Fender Hot Rod Deluxe

[bsco]

HI. Have a Hot Rod Deluxe and when powered up there is a lot of cracking and hum from the speaker. I checked for leaky filter caps for the hum problem but they test ok. I tested tubes as well..I do realise that testing tubes doesn't show exactly how the tube preforms in the actual ckt. If I remove the two 6L6 output tubes, the cracking and hum is still there. That leads me to suspect that the output transformer might be at fault?? The owner said that this amp blew a fuse and when they replaced it, they had no power still. When I checked the fuse it was also blown. My question is if i take an output transformer from an old Marshall running EL34's, would this be good enough just for a quick check? I only need to power the Fender up for a few seconds to see if this hum and cracking noise is still there...by the way, the DC resistance of the output transformer is 110 ohms form one side to center and 75 ohms for the other side to center at the primary. Thanking you in advance for your time.
Cheers,
Bernie

[soundguruman]

If you had a look under the circuit board, because the tracks can be damaged.
Make sure all the tracks are repaired...and the bigger power resistors have not flamed...
and then of course, well sure the transformer winding should read balanced on both sides.
is it something else? are the tube sockets burned?

I think I would replace the Illinois electrolytics.
Just because of personal preference.
If you are looking at the third one from the left, as you face the back of the amp...
the one above the two big power resistors.
Check that one...

Seems like the output tranny may be goofy, but is something else causing the reading?

[Enzo]

The transformer primary halves will be balanced only if the winding is double wound, but not at all if it is a simple wind with tap.

If noise gets through the speaker with no power tubes, you either have a failing output transformer OR failing flyback diodes. The only way to get noise into the speaker is to drag current through the transformer primary. Without tubes, the only remaining current paths are winding shorts or those diodes going leaky.

Frankly, bad diodes are more likely than bad transformer.

There is a push-on connector for the transformer wires by each power tube socket. There is a diode next to each of those. CR4 CR5. Unsolder and lift those two diodes and power back up without tubes. If the noise is gone, your transformer is OK and you need new diodes. If the noise remains without the diodes, your tansformer becomes the main suspect.

yes, for purposes of this test just about any transformer will work. All the test is doing is finding out if a different transformer stops the symptom. We don;t care about power transfer or distortion or tone. For the test, all we care about is whether that thing will do basic function without making that noise. I have an old fender Bassman output tranny I keep on the bench as my universal sub transformer for just such go/no-go testing.

While you are in there, look CLOSELY at the solder on the powr tube socket pins for cracks. Or just do what I do and resolder them all when you go in there.