Is My Power Transformer Toast?

[Jake H]

I have a Fender Bandmaster Reverb that is making a loud(ish) hum and produces a low output octave type effect when I play. The B+ voltage is sitting around 360VDC - from the schematic it should be around 435VDC. I pulled the rectifier and I'm getting roughly 360VAC coming out of the transformer. I figure it's toast, but maybe I'm missing something? Any particular reason why or how this could have happened?

[52 Bill]

I have a Fender Bandmaster Reverb that is making a loud(ish) hum and produces a low output octave type effect when I play. The B+ voltage is sitting around 360VDC - from the schematic it should be around 435VDC. I pulled the rectifier and I'm getting roughly 360VAC coming out of the transformer. I figure it's toast, but maybe I'm missing something? Any particular reason why or how this could have happened?

My crystal ball says that your power transformer is okay and that you probably need new filter caps and/or a new rectifier tube.

Welcome to the place.

[Enzo]

Agree^^^^


I have no reason at all to think the transformer is at fault.

[R.G.]

Why is it that people always think that their power or output transformer is broken? Next to the case and chassis, these are probably the two most reliable parts in the amp - at least in well designed amps.

I have to think this is a form of "OK, doc, give it to me straight. I can take it. I'm dying, aren't I?"

[The Dude]

Why is it that people always think that their power or output transformer is broken? Next to the case and chassis, these are probably the two most reliable parts in the amp - at least in well designed amps.

I have to think this is a form of "OK, doc, give it to me straight. I can take it. I'm dying, aren't I?"

The same reason people think they should just randomly replace every capacitor in an amp.

[Jake H]

Why is it that people always think that their power or output transformer is broken? Next to the case and chassis, these are probably the two most reliable parts in the amp - at least in well designed amps.

I have to think this is a form of "OK, doc, give it to me straight. I can take it. I'm dying, aren't I?"

Everyone, Thanks for the suggestions - I'll take a look at the filter caps again.

This is my first post... In regards to the above quote : is this how people normally respond to questions on this forum? I tested the output of the transformer with the rectifier unplugged and didn't know where to go from there... I'm here to learn, and grow. I've seen your name around on other forums and thanks for all the great work that you do, but I don't think I need to be looked down on in this scenario. Sorry if I upset your day.

[The Dude]

In regards to the above quote : is this how people normally respond to questions on this forum? I tested the output of the transformer with the rectifier unplugged and didn't know where to go from there... I'm here to learn, and grow. I've seen your name around on other forums and thanks for all the great work that you do, but I don't think I need to be looked down on in this scenario. Sorry if I upset your day.

Jake,
I don't think he was referring specifically to you- just making a side comment in general.

That said, given the symptoms you describe, the first thing I would look at would be the electrolytics. Given the age of the amp, you should test and/or replace them.

[Enzo]

Jake, RG was not picking on you. he is a most helpful person here.

Those of us who have been around a long time often wonder why it is that people who look for help with a problem almost universally decide it must be a transformer.


My mother was always convinced that ANYTHING that went wrong with her TV set must be a bad picture tube. "They haven;t perfected them yet" she'd always say. It may sound silly to us that she'd think a loss of sound could be the picture tubes fault, but to her, it was reasonable.

[Jake H]

Jake,
I don't think he was referring specifically to you- just making a side comment in general.

That said, given the symptoms you describe, the first thing I would look at would be the electrolytics. Given the age of the amp, you should test and/or replace them.

Thanks, I was just able to take a look. The filter caps were replaced with brand new ICs last year, they all look great... HOWEVER, on further inspection I gave a little tug on all the leads and one of the 100uF leads popped right out of the end of the cap - Must have been a stray manufacturing error or improperly handled when installed? Who knows. I'll replace it as soon as I can get the part and keep you posted.

BIG THANKS though, without your advice I would have probably tried out another transformer. Much appreciated!

[Jake H]

Jake, RG was not picking on you. he is a most helpful person here.

Those of us who have been around a long time often wonder why it is that people who look for help with a problem almost universally decide it must be a transformer.


