Hi Folks,
Schematic ^^
I'm repairing my bandmate's Fender Reverb tank reissue. We used it at practice, turned it off @ the end, and when he turned it on @ next practice, it wouldn't turn on.
The primary of the transformer is blown open, yet the slow-blow fuse did not blow. I've taken DC ohm readings of the circuits on the secondary sides, and while the heater / 6.3V winding is a bit low (~1.5 ohms @ rest vs. ~4 ohms would have I expected), I cannot find any shorts in those 2 circuits. Rectifier diodes are fine at rest.
I can put a new transformer in, but I'm wondering why a primary would blow open like that & not burn up the fuse. I'd like to find any faults in the secondary side that might have caused this.
My dad suggests putting in a fast-blow fuse, installing the new transformer, and putting a meter on the test points and looking for any low voltages that don't match the schematic's expected voltages.
Any other suggestions or insights?
Thanks,
boogieWoogie
An externally hosted image should be here but it was not working when we last tested it.
Schematic ^^
I'm repairing my bandmate's Fender Reverb tank reissue. We used it at practice, turned it off @ the end, and when he turned it on @ next practice, it wouldn't turn on.
The primary of the transformer is blown open, yet the slow-blow fuse did not blow. I've taken DC ohm readings of the circuits on the secondary sides, and while the heater / 6.3V winding is a bit low (~1.5 ohms @ rest vs. ~4 ohms would have I expected), I cannot find any shorts in those 2 circuits. Rectifier diodes are fine at rest.
I can put a new transformer in, but I'm wondering why a primary would blow open like that & not burn up the fuse. I'd like to find any faults in the secondary side that might have caused this.
My dad suggests putting in a fast-blow fuse, installing the new transformer, and putting a meter on the test points and looking for any low voltages that don't match the schematic's expected voltages.
Any other suggestions or insights?
Thanks,
boogieWoogie
A fuse won't blow if the winding opens.
Are you sure the primary is open and everything is fine with the switch, fuseholder, voltage selector...?
Apart from that, checking windings with an ohmmeter is always problematic and often of little value, except you find an open winding.
If the primary is really dead, you have to replace the transformer or have it rewound.
Cheers,
Georg
Are you sure the primary is open and everything is fine with the switch, fuseholder, voltage selector...?
Apart from that, checking windings with an ohmmeter is always problematic and often of little value, except you find an open winding.
If the primary is really dead, you have to replace the transformer or have it rewound.
Cheers,
Georg
Hi Georg,
Yes, the primary winding seems open. I removed the power transformer & resistence-tested the primary & secondary windings. The primary is an open circuit. The 2 secondary windings are close to the replacement tranformer's specs:
6.3V winding = 0.5 ohms (replacement tranny specs: 0.24 ohms)
high V windings = 236 ohms (replacement tranny specs: 185 ohms)
Thanks,
boogieWoogie
Yes, the primary winding seems open. I removed the power transformer & resistence-tested the primary & secondary windings. The primary is an open circuit. The 2 secondary windings are close to the replacement tranformer's specs:
6.3V winding = 0.5 ohms (replacement tranny specs: 0.24 ohms)
high V windings = 236 ohms (replacement tranny specs: 185 ohms)
Thanks,
boogieWoogie
Sounds like there is a thermal fuse in the transformer. This will blow if the transformer over heats. It is a safety that prevents fires. Not the same thing as a fuse. The low resistance readings are normal.
Disconnect the transformer leads and double check for an open primary. If needed replace the transformer. I suggest you then take apart the bad one. You might learn why it failed.
Disconnect the transformer leads and double check for an open primary. If needed replace the transformer. I suggest you then take apart the bad one. You might learn why it failed.
+1
In some cases a defective thermal fuse can even be replaced, but this requires closer examination of the transformer.
Thermal fuses are also not 100% fail-save, in rare cases they blow without thermal overload.
In some cases a defective thermal fuse can even be replaced, but this requires closer examination of the transformer.
Thermal fuses are also not 100% fail-save, in rare cases they blow without thermal overload.
Image link URL is broken; try http://i.imgur.com/CY3lqgc.png
The heater load is around 6V 1A or 6 Ohms; I would expect the winding to be "small" compared to this or nearer 0.6 Ohms than 4 Ohms.
No internal fuse is shown in plan; though it may be there.
Windings can fail "for no reason". One fault is the wire was pulled too tight in winding and (through thermal stress) eventually pulls apart (like my sewer line...)
If very sure the PT is bad, get another, but start it on a lamp limiter with no tubes. I would even disconnect the secondaries. A shorted rectifier is a dead-short on the transformer. If limiter lamp is no-glow secondaries open, but goes full bright when rectifier is connected, you found another fault.
The heater load is around 6V 1A or 6 Ohms; I would expect the winding to be "small" compared to this or nearer 0.6 Ohms than 4 Ohms.
No internal fuse is shown in plan; though it may be there.
Windings can fail "for no reason". One fault is the wire was pulled too tight in winding and (through thermal stress) eventually pulls apart (like my sewer line...)
If very sure the PT is bad, get another, but start it on a lamp limiter with no tubes. I would even disconnect the secondaries. A shorted rectifier is a dead-short on the transformer. If limiter lamp is no-glow secondaries open, but goes full bright when rectifier is connected, you found another fault.
Hi jjman, the fuse matches the schematic: 0.5A.
Hi PRR, thx for pasting the direct link to broken link. If there's a thermal fuse in the transformer, it's really buried inside. The tranny looks cheap as heck, and doesn't have extra, external wires. Good tip on the bulb limiter, gonna make one this weekend.
Hi PRR, thx for pasting the direct link to broken link. If there's a thermal fuse in the transformer, it's really buried inside. The tranny looks cheap as heck, and doesn't have extra, external wires. Good tip on the bulb limiter, gonna make one this weekend.
Thermal fuses usually are poked down deep inside, that is where they can best feel the temperature of the unit. The connections would be internal. Thermal fuses are not something intended to be changed like a current fuse in a holder.
I've seen many thermal fuses right on the outer layer under the paper. You can jumper them if bad and take current readings and if all is well they can be replaced.
In a properly designed power transformer a thermal fuse if present will be placed in approximately the hottest part of the winding in normal operation which is generally somewhere near the middle of the winding and it won't be accessible.
I have also seen them in the outer layers of the windings where they sometimes do not open until a winding short occurs due to insulation failure. lol
I have also seen them in the outer layers of the windings where they sometimes do not open until a winding short occurs due to insulation failure. lol
I do not recommend testing for voltage on the AC line with your tongue. Also it may not be a good idea to short out any safety device such as a thermal fuse.
But it really is your choice. As the advice goes "Choose wisely my friend."
But it really is your choice. As the advice goes "Choose wisely my friend."
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