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Topic: checking power transformer |
Jack Wilson
From:
Marshfield, MO
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Posted 3 Jan 2013 4:18 pm |
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| Hi to all you amp gurus out there. I bought a late 70s or earlier 80s Fender Deluxe Reverb that has had a very hard life. The seller said all it needed was a rectifier tube, which I doubted, but the price was right so I took it. I have taken the amp apart and found the 2 amp slo blow is blown, so I'm going to check the power tranformer while it is apart. It takes a 5U4 rectifier according to tube chart on the cabinet. I have discharged the filter caps and removed the 6V6s. My question is what should the readings be on the windings after I power it up without the 6V6s. I know the filament is 6.3 acv but I don't know what the other windings should be. |
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Matthew Dawson
From:
Portland Oregon, USA
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Posted 3 Jan 2013 5:07 pm
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I'm no expert, but what I would do is de-solder all the leads and check to see if there is continuity between any of the individual windings. For instance: with one lead from you ohm-meter hooked to the 6.3 volt winding and another hooked to the high voltage, there should be infinite resistance.
If there are no shorts between the windings, hook up the mains and let the transformer "bake" awhile, making sure to keep all the leads where they will not conduct into anything or anyone (taping them individually to a non-conductive surface works well). If the transformer is bad you will smell it.
Next CAREFULLY measure your voltages. They will be about 10% higher not attached to any load.
Finally, listen to Tim Marcus, Ken Fox, Dave Grafe, Brad Sarno or a number of others if they say to do anything differently because they know more than me! |
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Ken Fox
From:
Nashville GA USA
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Posted 3 Jan 2013 6:38 pm
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I have no idea what the primary or secondardy windings would read, as an ohms measurement is pretty much worthless for evaluation of the primary and secondary windings. I use a true impedance meter to test such things. However I would not use one to normally test a power transformer.
First thing I would do is remove the rectifier tube and all other tubes.
Apply power with the standby switch off and see if the fuse survives. I also bring the amp up thru a Variac and a current meter on the AC line side when doing all this. That can save a lot of 2 amp fuses. I also use a series 60 watt lamp on the line side if I suspect a shorted transformer. It will light full bright and save the fuses as well.
At that point the only load on the secondary is the bias supply diode and cap as well as the lamp, remove it and try again. Also a shorted lamp holder will result in fuses blowing.
Next turn the amp standby to on. If it blows a fuse you likely have shorted power supply caps or a bad choke coil. Also it is supplying voltage via the primary winding's center tap to the output transformer, a bad O.T. can blow a fuse as well.
If all that survives plug in the speaker and put the power tubes in.
Basically you keeping adding loads until you arrive at the problem.
This is a good job for an experienced tech, not someone unfamiliar with electronics. Unless you have a pretty good understanding of tube amps and safety this might be best passed on to someone with the proper knowledge. |
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Stephen Cowell
From:
Round Rock, Texas, USA
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Posted 4 Jan 2013 1:01 pm
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Between 'At that point' and 'Next' Ken forgot to tell you to put the new rectifier tube back in. Good luck!
_________________
Too much junk to list... always getting more. |
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Ken Fox
From:
Nashville GA USA
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Posted 4 Jan 2013 1:26 pm
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| That will really help a lot! Thanks for pointing that out. |
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Paul Arntson
From:
Washington, USA
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Posted 4 Jan 2013 10:41 pm The best advice of all...
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| "...a good job for an experienced tech..." |
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Jack Wilson
From:
Marshfield, MO
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Posted 5 Jan 2013 9:05 am
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| Matthew & Ken thanks so much. I did the continuity check that Matthew posted and then I followed Ken's step by step post. PT tested OK. Good information for my trouble shooting notes for the future. |
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