My mother was always convinced that ANYTHING that went wrong with her TV set must be a bad picture tube. "They haven;t perfected them yet" she'd always say. It may sound silly to us that she'd think a loss of sound could be the picture tubes fault, but to her, it was reasonable.

Enzo, I hear you. I guess what I'm saying is I came here to learn (which I sure did!) and not be discouraged from asking a question that I didn't know the answer to. If I didn't have a doubt that it may not be the transformer I never would have joined this forum and posted this thread. I had a inkling and I needed some help connecting the dots - so I came here.

I know that RG is helpful and knowledgeable. It's pretty amazing all the time and effort all of you have put in to furthering peoples understanding of these complex issues. I'm just giving some feedback. I appreciate you taking the time to respond!

[eschertron]

Thanks, I was just able to take a look. The filter caps were replaced with brand new ICs last year, they all look great... HOWEVER, on further inspection I gave a little tug on all the leads and one of the 100uF leads popped right out of the end of the cap - Must have been a stray manufacturing error or improperly handled when installed? Who knows. I'll replace it as soon as I can get the part and keep you posted.

BIG THANKS though, without your advice I would have probably tried out another transformer. Much appreciated!

Could be manufacturing or handling error. However, please make sure that the caps were replaced with properly-sized components. In this case the caps need to have a voltage rating higher than the unloaded B+ voltage (I think you said 435vdc?). If the replacement caps are not rated AT LEAST at what's on the schematic, they should be replaced.

[R.G.]

This is my first post... In regards to the above quote : is this how people normally respond to questions on this forum?

No, and it's not even the normal response from me. You happen to have walked into a situation where I had been pestered about three "it's my transformer, isn't it?" questions by email in the last week. So - sorry; no slight to you intended. The comment was not passive-aggressive, it really was just as it read.

I tested the output of the transformer with the rectifier unplugged and didn't know where to go from there... I'm here to learn, and grow. I've seen your name around on other forums and thanks for all the great work that you do, but I don't think I need to be looked down on in this scenario. Sorry if I upset your day.

Not looking down on you - just really wondering what it is that makes people do that.

And there was a bit of info for you, even hidden as it was. The transformers really are about the most reliable parts in an amp. This is why you can 'harvest' them from 50s PA amps and such when everything else in the amp is toast. It's possible to kill them, all right, but other parts are by far more likely to be bad.

There is a definitive set of tests which tell you to many-9's certainty whether a transformer is good if it has ever worked. You use an ohmmeter to verify that no windings are open, and then that no windings are shorted to one another. Then you put a voltage on it - any AC voltage within its frequency band and less than or equal to the rated voltage, so generally a heater winding from another transformer will do - and see if the voltages on each winding are in the right ratios, and if this it true, the transformer is good excepting only the quite-rare situation of an intermittent failure under heat and/or vibration.

[eschertron]

I have a Fender Bandmaster Reverb that is making a loud(ish) hum and produces a low output octave type effect when I play. The B+ voltage is sitting around 360VDC - from the schematic it should be around 435VDC. I pulled the rectifier and I'm getting roughly 360VAC coming out of the transformer. I figure it's toast, but maybe I'm missing something? Any particular reason why or how this could have happened?

One more bit of info about transformers - if you haven't already searched and found this - is the relationship between RMS AC voltage and peak AC voltage, and the resulting DC that can (theoretically) be gotten from rectification. The peak AC is 1.414 (radical 2) times the RMS voltage. The RMS is 'very probably' what your meter measures when looking at the AC sine wave on any line voltage. So 1.414 * the 360vac that you measured = 509 volts PEAK and can be rectified to produce the B+ that you want. So the AC voltage on the secondary is not low, if that's what led you to suspect the tranny in the first place. Do some looking, there are resources that will go into whatever depth you want regarding transformers and power supply design.

Welcome to the forum, and I hope you stay!

[Jake H]

Just wanted to give a quick update that I replaced the 100uF power filter cap and the amp is back up and running again!

I'll definitely take a deeper dive into the world of transformers to get a better understanding after all this. Thanks again for all your help!