Happy new year all,

I am repairing a Legend guitar tube amplifier for a friend and would like some advice. This is a Legend Model G1250 SC amplifier with 3 12AX7A tubes, and diodes and transistors. My friend had it previously evaluated by a repair shop and was told that the power transformer was 'bad.' There are 2 power transformers and the higher voltage rectified one is only putting out 80 volts DC to the plates of the tubes. But, I don't have a schematic to tell me the proper voltage to apply to these tubes - it can be 200 V or 280 V DC. So, I don't know what rating transformer to recommend that he purchase; and, I don't know if there is a voltage doubling DC power supply circuit that has a bad diode that would be simplier to fix.

Is anyone familiar with similar amplifier designs, or does anyone have a schematic I can borrow?

Thanks, Steve

Hi Steve

I don't have your schematic but there is an easy way to check the power transformers. Disconnect the rectifier diodes and turn on the power. If the transformers put out any voltage and do not heat, they are good. A bad transformer will have a shorted winding and overheat is a few minutes.

If your transformer is putting out 80 volts and not overheating, it is good.

Norm

: Happy new year all,

: I am repairing a Legend guitar tube amplifier for a friend and would like some advice. This is a Legend Model G1250 SC amplifier with 3 12AX7A tubes, and diodes and transistors. My friend had it previously evaluated by a repair shop and was told that the power transformer was 'bad.' There are 2 power transformers and the higher voltage rectified one is only putting out 80 volts DC to the plates of the tubes. But, I don't have a schematic to tell me the proper voltage to apply to these tubes - it can be 200 V or 280 V DC. So, I don't know what rating transformer to recommend that he purchase; and, I don't know if there is a voltage doubling DC power supply circuit that has a bad diode that would be simplier to fix.

: Is anyone familiar with similar amplifier designs, or does anyone have a schematic I can borrow?

: Thanks, Steve

Hey Steve,

A couple of notes on the power transformer.

1. Check the primary (without power) with an

ohm-meter. Check across the line (be sure

the fuse is not blown) for about 1-2 ohms

and from either side of the line to the

chassis. That better be read an open circuit.

If not, you've got a primary leakage problem.

2. As Norm mentioned, disconnect ALL secondary

leads of your transformer (a short on one

secondary winding will load down the entire

transformer). Then check with an ohm-meter

for a short from any secondary wire to chassis.

3. Apply AC power to the transformer primary

and turn ON the power switch with all

secondaries wires OPEN. (all disconnected from

anything) and see (as Norn noted) if the

transformer overheats. You'll know in about 2 mins.

4. Remember the voltage across the secondary

of a transformer is always AC. You ought

to read anywhere from 80-120 VAC. May be

higher.

Please report your progress and good luck.

Phil Sussman

Clayton, Ohio

PS. Bubbling, gurgling, smoke, bad smells,

heating, or dripping wax from your

transformer are not good signs.

----

: Hi Steve

: I don't have your schematic but there is an easy way to check the power transformers. Disconnect the rectifier diodes and turn on the power. If the transformers put out any voltage and do not heat, they are good. A bad transformer will have a shorted winding and overheat is a few minutes.

: If your transformer is putting out 80 volts and not overheating, it is good.

: Norm

:

: : Happy new year all,

: : I am repairing a Legend guitar tube amplifier for a friend and would like some advice. This is a Legend Model G1250 SC amplifier with 3 12AX7A tubes, and diodes and transistors. My friend had it previously evaluated by a repair shop and was told that the power transformer was 'bad.' There are 2 power transformers and the higher voltage rectified one is only putting out 80 volts DC to the plates of the tubes. But, I don't have a schematic to tell me the proper voltage to apply to these tubes - it can be 200 V or 280 V DC. So, I don't know what rating transformer to recommend that he purchase; and, I don't know if there is a voltage doubling DC power supply circuit that has a bad diode that would be simplier to fix.

: : Is anyone familiar with similar amplifier designs, or does anyone have a schematic I can borrow?

: : Thanks, Steve

I have a Legend Amplifier that is 22 years old an is blowing the 3/4 amp fuse and the master power switch is
flakey (light does not illuminate properly).

Any Suggestions?
Thanks,
Geoff

: Hey Steve,

: A couple of notes on the power transformer.

: 1. Check the primary (without power) with an

: ohm-meter. Check across the line (be sure

: the fuse is not blown) for about 1-2 ohms

: and from either side of the line to the

: chassis. That better be read an open circuit.

: If not, you've got a primary leakage problem.

