Fender 'The Twin' red Knob no sound, filaments ok, help with the chaos!

[OwenM]

Hi there

I have a Fender 'The Twin' Red Knob that popped and emitted a smoke smell. Now theres no output at all, no hiss or anything. The standby LED is on and filaments get power. I am going to go in and test the power transformer and B+ voltages when I get a spare moment but wanted to ask a few questions I have come across already when doing visual diagnosis and pouring over the schematic for an hour.

I'm not green as a hobbyist, but still pretty new to this, so this amp represents a bit of a beast to get my head around..

Under basic tests, the power transformer isn't shorted prim-sec and the secondary resistances seem normal. The problem is on the 3rd coil on the secondary (at least on a basic check), this is the one that does bias I believe for parts of the power tubes and provides various power to the switching circuits. It also powers the lamps in the LDR's that control resistances in the signal path. The resistor R306 has blown, its one of the rectangular white blocky cement resistors. Visually, most else checks out, though I couldn't get a good reading of a C303 with a component tester/ESR meter so that may be suspect too.

Being a high watt and specifically fire-resistant resistor I don't want to chuck another in without knowing if something else along the way may be the cause. But, the trouble is, I'm confused by what a lot of the rest of that circuit is doing.. firstly WHAT are the LDR's for? Generally, theres not a whole lot else in that windings path.. the stuff for the power tube bias perhaps, and once we get down to the switching circuit, the IC (lm339n). Theres quite a bit that could be tested on the other side of the IC in the fotswitch circuit but I don't feel that's priority to check (just a hunch)

Anyway, I'm not looking for someone to throw out a tutorial on these particularly complex amps but a bit of insight into what all those LDR's are for may help, as I HAVE lost LED voltage so those LDR's WILL be in a high-resistive state. Also, anything that could have made that resistor go that's easily visible would be appreciated cos I can't see it, save the PT!

Also worth noting, the BIG 10w cement resistors R128 and R129 on the B+ have been replaced before in a fairly hack fashion with a load of tied together older green resistors, it has what looks like a weird bit of solder wick looped around a leg... It looks super dodgy as it definitely could form some erroneous connections. I have no idea if that's to do with anything but I think I'll be sorting that out too if I can find those resistors.... but anyone have an idea what the previous 'tech's' thinking was? (photo attached)

All the best and thanks for any input!

[OwenM]

Ooookaaaayyy....

So the loop of copper braid is around a leg of R129. It's near R131, also on the B+ and likely to cause issue but not explosions (IME). BUT the OTHER resistor it can reach is R132, which is down on the left of the circuit on the schematic, on the wire above the switching and IC circuit. The amp hadn't been recently moved so no cause to assume that this loop thing had shifted but it IS possible to make the braid loop connect R129 to R132, putting a healthy dose of B+ on the circuit very close (read, directly next to with only a forward biased diode in between) to my blown cement resistor :O

I'm at a loss to what that braid loop is for, let alone the decision to leave it flapping around in the amp...

EDIT - actually, the braid can bridge very comfortably R131 to R132 just by lying on the circuit board...

[Enzo]

Let us be more practical. I'd bet my lunch money that that loop was originally tied around the three power resistors to hold them to one another, it was never intended to be electrically involved. Note the rounded triangle bends in it? Should they have done it a different way? Sure. Or at least a drop of glue to keep it in place.

The LDRs are all switching elements involved in channel switching or at least feature switching.

[OwenM]

Let us be more practical. I'd bet my lunch money that that loop was originally tied around the three power resistors to hold them to one another, it was never intended to be electrically involved. Note the rounded triangle bends in it? Should they have done it a different way? Sure. Or at least a drop of glue to keep it in place.

The LDRs are all switching elements involved in channel switching or at least feature switching.

Ah yeah, I see it, and it fits nice and snug around those three resistors. At least, snug enough that it wouldn't vibrate free unless a it was mechanical component in a supremely loud amp .. oh for a cable tie..

So that was loose when I opened the chassis, may or may not have been the fault but certainly risky in it's current 'placement'.

