cambridge a500 problems

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Chrisr3521

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Joined 2006
yeah both leds are lit and no theres no voltage on the base pins on the right channel , also i have managed to get the left channel going but the heatsink gets mildly warm on modest volume is this normal? however no such look on the right side, ive had smoke form the r257, c224 area and theres dc on the speaker terminals, and no theres no voltage on the base pins

thanks
 
jaycee

jaycee

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Joined 2004
If R257 is smoking then it's likely that the VAS is damaged, Q218/219 and possibly Q217. You might want to check the zener diode D216 too.

Be aware right now that the outputs are probably toast too.


What did you do to "get the left channel going" ?
 
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Chrisr3521

Member
Joined 2006
well i bought complete new trannys for both channels, and replaced the left first, soldered them on but must have overheated some of them! anyway as i said its now playing music from the left, but the heatsink is getting warm at low volumes, i dont think that the idle current is adjusting properly i cant seem to get it to measure 13mv across the s and e pins however i did measure the current to 06.0on the 200m seting of my meter is this right for the 60ma?

also whats the best way to solder transistors ive heard you can use a crocodile clip to take the heat away?

thanks
 
jaycee

jaycee

Member
Joined 2004
A croc clip will work but better still is to use the right kind of soldering iron (no more than 18W for this sort of work) and be quick about it.

What transistors did you replace? Just the outputs? All of the transistors?

Check that RV201 still works right. Measure it with a resistance meter. You may have to remove C238 to do that.

If theres a 2V scale on the meter, then you should see 0.013 measuring across S to E. It'd be better if there was a millivolt scale on it. Using the current measurement mode wont work as that requires the meter to be IN the circuit, not across it.

How was the amp damaged in the first place?
 
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Chrisr3521

Member
Joined 2006
hi they all got repalced on the left channel, only cost £10 off ebay so a bargin.

i think ive managed to ajust the left pot, am i right in assuming that 13.0 on the 200m setting is correct? my knowledge is very little hence why i ask.

as for the right channel would it be best to just replace all the trannys and zeners check all the resistor values are correct. but im thinking of leaving the saps in place just in case they have'nt gone of is it best to replace these any way?

thanks
 
jaycee

jaycee

Member
Joined 2004
To be honest i'd replace them as theyre the most likely thing that got smoked.

The overheating resistor you mentioned points to a fault with the VAS. A blanket replacement of all transistors will certainly cure it, although its a bit of a pricey way to do it IMO!
 
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Kajman

Member
Joined 2006
I know you guys are taking the 13mV bias setting as advised in the service manual but this translates into 60mA of current which in my opinion is not healthy for these devices. Having looked at a few of these Amps I have already noticed that you will need to crank the volume up a lot more than if you reduce the voltage to around 8mV which is about 40mA (Read the SAP15 datasheet).

I would nt waste hours trying to measure which component has failed. You will straight away need to change the 2 SAP15s, RV201 and all the resistors that have been smoked. I have seen this happen all too often so its only 4 resistors and maybe 3 electrolytics with the drivers MPSA42/92 to change.
 
jaycee

jaycee

Member
Joined 2004
It will increase as the amplifier warms up. They don't really tell you how to do a correct bias setup in that manual.

Adjust it to 13mV, leave it on to warm up (could take a good half hour) and readjust to 13mV when warm. it might help to use clip leads to do this.
 
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