Blues Junior Rev. D Blowing fuse

[wsscott]

Stratocaster-to take out the PI, would I jumper the grids across R22 and R23 on the side where they connect to the grids of V3?

You can't do anything with the cables or the tube sockets to jumper the grids.

[stratomaster]

The grids of the power tubes need to be jumped, not anything at the input to the PI. You're already muting that by turning down the master volume.

Take an alligator cable and jump across the grid stoppers. Looks like R33 and R34, but hard to read the numbers on the schematic you posted. Basically what the jumper does is mimic a post phase inverter cross-line master volume control turned all the way down.  If there are still noises and pops then the problem is in the circuitry supporting the power tubes.  Could be the tubes themselves or ribbon cables, PCB issues, etc. If the jumper silences the noise then the problem is in the phase inverter.

[wsscott]

Sorry, your sentence was clear, I just had a brain fart.

[wsscott]

Ok.  I just jumpered the grids and the noise is still there.

So we know the problem lies between the OT's and the Speaker.

[wsscott]

I took the EL84's out of their sockets, and thought I might test the pin sockets to see if there is continuity between the pins.

Pin 3 is the cathode, and pins 4 & 5 are the heaters.

There is continuity between all 3 of those pin sockets, ie. the cathode and heaters of each tube.

There is also continuity between the 2 tubes through those pin sockets.

The 6.3 VAC filament wiring goes from the Main Board to the Tube Board, and connects to it through R54 and R55.  They are identified in the schematic as Fp47 1/2 watt.  What does Fp47 stand for?

When I check the resistance of each, they each read 23 ohms.

Also, when I check their continuity from pole to pole they both show continuity.  The meter beeps.  Is that correct? Or should it show OL?

Now I've just found that R36 shows external damage to the case.  Its companion R35 doesn't show any damage.
They are identified in the schematic as Fp100 1/2 watt. Here's that "Fp" again. These are the screen grid resistors.
They are each 100 ohm, 1 or 2 watt resistors, and they actually read 97.3 ohms.
When I check the continuity for each resistor from pole to pole, they each show 97.7 ohms.  Should they each read OL when the meter is in continuity mode?

I know these are a lot of questions, but seems suspicious to me.  Am I barking up the wrong tree?

Thanks.

[tubeswell]

FP = flame proof. It means the resistor is designed to fail without causing a fire. It may look burnt but it shouldn’t ignite (unless someone already replaced the FP with an ordinary resistor).

[wsscott]

It doesn't look burnt, just some of the outside coating is cracked and broken off.

[stratomaster]

I took the EL84's out of their sockets, and thought I might test the pin sockets to see if there is continuity between the pins.

Pin 3 is the cathode, and pins 4 & 5 are the heaters.

There is continuity between all 3 of those pin sockets, ie. the cathode and heaters of each tube.

There is also continuity between the 2 tubes through those pin sockets.

The 6.3 VAC filament wiring goes from the Main Board to the Tube Board, and connects to it through R54 and R55.  They are identified in the schematic as Fp47 1/2 watt.  What does Fp47 stand for?

When I check the resistance of each, they each read 23 ohms.

Also, when I check their continuity from pole to pole they both show continuity.  The meter beeps.  Is that correct? Or should it show OL?

Now I've just found that R36 shows external damage to the case.  Its companion R35 doesn't show any damage.
They are identified in the schematic as Fp100 1/2 watt. Here's that "Fp" again. These are the screen grid resistors.
They are each 100 ohm, 1 or 2 watt resistors, and they actually read 97.3 ohms.
When I check the continuity for each resistor from pole to pole, they each show 97.7 ohms.  Should they each read OL when the meter is in continuity mode?

I know these are a lot of questions, but seems suspicious to me.  Am I barking up the wrong tree?

Thanks.

Checking continuity is likely a red herring unless you have a deeper understanding of the underlying circuit and how exactly your meter tests.

Cathode is at ground.  Each leg of the filament is 47 ohms from ground through the artificial center tap.  Is 47 ohms low enough for your meter to consider it continuous?  Read the specs and manual for the meter to find out.  When the amp is running current flows much differently than when you're doing in-circuit continuity testing. The 23 ohm reading is a testament to this as your meter placed across each resistor results in a circuit that the meter views as the resistors in parallel. 

Measure the resistance at the pins you're concerned about and cross reference the schematic. Draw little current paths if you need to for clarity on what you're actually measuring.

Also, now that you've shorted the grids to each other and eliminated the PI as a noise source, it's time to take a serious look at the tubes, the reservoir and screen nodes, screen resistors, EL84 sockets, PCB (pads), and the ribbon cable to the EL84 sockets.

[wsscott]

PROBLEM SOLVED!

I kept getting all this crackling and popping sounds, and with the amp off if I wiggled the tube the socket was wobbly.  This was true for both V4 and V5.  Upon closer inspection I found that when I did that, the solder connections on the pins moved around too.  So the solder joints to the tube sockets and their traces were bad.

I re-soldered all the pin connections, and V4 soldered up okay.  On V5 pins 7 and 9, cathode and screen wouldn't accept solder.  I found that the trace was broken at their pins.  The ribbon cable connection to the components was fine, but those pins didn't connect up to the ribbon cable.

So I decided to run jumper wires directly from the pin sockets to the respective components on the main board.  Attached is a photo showing the Yellow jumper wires I installed.  Not pretty but it works.

So once again, it was something simple but hard to see.

Thanks for all your help. Best.

[tubeswell]

Excellent, know you know what it is, sounds like you could use one of Doug's BJ socket board upgrades for improved reliability

[wsscott]

That’s what I’ve told the owner. Hope he decides to do it.