What two points were the probes on for these measurements? The numbers are meaningless to us without knowing that.

Hi Bob , those readings are between the ' zero output' point that is marked on pic from Dave . and each contact of the 2 transistors . positive lead on ' zero output' and neg lead on each tranny contact. hope thats clear

 

Hi Bob , those readings are between the ' zero output' point that is marked on pic from Dave . and each contact of the 2 transistors . positive lead on ' zero output' and neg lead on each tranny contact. hope thats clear

I think the supply is wrong..

 

Okay to measure the supply, so on DC, put your Negative lead on ground and measure the Positive side on the capacitor, and keep your Negative lead on ground now measure the Negative side, they should be around the same voltage.

To check the diodes turn the power off, and put your multimeter on diode test and check across each Diode, it should read open circuit one way, and around 0.7V the other way.

ok Dave . using ground contact .........I have 0.5 volts on both neg and pos caps .
Im still trying to figure out diode testing on my multimeter . bare with me

 

I think the supply is wrong..

do you mean the transformer is shot

 

do you mean the transformer is shot

To check the transformer output, pull the fuses out and put your meter on AC and measure between the green wire and each yellow one , they should be equal approx.

I would check the diodes they might be faulty.

 

ok Dave . using ground contact .........I have 0.5 volts on both neg and pos caps .
Im still trying to figure out diode testing on my multimeter . bare with me

I'm getting 0 .658 then swap polarity I get 0.834 . the other three diodes are similar readings

 

If these are the diodes you are testing, then it sounds like they are faulty, i would remove one end of each diode and check to confirm it.

 

To check the transformer output, pull the fuses out and put your meter on AC and measure between the green wire and each yellow one , they should be equal approx.

I would check the diodes they might be faulty.

Got AC 29.3 volts on each wire . both fuses blew when I powered up without speaker connected .

 

The first thing to do is to disconnect the speaker so that it is not destroyed by burning the drive coil.

Given that there is no schematic available to us, and the TS has only a voltmeter, the analysis must be based on considering what the symptoms are. Because of the DC on the driver coil and the loud hum, we know that the direct coupled drive has become unbalanced, and we know that the supply current has become excessive.
A puzzle is that in the picture in post #1 I see only two wires from the power transformer to the PCB, while three wired connections show onto the board. There is a wire to a lug bolted to the transformer frame, is that the supply common??? Is the transformer center tapped?? Others seem to think it is.

If the fuses in the transformer supply both failed when the speaker was disconnected then it appears that the voltage went way to high and something suffered a breakdown.
And even if half of the power transformer were open there would still be both supply voltages. So the transformer would appear to be OK.

 

will do . thank

If these are the diodes you are testing, then it sounds like they are faulty, i would remove one end of each diode and check to confirm it.


will do .thanks Dave

 

The first thing to do is to disconnect the speaker so that it is not destroyed by burning the drive coil.

Given that there is no schematic available to us, and the TS has only a voltmeter, the analysis must be based on considering what the symptoms are. Because of the DC on the driver coil and the loud hum, we know that the direct coupled drive has become unbalanced, and we know that the supply current has become excessive.
A puzzle is that in the picture in post #1 I see only two wires from the power transformer to the PCB, while three wired connections show onto the board. There is a wire to a lug bolted to the transformer frame, is that the supply common??? Is the transformer center tapped?? Others seem to think it is.

If the fuses in the transformer supply both failed when the speaker was disconnected then it appears that the voltage went way to high and something suffered a breakdown.

 

does that pic help ?

 

Yes it's a 28-0 28 transformer, so it uses a split supply like i said , so you should be getting around 40V +/- across the capacitors.
I've marked the supply rails and the ground point, check the DC supply first and also the diodes, .

 

Hi Bob , those readings are between the ' zero output' point that is marked on pic from Dave . and each contact of the 2 transistors . positive lead on ' zero output' and neg lead on each tranny contact. hope thats clear

Don’t know why Dave marked that spot, it does not look like a useful measurement to me.

 

Yes it's a 28-0 28 transformer, so it uses a split supply like i said , so you should be getting around 40V +/- across the capacitors.
I've marked the supply rails and the ground point, check the DC supply first and also the diodes, .
View attachment 304751

ok. gotta go find some fuses in town first. so may be a while ok

 

Disconnect the loudspeaker.
Unsolder and remove the two large 2N3055 transistors on the heatsinks.
Now measure the voltages across each of the two large blue capacitors.

 

ok. gotta go find some fuses in town first. so may be a while ok

so I snipped the diodes . got readings one way of . 0.63 0.67 0.67 0.684.

 

Disconnect the loudspeaker.
Unsolder and remove the two large 2N3055 transistors on the heatsinks.
Now measure the voltages across each of the two large blue capacitors.

Hiya ..........with power and fuses fitted ?

 

Hiya ..........with power and fuses fitted ?

Yes unsolder the two transistors on heatsinks, and with the fuses and power connected, measure the voltages across the capacitors.
You can test the transistors on diode test, across the collector and emitter terminals, it should read open circuit, .

 

If you don't want to go to the trouble of removing the two transistors, remove and replace the four diodes (1N4001-1N4007) and two capacitors (1000μF/50V). Make sure you get the orientation of the diodes and capacitors correct.

Of course, you cannot test the amplifier with no power and no fuse.