Double Trouble

GavJ

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Hi all, double blow..... my amp went pop and it took one of my speakers with it  :(

I was using a Belles 150a for bass & mid range and my Linn LK140 for treble..... really enjoyed the sound through a set of Castle Knight 5's.

Sound went off a few days ago, thought it was just a bad recording..... then heard some clicks through the left speaker. On switching off each amp in turn I realised that the left channel on the Belles was gone......

Reconfigured my wiring to have all drivers run from the Linn, sounded much better.

Done some basic tests with my multi-meter, across the output / speaker terminals I had no voltage on the right channel but 64v dc on the left? This was with no input connected.

Sent a mail to a company in London who deal with Belles, they reckoned that if I was getting DC on the left my speaker would have blown...... tweeters running ok, mid range running but lower driver dead on the left speaker. Locked solid, no movement at all.

I dont want to have to send the bulky / heavy amp to London, anyone recommend a repairer near Glasgow?

I have mailed Castle for a price for a new driver.... toes are curling up waiting for the cost. I have seen a company online advertising speaker repairs may try them as well.. any recommendations?

Im also a bit concerned that the crossover may have been damaged.... dont know how to check this?

Any help appreciated.... just a thought, would it be a good idea / upgrade to change both low range drivers?

 

Juancho

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Hi, if by the other driver you mean in the other speaker that should be OK if your DC offset is one channel only. Drivers are not that expensive so it will probably be OK. Normally the driver will go well before the bass inductor on the crossover (again not too pricey).

Measure the DC resistance across bass terminals. It should be of the order of 0.4 ohms and more importantly same for both channels. If same, crossover should be fine. I don't know your amp but massive DC offset (you've got what looks like the full rail there) can be output or driver or input transistor failure or the servo opamp. Good Luck!

And sorry don't know nay Glasgow repair folk

 
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GavJ

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Cheers Juancho, went ahead and ordered new low range driver today, £65... should get it tomorrow. Stripped out failed unit last night and put the good low unit into the blown units place and turned on.. worked ok and sounded fine so Im taking it that the crossover is ok.

I found a repair shop in Hamilton.. not too far from me, have tried mailing them, no response... phoned today and got the receptionist, she promised a call back and took my details... still waiting. Hoping I can get an economical repair for the amp, probably worth £250 working so could be written off?

Really would like to repair the amp myself but don't have the knowledge, if I could get it diagnosed I would be confident to de-solder & re solder components..... pic of offending board attached.

DSC_0558.JPG

 

Juancho

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I'll see what I can find tomorrow on this amp. I have one in for repair and the input J Fets which failed and caused the problem are now obsolete. So I'm replacing both output stages with Hypex modules, quite reasonably priced and good for 400 watts per channel, with great sound quality that I would go for myself if I wasn't into valves

 

Juancho

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Hi,

There's not much on line. I found out it's a differential J Fet input pair followed by cascaded drivers and MOSFet output transistors-so all fairly standard. Can't see how bias is set-there's no visible op amp or pot, but they may replace the pot with a fixed resistor. If you want to have a go yourself I would:

1. Replace the electrolytic caps (i can see one on the board)

2. Re-check dc offset if still there proceed to 3.

3. Check all readings across active and passive components against other channel. Change anything that differs

4. If there's still DC offset there I would swap the J Fets and driver transistors (i.e everything except the output transistors) and whilst they are out re-check all components for value. The transistor set shouldn't be more than £30

It seems impossible to get a schematic so the only option would be to draw one up yourself but the above should work 

Good luck!

 

GavJ

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Thanks Juancho, the repair shop called me back today and only wants £20 do diagnose & quote for repair so will let them do that..... will decide to go for it myself or not on their findings.

 

GavJ

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Latest update, had a call from the shop this morning.... £130, they recon its the power supply? The guy didn't sound too sure but said that he couldn't get a replacement so would have to replace components...

I hesitated as I'm not sure if I'm sending good money after bad? He then said "I think your right not to go ahead, you can buy a better amp in Richer sounds for the sort of money you would spend to repair"... I wonder what David Belles would say to that :).. he then asked if I wanted them to dispose of the amp for me.... I will collect it next week..

