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Posted: Sun Nov 14, 2010 5:10 am
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stratele52 wrote:
The death cap ... you mean the by- pass cap on AC line. If you want to replace it and go to 0.002 μf ( not bigger) at 1Kvolts to 5 Kvolts


Yes that one - the one shown here:

http://www.unclespot.com/3prongconversion.JPG

where it is removed completely.

Is this an advisable mod?


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Posted: Sun Nov 14, 2010 7:22 am
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Yes, I recommend removing the "death cap." Gently, or it may bite! 8^)

Remove any ground switch, as in diagram. Be sure "hot" line of AC is going to fuse, after rewiring. I solder the green, ground line, to the closest open area on the chassis, right after the cord grommet.

Good luck with the new addition!


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Posted: Sun Nov 14, 2010 7:24 am
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You are right BMW2002T, it's better without the bypass cap like your picture show


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Posted: Wed Dec 01, 2010 9:47 pm
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aclempoppi wrote:
Chris, I'll offer a few suggestions: 1) replace the bias cap with a 50uf/100vdc; check bias res 470ohm1watt; 2) replace power tube screen resistors with 470ohm2watt flameproof; 3) check the 1500 swamper resistors on the power tube sockets. I would build a light bulb in series current limiter, for initial power-up and checking for shorts in the amp. I'm not computer savvy enough to draw this out and post, but it should be available on some site. It's very simple and effective. As for the Caig cleaners, I use a 2-step method : D5 to clean and S5 as a preservative( not much is needed), seems like Caig changes the names quite often, so use your own best jugement. You can Email me if you can't find a diagram for the current limiter and I'll snail-mail a copy. Art


Hi Art (and everyone).

Finally, I have done the supply caps replace and built my light bulb current limiter and cleaned my pots and replaced bias cap. So I plugged in the current limiter and tested it - no problems with it. Then I plugged the amp into the limiter with a 25w bulb and the light glowed quite brightly, which, as I understand it, means I have some trouble. The amp, before my upgrades did work, but through the limiter, nothing, including no power/pilot light. No smoke, no sparks no flames, no glowing tubes. Fuse looks OK.

Suggestions? I visually checked all connections, solder joints etc - nothing untoward.

Regards, Chris


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Posted: Thu Dec 02, 2010 3:58 am
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You should use 100 watts bulb. Where do you read 25 watts is ok ? this 25 watts is not enough power full. It become full bright too easely OR you plug it in parallele by mistake not in series.

Do you see my sketch about this current limiter ? It's on another post. Il try to show you

It's normal that the lamp glow ( not full bright). You have litte current goes trought to the amp.

If the lamp ( 100 W) glow almost full brightness then you have a short in the amp.
If it glow dim good chance your amp is ok. you can plug it in wall outlet with the proper fuse .
Image


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Posted: Thu Dec 02, 2010 5:16 am
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stratele52 wrote:
You should use 100 watts bulb. Where do you read 25 watts is ok ? this 25 watts is not enough power full. It become full bright too easely OR you plug it in parallele by mistake not in series.

Do you see my sketch about this current limiter ? It's on another post. Il try to show you

It's normal that the lamp glow ( not full bright). You have litte current goes trought to the amp.

If the lamp ( 100 W) glow almost full brightness then you have a short in the amp.
If it glow dim good chance your amp is ok. you can plug it in wall outlet with the proper fuse .


Hi Stratele52,

Thanks for this - all guidance on the current limiter says to start at low watt bulb to catch issues. But some said full glow was a problem - even at 25W so I was concerned. So I put a 75W bulb in and first bright, then dim so looked ok. So then I plugged into wall and 5U4 rectifier showed some flash arcing so I turned off. Then I turned back on and all tubes heated up and no smoke, no sparks :-) But also no audio although a very very very slight hum from speakers.

So now the troubleshooting begins .....

Chris


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Posted: Thu Dec 02, 2010 5:36 am
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For test your 5U4
1- Is arcing in Standby mode ? If yes 5U4 may be defective. Can you take a mesure of voltage ? be careful; lethal voltage.

If no:

2- With all your power tubes removed, power ON , Standby at play, is 5U4 arcing ? Yes; test your power supply circuit

No arcing ; test your power tubes

Good luck


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Posted: Thu Dec 02, 2010 5:57 am
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stratele52 wrote:
For test your 5U4
1- Is arcing in Standby mode ? If yes 5U4 may be defective. Can you take a mesure of voltage ? be careful; lethal voltage.

If no:

2- With all your power tubes removed, power ON , Standby at play, is 5U4 arcing ? Yes; test your power supply circuit

No arcing ; test your power tubes

Good luck


Thanks - the funny thing was arcing only happened at initial turn-on and only once and no arcing at all now. But all tubes glowing and warm/hot.

Chris


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Posted: Thu Dec 02, 2010 6:04 am
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IMO tube are ok. Go ahead


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Posted: Thu Dec 02, 2010 7:46 am
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stratele52 wrote:
IMO tube are ok. Go ahead


Cool - the new adventure, phase 2 begins!

Thanks for the support - great place to hang out :-)

Chris


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Posted: Thu Dec 02, 2010 7:53 am
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You are welcome Chris. You have a great project.

Louis


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Posted: Thu Dec 02, 2010 10:49 am
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OK - so I started doing some voltage measurements as indicated on the schematic/layouts for the AB763.

1) Got voltages within Spec at pins 5 and 4 of rectifier (5u4GB)
2) Got no other voltages anywhere .....

All of the voltage points on the GLG6's - nothing, 12AX7/12AT7/7025s - nothing.

So something is pretty wrong :-) When I replaced the 470 ohm bias resistor I heat sinked the diode, but it was still hot - if I destroyed that one what would be the impact?

Also, does anyone have a picture of the orientation of the BIG caps in the doghouse? I replaced them in the orientation that I found them but when I look at the schematic, I can nearly convince myself the middle one (between R1K and R4700) is backwards ... But I also would have expected to witness a cap explosion if it had been reversed, wouldn't i?

Any suggestions about how to proceed when I have not much happening?

Regards, Chris


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Posted: Thu Dec 02, 2010 11:00 am
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What are you showing at pins 2 and 8 of the rectifier?

Those are the filaments and they need 5 volts.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Posted: Thu Dec 02, 2010 11:06 am
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Retroverbial wrote:
What are you showing at pins 2 and 8 of the rectifier?

Those are the filaments and they need 5 volts.

Arjay


Hi Arjay,

2 is 4.3v AC and 8 is ZERO for rectifier.

Chris


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Posted: Thu Dec 02, 2010 11:16 am
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I think the rectifier is hosed.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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