Stumped Again

[shcaz]

Update from shcaz - new build on Stereomour.  I had resistance problems traced back to bad output transformer.  Received new transformer last night and installed.  All resistances checked good.

Per advice from astroimage2002, I checked orientation on 2A3 sockets and indeed, discovered that one was installed incorrectly.  I changed the orientation of that socket so that pin sockets 1 & 4 were closest to the hum pots.  Followed next steps to install the 12AT7 and 2A3 tubes and sparked in up.  No glow at all in the tubes, but a little smoking in the Active Load board area.  Turned off and checked the fuse, not blown.

At this point, only option I can see is to progressively dissasemble the unit, checking wiring and connections along the way.  I did not check voltages based on the fear that I would do damage to components by leaving the unit powered up.  Any suggestions?  I can only surmise that I have shorted a connection or incorrectly installed a component.

Thx for any suggestions.
shcaz

 

[JC]

I don't know the circuit, so I can't be of a lot of help.  But, the first thing to look for, IMO, is incorrect orientation of parts where orientation matters.  Diodes and electrolytic capacitors are at the top of that list.  Thing is, under the right set of circumstances, a resistance check might not catch one of those being installed "backwards".

I often like to enlist a second pair of eyes for visual inspections.  They catch things that I overlook "because I know that part's OK."

 

[shcaz]

Additional troubleshooting:

-As mentioned previously, slight smoking around the active load board when powered on
-Initial check of connections seemed fine, so I powered up again
-The 5W resistor on the A side (5W4K7J) connecting terminals 6 & 16 was running very hot compared to C side identical part
-Removed active load board, checked all connections and joints, re-soldered several joints just to be sure, but found nothing miswired

Powered back up and same result - 'A' side resistor RW4K7J running very, very hot.  'C' side does not.  Something clearly wrong.  High temperature suggests too much current across that resistor.  Still looking.

 

[JC]

See, now you've moved into specifics that I have no way of addressing.  Hopefully, someone familiar with the amp will see this and chip in.

Best Luck!

 

[Grainger49]

I'm looking at the Eros manual and thinking the power supply is similar.  The first check I would make is the incoming voltage to the A side of the active load board versus the B side.  That is if IA is not jumpered to IB.  That means the voltage has to be the same.  If they are jumpered then there is something else wrong.

I think the other side is the B side but there is a screw hole in the middle of the B making it unreadable.

I am guessing here, the active loads are for the 12AT7 driver tube.  If so then they feed the plates of each triode in the 12AT7.  Those should be tube pins 1 and 6.  The tube pins are numbered clockwise, viewed from the underside of the socket, with the gap at 6:00 tube pin 1 is at 7:00.

 

[shcaz]

JC - Without doing complete disassembly, I took your advice and checked orientation on all diodes and caps, as well as wiring/terminal connections in general.  I also had a partner check themselves, all appeared to be fine.

Starting with the basis assumption that there must be something wrong, I'd like to start checking voltages as grainger suggests.

Basic question - given that something is amiss, can I safely power up for short spurts to check voltages without too much risk of damaging additional parts?  Also, can I/should I leave the tubes out?  Thanks for any guidance there.

 

[JC]

OK, before going on, let's re-cap:  After the initial re-work, your resistance checks ALL are now within tolerance, correct?  And, on the short time you powered up, the fuse held, correct?

This would be a good time for the "light bulb test", if you think you can manage it.  It involves devising a way to safely insert a light bulb of 100 Watts or so into the path of the "hot" lead going to the AC mains input of the amp.  Then, when you power the amp, the bulb should glow briefly on the initial turn-on, then dim out.  If it stays relatively bright, you know something is drawing too much current, but the bulb drops enough of the AC mains Voltage to keep the amp safe generally.

DO NOT TRY THIS IF IT IS NOT CLEAR OR IF YOU CANNOT DO SO WITHOUT EXPOSED AC MAINS VOLTAGES!

Failing that, if your answers to the questions above are "Yes", I think I would suggest making your initial try without tubes, with the understanding that many Voltages will by very high from spec.  There is no way of knowing if any other part may be damaged at this point, but having the tubes out may reduce the chances.

Also, keep in mind that I don't know the circuit; there could very easily be some risk(s) that I am not aware of.

I sure hope someone with a schematic happens along!

 

[Paul Joppa]

The high current suggests that the grid of the 2A3 (pin A3) is not grounded through the 249K resistor as it should be. The first possibility that comes to mind is a broken 220 ohm carbon composition resistor, mounted at pin A3 to T4. If that is intact, then check the 0.1uF coupling capacitor, T4 to T2 to be sure it has infinite DC resistance. You may have to disconnect the capacitor from T2, and/or the C4S board from T2, to make that check accurately; the measured DC resistance will drift upwards as the capacitor charges but should eventually get up to infinite.

 

[shcaz]

Stumped no more.  Thx for replies.  I believe I found my issue - terminal 16 & 17 were shorted through a wire that had insulation stripped too far back.  Straightened that out and continued testing on voltage.  So far so good, voltages checked within reasonable tolerances; just about to hook up and adjust hum.  Hopefully, my last post.

Thank you all for suggestions and help.  Fun project so far, even the troubleshooting.  I think I'm hooked..

 

[shcaz]

Beautiful music now playing.

Thanks for all the help

 

[JC]

Good deal!  Personally, I find the troubleshooting part of this hobby to be almost as much fun as the building, and usually a good deal more instructive.

That base is very pretty; care to share your recipe?

 

[myrun]

Nice base Steve and very pretty amp....we're all glad its singing now!
Mario

 

[shcaz]

Absolutely - Base is stained with TransTint red dye mixed with denatured alcohol.  Topcoat is General Finishes Oil & Urethane Arm-R-Seal, all available from rockler.com.  Picture loses some of the gloss, but I used semi-gloss finish.  Also planning to paint bell cover with similar color.  A little funky, but I wanted to try something atypical.

Thx, Steve C.

 

[jrebman]

Steve,

I'm blind so can't see your base color, but I used a couple of different trans-tint reds on my stock alder base, and although I also usually use arm-r-seal, this time I used minwax wipe-on poly.  Looks great, and I hope to post finished pics of my Crack amp soon.

My stereomour base is made of a fantastic piece of spalted birch with 1/8" walnut trim pieces top and bottom, has radiused sides like a bombay box, and just got it first coat of
General Finishes Seal-a-Cell before the Arm-R-coat goes on.

Now I'm trying to decide between polished brass plating, satin brass plating, and show chrome for te top plate.

Can't wait until I can show this one off as the comments from friends who have seen it are extremely positive.

-- Jim

 

[2wo]

Hi Jim,

If I remember yours correctly, it was a fairly vivid, opaque red. Not quite fire engine but bright. Steve's is a deeper red, hints of wine and somewhat transparent, the grain shows through .

Hope this helps...John 

 

[jrebman]

Hi John,

Well, I'll just post some more pics when I get it all finished.  I actually went back and added a couple more coats of different reds after the initial dyeing, but I'm sure it's still a somewhat darker red and not a fire engine red.  The General finishes poly/oil stuff seems to bring out more grain than other similar wipe-on finishes, and the seal-a-cell first coat tends to pop the grain even more.

I'll try to post some pics today.

Anyway, it's back to electronics work now as we had an extremely extended summer and had temps in the upper 70s and lower 80s as of a little over a week ago and I wanted to get as much woodshop time in as I could and so I saved the indoor stuff for the colder weather.

You just can't go wrong with a little red on an amp .

-- Jim