Blues Junior Rev. D Blowing fuse

[wsscott]

A buddy has this amp and it's blowing the 2A fuse as soon as its turned on.  Visual inspection doesn't show any obvious faults.

I have a Blue ESR meter and used it to check the electrolytics.  The only cap that tested "bad", ie. reading 20, is C4, 22uf/25V as shown on the attached schematic.

Is it possible this could cause the fuse to blow?  It's coming off the V1/12AX7's cathode.

I also checked the diodes in circuit.  They all read .5 and OL except for D11 and D12 which both read .98 and OL.  Those numbers seem high, but maybe it's because they are in circuit.

Thanks for your help.

[mresistor]

Check the tubes,.,  sub good ones in for each 

[shooter]

that PCB is great for loosing the bias voltage to the PA tubes, which leads to blown fuses, many hours of bad connections, solder traces and just loads of fun 

[mresistor]

pull the power tubes and check if you have -DC voltage (bias voltage) on TP24   also on pin 2 the grids at the power tube sockets    what does it measure?

[wsscott]

Thanks for the leads.  First, I've got to find a fuse!

[wsscott]

I thought I would post the attached photo of the B+ cap setup.  The schematic shows it as a 47uf/450VDC.  The photo shows the original, I assume, gray IC cap that's 47uf/450, and wired in parallel with it is another 47uf/450VDC cap.

Am I correct that if both caps are good and reflect their full value of 47uf each, then this would be equivalent to a 94uf cap at 450VDC.  Would being twice the size be a problem?

But if the original cap is good, why add another cap of equivalent value to begin with?

However, what if the original cap was bad?  Could using this method to add a new cap create a short that would be the cause of the blown fuses?

[shooter]

no photo
since it's a diode bridge size doesn't matter, typically done to "stiffen" a PS
anyone who tacked in a cap for testing and left it, should "know better" than to leave a suspected bad cap in circuit.


with all the tubes out does the fuse blow?

[wsscott]

I'll try the photo again.

Shooter, I don't have a good fuse yet, so I can't test it.

[mresistor]

I think I might unsolder the blue caps positive lead from the grey IC posaitive lead and check their capacitance again.

[shooter]

when you order fuses order 10EA, no self-respecting troubleshooter would not have a fuse 
any fuse < 2.5A with all tubes out will give you the quick answer.


want the real quick answer, install a steel fuse n follow smoke back to origin 

[tubeswell]

... want the real quick answer, install a steel fuse n follow smoke back to origin

Quote of the day! 

[Willabe]

... want the real quick answer, install a steel fuse n follow smoke back to origin

Quote of the day! 

Oh yeah!     

[shooter]

was given a steel fuse for graduation from my final Radar school by Master Chief Jahn, His words; "use sparingly"
got used about 4 time's in my 35year career

[wsscott]

Shooter's Reply #6 is correct about the extra cap.  It's for power stiffening.  A company called Formal Electronics makes a mod kit for this amp which includes a 100uf cap for "power stiffening", and also a bias board that replaces the bias resistor with a pot.  Looks pretty neat.

Looks like whoever worked on this amp before, removed those resistors (R51 and R52) and put in a trim pot that goes under the PCB.  Don't know what's under there

I don't see any TP's for test points on this PCB that the layout and mResistor refer to.  Where are they?

[shooter]

they're everywhere, look at the "bubbles n squares"
pic for example only

Don't know what's under there

monsters, just like under your bed as a kid

[mresistor]

The test points aren't marked as such on the board you have to find the components yourself on the board.


this is from the correct schematic

[wsscott]

Yea, I figured that out.  My only experience with them is where the component is mounted on the PCB so close that you can't touch the leads, and so they have separate test point pins to connect to.

[acheld]

Shooter's on target.   Look at the "daughter" board (for the tubes) and the associated ribbon cables -- traces are often burned there.

Alll bets are off, though, until we get a better understanding of what mods were performed.  Pictures of the boards, front and back, would be interesting.

[wsscott]

Ok, so I've installed a new fuse, and have it connected to my Variac and my Dim Bulb Limiter.

With all tubes removed, no glow and no blown fuse. 

With all tubes installed, at around 100VAC the bulb glows and popping sound, and I turn it off.  No blown fuse.

If I remove V5 tube, same effect.

