Blues Jr III no sound

[dpm309]

Working an a Blues Jr III with no sound. Tubes are not glowing and there is no power from the secondaries for the heaters. Getting proper voltages from the other secondaries. I am guessing that the PT is bad but I have never seen one with the heater secondaries not working. Any other solutions other than replacing the PT?

[tdvt]

Pretty sure there is a heater fuse in those.
Good?

[Dave]

https://schematicheaven.net/fenderamps/blues_jr.pdf


If this schematic is correct, I dont see anything that would go bad and prevent you from reading voltage at the transformer filament secondaries. TDVT said maybe a fuse. I don't see one on the schematic, but its the only thing I can think of other than a bad PT.


Dace

[tdvt]

There are lots of versions.

I have a III (not sure of version) but this schematic that is correct for mine.

[PRR]

Bad Joints!! The bane of post-1959 construction.

There's at least six things on the Dave-cited plan which could be joints so could be bad. The other plan is different, joy joy.

If you have really ripped-back to the transformer leads and no volts, and infinite ohms (power off), other windings OK, then it may really be bad. 

[dpm309]

There is a fuse (F2) for the heater secondary and it is not blown. I noticed some slight discoloration under the top prong of the fuse but am getting continuity from there to R54 and R55.

[dpm309]

Here is the schematic I am working off of.

[Dave]

Find the ends of the two secondary wires and test them for voltage. If there is nothing on the ends of the wires that come directly out of the transformer, then the transformer is bad. If you get voltage, then the next thing I would check would be that that fuse holder that has the burns on it is making a good connection to the board underneath.


Dave

[dpm309]

Dave, I pulled the two secondary leads off the board and measured them - no power. The fuse holder seems to be making a good connection. Need to find a new PT.

[jjasilli]

1.  The Blues Jr. & HRD is why I stopped working on other people's amps!
2.  IMHO the only solution is Doug's turret board for the Blues Jr.

[PRR]

....Need to find a new PT.

Or a 6VAC 3A heater transformer.

[dpm309]

OK, I made a stupid mistake and was measuring DC instead of AC voltages for the heater secondary. Turns out it was the fuse. I checked continuity between the fuse holder prongs with the bad fuse installed and it showed no short. I pulled the fuse and tested the continuity and it showed it was blown. Could not tell by looking at the bad fuse. Installed a new fuse and everything is working. Will bench test it for awhile. Still concerned about the discoloration on the PCB by the prong for the fuse (see picture). This heater fuse was added to the later versions of the BJ III so I am wondering if there were issues with the heater circuit prompting Fender to add the fuse in the later versions.

[sluckey]

Still concerned about the discoloration on the PCB by the prong for the fuse (see picture).

That is a sign of heat caused by a poor connection in a high current circuit. Most like a loose connection between the fuse and the holder and/or poor solder joint. I would check both.

[dpm309]

Thanks Sluckey, I'll definitely check that. Is the heater circuit considered high current?

[WimWalther]

Thanks Sluckey, I'll definitely check that. Is the heater circuit considered high current?

Chiming in..

When discussing PCB electronics, anything over a couple hundred mA is "high current".. and 3A definitely so. PCBs are really best at signal-level stuff, which runs in the few or tens of mA.

You start pushing a board to handle multiple amps and its just really easy for things to break down over time.

For example, if one end of that fuseholder develops even 1/10 ohm of resistance, it will drop 300mV. 3A @ 300mV is almost a full Watt of heat being generated right in the fuseholder terminal.

This will run hot enough to speed further breakdown and even more heat.. and by the time it finally fails its completely cooked.

ETA: And it's not just the heat, it's the heating & cooling (thermal cycling) that creates problems. Repeated cycles can crack solder joints, again leading to a snowballing effect that results in total breakdown of the board joint.