Hi there cambridge audio a4 faulty in need of the wizzes help (X PRO ) please - diyAudio
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Old 19th January 2009, 07:53 PM   #1
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Default Hi there cambridge audio a4 faulty in need of the wizzes help (X PRO ) please

have a bloody annoying DCproblem on output

Xpro help needed please
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Old 19th January 2009, 08:55 PM   #2
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erm im not xpro but the A series have problems with the sanken output transistors (the sap devices)

thanks
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Old 19th January 2009, 09:14 PM   #3
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Default cambridge a4

Hi reddish 75


Basically this amp had blown up on both channels,to such a degree you could see the internals of the sap15,s

I replaced the drivers,quiet a few resistors,all the little 100 uf caps the 2 10 uf idle current caps,diodes,bias pots, new red leds have purchased 4 new sap15,s for 12 pound including post,but i am not going to install them until i get rid of - dc on one of the sap15 terminals in board,the right channel had most damage,all other 3 output pins in board read correctly with dmm,only one dc voltage in centre terminal (collector)

I have been using p500 schematic as it is basically the same,
when i remove driver the problem dc disappears,collector terminal reads ov same as left channel,as soon as driver resoldered dc is fed from driver collector through a resistor straight to outer pin of sap15 position in board

Now you would think well shorted or leaky driver,so replaced with another and same,

All other voltages same on both channels,all components checked

At a loss
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Old 19th January 2009, 09:18 PM   #4
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Can you post either the schematic or a link to one so we can refer to the circuit by component numbers?
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Old 19th January 2009, 09:25 PM   #5
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Default cambridge audio

P500 schematic hopefully lol




problem area is Q219 (MJE340)

tHANKS IN ADVANCE
Page 2 and 3 of 5 page pdf....
Attached Files
File Type: pdf cambridge audio p500 schematic only.pdf (99.3 KB, 128 views)
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Old 19th January 2009, 09:51 PM   #6
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Right, here goes.

I'm assuming that you don't quite understand the working of the circuit and the function of each component.

The driver transistor Q219 which you refer to is in fact part of the VAS (Voltage Amplification Stage). Q214 along with the LED is a current source which loads the top of the stage. The current then flows through the bias diodes built into the output devices and into the Q218/219 pair which amplifies the output from the LTP input stage. You are going to have great trouble fault-finding this amplifier without Q219 fitted and probably the output devices too because unlike some designs, this one will bias up correctly without them.

If you're worried about blowing up the new output transistors then the technique is to wire a 60-100W lightbulb in series with the mains input. This will limit the current such that a fault is unlikely to do any damage, not quickly anyway. Usually it will allow the amp to power up sufficiently to make measurements.

In order for me to help you more I need more detailed measurements. Just saying that there is DC on a pin doesn't really help, an actual figure would be much more useful.

Another option might be to temporarily fit a resistor in place of the bias diodes to allow the VAS to function without the output transistors.

Hope that is of some help.

Chris
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Old 19th January 2009, 10:02 PM   #7
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Default cambridge audio a4

Hi there thankyou for reply,yes you are correct with the drivers actually being q216 and q215.....whoops ...wood for the trees and all that

I get main rail - dc volts on base of U206 ...

This voltage is present on collector of q219 then flows through r254(100 ohm) and r256 (47ohm)

When q219 is removed no suspect voltage is evident on collector trace on pcb...

Thanks in advance

Ps yes did have the series bulb current dump trick ,but have mislaid it...
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Old 19th January 2009, 10:17 PM   #8
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Quote:
yes you are correct with the drivers actually being q216 and q215
No, those are for overload (current) protection. Drivers are built
into the SAP's (all in one darlington with thermal diodes)
to fix the problem, measure and post voltages on schema for
(us ??) ..take a screenshot and edit text/numbers in graphics
program (paint shop pro..etc.)

Such a simple amp, would be a snap to point you
in the right direction.
OS
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Old 19th January 2009, 10:19 PM   #9
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Sorry if this comes out a little condescending but you're really not reading the schematic properly.

Q215/216 are purely part of the current limiting circuit and play no part of the signal path. They are there to turn the output devices off if excessive current flows through the emitter resistors of the output devices. Basically, ignore them. The only driver transistors are built into the Sanken output devices, which are driven directly from the voltage amplification stage.

You still don't say if the DC voltage you are seeing is the positive or negative rail, it makes a big difference.

I still think you're not going to get meaningful results without either the output devices in place or something to emulate them.

Without the output devices in circuit I would also expect you to be getting the positive rail voltage at the base of the positive output device U205 because it is just connected to the output of the current source. You need to follow the current flow from the +ve rail, through Q214, the two 100R and 47R resistors, the bias diodes and variable resistor, the other two 100R and 47R resistors then through Q219,218 to the -ve rail.
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Old 19th January 2009, 10:59 PM   #10
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Default cambridge audio a4

Thankyou very much for help everyone

I believe i have found the fault

Firstly the tips on probes of my meter were caked up

just wire wooled them

Now i get + vdc on bases and collectors of u205 (sap15n)

And if i short out base and emitter of q218 on left channel i now get

- vdc on base and collector of u206(sap15p) on both channels

so i suspect q218 on left channel was open circuit

shall post update when replaced suspect transistor

Usually i repair bipolar output stages and these sap15 seem to be

a clever circuit,however i have also heard they are very unreliable..

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