5150 II help

[Valvehead]

On the 5150 II scem, right after the 5A fuse and power switch, theres a part labeled TR1 . Ive never seen that symbol and its not listed in the parts list .

what is it?



I also have a cracked open cap that doesnt seem to be on the scem. Its connected between the big blue wire going into the power trans primary and the power on/off switch.

It appears to be labeled :
SCL 15
10005

its a big black ceramic disc type. measures 14 ohms on my meter, yet it wont register when testing capacitance.

I see the other 2 .01 orange caps in the circuit that ARE on the scem, but this one isnt.....



I dont know if that cap is the issue, but i have absolutely NO voltage coming out of the power trans secondaries--(yes the 5a fuse is good)

[Enzo]

TR1? It is an inrush limiter - a thermistor. A CL-60 specifically. Fender uses them on a lot of amps.

The part between the blue wire and the power switch? It is an inrush limiter - a thermistor. A CL-60 specifically. Fender uses them on a lot of amps.

If you look at the schematic, your mystery cap is in the position of TR1. It is not a cap, it is just disc shaped. It is the thermistor. Without it, there is no mains circuit through the PT primary.

You can get a new one from PV or Mouser. For testing, you can short across TR1 and that will let you power up the amp to see if anything else is wrong.

[Valvehead]

ok, i learned something new today


i shorted the place were that thermistor was , still 0v coming out of the power trans. This is not good....


I looked at mouser, what ratings would i need ? the scem doesnt list.


this one?

http://www.thermometrics.com/assets/images/pto.pdf

[Enzo]

Nope, those are PTC - positive temperature coefficient - or something like that. Hotter they get, the higher the resistance.

You need a NTC - negative coefficient. It starts out at something like your 14 ohms, then as it warms up the temperature drops.

Mouser 527-CL60 is the part. In stock, $1.39. That part links to the data sheet you linked, but it is the wrong sheet. CL60 is right part, PTC data sheet wrong data sheet.

When teh amp is cold, the resistance of the thing limits the current through the circuit, but it warms up in a second and the resistance decreases substantially until it can be ignored electrically. It makes a soft start.

Wel, what burnt the part out in the first place? The thermal fuse in the PT could have blown for exzmple.

Here is my trick. Turn the power switch on, and measure resistance between the two main prongs of the power cord. You should be seeing the series resistance of the AC power cord itself, the power switch, the thermistor, the fuse and holder, and the PT primary. ANy of those open and there is no continuity - and no voltage out the PT.

If you want the bad news right away, measure resistance between blue and black wires of the PT primary. Open or not?

If open, you can dig inside it for the thermal fuse, and if that is open, it can be replaced, or at least shorted across for testing.

[Valvehead]

i pulled the trans apart--i dont see any thermal fuse.


from hot to neutral on the ac ---tests open


across the PT primaries (blue and black) tests open .

i think she's cooked

[Enzo]

OK, so it is the what have you got to lose stage.

Inside the transformer, the insulated exterior wires are soldered to the enamelled winding wires on some terminals. You can usually see the brownish winding wires disappearing into the guts. Look closely and see if any of the bare looking wires are not enamelled but instead bright shiny tinned color. If so, that is probably the thermal fuse.

The fuse itself wil be a small cylinder with one end pointed. Looks like a diode or something. it will be tucked in between the winding layers. It can be fished out and a new one shoved back in.

You may have to slice through the stiff cardboard like stuff to get at it.

I'd be surprised if one were not there, I don't think the rating agencies like UL will approve without one.

[Valvehead]

Is this the one?

http://img.photobucket.com/albums/v2...o/P2090005.jpg


if thats it, i cant even see it ! its way in there, ok to cut the plastic cardboard stuff but not the copper shield....yes?

[Enzo]

Any fuse is underneath what you see in the photo.

Look along the left side, see the blue and orange insulated wires, and above them you can see the enamelled wires all hiding under the white paper. You will have to cut or unsolder the copper strap, and cut through the paper to get in there to see if there is a fuse. The fuse would be tucked inside between windings, so that is why I look for its shiny tinned leads.

[Valvehead]

It was way down in there, but i found it. That was the problem. Amp is great now !

thanks again

[Enzo]

Cool. Did you actually replace it or just short across the open fuse? What temp was it?

[Valvehead]

i just shorted it with a large wire, about an inch long.

didnt look at the temp