Soft Start Board Goof

[InfernoSTi]

On the second mono block, I made a big goof.  Reminds me to be patient, right?

I was measuring B6 and it was spot on (298 v DC) when my hand slipped a bit and I shorted it to B7.  Of course, I heard a loud POP and then my voltage when to zero. 

I'm going to have to figure out what I damaged/destroyed.  I feel pretty dumb...I had just looked up to take the reading when my hand slipped just enough to short the two. 

Any suggestions on putting my "parts list" together would be appreciated.  I'll be good at the soft start boards by the time I'm done with this project!

Best,
John

 

[Paul Joppa]

Pretty sure the 431 (B side) is toast. It probably did the fuse thing and protected the 5670. The 220 stopper at B7 is probably a goner as well. Don't know about the "A" side MJE5731A, it may have seen an excess voltage.

When you say "voltage went to zero" - give us a hint, what voltage? Is the whole power supply dead? Or just B6? Any LEDs glowing? Any tube filaments glowing?

 

[InfernoSTi]

Hey Paul,

Thanks so much for the quick reply.  

The filaments on the 5670 are lit.

None of the LEDs are glowing now.

However, when I plugged it back in, after about 30 to 45 seconds, as second louder pop (with blue flash) happened and my MJE gave up the ghost (it must not have been the first pop).  Very dramatic the way it has taken itself apart:

I guess I'm a bit nervous about taking voltage at this point...hmmm, I built it right, I'm sure but once I shorted it, things are going south fast.  If there are specific reading that I need to take, let me know.  

Best,
John

 

[Grainger49]

John,

You might want to go for a new board.  There is no telling what was molested on the old board.

 

[InfernoSTi]

Thanks Grainger.  I have sent a PM to Doc and Queenbottlehead....

Best,
John

 

[Paul Joppa]

On second thought, I agree with Grainger. Get a replacement board and all the components, then you know there are no damaged parts.

 

[InfernoSTi]

As ever, team Bottlehead has come through...the new board and parts arrived today (and I've got the old one out and everything ready to go).  What great customer service! 

I just need to get a free evening to concentrate on this last step. I feel pretty good because the previous build had me spot on the first reading (and the same with the mono block I didn't blunder up).  I will be taking my time on this one, though.

Best,
John

 

[InfernoSTi]

Well, I guess when one thing goes wrong, chances are something else is off, too.  That happens to be the case with this project. 

I did get the replacement board built and installed.  All heaters light up and all four LEDs light up and my B6 was 301v (expected 300v).  That was the good news. 

The next reading was A2 which was very low: 4v and falling (meaning it just kept dropping on the meter).  Expected was 465v (that is what I had on the first build).  I think it must have been residual charge in the capacitor, perhaps?  It behaved that way, anyway.

Then I checked T16 to make sure I had power going in and it was 563v (expected was 464v).

At that point I turned things off and decided to wait until I had a little more time to take a look at things.  I could be missing a simple connection.

So what would cause me to be too high on the input to the soft start board and too low on the 2A3 side of things?  I will do a complete list of the resistance and voltage readings but any early hints of where to look would be great!

I'm thinking I could also just set up a "mono" system with my one good unit, right?

Best,
John

 

[Grainger49]

John,

When a tube is not conducting everything from the plate back in the power supply floats high (because the tube is not conducting).  Try swapping tubes with the good side.  

B6 says the voltage regulator and the 10mA CCS are working.  

A2 is the plate of the 2A3/300B and it is fed through the plate choke.  4V is awfully low for the plate.  What is the voltage on the other side of the plate choke?  That 400+V has to be somewhere and disappearing before the plate.

 

[4krow]

I have an alarm clock that talks. People laugh, but I don't squint at night. Now you might know what I am thinking here,  a meter that talks when asked to. "99.9 Volts sir" (I added the sir part)

 

[InfernoSTi]

Grainer,

The tubes are fine...did the double swap (both the 5670 and 2A3) to make sure.  I haven't put the tubes back in because I wanted to check the resistance figures again.  So perhaps I have found the start of the solution: since the previous situation, the resistance has changed on the following points:

Expected/Actual

T9: 124K/280K
T19 128K/280K
B4 130K/280K
B6 177K/280K

I'm haunted by 280K!   Hmmm.....

John

P.S. Hey Greg, a talking meter would have (most likely) saved my bacon!

 

[Grainger49]

John,

Ok, so not a tube problem.  Just checking.  Moving on to resistance, no tubes installed...

T9: 124K/280K
T9 is connected to the grid of the output tube (is it 2A3 or 300B?).  The grid will not read to ground.  The resistor in series to the grid will not come into play here.  So... looking back into the plate of the driver (looking at the schematic) there is a 300k ohm resistor, RLd, to ground.  That is awfully close to 280, I think that is good.  The manual must be wrong.  Or I'm missing something.

