Hi,
I found this old Realistic amp in a loft and thought ....... why not have a look. This cannot be right. Main transformer input at 1ohm!
The sign on the back states it draws 600w. Am I right in thinking this is slightly dead?

 

primary side of a transformer is just a coil. so you are only measuring resistance of the wire. this may be ok.

 

this may be ok.

I am not very good at the maths bit, but I think if the amp is supposed to draw 600 watts at 240v this should work out at 2.5amp.
If I raise the mains voltage slowly with a variac , I am only putting 120 volts in when I hit 2 amps. This is with all the transformer output fuses removed so none of the PCB's are connected to the PSU.

 

Resistance and reactance are two different things.
Your measured resistance of 1Ω is the resistance to DC.
A transformer has inductance which will present reactance at 50 Hz AC.
If you are hitting 2 amps with nothing connected then there is something wrong.

 

Remove fuse F5 mains input to the transformer, and measure the resistance of the transformer primary winding directly at the transformer terminals, what reading do you get?

 

If you are hitting 2 amps with nothing connected then there is something wrong.

I was thinking the same

 

Remove fuse F5 mains input to the transformer, and measure the resistance of the transformer primary winding directly at the transformer terminals, what reading do you get?

I was getting my reading from the switched outlet socket. This did put the fuse in the circuit I was measuring but I figured it would make the reading go higher if anything. If you think it might change something, I can have a go at that reading tomorrow.

 

I am not very good at the maths bit, but I think if the amp is supposed to draw 600 watts at 240v this should work out at 2.5amp.
If I raise the mains voltage slowly with a variac , I am only putting 120 volts in when I hit 2 amps. This is with all the transformer output fuses removed so none of the PCB's are connected to the PSU.

2.5Amp is correct with full load (max output) if the 600W is rated to operate at 240VAC.
But drawing clearly shows that this is 120VAC mains...

Is something is plugged into one of the AC outlets...?

 

Input resistance is a tad bit low. Put it on an isolation transformer and Variac and slowly bring the voltage up to spec. Don't get in a hurry as any old capacitors need to reform and watch out for the magic smoke as you slowly bring it up. More concern than the primary winding is old electrolytics gone dry and shorted out. Lots of folks want to change them out BEFORE bring it up to voltage. YMMV

 

Do tell us the model number of the amp. Post a copy of the schematic if you have it.

 

But drawing clearly shows that this is 120VAC mains...

I could only find the schematic for the US model and it does show the primary as being 125VAC. The one I have is for the UK as it has 240 shown on the back.
Nothing plugged into the aux power outlets.

The model is a Realistic ST-2100. I did find the schematic and I will post the power circuit tomorrow.

 

I lied, I will post the PSU tonight.
I have removed F3, F4, F2 & F1 already and still pulls 2 amps. If the 1ohm reading on the primary is not ridiculous, my next step would be to find and disconnect the secondary at the bottom to D1410 and see what happens.

My thinking is that 1ohm is too low :-(

 

You are measuring the resistance of your meter's leads.

 

I have removed F3, F4, F2 & F1 already and still pulls 2 amps.

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This would indicate that You are testing a bonafied-Boat-Anchor.
.
.
.

 

Life continues to make me wonder if I know anything!

Today I removed the primary wire to F5 and checked the coil resistance between the wire just removed, and the other side connected to the switched AC outlet. I get 4.2 ohm.
This would indicate that I have a problem across the AC outlets but no.
Even after soldering the wire back on the fuse holder, I am still getting 4.2 measuring in exactly the same place as yesterday. I don't understand it??????

Anyway. This allowed me to get more volts into it. I slowly raised the input back to 100 VAC and it was pulling less current than yesterday. I slowly went up to 200 VAC while still keeping under 2.5amp and at this point the main capacitors told me that they were not happy.

Tomorrow I will see if I can find some temporary capacitors to bodge in place and see if I can make it to mains voltage.

 

Which model is it, Realistic STA-2100 or STA-2100D?
Presumably, STA-2100D has Dolby FM.

 

I already said that the value of the DC resistance of the transformer is not significant. If the unit is drawing a lot of current (more than 1A) it is very likely that the power amps are bad. This is where you need a light bulb test circuit while looking for short circuits.



Note that the lower winding of the transformer powers the Left and Right Channels of the power amplifier.

 

Which model is it

No Dolby for me. It is the 2100

 

This is where you need a light bulb test circuit while looking for short circuits.

I have owned the components for a current limit / test box for a few years but never found a reason to build it. Until now, so lunch breaks at work are spent cutting holes in the case to mount the meters and lamps. It should be ready by the weekend.

This morning I did separate C1405 & C1406 to put a tester on them and they both test good. I suspect the smoke has come from the main amp boards. Stupidly, I did not disconnect them before I ramped up the volts in.

I will update next week.

 

Test for shorted output power transistors in both left and right channels.