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Old 20th December 2006, 12:37 AM   #1
Kevster is offline Kevster  Scotland
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Default Dead Copland CTA-501

Hi guys.

Nothing dramatic happened, but the music just stopped.
After letting the amp cool down I had a look inside.


See pic:

Blown tube, a blown fuse and a frazzled 20 ohm resistor. Any ideas what might have caused this?

Replaced the tube today without noticing the blown fuse and resistor. All 7 tubes are glowing, but is it possible there's something more seriously wrong?

Happy to go to Maplin for the resistor and fuse and put them on.

Does this seem like a limited damage scenario here?

Your help would be appreciated, because my electronic knowledge is not extensive.

Cheers!
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Old 20th December 2006, 12:54 AM   #2
kevinkr is offline kevinkr  United States
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It might be a limited damage scenario, sounds like the output tube failed.

Note that these should be replaced as matched pairs, and given modern tube quality you might consider buying some that have been burned in by the vendor for at least 24 hrs before matching.

Next step is to replace that frazzled resistor and the fuse - make sure the fuse is the same type and rating as the original.

Also be careful to make sure the supply caps are fully discharged before you replace any parts. Use a 1k - 10K resistor to discharge them. (If you need more detail than this I don't think you should attempt this repair at all.)

Please also read the tube safety and newbie threads for tips on how to be safe.
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Old 20th December 2006, 02:20 AM   #3
kevinkr is offline kevinkr  United States
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You might also have a bad coupling capacitor, if the previous recommendations do not fix it I recommend finding a competent technician specializing in valve audio gear.
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Old 20th December 2006, 10:07 AM   #4
Kevster is offline Kevster  Scotland
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Thanks kevinkr

I will go as far as replacing the fuse and resistor, then seek pro help.

So how do I discharge the supply caps with the resistor, just bridge the terminals?

Thanks for input.
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Old 20th December 2006, 10:43 AM   #5
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Quote:
.............So how do I discharge the supply caps with the resistor, just bridge the terminals?..........
You qualify for this :
"......(If you need more detail than this I don't think you should attempt this repair at all.)......KevinKR ."

However I know you will still do something , so .......

1. Yes you bridge the terminals but ...
2. The resistor must be held with an INSULATOR than can withstand the voltage and not leak it through you to ground.
In that case you might get more 'frazzled ' than the resistor in your amp !
3. Depending on the resistor wattage ( at least a couple of watts ? ) , it will heat up .
4. In no case should your hand touch anything before you do this. Check all high value caps with a voltmeter just to be safe even after you discharge the main cap.

You asked " the " question at the beginning of this post. ........
You " need " to be extra careful !
Cheers.
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Old 21st December 2006, 12:35 AM   #6
Kevster is offline Kevster  Scotland
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Hi Guys

Took all precautions apart from putting on my wellington boots, and surgery was partly successful.

Replaced blown tube and fuse and resistor* and got sound from both channels.

* Despite my best efforts to buy the correct resistor, I ended up with the wrong one...

Red Black Black Gold Red - I assumed this is 20 ohms (2-0x10) and indeed the other 3 working ones measured 0.01 on my multimeter whose lowest setting was 2K ohms. I bought a few at Maplin (UK's Radio Shack) which appeared identical, but appear to measure 40 ohms.

Would this explain why although I now get sound out both channels, one is barely audible?
I only switched in on for a few seconds, and was relieved to get this far.

Any advice on the resistor debate?
Anyone got a schematic to check?

Cheers
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Old 21st December 2006, 02:50 AM   #7
kevinkr is offline kevinkr  United States
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Unlikely, sounds like there has been some damage elsewhere in the amplifier or only one of the two output tubes is doing anything on that channel.

The next logical step requires much better test equipment than you have at your disposal so I would recommend findin a good tech at this point.
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Old 21st December 2006, 03:32 AM   #8
Kevster is offline Kevster  Scotland
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Ta-da!!

Just out of curiosity and as a final test I switched the L+R EL34 valve pairs over, and I am delighted to say it is now back to full output. Maybe a badly seated valve after the replacement I made?

It's now 4:30am here, so unfortunately I can't give it a prolonged test, but all seems fine, despite the resistor value imbalance, which I will get to the bottom of and rectify.

Thanks for all your help, from one Kev to another
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Old 21st December 2006, 03:44 PM   #9
kevinkr is offline kevinkr  United States
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I'd go back and check the traces and soldering around that 20 ohm resistor. Things don't start working for no reason unfortunately..
You may have a cold solder joint or possibly a bad socket contact. Check to make sure the contacts are tight, and if not tighten slightly using a jewelers screw driver. Again of course make sure all caps are fully discharged before doing this.

Good though that it is working.

OT: I would invest in a meter that at least has a 200 ohm range minimum as well and can measure 1KV dc or ac.



edit:

One other thought, check the pins on the new tube for good solder joints. I have had more than one that was intermittent due to this reason. Resolder if necessary.
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