If I remove V4 tube, but reinstall V5, then no glow, no pops, and no blown fuse and holds at 120VAC.

Seems like something going on with V4 part of the circuit.

I've attached a photo of the ribbon cable going to V4 and V5.  I took it even before I saw the suggestion.  And you can see the scorched areas on the far left side of the cable.  That is connecting to Pin 7 of V5, which is the plate, of the Groove Tube EL84.

So is it the tube or something else in the circuit?  I had thought the problem was in V5 because of the scorched cable, but it seems like it's in V4.

Thoughts?

[wsscott]

The R3000 diodes, D11 and D12, that connect to the plates of the Output Tubes, both read .981V in circuit with my meter set to diode mode.

Is this correct voltage?  The other diodes in the circuit, which are 1N4006 or 1N4003, read .51V.

[shooter]

that scorch, looks like someone with NO soldering skills was poking around with a hot pencil


time to do the hard work, pull the boards, look for burned traces, bad solder joints, and ya, a spare set of PA tubes, but......
if this is a "keeper" then time n materials says you're far better off starting over with one of Doug's boards


the reason, you fix this problem n 1 day or 2 weeks later something else gives, then something else then....I quit servicing them way back


my line every since;
"can't you just look please, nope, it's a throw-away amp...I can gut it and build with quality otherwise, buy another amp"

[wsscott]

Shooter-that's a good point.  I've heard these amps are a pain to service, and that pulling the main board is very time consuming.  Maybe it's just a bad tube, although it looks okay.  Of course I don't have access to any EL84's.

[wsscott]

I may have found the cause.

Here's a photo of V4's solder connections.  As you can see it appears that one pin in the socket, Pin 2, has no solder.  I think that's the Control Grid of the EL84.

Obviously that needs to be soldered.  But why would that cause the fuse to blow?

[wsscott]

I think I know the answer to my own question.

It's because that's where the Negative Bias feed point is located.  TP33 connects to TP24 which feeds the Control Grid of V4.  And since there's a bad solder joint at that point, it loses bias.

Is that it?  Or sorta?

[stratomaster]

Bingo

[shooter]

 
that's problem 1
problem 2 will probably be a fried tube since, well, bias is what keeps it in check
problem 3 to infinity have yet to rear their ugly heads 

[wsscott]

Well that wasn't the problem.  I thought I had it fixed, and I checked the negative grid bias reading without either of the output tubes installed and it reads -11.7VDC.  I put in the output tubes and the same problem on V4.

So I've got negative bias, but still an issue.  I don't have access to a tube tester for a few days, and don't have another tube to try.  But, could it just be a bad tube?

[Willabe]

But, could it just be a bad tube?

Sure it could be just a bad tube.

Have you checked for screen dcv? Tube could have taken out a screen grid R, shorted through. Then cooked the tubes screen grid.

Measure the screen grid R's for ohms and measure the screen grid dcv at the tubes socket.

[wsscott]

The screen resistors are R34 and R35 at 100 ohms each.  They both read 97.8 ohms.  So they're good.

I can't read the screen voltage at pin 9 since I don't have the Output Tubes installed due the problem.

The Output Voltage on the OT is reading 350VDC at both terminals.  And each terminal reads 98 ohms between Red and Brown and also between Red and Blue.  So I think the OT is fine.

[wsscott]

Its the tube.

I took one of the EL84's and put it into V5 (no tube is in V4, the suspect socket), and it acted badly as described.
I removed it and put in the other EL84.  NO problem.

Then I put that tube in V4 and no problem.
Finally I put the first tube in V4 and PROBLEM.

So the problem goes with the tube.  Sorry to have wasted everyone's time.  I should have thought of swapping the tubes earlier.  The "bad" solder joint was a red herring for me.

When I get a new tube I'll check the bias.  If its like other Blues Junior amps is probably too hot.  Maybe that's what killed the tube.

[stratomaster]

The "bad" solder joint was a red herring for me.

No. That joint absolutely was a problem.

Sometimes you have compounding problems and repairing one allows you to find the others.

[wsscott]

I was wondering if there is a way I could run a 1 ohm resistor from each of the plates to a ground to use as a test point to read the bias current.

From the circuit and layout it looks like a connection somewhere between the plate pin 7 and the input of the D11/D12 diodes would be one place to connect.

