Can Anyone Identify This Component in Meridian 103/5 Power Amps Please?

[Thomo]

I have a Meridian 103 power amp that doesn't work, and was wondering if anyone could identify this component that is superglued (or similar) to the heatsink.

An externally hosted image should be here but it was not working when we last tested it.

I'll post a pic of the schematic shortly once I've drawn on where this part is connected.

Lee.

 

[Thomo]

Here's the schematic for the 103 with arrows drawn to show where it's connected.

Cheers, Lee.

http://www.diyhi-fi.co.uk/pdf/Service%20Manuals/meridian/103_Circuits_edit.pdf

 

[AndrewT]

can you identify the 2off 10V Zeners on the PCB, or is that them glued to the heatsink?

 

[PhaseLockLoopy]

Is that not just a thermal cut out? -- the wires are both the same colour and go back via two rails that come up through the centre of the PCB on fairly hefty track

 

[cliffforrest]

Is that not just a thermal cut out? -- the wires are both the same colour and go back via two rails that come up through the centre of the PCB on fairly hefty track

But if that were the case it would short out the rails, which is a bit drastic ....

It would work though!

 

[Steve Dunlap]

I may be missing one transistor in the picture, but the schematic seems to have 19 and I see only 18 on the board.

 

[Andrew Eckhardt]

It has to be a normally open thermal switch. (or a big NTC thermistor, you'd have to look much closer into the circuit to see what that would do, but measuring the part is easy) When the sink rises to the trip point it clamps off the supply to voltage stage through the isolating resistors.

 

[tvi]

Can you post a pic of the copper side of the board,
might help?

Rgds
James

 

[Andrew Eckhardt]

I tried to edit that post 15 times. It's the input stage left after the resistors, the stage that handles voltage gain is before it, and those aren't just isolation resistors, they're dropping resistors for the zeners. An NTC on there would have to seriously drop the supply to the input section before the bias regulator dropped out, so it probably wouldn't be any more effective than a thermal switch. An ohm meter will give you the answer.

 

[Thomo]

Hi guys, thanks for your answers. I too thought it was a thermistor or some sort of thermal cutout. I've measured it with an ohm-meter, and it's open circuit. I'll try heating it slightly with a hot-air blower and see if that changes anything.

Cheers, Lee.

 

[Thomo]

I had a look at that device in my 105 mono amp and it's a Hamlin reed switch of some sort. So it definitely looks like it was overheat/protection.

Cheers, Lee.

 

[Thomo]

So, now I have another question regarding this repair attempt. I hope you guys don't mind.

I've removed all the transistors and tested them with the diode test of my multimeter and they all worked fine for that. Is it a reliable enough test for this situation? It seems not to me, as the amp doesn't work.

Lee.

 

[AndrewT]

doing a diode test on a BJT requires 6 measurements.
4 will not pass current and two will pass current showing 500 to 600mVdc.

Try to set up a 5V or 6V battery supply and a couple of resistors to allow you to measure the gain of the transistor. Look up Pass or ESP to see the test circuit.

 

[Thomo]

Hi Andrew, thanks.

I did the the six tests on all the transistors and they seemed ok, but I'm sure a couple of them showed ~720mVdc, which didn't seem significant, but they all only passed current on two tests.

I'll have a read as you suggest and build the gain measurement circuit.

Thanks again, Lee.