Transformer Diagnostic.

[MyLastWord]

Yep! new Diodes in for D6, D7 (1N4004), new 10R/2W resistor, new 2200uf/35V for C23 doesn't get the 24VDC. Give up..

 

[Watthour]

Your previously reported AC voltages were:
Pin12 &13 about 33VAC (38V)
Pin 5 & 8 about 32VAC (38V)
Pin 6 & 8 about 32VAC (38V)
Pin10 & pin11 about 88VAC (102V)

All the secondary voltage seem to be about 15% low, which does not indicate any secondary winding problem.

What is the transformer AC input voltage between the gray and white wires?

 

[MyLastWord]

I have no Grey, but Between White (pin2) and Brown (pin31) is about 16VAC off the secondary. The other White and Brown off the TF is the Primary.

 

[Watthour]

The primary is what I am curious about.

 

[MyLastWord]

101VAC

 

[Watthour]

DING! DING! DING!

That may be the whole problem. That's more than 10% below the 117V original spec, and could be why the 6A line fuse failed in the first place.

 

[MyLastWord]

Thanks Watt.. I think that's what I originally suspected when I check the resistant value. I don't think I will be able to find a reasonable (price-wise) replacement for this TF.

 

[merlynski]

What is the voltage coming from the wall socket? You are not, perhaps, still plugged into a DBT are you? The voltage across the primary side is not controlled by the transformer primary, but by the power company, unless you are plugged into something else, or there is something else in the primary side circuit.

 

[MyLastWord]

It's plugged into to DBT, to normal wall socket (Canada so 110-120v).. Not sure what else could be in the primary circuit..

 

[merlynski]

It's plugged into to DBT, to normal wall socket (Canada so 110-120v).. Not sure what else could be in the primary circuit..

The DBT is what is reducing your input voltage. It is to be used to protect the amp from overload like shorted output transistors, shorted rectifiers etc. during troubleshooting. Once you can power on with the bulb going bright then dimming down you are done with the DBT. You MUST remove it so full power can flow in order to get full voltages on the secondary side.

EDIT: more info

 

[MyLastWord]

You are right, straight from the wall it's 117VAC, but the 10R/2W resistor smoked immediately..

 

[merlynski]

You are right, straight from the wall it's 117VAC, but the 10R/2W resistor smoked immediately..

OK, then the problem that causes the resistor to smoke has to be figured out first, before going back to full power OR worrying about other things.

 

[merlynski]

Pull and test Q5 and Q6, and C13. The problem has to be downstream from the smoked resistor, pulling more current through it than it can handle.
I believe your transformer is just fine.

EDIT: more info

 

[MyLastWord]

With the 10R/2W removed, D6/D7 is at 45VDC... only two C10 0.010uF and C23 2200uF/35V in this circuit.

 

[merlynski]

With the 10R/2W removed, D6/D7 is at 45VDC... only two C10 0.010uF and C23 2200uF/35V in this circuit.

With no load on it the caps will charge up to the peak rectifier input voltage of the secondary that feeds the rectifiers. That does not mean that it is a bad transformer, just that there is no load on the circuit.

 

[merlynski]

Do you have another 10Ω 2W resistor to replace the smoked one? Did you test the transistors and cap?

 

[MyLastWord]

I have another 10R/2W resistors, and in the process of removed the Q5/Q6. Forgot to discharge the C13, sparks fly everywhere while remove it.

 

[merlynski]

What was the initial problem with this unit? Before you replace Q5/Q6 measure the resistance to ground of pin 16 on the power supply board. That is the +13vdc supply and it goes lots of places in the receiver, but normally does not draw much current, schematic shows 200ma. That would be a max of 0.4 watts on the 10Ω resistor.
EDIT: correct math error

 

[MyLastWord]

Pulled Q5 to test via diodes function show good.. Q6 is bad, no voltage (not sure it was shot when sparks fly or before). resistance between pin16 to ground (chassis or pin15) show around 100R. This unit I got from someone saying the TF is bad, need replaced and there were no fuse in FU5, FU6. So put in the new fuses to check around the TF but it keep blowing up FU5 & FU6 fuses.

 

[merlynski]

Pulled Q5 to test via diodes function show good.. Q6 is bad, no voltage (not sure it was shot when sparks fly or before). resistance between pin16 to ground (chassis or pin15) show around 100R. This unit I got from someone saying the TF is bad, need replaced and there were no fuse in FU5, FU6. So put in the new fuses to check around the TF but it keep blowing up FU5 & FU6 fuses.

I believe they were wrong about the transformer, looks like a power supply problem instead. If Q6 is bad that leads back into the rest of the voltage regulator. Do a diode test on D8 (14v zener).