Any ideas?

[shooter]

bias is a VOLTAGE without current. (simplified)


put the 1 ohm between cathode and ground, that'll tell you TUBE current, which is directly related to BIAS voltage

[wsscott]

Yes, I should have said cathode, pin 3.

Would soldering one end of the resistor to the pin 3 solder point on the socket, and the other end to a lug sitting under one of the lock nuts on the tube socket board be a "good" option?

[stratomaster]

More trouble than it's worth. Just brush up on the output transformer resistance/voltage drop method. Safer than the shunt, and no shoehorning resistors on PCBs.

[tubeswell]

More trouble than it's worth.

Cheap stuff designed to just outlast the 12-month factory warranty period.


Not worth 'fixing' IMO - as soon as the fixed amp is returned to the customer, something else goes wrong with it and they bring it back and say 'I paid you to fix this and its not fixed etc'. More trouble than its worth

[shooter]

echo 

....I quit servicing them way backmy line every since;"can't you just look please, nope, it's a throw-away amp...I can gut it and build with quality otherwise, buy another amp"

wherever you decide to put the 1-ohms, make sure they are easy to attach gator-clips, you want one hand on the power switch, one adjusting bias so holding meter leads with your teeth might be an OSHA violation 

[Willabe]

wherever you decide to put the 1-ohms, make sure they are easy to attach gator-clips, you want one hand on the power switch, one adjusting bias so holding meter leads with your teeth might be an OSHA violation 

                  

[wsscott]

I think I'll drop that idea.

I guess I can get Plate voltage for the tubes at the OT's leads to each plate on one end, and connect to a chassis ground on the other.

I can get the Negative Bias voltage reading at C- where the trim pot has been installed in place of the R31 and R32 220K resistors.  And I can adjust the bias at that point on the trim pot.

I guess I need to get the Cathode voltage so I can use it in calculating the current.  Looks like the only way to get it though is by reading the voltage directly from the tube socket, pin 3.  I don't see any other way to read it.

[shooter]

fwiw
I've worked on a few "vintage money amps" that the owner does not want modified, so I install the 1-ohms to set tube current/bias voltage then remove before it goes out the door.


my contention, any well designed system should have a way to measure n monitor ANY "critical function" so it doesn't become critical.

[wsscott]

Yes.  Agree.  This is not a money amp and the owner has had it modified, so that wouldn't be an issue.  Its just finding a way to connect the resistor to the cathode pin is the problem, since its not directly accessible like on a standard tube socket.

[stratomaster]

I can get the Negative Bias voltage reading at C- where the trim pot has been installed in place of the R31 and R32 220K resistors.  And I can adjust the bias at that point on the trim pot.

You should not have a trim pot in place of those resistors. Those are the bias leak resistors and need to be a rock solid connection. To adjust the bias you need to adjust the voltage divider formed by R51 and R52.  I'm somewhat worried that you surmised or were advised to make the grid leaks adjustable as this indicates a lack of familiarity with amps and could indicate more harm than good can come from continuing to work on this amp.

I guess I need to get the Cathode voltage so I can use it in calculating the current.  Looks like the only way to get it though is by reading the voltage directly from the tube socket, pin 3.  I don't see any other way to read it.

I told you above how to do this.

Just brush up on the output transformer resistance/voltage drop method.

Take the tubes out and adjust your bias pot to give you the most negative voltage at the tube grid. Put the tubes in and let the amp warm up for a few mins.

Turn the amp off, unplug, and drain the filter caps. (I use a set of alligator clips with a 220k - 3W resistor spliced in across the reservoir cap--not the fastest drain, but also won't harm anything if I accidentally leave it connected and power up). 

Confirm voltage to ground at the output transformer center tap is near 0. If so, proceed to measure the resistance between the center tap and the two primaries.  Red to blue then red to brown. Record these resistances.

Remove your drain resistor and power the amp up.  Switch meter back to voltage and measure voltage to ground at the center tap, then blue and brown primaries.

The difference between the voltage at the center tap and each primary divided by the resistance between center tap and each primary will give you plate current.

Multiply this by plate voltage and you now have plate dissipation.  You can now adjust your bias pot accordingly. 

