help please

[bjorgens]

Well.

Build for the first unit went off fine, but I am struggling with getting #2 up and running

Resistance checks all look normal with exception of pins B8, B9 .  Manual states these can vary, but mine are reading 0 ohms (though the autorange does go all the way out.)

Given my parenthetical above, I elected to try the voltage checks.  First attempt blew the fuse.  On checking the fuse, it was a fast blow so I attributed the failure to this.  On replacing the fuse with a slow blow 1A 250V, same failure mode.

I set about re-checking solder joints, etc. as well as continuity for all the wire connections to see if perchance I had a broken lead someplace

[vetmed]

At the risk of pointing out the obvious you have one amp that works and therefore an excellent point of reference. For a start try looking at the two side by side and perhaps something may jump out that would not otherwise be evident.

               Regards
                   Robert Lees

[Paul Joppa]

I would suspect the high voltage power supply based on the available information. I don't think any misbehavior of the 5670 would blow a fuse, with the possible exception of a shorted filament winding - but the initial glow suggests that was working until the fuse blew.

Check the orientation of the diodes, especially the high voltage ones, and all the electrolytic caps on the power supply board. Better check the 300B cathode bypass as well (47uF/450v).

[bjorgens]

Thanks Paul
Diodes and electrolytic caps are properly oriented.  Pulled power supply board (that was a bit of a pain) to verify nothing shorting underneath... I re-trimmed a few leads, but nothing apparent.

Visual comparison to my working other unit yielded nothing I could see. No material differences in resistance checks.

Is swapping the known good 5670 to the non working unit a useful experiment?

Bryan

[Grainger49]

Tube swaps are always a first step.  Even before resistance and voltage measurements because it is just so easy.  Give it a try and see.

[bjorgens]

Thanks Grainger...

No dice with the good tube.  Still blows fuse.  I have to keep looking. 

Bryan

[Paul Joppa]

OK, the next step is to pull the tubes and fire it up - if the fuse blows, you will at least have narrowed it down; if not, check the voltages - they will NOT follow the manual - and report the results. That may tell us something.

At this point, we are just looking for more data = more clues. I hope you got a 5-pak of fuses, we may be going through them until we find the problem!

Just in case, measure the DC resistance of the primary and HV secondary of the transformer. Transformers are hardly ever the problem, but pretty often they are suspected, so might as well eliminate that possibility.

Yes, I hear you about the difficulty removing the PSU board! In the next update we'll shift to solder lugs on the power transformer, with enough lead length so you can just unbolt the damn thing and turn it over. What can I say, it seemed like a good idea at the time...  :^(

[Doc B.]

If you are getting a short reading where there should be some resistance and you cannot isolate the short you might have to disconnect some of the circuit where you are testing to further isolate the problem. Maybe disconnect the cable from the pc board again, and then measure across those pads on the pc board to look for the short, and also across the disconnected ends of the stp to look for it.

[bjorgens]

Thanks gentlemen... Will do... In the morning as I am most of the way through a bottle if wine...

Again thanks, and will follow up with more data...

B

[bjorgens]

... And just to ensure I'm not a complete idiot, I have been using 1.0A 250V slow blow fuses.  That is the correct value is it not?

B

[bjorgens]

Curiosity got best of me

No tubes and the fuse still blows...

Apologies in advance for my ignorance... To measure DC resistance of primary and HV secondary, I should be probing at which two points?

B

[Paul Birkeland]

Boy, 99% of the time this is a backwards cap or diode (the silver bands on the 1N5818's and 1N5820's can be tough to see)

There are three sets of wires leaving the power supply PCB.

1.  Going to the IEC power inlet
2.  Going to the FC-1
3.  Going to pins 1/9 on the 9 pin socket
4.  Feeding the PC-2/9 pi socket

One at a time, remove #2, turn the amp on, check the fuse, the remove #3, turn the amp on, check the fuse, then remove #4, turn the amp on, check the fuse.

If the amp does not blow a fuse when you've removed one of those connections, you will have narrowed down the symptoms properly. 

On the STP issue you mentioned, the 6.3V winding has a center tap that is grounded, so you may get a few Ohms of resistance between 1 and 9 to ground, with the continuity beep.  If the readings of pins 1 and 9 on the 9 pin socket are different, please report that to us.

-PB

[Paul Joppa]

This is good news diagnostically - we now know the problem is in the high voltage power supply.

Just a thought - make sure the power transformer is oriented correctly (not rotated 180 degrees), and likewise that the board is correctly oriented.

To measure the primary resistance, use the power cord (with the power switch on). It should be about 7.5 ohms - note that many meters read around 0.5 ohms with the leads shorted, so subtract that from the reading to get the actual resistance.

The high voltage secondary connects one end to the ends of the Cree Schottky rectifiers nearest the middle of the board - both rectifiers are tied together at that end, on the back side of the board. The other end of the winding connects to the short jumper near the front of the board. It should measure about 31.5 ohms.

[bjorgens]

Fuse blew after disconnecting FC1 from PS board.  No tubes populated.  1.0A 250V fuse

Diodes and caps are properly oriented (see pic). I have not again verified orientation of the PT, as I'm reticent to desolder the PS board again unless necessary.

B

[bjorgens]

Likewise for the other two connections.  Only wires that remain connected to ps board are to IEC and ground on STP going to 17U

Fuses still blow (and now I need more.