Hi ALF,

The measurement on 7R1 should be an AC voltage measurement, can you confirm if that was the case?

The anode of the diode 7D2 should always be at -23v or thereabouts. The cathode should be -23v in STANDBY and -02v (approx) during ON.

Do you still have the -15v while in STANDBY? If you do do you have -23v at P51(2)?

Regards

Lonnie

Apologies Lonnie,

dived straight into verifying your suggestions - should have looked at the circuit diagram....didn’t realise that 6TR18-C is against ground....

you can’t measure ground against ground !

I have to revisit the measurments and will report back once done.

ALf

Hi Lonnie,

at 7R1 I get about 27VAC in STBY and about 2VAC in ON mode.

no, I don’t  have the -15V anymore in STBY.

I can not verify the measurments at 6TR18 you suggested ?!

ALF

Ok,

I checked and trsted off-board 6TR18 once again - no adverse findings.

as per circuit diagram its collector is connected via P51- pin1 to 7RL2.....

here is where the problem may be hiding?

took out RL2, checked the resistance of its coil.....OL !!! Meaning its coil is burnt 😳😢

now the tricky part: 

where do I get a replacement relay ?

any suggestions are most welcome - thank you

ALF

Hi ALF,

The replacement relay I used was:

SPST, Coil: 24VDC,  Switching Current: 10A@240VAC,  Coil resistance: 880ohm.

I found some 5A relays with a max current rating of 7A but decided to go with the 10A instead. The 5A is probably ok for the 240V version of the amp given that the fuse F2 is spec'd at 4A.

You can buy it here:

https://www.ebay.com.au/itm/24V-10A-RELAY-SPST-TYCO-EBERLE-SHRACK-AUS-STOCK-/180538893095

I've had a look at the power supply ON sequence and realised that the -23v is only going to be available when the the amp is switched out of STANDBY to ON. The C of 6TR18 will be 0v at standby and around -0.2v when ON. It will be easier to see if the relay engages once you replace it rather than measuring the voltages at C of 6TR18 again.

Regards

Lonnie

MANY thanks Lonnie,

I ordered the one you suggested - I wonder if that would be suitable as a RL1 replacement as well....just in case ?!

will report back with further progress.

best

ALF

Ok,

back on the project:

it turned out both of the old relays RL1 & 2 were dead. Today was the big day .....well and truly !

two new Shrack relays in.....and both relays clicked 😁. Nice, but short lived !

after adjusting the no-load current and connecting two speakers the BM suddenly disappeared in a cloud of stinking smoke coming from board #7 - the start-up circuit ! 😡😡

what happened was C1 - the 330nF/275V cap - blew up in smoke - a miracle the smoke alarm wasn’t triggered.

now, the big question is: was this cap pre damaged and could not perform the intented task (as both relays were dead to begin with, reasons unknown)

or is there another likely cause ?

that cap seems difficult to find, unless I order 5 or more, which is a small problem in the scheme of things.

Could I risk a 470nF / 275V cap for the job as well or is this similar to changing a fuse for a higher spec one - an absolute NO !

Any thoughts from the crew of experts ?.......the uP still tests ‘OK’

thank you

ALF

Well, an additional question for 7C1 replacement would be:

Kemet paper cap or Polypropelene cap ??

ALF

The two relays are not identical!

Martin

The suppression capacitor MUST be a Class X!

Martin