Hi there cambridge audio a4 faulty in need of the wizzes help (X PRO ) please

[jaycee]

You might not like it, but blanket replace all the transistors in that channel. They are cheap enough to do this if you really just want it fixed.

What voltage is at the base of the MJE350 with the zener on it?

 

[homertooties]

Hi there jayceee


right as follows

vdc readings on mje340 (q 219) n channel

base= -36vdc
collector= -0vdc
emitter= -26vdc..this is on left channel (faulty one)
zener gets rail voltage on neg side -39vdc
on positive side of course same as base reading of q219

on right channel b,c,e all - 39 vdc....39vdc both sides of the zener

 

[Chrisr3521]

looking at the schematic c225 should connect directly to the collector of q219, is there voltage on that side of the cap? also is there voltage on r254?

regards

 

[homertooties]

hI THERE


C225 THERE IS -39 VDC ON BASE OF Q218 AND EMITTER OF Q219

HOWEVER COLLECTOR IS ABOUT 20MV

 

[Chrisr3521]

have a check around q216 see if every thing checks out

 

[DRC]

I'm a bit confused by your measurements, for example you stated -39v both side of the zener on the working channel

anyway ...

here's what i think the quiescent conditions should be, assuming 45V rails and 2.3V across the LED :

8mA into the LED (42V across 4K7 - R245)
2mA into the LTP (1.7V across 820R - R243)
4mA into the VAS (1.7V across 390R - R390)

this should give 0.65V across R237 and R257

D216 should have 3.3V

about 180mV across R252 (4mA into 47R)
about 400mV across R254 (4mA into 100R)

dave

 

[homertooties]

Thankyou for reply

i am fault finding with output stage not present

i shall be installing output tomorrow with resistors in fuse holders..

 

[Chrisr3521]

why not go out and buy the bits for a light bulb tester? surely an easier and safer option than resistors accross the fuse holders?

regards

 

[homertooties]

Yes point taken i have soldered in new sap15,done usual cotinuity tests for pins to board and pins to sink,going to purchase a
series light tester then restest and post voltages for both channels,

Thankyou everyone who has helped so far and shall post an update soon


Ps great easy smelly way of removing s,grease from heatsinks and
insulating washers is quick squirt of carburettor cleaner,dissolves on contact buff up with kitchen towel available from motor factors

 

[ceharden]

Isopropanol (IPA) is also fine for cleaning silicon grease off equipment and probably nicer to use. Also good for lots of other cleaning jobs wrt electronic equipment.

Hope all goes well when you fire it up.

 

[homertooties]

Hi everyone,

Well made my 100 watt series current dump for 240 volt mains input,

New output stage fitted to amp

powered up 100w light bulb flashed then went out good good

connected up speakers Right channel fine ,left channel very quiet and distorted....

Done a quick check on both channels with dmm

Voltages on output stage read good bias 1.1 volts on left 1.3 volts on right.

To me sounds like blown output but doubt it as new just fitted,

Would a diode be responsible for this problem...ie 3.3 v zener

or clamp diodes 1n4004....Thanks in advance

 

[Chrisr3521]

first thing i'd think would be one of the mje devices

thanks