Audio amplifier Cambridge Audio Topaz AM10 no output

[RandomGrinch]

Here are some photos of main and input PCBs front and back, let me know should these be inadequate or if more are needed before box is reassembled for further testing.

Thanks, the photos are good!

No obvious signs of shorts, overheating, dry joints, bulging caps, etc.
The only interesting thing to me is (and it's almost definitely nothing!), what is the pink stuff on C10?

 

[Zjho]

Thanks, the photos are good!

No obvious signs of shorts, overheating, dry joints, bulging caps, etc.
The only interesting thing to me is (and it's almost definitely nothing!), what is the pink stuff on C10?

Thanks for the compliment re. photos
Pink stuff is likely remnant of red silastic from nearby connector. Some connectors have received tan coloured hot glue instead but being easier to remove should be less apparent than silastic.

Started by diode testing pins of transistors Q1 to Q6 on main pcb as may be part of voltage regulation, none are showing short so maybe okay.
There are no other chips on that pcb besides audio output ics, maybe a resistor in a voltage divider is out of spec ?

 

[freddiemercurystwin]

My Cyrus III and its power supply has been in the loft for about 8 years 'awaiting repair' at a cool £600 the last time I checked.

 

[RandomGrinch]

Pink stuff is likely remnant of red silastic from nearby connector.

Thought that would be the case, but just couldn't see it anywhere else on the board!

Started by diode testing pins of transistors Q1 to Q6 on main pcb as may be part of voltage regulation, none are showing short so maybe okay.

Checking the Zeners and resister values in that region would also be a good idea - from the PCB trace, that is the next area along from the bridge rectifier and capacitor bank.

 

[Harry Bloomfield]

Thanks, the photos are good!

No obvious signs of shorts, overheating, dry joints, bulging caps, etc.
The only interesting thing to me is (and it's almost definitely nothing!), what is the pink stuff on C10?

I agree, no obvious issues, apart from the electrolytic cap number - it does -NOT - look like a 'C10' to my eyes, unless you have a diagram? Top left, the cap adjacent to C12/ to it's immediate right.

Sorry, missed the NOT out

 

[Zjho]

Thought that would be the case, but just couldn't see it anywhere else on the board!

Checking the Zeners and resister values in that region would also be a good idea - from the PCB trace, that is the next area along from the bridge rectifier and capacitor bank.

So far all ten zener diodes on main pcb on diode test in one direction are between .6v and .8v and other direction 1.7v upwards, other than D17, found nearest the relay (not the regular diode half visible right next to the relay or the zener half visible and labelled D18 a bit nearer the corner of board) which only has .6v and 1.3v. So the zener in region near what i assume are two sets of discrete bridge rectifiers appears okay.
Resistors will take a bit more time because although none on entire board are showing shorted for diode test their value in-circuit is not easily found by my resistive based meter due to parallel components' capacitance or inductance. I may end up desoldering some if they're way off their colour code.
Hope to put up some more photos of the main pcb topside this afternoon if possible as clearly quite a few components and white labels are obscured.
Not that am personally held up by the public holiday, actually finding this a lot of learning, but understand others might not be responding to this thread for some time.

 

[Zjho]

More photos





















These next two have been cleaned with alcohol to try to remove most of solder flux residue

 

[Zjho]

Am having a problem with R9, R33 and R34 from main PCB, my logic must be wrong.
Meter reading of these in-circuit resistors is 392, 392 and 394 ohms so fairly consistent and they all have same colours, but orange white black black brown should be 301 ohms with 1% tolerance ? Am assuming other components would only reduce figure so had expected 301 or less.

 

[RandomGrinch]

White is 9!
390Ω is perfect
Put the meter down and enjoy Xmas!

 

[Zjho]

White is 9!
390Ω is perfect
Put the meter down and enjoy Xmas!

So it is ! Ha, completely missed that one i did, as do for Xmas but each to their own. Thanks anyway. Won't interrupt further 'till tomorrow with main pcb resistance test results.

 

[Harry Bloomfield]

It is least likely to be a resistor, most likely to be an IC or other active component which has failed.

 

[Zjho]

It is least likely to be a resistor, most likely to be an IC or other active component which has failed.

Beginning to look that way for the main PCB resistors, most read a fixed value close to their code. The only three that don't are giving moving readings, the two biggest resistors on board, R27 and R28 are coded 2.2 and read falling from .3 ohms but have two large coils L2 and L1 in parallel, one for each. R35, lying within a cluster of three other similar shaped resistors is coded for 470K and reads rising from 75K. All three look in perfect condition.

 

[Harry Bloomfield]

Beginning to look that way for the main PCB resistors, most read a fixed value close to their code. The only three that don't are giving moving readings, the two biggest resistors on board, R27 and R28 are coded 2.2 and read falling from .3 ohms but have two large coils L2 and L1 in parallel, one for each. R35, lying within a cluster of three other similar shaped resistors is coded for 470K and reads rising from 75K. All three look in perfect condition.

Readings increasing would suggest capacitors charging up - the best you can do with in circuit testing, is make an judgement based on experience.

 

[RandomGrinch]

R35, lying within a cluster of three other similar shaped resistors is coded for 470K and reads rising from 75K.

Readings increasing would suggest capacitors charging up - the best you can do with in circuit testing, is make an judgement

As above, in circuit testing certainly has its limitations and challenges!

As you already know / have guessed, multimeters apply a small current to the circuit when measuring resistance; this can charge capacitors and give you the varying readings.
Measuring the resistance of a capacitor is one of the simplest (but far from the best!) ways of checking it is working, while in circuit.

It is least likely to be a resistor, most likely to be an IC or other active component which has failed.

It might not be likely to be a resistor, but I have come across resistor failures, in a couple of VCR PSU's!
...and staying with the theme, they are one of the easiest components to check in-circuit. There's no harm in ruling them out!

Going back to the original post:

Noticed the voltage rails are about 50% higher than stated on their labels e.g. 5v rail shows 7.5v and 8v rail 12v.

The 8V rail is a 9V rail, just poor silk screening! The voltage is still higher than expected, but a lot of components that can take 9V, can also take 12V without issue.

it does -NOT - look like a 'C10' to my eyes,

I was going with C11 to 19 already being taken!

 

[Zjho]

Discovered one of my multimeters had a low batt and being the type that cost a fiver it didn't have a warning other than if had noticed some of info such as HV and decimal points were not properly displayed ! Also have a better quality meter to verify readings and now know there are no overvoltages anywhere. Big caps on main PCB, presumably for smoothing are not not showing any appreciable mains ripple and are balancing voltage at 33v above and below ground so believe they are okay. So no discrete faults yet detectable.