i'm on a roll... the kgsshv

[eggil]

 

 

The 2 50k resistors pads under the 2sa1486 is just one 50k crossed over to each side of the 50k pads, 2 of the silkscreened 50k's bridged across is a mistake?

 

Edited May 22, 2014 by eggil

[Pororo]

A short was my first thought... But the fuse didn't blow, so I thought the fets might be the problem.

I'll have to take the bracket and heatsink off again, a major PITA. In retrospect I should have picked the onboard version for my first build.

[DefQon]

Thanks eggil that's what my post was referring to when I first saw the pictures.

Correct me if I'm wrong the onboard and offboard share the same parts and similar trace layout only difference is external mounting compared to the onboard heatsink mounting style.

[Inu]

2SC1815 ECB

2SC2705 ECB

2SC4686A BCE

2SA1968 BCE

Edited May 22, 2014 by Inu

[johnwmclean]

Double check the data sheets.

Confirm whether the pinout diagram is viewed from the top or from the base.

[Kerry]

You'll need to test your transistors. If they were in wrong or something around them was wrong there is a good chance they are blown.

Edited May 22, 2014 by Kerry

[Pororo]

John, I double checked the datasheets and you're right top/bottom was misread and as Inu noted, 2sc2705 and 2SC1815 are both ECB.

 

So these 2 were initailly in the correct orientation and I still had the problem.

 

Thanks guys, I'll start testing transistors and resistors.

[spritzer]

Better yet, just get some of the Japanese models to begin with.  You have probably done a lot of damage doing something wrong so the best bet would be to scrap all the small  transistors.  Also clean the flux on the output devices.  It can short...

 

Spritzer - On the latest rev 16 offboard amp silkscreen there is an extraneous "A" next to the cathode side of the upper right LED.

 

You mean upper left?  That is supposed to be there as the LED is part of the 2SA ccs. 

[Pororo]

^^^  Yeah, that's what I should have done from the beginning; getting the fets from Bdent instead of ordering everything from Mouser.  I'll troubleshoot these later when I have time and just stuff 2 new boards.

[spritzer]

Those are BJT's...    I I doubt you've killed a resistor while doing this so simply swapping in all new transistors should clear it up. 

[Pororo]

Ahh, good to know. Thanks

[Pororo]

I'm trouble shooting some more on the amp board.  Right now, all 4 LEDs are lit, and the balance is adjusted to zero.

But the the offset is stuck at -41V.  The pot is set to maximum resistance.  Lowering the resistance gives slightly more negative offset like around -43V.

 

So I tried increasing the value of the 2K resistor in series with the pot and it helps a small amount, maybe getting to around -38V then if I bump up the resistance above a certain threshold the offset climbs sky high to 452VDC!

 

The heatsink is warm, but not hot.  The PS is putting out +/- 455V and +/- 15V.

 

Before I could even adjust the balance and offset though, I had problems with arcing and LOUD popping.

 

Once there was a short underneath one of the 0.1uf 1000V caps and the other 3 or 4 times I assumed the solder joints of the outputs were shorting.  I cleaned them all top and bottom and there is no trace of flux and no more arcing.  My assumption is that the outputs were shorting because there was no apparent damage anywhere else and the arcing/popping stopped after cleaning the flux.

I've got 1968A and 4686A installed.

 

EDIT: I forgot to mention: the output current is set at 10mA with 100ohm resistors.

 

Do you guys think one or more of the outputs are damaged?  Any other ideas?

Edited May 30, 2014 by Pororo

[spritzer]

Do diode tests on the transistors, that's the only way to check while in the circuit.  Other than that, just change them out. 

[Pororo]

Thanks for the response Spritzer.

 

Here's the results of the diode test.

They're all good, right?

 

0.689   OL

0.607   OL

 

OL   0.661

OL   0.625

 

OL   0.658

OL   0.623

 

OL   0.651

OL   0.620

 

OL   0.650

OL   0.618

 

OL   0.658

OL   0.619

 

OL   0.658

OL   0.618

 

0.688   OL

0.606   OL

[spritzer]

Looks good, one thing to check though, is the pot dead.  That has happened once to me. 

[luvdunhill]

You need to check both probe orientations. 6 measurements per device.

Edited May 30, 2014 by luvdunhill

[Pororo]

You need to check both probe orientations. 6 measurements per device.

 

Here's the C - E measurements:

2.04  OL

OL  OL

OL  OL

OL  OL

OL  OL

OL  OL

OL  OL

2.04  OL

 

Are both 1968A bad?

 

EDIT:  I checked 4 new and uninstalled 1968A and they all measure OL OL for C-E... so I guess that means I've got 2 bad 1968A?  Does it make sense that that would cause -41V offset that doesn't budge?

 

The pot I checked before installing. It was centered so I put it at max resistance.  I guess it's possible it failed under load or got zapped either directly or indirectly from the arcing.

Edited May 30, 2014 by Pororo

[kevin gilmore]

balance pot has to be set in the middle or bad things may happen.

normally the pots are set to middle out of the box.

 

its usually pretty hard to do diode checks in circuit and get reliable info

 

if all the leds are lit it is working and maybe the one pot is shorted

[palchiu]

Just finish my PSU, everything working fine but I've questions about measuring Bias voltage.

 

There is measured 576V@Bias TP(570V before 4.7M resistor), but 380V@Bias out. (I use Mini PSU board)

 

Is that working correctly?

 

Thanks!

 

 

Pal

Edited May 31, 2014 by palchiu

[Pororo]

balance pot has to be set in the middle or bad things may happen.

normally the pots are set to middle out of the box.

 

its usually pretty hard to do diode checks in circuit and get reliable info

 

if all the leds are lit it is working and maybe the one pot is shorted

 

Thanks Kevin.  Now that I think about it, it seems unlikely that both 1968A would fail and give the same measurement.

 

Yes, all 4 LEDs are still lit.  I'll try swapping out the pot when I have time on Monday.

[insanity]

Just finish my PSU, everything working fine but I've questions about measuring Bias voltage.

 

There is measured 576V@Bias TP(570V before 4.7M resistor), but 380V@Bias out. (I use Mini PSU board)

 

Is that working correctly?

 

Thanks!

 

 

Pal

This is normal, if you do not use a high impedance voltmeter. Thats what the TP is for. 

[palchiu]

This is normal, if you do not use a high impedance voltmeter. Thats what the TP is for. 

 

Hi insanity, Thank you for your help!

[Pororo]

balance pot has to be set in the middle or bad things may happen.

normally the pots are set to middle out of the box.

 

its usually pretty hard to do diode checks in circuit and get reliable info

 

if all the leds are lit it is working and maybe the one pot is shorted

 

I had some time, so I swapped the offset pot and the problem remains... -38VDC that doesn't budge.  All 4 LEDs are still lit.

Is the next step pulling the 1968A and testing them?  Any other ideas?

[spritzer]

First thing first, you had shorts so were all of them properly isolated and cleaned?  All resistors measured out of the circuit (one end lifted up)?  With a short you can even damage the PCB. 

[Pororo]

Yes, all shorts isolated and cleaned.  I have not measured any resistors out of circuit, except for the 2k one in series with the pot.  There doesn't seem to be any damage to the topside of the PCB; haven't checked the bottom too closely due to the angle bracket.