Broken Amplifier help! Technics SU600 (not a stompbox, sorry!)

[Rob Strand]

Because you don't have a grounding wire on the mains plug, the AC voltage ought to be measured between L & N at the transformer's primary input. However, Neutral is grounded somewhere back in the supply feed but it doesn't prove the plug neutral is connecting if you measure the live from the chassis ground.

Yes, it's not a reliable mains measurement connecting the DMM to the chassis.

*Brits should probably say "earth" for the plug/socket safety pin and not "ground" but the rest of the world seems to understand ground better.

In au we also use "Earth" for mains earth.   Ground is more for circuits.

If you look at the pic of the back of the unit you can see a symbol with two encircled squares.  That means "double insulated" and typically the mains earth isn't connected on such equipment.



For those not into HiFi the "GND" terminal on the back of the unit is normally used to connect a ground lead from the turntable (ie. Phono Input).  That normally connects to the chassis ground at the amp.

[anotherjim]

If you want a safely grounded chassis, get a 13A plug and some green/yellow wire and terminate one end to the lug earth pin and take out the fuse. At the other end of the wire fit a croc' clip. However, also keep one hand away from the chassis when probing around high voltage parts. Electricity discharged to earth via your hands is likely to interrupt your heart!

Any voltage you measured using a floating (disconnected) chassis for the meter return can be stray/induced. The meter has a high impedance so will pick these voltages up even though they are feeble and can't do any work.

The power transformer primary only works of the voltage it's given, so that's where you want to measure -  both probes across the primary feed. Only then will you know the actual supply voltage to the primary.

When something that did work stops working, don't ignore stupid simple causes such as the dog chewing through the speaker wires etc...

[Rob Strand]

Don't forget the unit has its own fuse, the one right near the mains inlet PCB.  (pull the mains plug when looking at this one.)

... and maybe that fuse isn't connecting to the fuse socket.

You can also check the switch contacts are closing. (mains plug pulled out if using ohms/conductivity.)

If you can connect the neutral correctly you should be able to get 240V all the way upto where the transformer connects to the the mains inlet PCB.  If not backtrack until you find where it stops.

[moid]

Some good news / closure to those who were wondering what happened to the amplifier (OK probably nobody, but I thought I'd leave the fix here in case anyone else needs it in future). I tried testing the amp and measuring the voltages on the transformer, then tried to get underneath the PCB to measure the big power capacitors (which would require a complete dismantlement of the chassis) and spoke to my brother in law (who is an electrician) who, after I described what I was proposing doing, strongly suggested that I either check my life insurance policy or maybe get someone else to look at it (not him of course, I mean why would he help...). Anyway my wife suggested that perhaps it was worth someone else taking a look at it (because the life insurance policy expired when we paid off our mortgage a few year's ago...) so I found a shop an hour's drive from me and took it in and they fixed it in an hour!

The fault (there were two) was a dry solder joint on the power jack connection to the PCB (which meant that if you tilted the amplifier slightly at certain angles power would go through, and not at other angles!) and another similar problem with the two brackets/ grips (no idea what these are called) that hold the time delay fuse on the inside power PCB - those were also loose, so he adjusted them and re soldered the connections to the PCB. So nothing had actually died; just the solder had given out! And the repair bill was £36, which was a bargain. So all's well that end's well!

If you're in the UK and within driving distance of Bedford and need an amp repairing, I can totally recommend Mick Newman at MC Electronics. 

[Rob Strand]

Some good news / closure

Good news indeed, was bound to be something like that.
Funny that you got two doses of trouble.

[moid]

Double trouble seems like my style! I doubt I would have ever found those problems myself, I guess that's why it is totally worth paying someone with years and years of experience (and the added bonus of not killing myself of course!)

[Rob Strand]

I guess that's why it is totally worth paying someone with years and years of experience (and the added bonus of not killing myself of course!)

Very true.   Unfortunately a lot of debug problems which come up on the forum can be fixed easily but there's no one you can take it to - so you end up with very long threads solving very simple problems.