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Dub Defector View Drop Down
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Joined: 08 January 2008
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    Posted: 05 March 2008 at 4:43pm
We have a Hill Audio 1200 watt amp that seems to have a small fault. The clipping light on one channel is permanently on. Now, we have taken it to our local old school amp guy and he cant diagnose a fault but says there is a small amount of DC current leaking in to the channel. He feels this will not damage anything but we are not so sure. The last time we used the amp we knackered two black widow drivers through overheating somehow... could this be related? We dont really want to run the risk of using it again and blowing more speakers.. what can we do?

Cheers

Edited by Dub Defector - 07 March 2008 at 10:19am
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tweeter box View Drop Down
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Get a second opinion!
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Dub Defector View Drop Down
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Points: 801 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Dub Defector Quote  Post ReplyReply Direct Link To This Post Posted: 05 March 2008 at 6:39pm
Thanks tweeter, will do exactly that some point soon, but wondered if it was indicative of some fault others have experienced?
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Dub Defector View Drop Down
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Points: 801 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Dub Defector Quote  Post ReplyReply Direct Link To This Post Posted: 05 March 2008 at 7:52pm
anyone know of a good person / company that could look at this fault in Devon? Torbay or Plymouth area?

Cheers
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Dub Defector View Drop Down
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Points: 801 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Dub Defector Quote  Post ReplyReply Direct Link To This Post Posted: 07 March 2008 at 10:17am
OK, so i got this response from a company, does it make sense to anyone?

Quote I've had this problem before on other Hills', including the LC800, but
fortunately not my own Hills'.
I did however, some time ago, fry my new Celestion Frontline 15" 600 Watt
Bass spkr on someone else's Hill by using
the Celestion as a 'Load' whilst I was testing.

I'm 90% sure it would be the Ballast Resistors located inside each of the
Ram-Air heat sinks ( o/p stages ). There's
2 per o/p stage. If these go open-circuit then it is possible to have >50
Volts DC across the Spkr v/coil. I've
never thought it necessary to do a Current test as the Celestions'
voice-coil was open circuit in around 90 seconds
at which point I could smell the room fill with burning wire + lacquer!

The original Resistors are no longer available according to my sources, one
of which is an ex Hill employee.
My own solution is the fitting and wiring of chassis mount Resistors of a
higher wattage value in place of the
originals. It means a total strip down though of that Channel in order to
drill the 4 holes needed for fixing the
Resistors. This is a rather 'sensitive' area of the chain as it's the last
part whereby the efforts of the Musicians, Equipment and
Engineers finally hit the Speakers! Changing a failed Amp is easy but when
the Amp fries the Spkrs as well it's another issue!

The cause of this happening on Hills', primarily, is the failure of 1 or
more of the 10 o/p transistors ( 10 per side ). Since
these are quite chunky with a 20 or so Amp rating I believe their failure
may be premature due to dry thermal heat-sink
compound between them and the Heat sink. It's D.O.M would be around late
80's / early 90's which is enough time for
the original thermal paste to completely dry out and become powder, even the
silicone type reduces in effeciency.

The Hill can be repaired for £75 or totally refurbished and modified on
both Channels etc for £150 inc parts.
I can mail you a detailed process which involves biasing and matching both
channels so that only 0.003 Vdc is
present across the Spkr o/p's at full load with given i/p conditions. Quite
a difference from 50!
De-soldering and removing all 20 o/p transistors( 2 nuts /bolts each ) for a
clean, current gain test, new heat-sink paste
etc can become quite laborious but non the less important. They take about 2
whole days from start to finish as there are
many solder joints to check over as well on both the front i/p board and
rear supply board.: Unfortunately one of
Malcolm Hills production 'slacks' that ultimately cost the business.
The next whole day is Calibrating and soak testing flat out with dummy loads
etc. Basically 'red-lining / fine tuning it.


Looks to me like the fix is going to cost more than the amp is worth i recon..

opinions?


Edited by Dub Defector - 07 March 2008 at 10:19am
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canonrich View Drop Down
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Points: 4 		Post Options Post Options   Thanks (0) Thanks(0)   Quote canonrich Quote  Post ReplyReply Direct Link To This Post Posted: 28 May 2008 at 7:14pm
Hi Dub defector,

I think the fix is more than the worth of the amp. That is a pitty.
the story make sense cause if the power transistors have a fault in the gate, a dc voltage will be added to the line.

Did you fixed it or got yourself a new amp?

 
a friend of mine gave me a LC800 for free last January.
it has two problems, a 50hz hum and a small, 0,5 volt DC voltage on one channel.

