broken amp

madmadra · 22nd February 2006, 03:53 AM

Hi folks - don't know if i'm in the right section but i bought a technics SU-7700 integrated amp recently on ebay and it's died already and i haven't a clue what the problem is with it. It's impossible to find anyone in my part of the world who would be able to fix it and i was wondering if anyone had any ideas. I was using it this time last week and the speakers went dead - it kicked back in a few minutes later and repeated this a couple of times over the next 24 hours and then died for good. The lights in the VU METER windows stayed on and the very low hum that's audible when it's on is still there, it just doesn't seem to be delivering to the speakers. i checked what i could re fuses and connections and couldn't find anything though my knowledge is pretty basic in this dept. It did normally take a couple of minutes to warm up in the morning and had recently been making static noises when adjusting the volume though i understand this isn't unusual in old amps. I'm really fond of this amp and would hate to see it go to waste esp if the problem is something simple. Can anyone help me? Many thanks

gmphadte · 22nd February 2006, 10:46 AM

U have dry solder problem due to heat.
Search for dry solder on DIYAUDIO, u will find so many answers.

Gajanan Phadte

jaycee · 22nd February 2006, 02:24 PM

... or theres a fault in the speaker protection circuit which has caused it to disconnect the speakers permanently. There's usually a startup delay which is a simple RC timer, if the capacitor dries up, it kills the protection circuit which makes the speakers stay switched off.

richie00boy · 22nd February 2006, 02:37 PM

Quote:

Originally posted by madmadra
very low hum that's audible when it's on is still there, it just doesn't seem to be delivering to the speakers.

Is this hum from the amp itself or coming out the speakers? If it's coming out the speakers it cannot be the startup delay.

madmadra · 22nd February 2006, 03:51 PM

Thank you - anyone know where this problem is usually located? and any advice on fixing it - i'm not a techie and would need it explained to me like i'm 5

madmadra · 22nd February 2006, 05:17 PM

No the hum is from the amp, barely audible. i'll attach a photo - if it's a dry solder prob can someone point out the location to focus on - many thanks

madmadra · 22nd February 2006, 05:17 PM

sorry forgot the photo!

Bobken · 22nd February 2006, 06:05 PM

Hi,

I don't know anything about this particular amp., but from what you say, this could be a problem with the 'wiper' going open circuit on the vol control, especially if it is as old as you suggest.

This could explain the intermittent static noises you mention when operating the vol control, and would more than likely give rise to some hum as the input to the first stage would probably then be open-circuit.

However, I assume it is a stereo amp, and it would be unusual for the same problem to occur (at one and the same time) in both channels.

Did both channels work OK when you first tried it out, and immediately before it finally died?

Edit. I just realised you later said the hum was from the amp, not the speakers, but a slight hum from an amp is not unusual, anyway. However, if the problem is with both channels, this is significant as it is unlikely that both channels of the amp boards would die together.

You would then need to consider any 'shared' parts of the circuit more like the power supply, perhaps.

madmadra · 24th February 2006, 01:51 AM

ok thanks Bob - i'm going to have to try to get someone to have a look - i wouldn't know where to start - it's very difficult to find anyone around here who fixes electronics that are not new or under warranty - i just thought someone here would be able to point me in a def direction where i could go at it myself - many thanks to all who offered their help - it was much appreciated

ps, if it's a dry solder problem, where would it most likely occur in this instance?

clem_o · 24th February 2006, 02:59 AM

Dry solder joints can occur anywhere, there is no "more likely place". Unfortunately, any bad connection on the amplifier board can cause the symptoms you've described.

Could even be a bad coupling capacitor. It could introduce a DC offset at the output of one of the amplifiers, causing the speaker relay circuit to cut out (protection).

If you have a DVM handy you could try measuring voltages; that would be a starting point but you'd have to have a schematic and know how to read it (i.e. people here could tell you where to measure on the schematic, but you'd have to have a basic electronics background to be sure you're measuring the right point on the actual amplifier...

Cheers!