Anyone fancy teaching me electronic repair with a correspondence course :) .. would be a brave man that would accept that challenge....  I am going to strip it down and see what I can teach myself, last resort I fancy using the chassis and heatsinks to construct my own amp....

Forgot to say, i asked if he could fit an alternative power supply.... he said something about the boards not being able to talk to the processor? I didnt even know there was a processor?

 

GavJ

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Juancho, are you on line? I have retrieved the amp from the repair shop and stripped it down...... early indications are that you are spot on.....

The Capacitor looks suspect, the aluminium cap on the end is dished out and the body has moved out from the circuit board as if its innards have been pushed out..

The text on its states - 1000 uf / 6.3 v, its 20mm long and 10mm diameter, is the link below the item I need?

http://www.ebay.co.uk/itm/3x-UFG0J102MPM-Capacitor-electrolytic-THT-1000uF-6-3V-10x20mm-FG-5mm-/201627276422?hash=item2ef1ec1486:g:tQwAAOSwbsBXjfyZ

 
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Juancho

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Hi, I'm on hols! I would buy an Elna Silmic and replace both caps for both channels. You need the same spec capacitance and voltage as before. 6.3v seems too low so double check that. Also, make sure there's enough room. Elnas are available from hi fi collective. Even if previous one 6.3v I would up the v rating if space permits

 
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danilo

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Uhhh that's not good.

 Bit Odd though that the bass/mid driver toasted as they are far more  durable in the face of DC input than tweeters.

 That's some Wonky Amp.. Mate. Brand name be damned.

 When I built MY bespoke Active crossovers I fitted 40Uf "DC insurance" film caps on the Actives' output. Does the trick with No adverse effects.

 

GavJ

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Danilo, the tweeter was saved because i was bi-amping, like the sound of the "DC Insurance", even if I get favorable readings after changing the cap Im still wary of connecting up again. I was going to buy a cheap pair of speakers and run the amp with them for a while before allowing it back into my system.......

I assume you fitted the protection to the crossover on the speakers? Is this something i could add into the amp?

 

GavJ

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Replaced the cap this morning, still getting 64v dc at the speaker terminals......... plan B....

Is it possible to replace both boards with alternatives? I have seen  some Quad clones on eBay but the input voltage doesn't match my power supply..

 

Juancho

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you can replace the boards but the input voltage needs to match your measured psu volts

 

GavJ

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Im struggling to find boards to match.. if I have my facts correct. The power supply board has two large 64v capacitors... both channels are fed from the same posts so I assume both caps are used for each, when measured I am getting around 130v at each channel. A lot of the boards I am seeing use 64v (60 - 70v). The power supply appears to be unusual?

 

Juancho

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Jan 22, 2014
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Hi Gav, you are measuring the power supply incorrectly. There should be + 65v -65  and a central 0v rail. You need to establish if these measurements are correct for both channels. If your psu is going 65v each side the caps should be rated at at least 80v and one or more have likely failed. a + and - 65v supply with a 0v rail is completely conventional for this type of amp. Let us know what the v readings are. Speaker neg is connected to the 0v rail usually if you have any difficulty finding it

 

GavJ

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Thanks Juancho.... you have lots of patience :)

The power supply has a 0v rail as you say. The voltage between the middle "GND" post and the pos & neg posts is +65v and -65v. 

The circuit boards are fed from the Pos & Neg posts and as you say the speaker neg is connected to the GND post. 

I have photographed the boards..... so why does the meter read 130v? I assume im getting mixed up with AC and DC? Are the boards fed with AC current?

DSC_0624.JPG

 

Juancho

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if you  measure between + and - you will measure the difference between +65 and -65 i.e 130v. One things is you've proved the psu works correctly so your repair  shop was having you on. There is definitely a servo or other fault on the board. Difficult to advise without a circuit diagram but I would go back to my original step 3 and try and find any difference between the working and non working channels

 

GavJ

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Cheers, I bought a second LK140 tonight so determined to fix the Belles for a future second system. I metered for resistance across all of the resistors & diodes and all seem to be identical on each board. Cant get to the transistors without removing boards from the heat sinks.. so that's next...

 

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