Although it will be quit some work, I’ll try to fix it. Just for fun.

Can you give me the e-mail address from the company who gave you all this information?

I would like to sent him a mail and ask some questions about the faults in mine. Maybe he even has a schematic for the amp. Now I need to go without.

And I am interested in his detailed process for biasing

 

Thanks very much.

 
Regard Richard

 

 
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joseph View Drop Down
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Points: 228 		Post Options Post Options   Thanks (0) Thanks(0)   Quote joseph Quote  Post ReplyReply Direct Link To This Post Posted: 29 May 2008 at 11:21am
yes bro
i had a lc 400 for tops
got same problem and did sell it to a electyronic man .. dont know what happend since but was too frightened to test it again :)
strange cause hill audi, for my own experienxce, are wikid amps, very good sound and solid equipement .
 
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canonrich View Drop Down
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Points: 4 		Post Options Post Options   Thanks (0) Thanks(0)   Quote canonrich Quote  Post ReplyReply Direct Link To This Post Posted: 29 May 2008 at 7:51pm
The guy who gave it to me is really enthousisatic about hill.
great sound and solid amp. (it weighs about 16 kg I guess).
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amptech View Drop Down
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Points: 82 		Post Options Post Options   Thanks (0) Thanks(0)   Quote amptech Quote  Post ReplyReply Direct Link To This Post Posted: 05 June 2008 at 10:20pm

yeap the 'clip' led is telling you there's a dc fault with in the unit and get it fixed befor it gets out of hand and things shut down and burn out.
any tech should of told you guys not to stress the amp by keep using or if you needed it disconect the faulty side and use it as a mono block.
dc faults start at the drive stages due to leaky componets, you can use the good channel to take voltage tests and take note to aid in repair.
a quick test is to connect your ddm meter across the red and black post and power up, your meter range should be on 200v dc. slowly drop the volt range to 20v dc,a working unit will be below 1v ie mv range if not do not link speakers to it or by by speaker! whats burning!......
if it works leave it be!
one flash and your ash.....
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canonrich View Drop Down
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Points: 4 		Post Options Post Options   Thanks (0) Thanks(0)   Quote canonrich Quote  Post ReplyReply Direct Link To This Post Posted: 16 June 2008 at 7:09pm
Hi all,

is there somebody who can give me a good direction to search the fault.
it's been a few year since I was working on electronics.
the main issue here is that the amp has a 50hz hum (yeah yeah, europe;-)
i would like to get it out and get the amp back in use again.
the thing is i'm lost and don't have a clue to continue
I tried to find the schematics but had no luck there either


the amp makes a + and a -85V for the main stages and + and - 18V for the i/p board and for use on the o/p board.
the 85V is made via a bridge rectifier and 68000uF capacitors
the 18V via diodes and 6800uF capacitors and a 7918 voltage regulator

I have connected this amp to a scoop and got the following 'image'.
http://www.picasaweb.google.com/canonrich/HillAudioLC800

my interpretation is ,it looks like only when the main capacitors are charged somewhere on the board there is generated a high frequency distortion.
i've measured the 68000uF capacitors and they look fine they reach their value. i replaced the 6800 for new ones.

does it sound like an earthing problem because the only moment this visible is when there is a current running to the capacitors?
or is it a problem with the main capacitors. even when the measure correctly?
 

on the i/p board ther is mounted a TDA2030 as well i don't know what its for.
any ID's?

thanks
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amptech View Drop Down
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Points: 82 		Post Options Post Options   Thanks (0) Thanks(0)   Quote amptech Quote  Post ReplyReply Direct Link To This Post Posted: 17 June 2008 at 8:49am
Hi,

Some old amps develop this fault due to 'old' cap's drying out and it's not to long befor you get ripple on supply line's and hear the dreaded hum at low level. replace all smoothing and decoupling caps check board for dry joints,if you have one use a signal generator and scope the o/p stage with no speaker load via a 1khz sine wave.
proline amp's develop this fault due to no head room on the main filter cap's because the supply line's at power up went over the 82 volts dc and the cap rating was 80v.....
if it works leave it be!
one flash and your ash.....
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600ohm View Drop Down
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Hi,
 
The TDA chip drives the DC cooling fans, speed is dependant on amp temprature. A fault can occour in this circuit which superimposes a high frequency 'buzz' on the supply rails. This can be clearly (!) heard on both channels. I have seen (heard) this problem on 3 LC800's, I cured mine by replacing the fans with AC mains versions, works a treat!
Hope this helps.
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