Fender Dlx Rev build- last Q b4 fire-up.

[Sea Chief]

So, is the hum on the reverb intermittent? Has it come and gone with no circuit changes to the amp? If this is the case you must have an intermittent open ground. Probably on the connecting cable since that is what gets moved around the most. If you have other shielded RCA cables this is pretty easy to check.

Not intermittant- stuck firm now. It was fine just y'day, and there has been nothing done at all to amp since- not even amp out of cabinet or even amp moved from where it sits. Most peculiar.

I think Ive ruled out ftsw too havent I, if once ftsw plug pulled it remains the same?

Cable is new and altho a dead cheapo rca as I said, the reverb was just fine & finally sorted w'out the innitial feedback prob. Just a mild perfectly normal hum typical of a reverb. Now this V bad hum springs from nowhere.. which yes sounds like simply a bad earth/ or completely undone earth.. but how/ where/ why is perplexing.

Rca connections into plugs underside of amp- fine/ firm/ as they were.
Rca's into pan- fine/ firm/ as they were.
Pan is new- not dropped, or pulled or anything.

[eschertron]

Not intermittant- stuck firm now. It was fine just y'day, and there has been nothing done at all to amp since- not even amp out of cabinet or even amp moved from where it sits. Most peculiar.

And it's the same with no guitar plugged in (nothing in the input jack)? DO you have different lights on today than you had on yesterday? IS the amp closer to a TV or computer monitor? did anything in the environment change?

oh.. I see you said the amp hasn't moved. there's one check.

[Sea Chief]

Yes, buzz/hum remains with no gtr plugged in.

Of all the puzzling whys of amp-making Ive had.. this is the most confounding, even if it may be a simple fix (should I find it).

[guitician]

How are the RCA cords grounded? Each cable should be grounded only at one end.

[Sea Chief]

Well, theyre simple cheapo rca cords (that I didnt expect to be any good- they are, as the reverb circuit/ tank has been fine -albeit just the fdbk issue/ fixed by taking rev away from spkr- and dandy, with no cause to replace the rca cords at all tbh).

I suppose the answer though, is both ends. If this was an issue tho.. the prob would have been from the start- but its not.

[guitician]

Ground loops can come and go. I have usually seen only the one end grounded in reverb lines. And cheap rca wires may be more likely to get loops because the shield wires are thinner.

[g1]

Try some other RCA cables. New means nothing. Especially cheap stuff.

[Sea Chief]

Try some other RCA cables. New means nothing. Especially cheap stuff.

But I ruled these out as they worked perfectly y'day, & before too (albeit with fdbk). It worked in the head cab, where it sits. Theres no physical way the rca cables could have degraded in 1 day.. besides..

I just took the whole shebang out again onto workbench nearby. Cleaned all contacts, normal hum. Ok so good, so that must be it then > back in head, back where it was. Loud buzz/hum back. Christ on a flamin bike. Ive ruled out position of head, plug socket, ftsw, adajcent PC, adjacent lamp. Every conceivable thing. Even moved it- same. As I said where it is anyway, it finally worked fine y'day w'out the fdbk which was the only issue up to y'day evening. Bizarre.

[g1]

When you find the problem, it will be something that "worked perfectly y'day, & before too".
The only way to rule it out is to rule it out.
I've had my share of new replacement parts that "couldn't possibly be bad" the were... bad. After you spend days looking for a problem only to find out it was that new thing that couldn't possibly be bad, you'll know.
But I guess that is something that you have to learn for yourself .
There is a quote by Enzo, you will hear him repeat it often, and others quoting it. Never forget it:
"never make up an excuse not to check something".

[g1]

So now it is ok out of the cab, but bad hum when re-assembled?

[Sea Chief]

I am going to start a new thread re the reverb hum. Clear my head.

Just before tho: I did have plate V at 425V (after the resisitor change at R59 to get a better bias range). Measured last ev & monitored whilst playing thru, as I did have an odd sound-went-quieter-with-bit-raspiness intermittant glitch after an hr playing away.. & I noticed V8 pin 3 voltage hovvering around the 439-440V mark, not the expected 425V.

Any idea in this V hike? any cause for concern? (bias pot defo same position). And was my figure of -20mA (-20mV measured across my added R's) a good average figure to land at? I didnt ask before, just read a few folks ~aimed for 20.

Thanks chaps- SC.

[Sea Chief]

So now it is ok out of the cab, but bad hum when re-assembled?

Yup indeed. But even when re-assembled day before y'day I had it fine, no bad buzz (& actually feedback finally gone too- this sorted at least it seems).

So, obvious conclusion of it being somehow assembled as cause is n/a. Need to start afresh on a new thread for this gremlin.