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Old 3rd July 2012, 06:42 PM   #1
Mote is offline Mote  Estonia
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Default Advice on cheap amplifier repair

I got this broken Auna AMP-7100 amplifier.
At first on powering up it only switched on internal cooling fan. After investigating a bit I found that SyncMOS chip was not in its socket properly and thats why amp did not power up.
After reinstalling SyncMos chip amp powered up but stayed in protection - the relays did not engage.

It is a 7 channel amp, 5 channels use TDA chips as output devices and 2 use output transistors. One of the channels with output transistors had different voltages than other. Do not remember exactly what the voltages were but on one channel one output had -30V, 6V and -30V DC, on the other channel -30V, 0V, -30V.

Seemed to me that channel with 6V was not in very good health. I pulled two small transistors from that channel and now amp powers up and relays click and other channels beside the one from which I pulled the transistors produce clean and loud audio.

Outputs and drivers and all other components seem to be in good health but it looks llike that they have been already replaced. I post photos of the board, hoping that it is some generic design and someone can provide a schematic or offer advice from where to look the defective component. Maybe amp already failed once and something in the protection circuit failed as well?
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File Type: jpg DSC00326.jpg (790.0 KB, 164 views)
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Old 3rd July 2012, 10:44 PM   #2
sreten is offline sreten  United Kingdom
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Hi,

Just by comparing the working left or right channel to the other one
and swapping components you should be able to fix the L or R channel.

Auna AMP-7100 Home Hifi 7.1 AV Receiver 2000W Amplifier at best price!

Click the image to open in full size.

rgds, sreten.
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Old 4th July 2012, 04:30 AM   #3
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Inspect for cracks in the solder.

Gajanan Phadte
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Old 4th July 2012, 12:34 PM   #4
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See this thread. vintage amplifier repair/upgrade manual
More fun than crossword puzzles and saves a useful product from the landfill.
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Old 4th July 2012, 01:00 PM   #5
Mote is offline Mote  Estonia
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Thanks, very good read.

What did I say that makes you think that I will turn this product into garbage?
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Old 4th July 2012, 03:25 PM   #6
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Quote:
Originally Posted by Mote View Post
What did I say that makes you think that I will turn this product into garbage?
Everybody does it. Most of my friends and neighbors think it very strange that I don't throw enough things away. They also think it strange that I would listen to music on LP's when I could be watching people shoot each other on blue ray movies.
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Old 8th July 2012, 07:47 PM   #7
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For now it works, -12V regulator had a bad solder, TL084 did not get -12V and one of its outputs had 10V on it.

I "suffer" from the same condition, I also do not throw out stuff easily and I own a collection of 600+ LPs, although they are not very old: 1997 - 2011.
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Old 10th July 2012, 10:15 AM   #8
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As soon as I fixed it I managed to break it again.
I checked if all channels, FM radio etc. works.
They did, then I assembled it and tried it again, I got only noises from AUX, FM, COAX etc, noises came from inputs which are on a smaller board for inputs.
7.1 inputs, DVD, CD work fine, these inputs are on the main board.

There were two wires with 8 pin plugs and I of course managed to connect them incorrectly. On one plug the 8th pin is GND on the other +12V so it was pretty bad mistake.

One wire was supposed to connect main board to additional inputs board, and the other wire, inputs board to FM radio board.

Now after connecting the wires correctly I can not get any sound from AUX or FM radio, MP3 RCA input. I can not check any other inputs. Inputs on the main board still work and all channels work.

At first I had two suspects - HCF4052BE or SM8952AC25. Display still works and channels on the main board are switching properly, so SM8952AC25 I thought was OK. I replaced HCF4052BE with another one from another location but that did not improve the situation.

Then I noticed that there is another chip: HCF4094BE its pins 1 (Strobe), 2 (Data) and 3 (Clock) are connected to SM8952AC25 ports through the same wire which I connected incorrectly earlier.

I removed the chip and now when I tie HCF4052BEs (that is in charge of switching between AUX, FM etc.) pins 9 and 10 to GND I get the amp to play FM radio through speakers, when I tie pin 9 to GND and pin 10 to +5V I get it to play AUX. I read from the datasheet that this is how it is supposed to work.

So probably HCF4094BE is faulty? Or can it be that SM8952ACs some ports are not functioning? FM radio boards chip gets its Clock etc also from SM8952ACs other ports and tunes and plays stations as it should.
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Old 11th July 2012, 05:16 AM   #9
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Your 4094 and 4052 have numbers that indicate they may be special (hcf) CMOS logic IC's, and hence available cheap. I think TI used HCF for some very fast parts. The base numbers look like RCA CMOS numbers but they were bankrupt before the HC parts came out. (Example TI 74HC00 etc). I have no clue on a SM8952. MOS is not tolerant of voltage out of spec, you you may have really damaged things with your plug misplaced. I try to mark any connector I take off with a sharpie marker as to orientation before I pull it, but even with that I have plugged headers back in one pin off and damaged up to 50 parts at a time.
If you can't find the part at the usual suspects farnell.com, mouser.com, digitech.com, alliedelec.com I have seen some obsolete stuff at consolidated electronics of dayton ohio ceitron.com. Haven't used them so I don't know how good their shipment practices are. I have bought some obsolete things from jameco.com: shipment is good but their website organization is bad.
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Last edited by indianajo; 11th July 2012 at 05:18 AM.
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Old 11th July 2012, 06:48 AM   #10
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HCF4094 is ST part, readily available and cheap
https://www.elfa.se/elfa3~ee_et/elfa...3-509-89&toc=0

The SM8952 is a PCT part:
SM8952AC25 datasheet pdf datenblatt - SyncMOS Technologies,Inc - 8-Bits Micro-controller With 4/8KB flash embedded ::: ALLDATASHEET :::
Even if I bought one it would be useless because it needs to be programmed first. I need SM8952 from another amp exactly like this one.

The 4094 I will buy and see if it will make the amp work if not then I will just use the 7.1 inputs and probably solder jumpers so that FM radio will work. But that way with any of these inputs: AUX, FM, MP3, COAX1, COAX2, OPT1, OPT2 selected the amp will still play FM radio.

HCF4052 have also a pin which when high disables all outputs of the chip. Maybe if I tie that to another 4052s A or B (pins which control which input is passed to output) then maybe I can make some switching happen, but I did not see any change in the As and Bs when selecting different inputs, thats why I think HCF4094 is faulty.
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