Fender Hot Rod Deluxe Died Suddenly

[Chuck H]

Truely a bummer. I hate that I have nothing to offer. I don't do service like some guys here so I'm not aware of any common failures in most production amps. I'm hoping someone with shop experience will step up having experienced this and have a solution.

[Steve Conner]

is there anything on the secondary side that can cause a primary winding to open?
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Yes, most of the stuff on the secondary side. It's not that the primary itself burns open. Excessive current draw from any winding will overheat the whole transformer generally, and pop the thermal fuse.

Particularly rectifier diodes, filter capacitors, power tubes. and heater circuit. Check all of those.

If the transformer has one of those thermal fuses, it's embedded deep inside the windings where you can't get at it. For two reasons: That's where it gets hottest, and they want to make the fuse impossible to replace, because the transformer isn't safe to use again after it's been overheated.

[Chuck H]

Thank you Steve. Although I've never had a primary blowing open as a lone symptom for diagnosis it obviously can happen.

I was eyeballing that 8200P cap across the bridge rectifier on the schem. Certainly check that too. But overall I would say that cap is there to squelch voltage spikes and that thermister on the primary is there for similar reasons. So it would seem to me that there is a design shortfall in this amp that the engineers were aware of and these circuts are a bandaid fix. So I'm wondering what might be done to make real improvement and prevent future failures.

[tmlesko]

Steve,
Thanks so much for your reply. My question now is. . .can I reliably check any of those components without a power transformer installed? I am "re-learning" electronics after 40 years in a different field so my memory is a bit foggy. Thanks again,
Tom L.

[pontiacpete]

you could check a lot with an ohm meter.

[Mark Black]

One thought - you may want to build one of these: http://www.geofex.com/article_folders/spo_test.htm

They can save a lot of grief & money when hunting for over-current problems, and give a quick visual for intermittent types. I have considered the one I built to be well worth the time & parts investment over the years.

By the way is/was the power fuse correct value & type in your amp?

[tmlesko]

One thought - you may want to build one of these: http://www.geofex.com/article_folders/spo_test.htm

By the way is/was the power fuse correct value & type in your amp?

Yes, the fuse is correct, thanks. I have seen these light bulb testers for use with antique radio repair and haven't built one yet but I will now for use when I get another xformer, thanks again!

Will ohm meter tests with the components "in the circuit" really be reliable? I have attached the schematic to one of my earlier posts.

Tom L.

[Jazz P Bass]

A light bulb limiter is of great value for saving the amp when there is a problem.
Now, you need to systematically troubleshoot this amp.
You are on your third power transformer.
Something is seriously wrong.
If you do not know how to troubleshoot the amp, may I suggest taking it to a tech.
Maybe they will let you watch.

[TomCarlos]

Tom L and others...

I too am a returning tech after many years of doing something else. So please know, I appreciate your time and offerings here.

I am looking at a Hot Rod Deluxe with a bad Power Transformer. I measured across the primary windings with an Ohm Meter and it's open :-( The secondary windings appear to be ok.

It's puzzling to think that this could happen. Why doesn't the fuse or Thermistor fail first? I'd hate to think I will replace the transformer and this could happen again?

Yes, while pondering the decision to replace the PT, I will look at everything mentioned in this thread and all others related to the Hot Rod Deluxe. Seems that we have several resistors which need to be upgraded, solder joints to check and redo with fresh solder. I might even change out the rectifier diodes. It makes sense to mount some of the components with some space between them and the PC board.

Oh, the Thermistor - it's reading about 10 ohms? Does that sound right? A 5 amp thermistor in series with a 3 amp fuse?

http://support.fender.com/schematics..._Schematic.pdf

Tom

[Prairie Dawg]

Sometimes transformers fail for no apparent reason and without a lot of fuss and bother. The PT in my Weber 5E3P died all by itself sitting there quietly. One day it was fine, the next day gone. Under normal conditions the fuse would have blown, I guess. Don't read too much into it.

[Jazz P Bass]

The thermistor value & current rating are fine.
The primary of the transformer probably had a "fusible link" wrapped in the winding (series connection).
If it failed you will then have an open winding.
It may have failed for a number of reasons.
You have to ascertain which failure mode could still be present.
If none is found then the failure can be attributed to something external of the amp. (consider the power transformer external)
You will then be good to go with a new transformer.

[TomCarlos]

Thank you JPB... I dissasembled the case and can see something inside the paper wrapping. So it looks like I need to remove the copper shielding to get to it. Will take photos and report back one way or another. Tom

[TomCarlos]

Ok... the process of dissassembling a transformer to replace the thermal protection device is a delicate one. After removing the case, cutting the copper shielding, and gently pealing away the protective layers of paper insulation, I found the thermal device, buried under the outer windings. This device runs from the top to bottom of the windings. It is encapsulated with a heat resistant sleeve. It's then soldered onto the windings and black external wire. Replacing this is tricky- but it can be done. Is it worth saving $58? Eh, doubt it. Reassembly and the fear nothing will short may not be worth it.

Just for the hell of it, I took this device out and put a jumper wire in it's place. Yes, the transformer works. But the Red/Red leads only yielded 370 volts- far less than the expected 440V. So... it will be a project transformer on something else.

Good idea though... and thanks. Tom

[olddawg]

I've repaired transformers like this many times. (Usually for a PT with no easy replacement) If you're careful, use heat shrink, insulator paper, and even clear lacquer fingernail polish it isn't a big deal. Just make sure it's well fused and there is no leakage to the core. If you do it for a customer you better have him sign a release.

[Chuck H]

Is that 370V read from one end to CTR? And is the 440V figure from the rectifier output on the schem?