If you follow these directions you'll be able to bias your amp (or any adjustable fixed bias amp) without special probes or added resistors, and you'll never have to measure across two high voltage points or worry about your meter shorting HT to anywhere it shouldn't be. On the Blues Jr you can take all these measurements at the quick disconnects on the main board.

[wsscott]

Someone else did all the mods including the trim pot.  I’ve changed nothing and don’t intend to, other than the tube and bias if needed.

[wsscott]

Sorry it was R51 and R52 that were removed by someone and replaced with the trim pot.  R31 and R32, 200K each, have not been removed.

[acheld]

Two suggestions:

1)  You are most likely going to have other issues with this amp.   The boards are not high quality, and BJ's just don't stand up to the test of time -- though with quality work on the bias circuit you can improve the run time.   So, my personal bias is the go with Doug's conversion, which is a solid and enjoyable take on the BJ.   I've built several, and have had no problems with them.  I've owned 5 stock  BJ's over the years and had problems with every one of them.   

But I know it's a lot of work.  So,

2) Doug also makes a BJ Tube Board.  As you've already experienced, the stock BJ tube board is prone to failure due to heat and mechanical stress.  One option would be to replace that board only, and hope for the best.   A lot less work (though not without some pain points!) and at least you won't have to worry about that.

[wsscott]

Thanks for the lead with Hoffman boards.  I'll see if the owner wants to pursue it.  I don't know how long he's had the amp or if he's had problems before, or if the amp had been modded by a prior owner.

[wsscott]

I had access to an EICO tube tester and tested the EL84's.  One was really BAD, and the other was right on the line.  I got a new pair of EL84's and installed them.  Now when I turn it on the Light Bulb Limiter doesn't glow and when it's removed, I can turn on the amp without blowing the fuse.  So the bad tube must have been the cause of that.

However, although the amp was fine for awhile, it started making loud crackling and popping noises.  I have all the pots turned fully CCW to off, and the noise still is there.  How can that be?  Sometimes if it's running quiet and I tap the tubes with a probe, it fires up with the cracks and pops. But only if either of the EL84's is stimulated.  The other 3 tubes are quiet.  The tube sockets for them are really snug, but the sockets for the EL84's are easy to remove the tube.

I have set the negative bias to -11.6VDC.

So I then tested the new tubes on the tube tester, and they read GOOD.  So the tubes are okay.

I've checked all the resistors and they read good.

I'm guessing it's either the EL84 tube sockets or the Power Board for the tubes.  Maybe that bad tube did something to those sockets or the traces on the back side of the board--which is not visible.

Any ideas left to try.  I really don't want to take out that board, especially if everyone says it's not worth fixing.

Thanks.

[shooter]

Any ideas left to try.

try'd 'em all, gave up 


every solder pad, ribbon-cables, connectors, sockets....etal


when hunting intermittence, you have to be "all hooked up" monitoring key-points in order to get a clear-shot at them, they are quick, destructive, and intent on ruin'n any sane persons day
happy hunting....

[stratomaster]

it started making loud crackling and popping noises.  I have all the pots turned fully CCW to off, and the noise still is there.  How can that be?  Sometimes if it's running quiet and I tap the tubes with a probe, it fires up with the cracks and pops. But only if either of the EL84's is stimulated.  The other 3 tubes are quiet. 

Sounds like your new tubes could be bad. Take a look at the schematic and you'll see the master volume control is a pre-phase inverter master. Turning it down shunts the signal from the preamp at the PI input. If you're still getting noise then it must be originating in the PI or after.

You can be strategic and put a jumper across the grids of the EL84 to eliminate the PI.

Unlike others here, I don't think the problems with the stock PCBs are insurmountable, and with careful/solid work you can coax those amps into serving admirably. That having been said, there is a possibility the ribbon cables will need attention--especially if there's evidence of prior work of low quality. And if your tube socket board is pretty well toasted, the replacements available here and elsewhere are worthwhile upgrades.

[wsscott]

Thanks for the input. 

As I said, I tested the new tubes with a EICO tube tester and they tested good.  So I don't think it's the tubes.  I'll try to find another amp that uses EL84's and see if they work ok in that amp.

All of my voltages, resistances, and OT resistance and output voltage to plates is fine.

So, I'll let it stew for a bit before deciding whether to dig a deeper hole, or fill it in.