x 16v) for good measure.
Connect the light safety contraption to the amp and fire it up
If light goes bright turn the trimpot down (anticlock wise) until you get a very faint glow on the light bulb, if it doesn't produce any change you have something else burned
If the light goes dim connect 2 wires to pins E and S of any SAP you replaced and set the idle current to 10 or 11mV and let it sit for 10/15 minutes, remove the light safety contraption and then set idle current to 13 mV (stated in the service manual)
In all the ones I repaired I never found any input transistors gone faulty it is always the:
Output transistors
Vas stage
VI limiting components
Good luck

PS: this is a resumed explanation I normally check other components on the board and set the idle current on both channels
 
Last edited:
#58
Test the SAPs like so. You need diode test on your meter.

* Test from C-S - you should see a diode drop one way, infinity the other.
* Test from C-B - same.
* Test S-E using resistance check. You should see about 0.22 ohms. If you read infinity, the resistor died.

I'll place a good bet that you have at least one short. Replace both pairs of them, using the STD03 as I described earlier.

Agreed, replace the bias trimpot (it usually gets damaged) and the capacitor across it too.

After you've done that, make some tests using the bulb limiter inline. If there's still a fault, the bulb limiter will prevent damage.
 
#59
I've already taken out all of the SAP15's. I took one out and that was gone so I thought I'd replace them all with std03's and add the external resistor like you said.

The transistors I tested were the small Q207 ones, A45 or something like that.
 
Status
Not open for further replies.
Top Bottom