: 2. As Norm mentioned, disconnect ALL secondary

: leads of your transformer (a short on one

: secondary winding will load down the entire

: transformer). Then check with an ohm-meter

: for a short from any secondary wire to chassis.

: 3. Apply AC power to the transformer primary

: and turn ON the power switch with all

: secondaries wires OPEN. (all disconnected from

: anything) and see (as Norn noted) if the

: transformer overheats. You'll know in about 2 mins.

: 4. Remember the voltage across the secondary

: of a transformer is always AC. You ought

: to read anywhere from 80-120 VAC. May be

: higher.

: Please report your progress and good luck.

: Phil Sussman

: Clayton, Ohio

: PS. Bubbling, gurgling, smoke, bad smells,

: heating, or dripping wax from your

: transformer are not good signs.

: ----

: : Hi Steve

: : I don't have your schematic but there is an easy way to check the power transformers. Disconnect the rectifier diodes and turn on the power. If the transformers put out any voltage and do not heat, they are good. A bad transformer will have a shorted winding and overheat is a few minutes.

: : If your transformer is putting out 80 volts and not overheating, it is good.

: : Norm

: :

: : : Happy new year all,

: : : I am repairing a Legend guitar tube amplifier for a friend and would like some advice. This is a Legend Model G1250 SC amplifier with 3 12AX7A tubes, and diodes and transistors. My friend had it previously evaluated by a repair shop and was told that the power transformer was 'bad.' There are 2 power transformers and the higher voltage rectified one is only putting out 80 volts DC to the plates of the tubes. But, I don't have a schematic to tell me the proper voltage to apply to these tubes - it can be 200 V or 280 V DC. So, I don't know what rating transformer to recommend that he purchase; and, I don't know if there is a voltage doubling DC power supply circuit that has a bad diode that would be simplier to fix.

: : : Is anyone familiar with similar amplifier designs, or does anyone have a schematic I can borrow?

: : : Thanks, Steve

Hi Steve, I'd follow Norm and Phil's suggestions about the transformer. You have mentioned some voltages but said nothing about smoke etc. I think there's a fair chance the transformer is OK though it could have a winding open circuit. If so make sure it's not a dry soldered joint at a terminal. It wouldn't be the first time someone has given the wrong advice just to get rid of a problem job. Of course one of the transformers may well be faulty. I don't know how old the Amp is but it sounds like it's basically a solid state unit with some tubes for "tube sound" and distortion effects. That 80 volts could be the right B+ voltage in this service. The problem might be the other transformer that I'm guessing is for the solid state circuits. It's possible the transformers have thermal fuses fitted between the windings. These are designed to open if the transformer gets too hot and often just fatigue and fail. You can usually find the fuse connections on the terminal strip in series with the primary as two wires
going into the windings where the fuse proper is located. Replacement fuses should be obtainable, it's usually possible to just short across the fuse at least for testing, it's then equivalent to any other transformer without such a fuse. Leaving it like this might cause legal violations for not conforming to original underwriter specifications. If this is the problem you can at least get it working and measure voltages, hopefully get replacement fuze and fit it. If you can't find a fuze locally I can give you a supplier in Australia that stocks them, you could order them from there. If you look down this forum to the 15th. of December '99 there are some postings for Bass Guitar Amp and several Guiter web sites are listed, you might be able to find some information on one of them for your unit. I'd check both transformers to see if they are both putting out voltage, I think only one would be faulty. Good Luck, I hope the transformers are OK. Don Black.

: Happy new year all,

: I am repairing a Legend guitar tube amplifier for a friend and would like some advice. This is a Legend Model G1250 SC amplifier with 3 12AX7A tubes, and diodes and transistors. My friend had it previously evaluated by a repair shop and was told that the power transformer was 'bad.' There are 2 power transformers and the higher voltage rectified one is only putting out 80 volts DC to the plates of the tubes. But, I don't have a schematic to tell me the proper voltage to apply to these tubes - it can be 200 V or 280 V DC. So, I don't know what rating transformer to recommend that he purchase; and, I don't know if there is a voltage doubling DC power supply circuit that has a bad diode that would be simplier to fix.

: Is anyone familiar with similar amplifier designs, or does anyone have a schematic I can borrow?

: Thanks, Steve

All of you - thanks for your tips. I've previously isolated the high voltage power transformer and tested the resistance and voltage of each winding. They all looked good to me, which is why I decided to look for more information.

Next, I'll check the rectifier diodes and power capacitors, and then the other power transformer. It might have fuses; never thought of that.

Steve