Fender Blues Deluxe Reissue

[mtlbasslad]

Completely dead amp coming from some other 'tech' who changed a preamp tube for who knows why...

Check main fuse - OK. Power on - nothing lights up, not even pilot light. I suspect power trans... check primary OK. Check power & standby switches OK...
Three secondaries to check - red wires are the high voltage (is that what we call B+ ?) I read 44 ohms. Brown is the 39V bias supply - 4.9 ohms. Green is the 6.5V heater & pilot light - zero ohms.
Interestingly the well hidden connections for the 6.5V are also hiding another fuse (which I don't see on the schematic) which tests OK.

So if the heaters aren't heating I guess nothing else will work - could it be the trans secondary is the culprit?

Thanks in advance, Lorne

[The Dude]

Measure AC voltage at the heater wires coming out of the main transformer before and after the fuse. I suspect the fuse. I've had to change a few. Continuity testing the fuse in circuit won't tell you anything because you are also measuring the transformer winding and the tube filaments. Measuring voltage will give you more info.

P.S. That fuse is a PITA to get at!

[mtlbasslad]

Measure AC voltage at the heater wires coming out of the main transformer before and after the fuse. I suspect the fuse. I've had to change a few. Continuity testing the fuse in circuit won't tell you anything because you are also measuring the transformer winding and the tube filaments. Measuring voltage will give you more info.

P.S. That fuse is a PITA to get at!

I tested the fuse right at it's ends... any idea the rating (I can't see it on the schematic) ? Don't know how I'll get in there without taking half the amp apart - what were they thinking?
No ideas about the secondary test results? As in zero ohms on that heater circuit?

[The Dude]

Test the AC voltage- not resistance. If you have voltage before the fuse and not after, you'll know the fuse is bad. If it is a bad fuse, the best way to fish a new fuse in there is to get the board loose so you have access. You don't have to remove it. Then, turn the entire amp upside down so that the fuse doesn't fall when you try to set it in the clips. Once you have it set into the clips, push it down to snap it in. Like I said, it's not fun. Whoever thought of putting a fuse there should be shot.

Edit: I don't recall the fuse value. Just look at the old fuse.

[mtlbasslad]

Test the AC voltage- not resistance. If you have voltage before the fuse and not after, you'll know the fuse is bad. If it is a bad fuse, the best way to fish a new fuse in there is to get the board loose so you have access. You don't have to remove it. Then, turn the entire amp upside down so that the fuse doesn't fall when you try to set it in the clips. Once you have it set into the clips, push it down to snap it in. Like I said, it's not fun. Whoever thought of putting a fuse there should be shot.

Edit: I don't recall the fuse value. Just look at the old fuse.

The probes on my Fluke can barely get in there (power off) to check things. I can't imagine doing that with power on. Tomorrow (after I do the snow clearing & before the guy comes to pick up his Mesa) I'll try to get the 2 90 degree spade connectors off without harming some tiny little grey wire connected to the speaker jack... who knows how I will ever get them back on with my 76 YO fingers...

Shot you say? - I'm thinking more slow & painful, as in suspended by some sensitive parts...

More news tomorrow.

[The Dude]

You don't have to remove the spade connectors. Just measure voltage from the top/solder side where they are soldered in.

[mtlbasslad]

You don't have to remove the spade connectors. Just measure voltage from the top/solder side where they are soldered in.

That's what I did to check the fuse - again I can't see me getting probes in there without sparks & magic smoke... I'll pull the spades & take out the fuse and live to see another day. Don't know the status of your fingers but mine are swollen & painful. They say these are my 'Golden Years' - uh, yeah.

[Mick Bailey]

No ideas about the secondary test results? As in zero ohms on that heater circuit?

It's likely that your meter doesn't resolve the fractional ohm resistance of the winding. It's heavy wire with relatively few turns. Heater windings rarely fail and unlike the other windings that have fly lead connections to their fine wire, heater leads are usually the winding itself that's sleeved, so no joint to fail.

[mtlbasslad]

So after prying off the 2 90 degree green spade connectors I got the fuse out - dead - it was marked T6.3 - a slo-blo - all I had was 6 slo-blo so put that in with medical hemostat.
Thanks to the dude for the install tips.
Hope I never do this again.

Another contribution to the forum coming.

[mtlbasslad]

It's likely that your meter doesn't resolve the fractional ohm resistance of the winding. It's heavy wire with relatively few turns. Heater windings rarely fail and unlike the other windings that have fly lead connections to their fine wire, heater leads are usually the winding itself that's sleeved, so no joint to fail.

Thanks Mick for that insight - never had a trasfo apart but it makes sense - low voltage but high amperage on the heater secondary.