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Old 25th June 2006, 04:54 PM   #1
mikee55 is offline mikee55  United Kingdom
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Default Amp with 40vdc on lts output

Hi all,I've been given a Realistic chassis which has been stripped.It used to be a reciever,but all that has been gutted and all thats left is the amp board and psu.It was used as a bridged amp for a sub.The bridge was a basic op-amp affair which has been removed.The amp worked fine for a while then justwent wrong.I put an old loudspeaker on it and the cone sucked in."Ooops",DC on the output.I replaced the speaker for me meter and on both channels it reads 40vdc.Nothing cooks,the relay doesn't cut out and I can't for the life of me know whats wrong.I did an ohms check to see if any of the output trannies are short,but they're ok.I have two IC's with heatsink tabs on them and assume they either are drivers or biasing circuits and I'm not sure how to check them.

Does anyone have a clue?

STA-2270 Digital Synthesized AM FM Reciever.

Cheers.Mikee55 I've got room in me bin for it
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Old 25th June 2006, 05:30 PM   #2
Netlist is offline Netlist  Belgium
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Default Re: Amp with 40vdc on lts output

Quote:
Originally posted by mikee55
I've got room in me bin for it
Hmm..after so much abuse, I'd rather consider it as a museum piece.

/Hugo
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Old 25th June 2006, 06:12 PM   #3
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Both channels have the same DC output? Then the same fault is on both. When you removed the op-amp bridge adapter, maybe you connected the inputs back up wrong?
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Old 25th June 2006, 06:34 PM   #4
mikee55 is offline mikee55  United Kingdom
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Default fault was there with bridge connected

Hi thankyou for replying.The fault occured with the bridge circuit attached.Thats why I now have the amp.I removed the bridge circuit to see if this made a difference.Thing is my JVC AX-2 went to heaven,and my Technics SU-V2has only one channel and until I build my gain clone Im stuck using the guts from a pair of PC speakers as an amp.I have 7watts per channel to play with.I understand this Radioshag has 75watts.

I wouldn't mind a burnt unit,but this thing looks innocent,nothing visual,I would have thought 40 volts output would give a burnt component or heat,smoke but not a thing.Didn't even blow its fuse.

Doh[/B]

Ah well,worth the try I suppose

cheers
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Old 26th June 2006, 03:12 AM   #5
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You you say it has 40 volts on the output? do you have the -40V on the other set of output's? if not, thats your problem. the other is a bad driver(s) or a part in the pre-amp thats gone and of course being DC coupled you get a good DC output.
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Old 26th June 2006, 03:07 PM   #6
jaycee is offline jaycee  United Kingdom
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Is it 40V on one output or both?

If it's both, check the -40V hasn't gone missing.

Don't bin the chassis - use it for your gainclone unfortunately that power supply is too high for a gainclone, though. Maybe you should build an ESP P3A instead
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Old 26th June 2006, 10:37 PM   #7
mikee55 is offline mikee55  United Kingdom
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Default Not sure

Hi all,not been well today,will check voltage again tommorrow and I'll get back to you.

Kind regards Mike
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Old 27th June 2006, 11:20 AM   #8
mikee55 is offline mikee55  United Kingdom
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Default Hello again

Hi,better today.I've taken some readings and I get + 40vdc on both channels,and I've measured the psu at the smoothies and it seems to be fine +/- appx 42vdc.I'm using an analogue meter at the mo,my DMM needs a new battery.

The voltage out of the speaker terminals is +40vdc as in the red terminal is + , there is no negative as the black terminal is ground at 0v.Does this make any sense?

So,Relay operates when its switched on,power supply is ok,output trannies ok and pure DC on the LS terminals,relay should shut the amp down shouldn't it?By the way the longest period I've left it on is less than a minute and I just connected the meter leads to the LS terminals without a load added!

Thanks Mike
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Old 27th June 2006, 11:56 AM   #9
sreten is offline sreten  United Kingdom
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Hi,

Output trannies short = blowing fuses or lots of smoke.

Possibly the one of the output trannies is open ?

Worth checking, should measure as a pair of back to back diodes.
(unless its a FET output stage)

/sreten.
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Old 27th June 2006, 12:36 PM   #10
mikee55 is offline mikee55  United Kingdom
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Default update

The upc1225H seems to be a 30 to 50 watt power amp driver.God knows how to test these!?
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