T19 128K/280K
T9 and T19 are connected by the red of the red/black twisted wires.  You should be ok there.

B4 130K/280K
B4 is the plate of the driver, attached to T9 and T19, well, T19 first.  So the same thing applies.

B6 177K/280K
HOLY COW ! ! !  I can't tell what resistance you should see there.  B6 is the regulated voltage on that 5670.  So since you get 1V off of desired at this point I wouldn't worry.  

The problem is the 4V and dropping at A2.  That is just not right.  So go back and see what voltage you get at the top of the plate choke, that is the end that is not attached to tube pin A2.

Like I said above, "That 400+V has to be somewhere and disappearing before the plate."

I have an alarm clock that talks. People laugh, but I don't squint at night. Now you might know what I am thinking here,   a meter that talks when asked to. "99.9 Volts sir" (I added the sir part)

I think that is Jim's meter.  I always miss that my meter says 0.703 K ohms, so 703 ohms.

 

[InfernoSTi]

Grainger,

This is built as the 2A3 version.

The A2 (edit: is the plate) and A3 (edit: is the grid)...A3 is 21.5v.

I traced this value back to the 300K resistor on the soft start board. The 300K resistor is attached at OB and reads 21.6v on the OB side of the resistor.  

I compared to the "good" amp and that reads 150.0v on the OB side of the 300K resistor.  So I would guess the issue is on the soft start board?  

Thoughts?

John

 

[Grainger49]

John,

A2 is the plate, A3 is the grid.  In the 2A3 version of the Paramount the A3 voltage is equal to the B4 voltage, they are connected.

In Paramount V 1.1 A3, the grid, A3, should be A2/3.  This voltage is adjusted by using the bias adjust  on the new driver boards.  Since this is a new build I expect this is the case.  It is a 10 turn pot so don't be too gentle.

IIRC, when you adjust the grid, the plate changes a bit.  So check back and fourth several times.

This might just get you there.

 

[InfernoSTi]

Thanks again, Grainger.  I can adjust the voltage at OB down (the wrong direction!) or up but it stalls at 21.98v so that isn't the solution yet.  I feel like it is something simple.  Is there a transistor or other semiconductor that I might have toasted that would cause this?  All four LEDs light (two..on the A side...are very bright, two...on the B side...are very dim but lit).

John

 

[Grainger49]

There is a regulator, three pin semiconductor regulator, that is in the driver tube cathode circuit.  Look at the schematic for the driver board.  This could be a problem but someone more familiar with this circuit should point you to a possible problem in the cathode area.  I'm pretty sure it is black magic!

Try this, remove the 2A3 and see if it still stops at 21.8V.  I'm just trying to cut the circuit in half to see which half it affecting your voltage. I doubt this will do anything but it is worth a minute to check.

 

[InfernoSTi]

Grainger,

Without the 2A3 tube, the OB is 251v (at the 300K resistor on the OB side). 

I also took the effort to change out the parafeed cap (3.3uf) and that didn't make a difference.  Just wanted to make sure it wasn't the cause since things are right around that point. 

John

 

[Grainger49]

John, 

We are chasing two things.  The OB voltage should be the 2A3 voltage divided by three.  There should be ~460V on the 2A3 plate so 153V would be the target.  251V available says it should be tunable.

Did you check the voltage on the power supply side of the 2A3 plate choke?  I keep asking because the plate choke might be a problem.

 

[InfernoSTi]

Oh, THAT voltage!  

The power supply side of the PC-1 is 564v, the same as T16.  The T1 side of PC-2 is a falling voltage that started about 9v and fell to 7v when I stopped measuring...it was continuously falling.  This was WITH the 2A3 installed.

John

EDIT: And without the 2A3 in, I get the same power supply side voltage on the PC-2 and I tried adjusting the OB voltage and was able to reduce it to the correct 150v.  

I also checked the "in" and "out" sides of the PC-2 on the good amp:  494v at the power supply side and 476v at the parafeed cap side (measurements taken on the PC-2 terminals themselves.

And I swapped 2A3 tubes again (this time listening to the one I swapped out and it is fine...it plays nicely on my other amp so I didn't inadvertantly swap a bad tube for a bad tube).

 

[Grainger49]

For some reason you are not getting the voltage on A2 that should see.  If the tube just wasn't conducting the high voltage would all be at the plate, A2. 

Right now the only thing I can think of is a bad plate choke or a bad connection.  You have the voltage coming out of the power supply but it doesn't get to the plate of the 2A3.  The only thing between there are two solder joints and the plate choke.

With the amps off measure resistance on both plate chokes, red wire to black wire.  Then check continuity from the lug with the black wire on PC-2 to Terminal 10 and from the lug with the red wire to Terminal 16.