Thanks, I think getting my head around exactly how the LDR's work individually may be beyond me for now, but I'll focus on checking the various voltages and get a new R306 and maybe those other two big power resistors if I can find them in the needed ratings. If not, I'll test them and glue/tie them.. I guess they may heat a bit...

[Enzo]

SO snip the braid off and blob some silicone on the resistors.

[OwenM]

SO snip the braid off and blob some silicone on the resistors.

Will do, as for R306, would 1k or 1.5k effect things so much? Finding it hard to find a supplier of 1.2k outside of the more industry-scale suppliers that would end up costing £8 for a resistor after handling and postage for a consumer level buyer like me!

[g1]

Being a 3W resistor, I think the current is probably enough that you need to be closer than 1K or 1K5. Can you make a series or parallel combination that will work?

You mentioned C303 seemed iffy. If it shorted, it would cause R306 to burn.

[Enzo]

Oh... A quick look and I see the D supply (what R306 powers) is to a 30v zener 5 watt D108 over lower left. That 30v supply is for the IC and various LED circuits. I'd probably feel OK with 1.5K if that is all you have.

If you can't find your 1.2k in 3w, nothing says you can't use a 5w resistor.

[OwenM]

Thanks guys, I've got a 1.2k 5w ordered, the cap was fine when tested out of circuit, here's hoping it is just that one resistor blown because of a short from that power resistor band-thing!

[g1]

Did you check resistance across the zener Enzo mentioned?

[OwenM]

No but just have, in circuit.. Weirdly I measure 36 Ohms both ways.. sounds def faulty right, or am I missing something? I can see any other path to ground the multimeter may be reading that has anywhere near as low a resistance as 36 so am I right in thinking that's probably just going across a shorted zener?

[Enzo]

The IC is right across the 30v, remember. It could be shorted.

Lift one end of the zener from the circuit and then measure it. If it still measures 36 ohms, then yes it is bad.

[OwenM]

Thanks, makes sense. With one leg pulled D108 reads the same, 36 ohms both ways and reads a voltage drop of 0.2v in BOTH directions in diode test mode. So I'll be getting another of those then!

I made a new schematic which shows where the B+ entered the switching circuits path and where it went. Red line is the voltage transferring via the braid thing, red circles are what it took out and blue lines to connect 'D' on the circuit. I know you guys commenting don't need to see this but just posting as I did it for my sake and thought anyone following of my level may want to see it.

TheTwin Annotated (dragged).pdf

So (hopefully this is the whole story), it erroneously bridged next to r132, went back towards the transformer on the schematic and blew the fuse (mines the alternate UK PT circuit). Went through diode D306 (which seems fine), fry'd R306 and then found ground through the zener D108, destroying it in the process. Worth noting the burned R306 is not open, its just a higher resistance so it was on the way out but obviously I turned it off before it was completely cooked.

Hopefully no more to find when these new parts arrive!

Side question, I know you shouldn't replace a ceramic fuse with a glass one as the ceramics have a greater protection against connection being maintained with arcing over an already blown fuse, but is it safe to put a ceramic of the same rating (T250ma 250v) where a glass one was? I already have those, and the fuse F305 blew, understandably as its on the ground path of the secondary winding of the power transformer that we are looking at. I could and should just buy a pack of glass ones, but I'm asking more for understanding.

Thanks so far with everyones help, learning a lot!

[OwenM]

Just realised this isn't in the repair subform, Mod's could you move it please if you see this?

[g1]

So (hopefully this is the whole story), it erroneously bridged next to r132, went back towards the transformer on the schematic and blew the fuse (mines the alternate UK PT circuit). Went through diode D306 (which seems fine), fry'd R306 and then found ground through the zener D108, destroying it in the process. Worth noting the burned R306 is not open, its just a higher resistance so it was on the way out but obviously I turned it off before it was completely cooked.

Most likely the zener diode D108 was the failure. Zener's usually fail to low resistance, which is what you found. The excess current due to the zener failure would cause R306 to overheat, and the fuse to blow.

No worries about moving the thread, it is ok here in this section.