Fender 75 - b+ fuses blow intermittently

[Mars Amp Repair]

Hello folks,
I am servicing a Fender 75 combo amp that has an unusual issue. Find Fender Twin II schemo attached. The power supply seciton in question is the same as the Fender 75 save not being fused. I hope the attachment makes it, I don't see it in the initial mail.
The problem is if the filters are discharged (even with the stby sw in stby) , these 2 fuses pop when the power switch is turned on.

Seems the initial surge to the caps draws more that 1amp, as 1amp fuses will similarly blow.

If the attachment doesn't get there, the B+ from the bridge goes to the upper cap....that cap is in series with another of the same value. There are 220K divider resistors across both caps. The lower one goes to ground. The center tap of the bridge winding goes to the point where the 2 caps meet. (perhaps someone can explain that arrangement, too). BTW, with the stby switch in stby, the B+ stops here & goes no where else.

I'm thinking of adding a surge resistor in series with the B+ line to the caps (or perhaps inseries with the fuses).

Any ideas? The filters have been replaced & the 220K divider resistor are what's in there.

Thanx, glen

[Amp Kat]

Schematic didn't make it but I found it on Mr.GH 3. Lots to go wrong here and you do have the power tubes out right ? Ultra Linear amp so is the high / low power switch circuit working right ? You may have to isolate the HV from the bias supply as it's all tied together even that 6.2 zener could wreak havoc but if nothing around that supply turns out faulty your going to have to start taking connections off and find the circuit that's dragging it down. RG's lightbulb tester is almost a most on this one to keep your fuse count down. If the bias is ok at it's set -56 then it's most likely further back. If you use your variac to check supply nodes so you can at least see if they are coming up. If they stay at zero or slightly above at least you know the problem is near.

[Mars Amp Repair]

Thanx Amp Kat,
I hear what your saying, however as I mentioned with the stby switch in the stby position, there is nothing connected to the bridge output but those 2 caps & 2 x 220k divider resistors across the 2 caps.

I'm not certain if your schemo shows the fuses, but they're in series with the PT windings to the bridge. With the stby switch in stby the path is PT to fuses to bridge to caps...that's it. So, power tubes in or out....it doesn't matter.

As aforementioned, the fuses only blow when I discharge those two caps (or leave it off long enough for them to discharge). Even with the stby switch left in the off standby position (IOW no standby) if the caps are not discharged, the fuses won't blow...they won't even bend.

It's just as if the insurge of current to the discharged caps is too much for the 630ma (or even 1amp) fuses to handle.

The only other unusual part is the centertap of an otherwise usually non-centertapped bridge supply from the PT is connected to the junction of the 2 caps. I'm not certain what that is accomplishing. I've disconnected the CT & nothing about the operation of the amp changes...HMMM.

The only thing I haven't done is substitute the bridge. But given the proper dc out, I find it difficult to buy that there would be an issue there.

Now I'm like most of us where you know there have to be thousands of these amps working correctly in the world, so why should I need to modify just this one....but my inclination given the limited number of components involved, is to just add a series surge resistor & be done with it.

I'll have to experiement to see if that even solves the blowing fuse issue. I'm thinking if the centertap didn't go anywhere, they probably would have just used one fuse in the bridge output as usual.

There just aren't that many variables here.

glen

[Enzo]

Well, are you using slow blow fuses? I would.

And what value fuse? Is that printed inside? Or is that just what was in there when you opened it? You mentioned 1A.

The schemo does not show secondary fuses, so I would be thinking you got an export version or at least a Canadian one. Maybe?

Discharged caps are a dead short to a rectifier for a moment. In fact, that is how I figure limiting resistors - look at rectifier surge rating, then consider the cap a dead short and make the resistor holt it to 100A or whatever.

The center tap is the lower voltage supply. If you switch the amp to low power mode, S1A selects the PT CT supply of 250v. If you disconnect the CT, it may not affect full power, but in low power, the current for the output tubes would not come direct from the transformer, but would have to come through the 100k resistor across the high side filter cap. That ought to make some very serious sag available, at the least.

I can't see the bridge at fault.

KB, the bias is not connected to the B+. The point where a cap from each "join" is ground.

[Amp Kat]

KB, the bias is not connected to the B+. The point where a cap from each "join" is ground.

Yeah I thought about it later after I posted that it was the case when I noticed the brown winding wire going to the diode. Anyway around if you disconnect the circuit right at the bridge and it doesn't blow fuses then something is pulling on it. Higher rated fuses and it cures the problem then that's just what you have to do. Higher rated fuses and it still blows then you have a problem even though it looks like you can't

[Mars Amp Repair]

Well,
Enzo great observation that this might be an import. I see now that it indeed is (the back is marked as 220v) & the power transformer apparently has been changed to accommodate the 110V.

BTW, the power supply in this amp is more like the Twin Reverb II found in the Gearhead disk #3, except this one has 4x6l6's (that's definitely original).

There is no dual power selection switch & the stby switch is a single pole. The centertap goes to the same point perhaps just to have a place for it to go?!! The Twin Reverb II schemo shows it going there, too.

The mains fuse rating at 220v is 3.15a. Since this is a 100watt (4x6l6) model, the fuse needs to be double that (6a) correct?

The only question is that regardless of the primary voltage, the secondaries should be the same as the 110v version, so I don't see that the fuses should be a different value. Slow blow fuses of the same value (630ma) still blew just as effectively as the fast blow. I can see the 220v model being dual fused at this point as possibly something to do with both ends of the ac input being 110V above ground & no neutral...for safety.

Since it appears that in the US the dual fuses are not required, I believe I'll probably just bypass those puppies or perhaps just increase those values & mark the back accordingly. But still.....

glen

[Enzo]

The 75 drawing is downloading from schematic heaven at the moment. I want to compare it to the paper copy in my file. It has the features I described. Yes, the drawing there is the same as mine, check it out.

[Mars Amp Repair]

Yeah Enzo, I have that schemo too.
I guess this amp must be a special version for overseas. It seems to be a combination of 2 different models.

The power supply is definitely is more like the Fender Twin II (actually except for no fuses, it's exactly like it) but with 4x6L6's.

The only changes I can see are the PT & the wiring to it. Everything else is remains untouched. glen

[Mars Amp Repair]

BTW,
The Twin Reverb II schematic also denotes what the 220v xformer wiring looks like (has 110v, 120v, 127v, 220v, 240v, & 260v taps). This model 75 only has a 115v primary. That leads me to believe this amp was designed exclusively as a 220v amp otherwise why change the xformer.

The Twin II schemo also denotes that the ground switch & receptical are deleted for the export model. This one of course has no ground switch or recepticle, too.

This also has a weird short stubby 8A fuse in line with the filiament windings. Very different looking amp in that respect.


glen

[Amp Kat]

Hope I never see one of those Glen. Keep that thing in your neck of the woods.

[Mars Amp Repair]

Sorry Enzo & Amp Kat,
my guy told me this was a Fender 75, but he mixed the tickets up! This one is actually a Fender Twin II, no wonder it looks more like that schemo!! It really seems to be a combination of the 2, as this one has the zener diode config. of the 75. Just plain weird.

Anyway the rest of the story is the same...thanx again Enzo for your observation that this might be an export model.

I still can't answer why those B+ bridge supply fuses would blow, regardless of being a 220v or 110v model, but what the hell...we don't require them, so now it doesn't have them & all functions fine. The Twin II also has no B+ fuse. Thanx again, glen