New Crack Build Voltage Problem [resolved]

[jminassin]

Doc,

The reading between pin B7 and B8 is 0.1 ohm.

PB,

I set the chassis plate on the base right side up, plugged it in, and both tubes were glowing after a few minutes.  However, when I removed the chassis from the wood base and turned it on its side, only the 6080 glowed.  The socket appears to be installed correctly and I can assure you that the plastic keyway is fully intact.  The wires going to the 9 pin socket appear to be securely soldered in place.  The LEDs are not lit.

I measured the voltages and they are way off as follows:

1.  205.1 V
2.  205.5 V
3.    96.6 V
4.  205.3 V
5.  197.2 V
6.      .3 mV
7.      .3 mV
8.        0 V
9.    92.6 V
10.  92.7 V

I disconnected the test lead and put the chassis back into the wood base.  The 12AU7 is not glowing.  The 6080 glows when first turned on and then goes dark after about 30 seconds.

Let me know what you think.  I do appreciate your help.

John

 

[Paul Birkeland]

I set the chassis plate on the base right side up, plugged it in, and both tubes were glowing after a few minutes.  However, when I removed the chassis from the wood base and turned it on its side, only the 6080 glowed. 

A loose solder joint will do that.

Since you have tubes lighting and not lighting, there is still an issue with the green twisted wires and their connections.

 

[jminassin]

Ok, it was a loose solder joint just like PB said.  Tubes are glowing.  LEDs are glowing.  All voltages look good except Terminal 7.  I have floating millivolts where I should have 90-115 V.

Any idea what I should be looking at?

John

 

[Deke609]

Double check that your meter is set to measure DC volts (parallel dashed and straight lines), not AC.

cheers, Derek

 

[jminassin]

Hi Derek,

Thanks for getting back to me.  I am definitely  on DC.  All the other voltages measure correctly.

Here's a picture of my wiring of the octal tube.

Best,

John

 

[Deke609]

Hmm. Just a shot in the dark. With the amp unplugged and cooled down, what resistance (ohms) do you get between terminals 7 and 3, when touching the probes to the terminal lugs themselves (not the leads of the 3K resistor)? 

 

[jminassin]

Yikes, what did I do?  Everything was working except for one voltage reading.  I plugged the Crack in and I saw a small flash where the UF4007 rectifiers are.  I shut it down and looked for shorts.  Not finding anything I powered back up.  The 270 ohm 5 watt wire wound resistor between 15U and 21U gets very, very hot.  I shut it down.

Is it possible one or more of the rectifiers failed?  I did initially have the polarity of one of them reversed.

Help!

John

 

[jminassin]

Deke,

I get 2.971k ohms between lugs 3 and 7 with amp cooled down and unplugged.  That reading is done by just touching the lugs.

John

 

[Deke609]

Post lots of detailed pics - overhead and from different sides.

If the first 270 ohm 5W resistor is getting super hot, but not the second one, then that seems to point to a short immediately after the first one - my bet would be between T15 and one of (a) chassis, (b) T16, or (c) T14.

A rectifier diode shouldn't be affected by being installed backwards - unless you ran a lot of current through it - but I think the fuse would prevent that.

cheers, Derek

 

[Paul Birkeland]

Look for leads poking through that should have been trimmed off, but are now touching the chassis.  Trim them off. 

Consider also posting some fresh build photos.

 

[jminassin]

I can't see any leads touching the chassis.  Here are photos.  Let me know if you want more.

Best,

John

 

[Deke609]

Some ideas:

(1) I think you could use some more heat on the 8 pin socket - and quite possibly on other joints. What solder are you using?  If it's Cardas Quad, you should see brown flux come out of the joint and settle on the surface of the solder.  If it doesn't rise to the surface, it's insulating the inside of the joint. If you have an adjustable soldering station, try cranking it up to at least 750F.

(2) You've painted the bottom of the chassis and I see that you've scraped the paint away from at least some of the terminal strip attachment points. But I can't tell the full extent of the scraping. I would make sure that the paint is scraped away from any terminal-strip-to chassis connection to which a black wire attaches.  But I doubt that's what causing your negligible voltage at T7 since you have the correct voltage at T9.

(3) Purely for the sake of future reliability, I suggest pushing the leads of the "internal" diodes (the ones closest to the power transformer) farther into the terminal lugs and then trimming the excess on the other side.

All that said, I think your best and fastest route to a working amp is to reflow all the joints, making sure that they get a good heating, and adding just a small dab of solder (unless there's a solder blob there already, in which case don't add any -- or better yet, remove most of it with a sucker or wick and redo the joint).

That's all I can think of. PB may have some better pinpoint suggestions.

cheers and good luck, Derek

 

[jminassin]

Hi Derek,

I’m using Kester 60/40 leaded solder.  I had the chassis powder coated.  The guy wouldn’t mask the underside for the $50 he needed to do it and the bell housing.  I saw PB’s instructions to someone else who had the plate powder coated.  I removed paint with a Dremel tool at the ground lug next to the IEC power inlet, and where the screw that holds the 5 lug terminal strip closest to the 9 pin tube is.  I also removed paint under both screws that hold in the octal socket.  I removed paint from the underside of the transformer bell housing where it meets the transformer too.

Thanks for your suggestions,

John

 

[Paul Birkeland]

What's your DC voltage at 21?

 

[jminassin]

PB,

DC Volts at lug 21 is 219.6.  Let me know if this is significant.

Thanks and have a great holiday,

John

 

[Paul Birkeland]

How about 15?

 

[jminassin]

PB,

0.3 mV at lug 15.

 

[Paul Birkeland]

Are you certain about that?  If this was the case, the 270 ohm resistor that runs across your power transformer either isn't connected on one end or would be burned up into pieces.

Because you have appropriately high voltage at terminal 21, I would suspect maybe you measured 16 or 14, both of which would be 0V.

 

[jminassin]

Sorry PB, I was on lug 16.  But I only get 27.1 mV at lug 15.

 

[Paul Birkeland]

Are you certain about that?  220V on one side of that resistor and 0V on the other side of that resistor would dissipate 180 watts and it would get so hot that it would fail quickly and horribly, your fuse would blow, etc.

It's possible that one end of the 270 ohm resistor that crosses over the power transformer isn't well connected on one end or the other, or that one of the black wires running around the 6 lug terminal strips isn't well connected. 

Can you power down the amp and let the DC voltage bleed out, then try measuring the DC resistance of the 270 ohm resistor that crosses over the power transformer?