22nd February 2006, 03:53 AM	#1
madmadra diyAudio Member Join Date: Feb 2006 Location: -	broken amp Hi folks - don't know if i'm in the right section but i bought a technics SU-7700 integrated amp recently on ebay and it's died already and i haven't a clue what the problem is with it. It's impossible to find anyone in my part of the world who would be able to fix it and i was wondering if anyone had any ideas. I was using it this time last week and the speakers went dead - it kicked back in a few minutes later and repeated this a couple of times over the next 24 hours and then died for good. The lights in the VU METER windows stayed on and the very low hum that's audible when it's on is still there, it just doesn't seem to be delivering to the speakers. i checked what i could re fuses and connections and couldn't find anything though my knowledge is pretty basic in this dept. It did normally take a couple of minutes to warm up in the morning and had recently been making static noises when adjusting the volume though i understand this isn't unusual in old amps. I'm really fond of this amp and would hate to see it go to waste esp if the problem is something simple. Can anyone help me? Many thanks

22nd February 2006, 06:05 PM	#8
Bobken diyAudio Member Join Date: Dec 2002 Location: UK	Hi, I don't know anything about this particular amp., but from what you say, this could be a problem with the 'wiper' going open circuit on the vol control, especially if it is as old as you suggest. This could explain the intermittent static noises you mention when operating the vol control, and would more than likely give rise to some hum as the input to the first stage would probably then be open-circuit. However, I assume it is a stereo amp, and it would be unusual for the same problem to occur (at one and the same time) in both channels. Did both channels work OK when you first tried it out, and immediately before it finally died? Edit. I just realised you later said the hum was from the amp, not the speakers, but a slight hum from an amp is not unusual, anyway. However, if the problem is with both channels, this is significant as it is unlikely that both channels of the amp boards would die together. You would then need to consider any 'shared' parts of the circuit more like the power supply, perhaps. __________________ Bob

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22nd February 2006, 10:46 AM	#2
gmphadte diyAudio Member Join Date: Jul 2003 Location: Dona paula, Goa	U have dry solder problem due to heat. Search for dry solder on DIYAUDIO, u will find so many answers. Gajanan Phadte

22nd February 2006, 02:24 PM	#3
jaycee diyAudio Member Join Date: Apr 2004 Location: Norwich, UK	... or theres a fault in the speaker protection circuit which has caused it to disconnect the speakers permanently. There's usually a startup delay which is a simple RC timer, if the capacitor dries up, it kills the protection circuit which makes the speakers stay switched off.

22nd February 2006, 03:51 PM	#5
madmadra diyAudio Member Join Date: Feb 2006 Location: -	Thank you - anyone know where this problem is usually located? and any advice on fixing it - i'm not a techie and would need it explained to me like i'm 5

22nd February 2006, 05:17 PM	#6
madmadra diyAudio Member Join Date: Feb 2006 Location: -	No the hum is from the amp, barely audible. i'll attach a photo - if it's a dry solder prob can someone point out the location to focus on - many thanks

24th February 2006, 01:51 AM	#9
madmadra diyAudio Member Join Date: Feb 2006 Location: -	ok thanks Bob - i'm going to have to try to get someone to have a look - i wouldn't know where to start - it's very difficult to find anyone around here who fixes electronics that are not new or under warranty - i just thought someone here would be able to point me in a def direction where i could go at it myself - many thanks to all who offered their help - it was much appreciated ps, if it's a dry solder problem, where would it most likely occur in this instance?

24th February 2006, 02:59 AM	#10
clem_o diyAudio Member Join Date: Jun 2005 Location: Manila	Dry solder joints can occur anywhere, there is no "more likely place". Unfortunately, any bad connection on the amplifier board can cause the symptoms you've described. Could even be a bad coupling capacitor. It could introduce a DC offset at the output of one of the amplifiers, causing the speaker relay circuit to cut out (protection). If you have a DVM handy you could try measuring voltages; that would be a starting point but you'd have to have a schematic and know how to read it (i.e. people here could tell you where to measure on the schematic, but you'd have to have a basic electronics background to be sure you're measuring the right point on the actual amplifier... Cheers!