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Old 5th November 2006, 05:54 PM   #1
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Default Please help: no sound and blown fuse

Hi

I have a problem with my valve amplifier, a Copland CTA501, and Id be very grateful if you could give me some advice. The other night I was listening to it, I went out of the room a few minutes and when I returned, the valves (4xEL34, 2xECC82 and one ECC83) were still on (their filaments glowing), but there was absolutely no sound. I opened the amplifier and I saw a blown fuse, F2 in schematic (the glass was blackened). I have bought several fuses and tried it but every time I turn on the amp, the fuse F2 blows with an spark, although I have changed all the four power valves. The other fuses F1 (mains transformer primary) and F3 dont blow. TPerhaps it is important to note that he amp is 15 years old and when it failed last night, it was several months after the last listening session with it and loudspeakers (these months I have listened to music only with headphones with another amplifier).

What kind of troubleshooting do you recommend me? If the problem is just a component failure, I think I can fix it by myself (I know to use a multimeter and Im good at soldering): I hope its just a matter of find the broken component (perhaps a too old electrolityc capacitor), desolder it and put an equivalent replacement in its place. I send a photo of the power supply section of the schematic with the blown fuse highlighted in red, if it helps (BTW, what does T.630mA mean?).

Thanks in advance

Regards
Jose
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Old 5th November 2006, 06:32 PM   #2
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HI Jose,

I seems like you have a direct short circuit in that power supply section since the fuse blows very quickly with a spark. I would use your ohmmeter to check the 4 diodes in the bridge (D1 through D4) plus the capacitor C32. The diodes should measure very low in one direction, and very high in the other. One, or more, may be shorted. (very low in both directions) If so, perhaps replace all 4. You can compair the readings to the other diodes D5-D8 for reference.

If the diodes all measure ok, then move on to the filter capacitors that follow. (C30, C37, C28, C27 & C17) There are other possiblities, but you should start here.

Depending on what kind of meter you have, some lower priced digital meters can't measure diodes well. Hopefully yours can or maybe has a low ohms position for diodes.

Oh yea, .630ma is the current rating of the fuse. It should be a slow-blow type for tubes. A amp would work ok too.

Hope this will help,
Victor
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Old 5th November 2006, 06:46 PM   #3
singa is offline singa  Singapore
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Hi Jose,
Please be aware the the F2 fuse is in the B+ or anode power supply.430V and further down 370V becareful and discharge the electrolytics before you measure for low resistance.You can use a 100ohm - 1K ohm 5W-10W resistor to disharge.Crocodile clips with wires would help alot.Also measure voltage across the capacitor after discharging to give yourself peace of mind.
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Old 5th November 2006, 09:07 PM   #4
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If indeed the fuse is for your B+ it should be of the ceramic type as glass type fuses can vaporize the filiment & leave a vapor deposit on the inside of the glass casing & still conduct leaving the following circuits unprotected.
It seems however that something else is occuring & the fuses have worked OK...for now.
___________________________________Rick..........
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Old 14th November 2006, 06:19 AM   #5
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Thank you for your answers and sorry about the delay, but I had to nearly dismantle the amplifier to gain access to the bottom side of PCB, where the traces and solder pads are, and I had little time.

I have soldered 4 new 1N4007 diodes in D1-4 positions and a new 1000pF/3kV capacitor in C32, but the fuse F2 still blows, so I'm going to replace the other capacitors C30, 37, 28, 27 and 17 (I had planned to do it anyway, because the amp is 15 years old).

The original big electrolytic caps are one 100uF/500v (C27) and two 220uF/500v (C28 & 30) and there is a new problem, because I don't have the exact 220uF value, but only 200 or 250uF. What is it preferable: the higher or the lower value? I'm thinking about to use the 250uF cap to allow more energy storage, but perhaps the added capacitance and the associated higher inrush when charging the bigger caps could stress too much the other components of power supply. Or perhaps the difference beetween the original 220 and the new 250uF is of no importance here, in these storage capacitors rated anyway at 20% tolerance. What do you recommend me: solder the 200 or the 250uF caps?

Thank you again for your help.
Best regards
Jose
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Old 14th November 2006, 07:40 AM   #6
kskr is offline kskr  Hong Kong
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Too sorry hearing that,

Hi, have you tried to change all ps caps(may test with cheap and higher ratings one, say, 700v caps) and
use a, say 1A slow blow type fuse?(1A=1000mA, which is little higher rating yet safe enough for protection)

B+ caps may charge too quick the moment you turn on, and
indeed the voltage and current are both much higher before it stabilizes. (especially you're using high capacitance {over 100uf} caps.)

Also, the moment you turn on the main switch, the filaments are
still cold, only after few seconds the B+ then starts flow though the tubes.

So, it's always recommended to put a 'discharge capacitor' at the ps circult (say, a resistor of 150K 5W, paralleled with c28).
And use higher ratings caps.


And I think the c32 can be omitted now for testing purpose.

Hope to have helped any
Kenny
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Old 11th December 2008, 02:48 PM   #7
Gagarin is offline Gagarin  Sweden
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Hi Spendorspain!

How did things work out with your Copland?

I have bought a broken 501 (fried rectifier circuit on R channel) thats coming to me in two weeks time.

Do you have a parts list for the capacitors?
I would like to replace those in the signal path.

Thanks in advance!
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