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Old 27th February 2008, 10:59 AM   #1
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Default cement resisters

hi, can anyone help me i am fixing a cambridge audio a500 amp and having fitted the new parts, but cannot get the bias current to stablize, the ony 2 differences in parts from the other channel are i'm using cement wirewounds, instead of just normal one's, and the new 100r pot

heres a link to the service manual

http://homepage.mac.com/WebObjects/F...Sharing14.html

thanks

chris
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Old 27th February 2008, 11:42 AM   #2
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Are you referring to R264, R261?

They have no effect on the DC stability, being outside of the feedback loop. In any case the actual resistor technology has no first-order effect.

Looks like you have a problem elsewhere.

More details on what you changed and WHY! Detail what happens when you try and set the Iq. What load (if any) are you using?

Do you have a 'scope?

Cliff
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Old 27th February 2008, 04:01 PM   #3
Corax is offline Corax  Germany
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Hi reddish75,

the picture in your link looks really nasty (not the picture itself but what's burned down).
If the bulk caps (C208, C209, C211 and C236) are "exploded" like a water fountain, I must agree with Cliff, that something horribly went wrong elsewhere in/with your A500.

Maybe a track underneath the burned spots is damaged?!

You better start checking all supply voltages if they're in the right range as shown in the schematics (±45V at the bulk caps and ±30V after the regulators). Only in case they're correct you can proceed and look for the issue in the amplifier stage itself.
Since you had to replace the 100R trimmer (RV201/RV202) there's a good chance that the output devices (SAP15..) have a base-collector (B-C) short and/or the drivers (Q206/Q214 and/or Q211/Q219, ...) are affected (i.e. blown) in addition. You might need to figure that out before you replace the bulk caps and risk to blow them again.
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Old 29th February 2008, 10:39 AM   #4
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FIXED, it turns out that cambridge or rather richer sounds service dept, sent out the wrong schematic i mean its the same one but resistors 240 and 258 are slightly different, now i just have one small issue the heatsink gets very hot at half volume, the sap15's are a lot cooler so its doing its job, but still very hot. i have also adjusted the bias current to 40ma not 60ma to try and cool this down but no real effect, will backing this off further reduce the temp? what would be the side effects of this, will the amp deliver full power earlier?
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Old 29th February 2008, 11:44 AM   #5
Corax is offline Corax  Germany
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Even if the values of R240/R258 are changed, the normal operation of the amplifier is not touched.

Those two resistors in conjunction with Q215/Q216 are just limiting the max. current through U205/U206 i.e. in case of an output short. This is done by monitoring the voltage across the pins S and E of the output trannies (the SAP15.. devices). In case the current exceeds a certain value the trannies Q215/Q216 start to conduct and partially take away the base current of the output devices, therefore limiting the max. output current. The limiting value will be in between the limits of the output devices but must be above the value of the fuse(s) (4A) of the supply voltage (±45V) to blow them off.

According to your statements I still doubt that everything is fixed in your amp. It appears to me that the biasing doesn't really work. As soon as the output devices are heating up the biasing should derate the B-E voltage because bipolar trannies have a negative temp coefficient (tempco).

You better check the complete path between the bases (B) of both output trannies to make sure it exists, because you said you had to remove/replace the 100R trimmpot RV202. If possible hook up a multimeter between the two B-pins and monitor the voltage when the output trannies start to heat up. The voltage must decrease! If it doesn't - something is still wrong.

By the way, if you encounter problem to get a steady voltage reading across the base pins I meant to measure it without a signal applied to the amp. So, measure it once when cold (right after power-up) and when warm/hot - after you've tortured it with some kind of audio signal and the heatsink is hot. The diodes inside the output trannies are actually responsible for the bias current regulation in this amplifier design.

With other output trannies an additional trannie is installed between the base pins and must be mounted on the heatsink to track the temperature of the output devices.

MOSFETs, as output trannies, for instance have a pos. tempco, therefore you won't have to mount a bias-trannie on the heatsink.
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Old 29th February 2008, 12:51 PM   #6
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i will try this but even the left cahnnel is as hot and this was fully working and i,ve repalced nothing on this, could faulty smoothing caps cause this?
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Old 1st March 2008, 11:22 AM   #7
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hi, again the voltage does indeed decrease when measuring between the b pins, i think that i am of the opinoin that these amps do indeed run hot, hence why so many broken ones on ebay?

does anyone have any ideas on how hot these things should run?

thanks

chris
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Old 3rd March 2008, 08:59 AM   #8
Corax is offline Corax  Germany
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OK, I agree - no faulty bias circuit though.

With just 60mA...80mA at ±45V the idle power dissipation is just 5.4W...7.2W. With a sufficient dimensioned heatsink this will increase the heatsink temp by just a few degrees - when idling!

Besides that I could not really see any design flaws so I doubt that this "high" temperature is an issue of all A500 amps.
I might have designed it similar.

Another thing I like to mention is that the maximum power dissipation of a class-AB amplifier (at a resistive load) is, as a rule of thumb, at approx. 2/3 of the maximum output voltage.
If this circumstance might be the case for your observations and your statement
Quote:
... the heatsink gets very hot at half volume, ...
then you're a lucky one and you own a working model of the A500

How hot your heatsink could get is a question of design and heatsink size. If it's an external heatsink (the ones you can touch at the outside of the case) I wouldn't let them reach temperatures above i.e. 50°C...60°C .
Internal heatsinks however, where they're prevented from getting directly touched, are often designed to reach temperatures above 80°C...90°C.
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