Blues Jr intermittent power

[Jake152436]

Hey,
First time posting here!

I have a friends blues junior that I told him I would troubleshoot before taking it to a tech. I have some experience working/building amps, but don’t have any scope or anything like that.

90% of the time the amp works fine, but occasionally when you power the amp the pilot light will flicker and go out. There is no power to the tubes when this happens. You can flip it on and off for a while with no luck, and eventually the power will come back.

I have checked the power switch and it’s obviously not the fuses or tubes. What else could be going on here? Could it be a bad ground somewhere?
Thanks!

[Leo_Gnardo]

I'd check the AC cable. Could be the molded plug or the cable near it going bad intermittently. Less likely, the cable closer to the amp. Those parts of the cable take the most flexing and occasionally break down.

[Jake152436]

I'd check the AC cable. Could be the molded plug or the cable near it going bad intermittently. Less likely, the cable closer to the amp. Those parts of the cable take the most flexing and occasionally break down.

Good thought!! Unfortunately when the amp is not lighting up there is still power at the switch on the amp. It is also passes power fine. I think it’s something further down the line...

[Jon Snell]

Blues Juniors have lots of spade terminations. Check the wire ends that are crimped into the spades and the spades that are soldered to the pc board. Check for bad connections and measure the resistance with the amplifier unplugged between the transformer primary tappings.
Work your way back towards the IEC socket looking for a break in the circuit.

[Jake152436]

Blues Juniors have lots of spade terminations. Check the wire ends that are crimped into the spades and the spades that are soldered to the pc board. Check for bad connections and measure the resistance with the amplifier unplugged between the transformer primary tappings.
Work your way back towards the IEC socket looking for a break in the circuit.

I can check all the spade connections easy enough, although it’s tricky because I rarely can get the amp to act up.

is there anything specific I am looking for on the transformer primary taps?

also, excuse the potentially ignorant question, but if the pilot light isn’t coming on, would that indicate that the issue is somewhere prior to the light in the circuit? Ie transformer

[Jon Snell]

Indeed, there is missing power to the primary of the transformer. If I knew the model of your Blues Junior, Mk1, Mk2 or Mk3 (Mk4 is too new to be affected), I would be able to tell you the wire colours but here is a diagramme from the Mk3.

Trace the wires and test for continuity, if all OK then follow the cirsuit and wires back towards the IEC socket until you find the break.

[Jake152436]

Indeed, there is missing power to the primary of the transformer. If I knew the model of your Blues Junior, Mk1, Mk2 or Mk3 (Mk4 is too new to be affected), I would be able to tell you the wire colours but here is a diagramme from the Mk3.

Trace the wires and test for continuity, if all OK then follow the cirsuit and wires back towards the IEC socket until you find the break.

It is a blues junior mk3.


when you say measure between the transformer taps, do you mean between red and green, green and brown etc?

A fellow on another form thinks it could be the internal thermal cutout in the PT malfunctioning.

his advice was:

‘the way I'd set up to test this is to connect an AC voltmeter to monitor for the presence of 120V at the black and white leads going to the power transformer primary (the white coming from the power switch, and the black coming from the fuse). I'd probably also monitor one of the secondary windings (not the 6.3VAC one, you already have a monitor on that in the form of the power LED). Say the two brown leads for the low voltage supply, at P13 and P14 -- should be around 21 VAC there during normal operation.

Then if the power light goes out, and the P13/P14 21VAC goes away, and the voltmeter on the black and white primary leads still shows 120V, then you can be sure it's the internal cutout acting up.’

what are your thoughts?

[Jon Snell]

OK, lets assume you are 110v
Have you mains between CP5/6 & CP10. If no have you mains between CP4 & CP9.
If you have mains between Black and White on the transformer, the transformer is faulty.

[g1]

Not sure why you would be looking at CP13 or LV supply winding. The heaters and pilot LED run off the heater winding.
That is what you need to check, whether there is 6.3VAC between the PT green wires when the light shuts off.