Hi Folks.
This is my first post to this forum. So it's nice to meet you all.
I have a Technics SU-A808 which sounds great most of the time. But sometimes the safety circuit kicks-in and resets the amplifier. Then it comes back on and works like normal again.
Now I'm an Electronics Design engineer by trade, but not in audio. By business is Temperature & Humidity Measurement and Logging.
I puchased the Technics Service Manual online, thinking it would give lots of information. But apart from the circuit and a parts list, it's lacking good description.
The fault occurs when cold or hot. I have heated with a hair dryer and it doesn't trigger the error, and cooled with freezer spray and no change.
I have removed the board, and re-soldered the SVI3205B Power Amp Modules. The soldering looks very dry and cracked, and I thought this was the issue. But it made no difference.
Both main rails look ok at -50 and +50V.
I guess the safety circuit is tripping because there is to much dc on the speaker output.
The manual talks about adjusting the amplifer so output voltage. This is done by injecting a signal of 1kHz, 100mV in to the tuner input. Set Volume to MAX and adjust VR203 (Lch) and VR204 (Rch) for 14.0Vac +/- 0.2V. But I don't think this is effecting the dc output level, just setting up the correct input level to the amp.
When the trip occurs it can happen a couple of times in a row, but something many hours between. To I can't see any errors.
I guess I could discount the speakers, then disable to safety circuit, and when the error occurs try and check the voltages. But I risk damage to the amp.
Any advice out there?
How reliable are the SVI3205B Power Amp Modules, and how available are they. I would hate to bin this amp, the sound is just perfect for my old ears. It sounds great when playing my DVD-Audio Queen music through it.
Thanks for responses in advance
sNick
This is my first post to this forum. So it's nice to meet you all.
I have a Technics SU-A808 which sounds great most of the time. But sometimes the safety circuit kicks-in and resets the amplifier. Then it comes back on and works like normal again.
Now I'm an Electronics Design engineer by trade, but not in audio. By business is Temperature & Humidity Measurement and Logging.
I puchased the Technics Service Manual online, thinking it would give lots of information. But apart from the circuit and a parts list, it's lacking good description.
The fault occurs when cold or hot. I have heated with a hair dryer and it doesn't trigger the error, and cooled with freezer spray and no change.
I have removed the board, and re-soldered the SVI3205B Power Amp Modules. The soldering looks very dry and cracked, and I thought this was the issue. But it made no difference.
Both main rails look ok at -50 and +50V.
I guess the safety circuit is tripping because there is to much dc on the speaker output.
The manual talks about adjusting the amplifer so output voltage. This is done by injecting a signal of 1kHz, 100mV in to the tuner input. Set Volume to MAX and adjust VR203 (Lch) and VR204 (Rch) for 14.0Vac +/- 0.2V. But I don't think this is effecting the dc output level, just setting up the correct input level to the amp.
When the trip occurs it can happen a couple of times in a row, but something many hours between. To I can't see any errors.
I guess I could discount the speakers, then disable to safety circuit, and when the error occurs try and check the voltages. But I risk damage to the amp.
Any advice out there?
How reliable are the SVI3205B Power Amp Modules, and how available are they. I would hate to bin this amp, the sound is just perfect for my old ears. It sounds great when playing my DVD-Audio Queen music through it.
Thanks for responses in advance
sNick
The SVI3205B as a generalisation could be favourite. I'm not familiar with your amp specifically, however a quick image search shows these to look like the well known 'STK' type modules... and those were always troublesome.
Its quite possible the problem is a dry within the module. Do an image search and you will see pictures of the internals. These are called a 'thick film' type module.
Found a shot here, post #7
http://www.diyaudio.com/forums/chip...-but-no-sound-no-relay-click.html#post4647360
Lets say its probably favourite although without more detailed tests it is not 100% guaranteed.
No idea on quality or authenticity of replacements... sorry.
Its quite possible the problem is a dry within the module. Do an image search and you will see pictures of the internals. These are called a 'thick film' type module.
Found a shot here, post #7
http://www.diyaudio.com/forums/chip...-but-no-sound-no-relay-click.html#post4647360
Lets say its probably favourite although without more detailed tests it is not 100% guaranteed.
No idea on quality or authenticity of replacements... sorry.
Thanks for the fast responses. I will check with my scope, and check the 15V rails.
If the SVI3205B is the issue, do you know of a supplier?
To refresh your minds, the SU-A808 was the amp with the Bamboo Caps and Variable Gain instead of a pot changing the amplifiers input level. To improve signal to noise ratio to 114dB. Mine was manufactured in 1999.
Thanks Folks
If the SVI3205B is the issue, do you know of a supplier?
To refresh your minds, the SU-A808 was the amp with the Bamboo Caps and Variable Gain instead of a pot changing the amplifiers input level. To improve signal to noise ratio to 114dB. Mine was manufactured in 1999.
Thanks Folks
I have just come across the same Model with a similar fault description.
In this case it was the low voltage power switch on the front panel was intermittent. Very slight movement of the switch and its mounting board would cause the amp to trip. The low voltage switch is similar in design to most push/push tape monitor type switches which often go intermittent/high resistance.
In this case the intermittent contacts were causing the mains power relay to cut which in turn powers everything down for a second while it restarts. This would give symptoms similar to protection relay cutting in.
Plus on this model it is always worth re-soldering the pins on the two hybrid ics and also the two regulator transistors that sit between them.
Hope that this helps
John.
In this case it was the low voltage power switch on the front panel was intermittent. Very slight movement of the switch and its mounting board would cause the amp to trip. The low voltage switch is similar in design to most push/push tape monitor type switches which often go intermittent/high resistance.
In this case the intermittent contacts were causing the mains power relay to cut which in turn powers everything down for a second while it restarts. This would give symptoms similar to protection relay cutting in.
Plus on this model it is always worth re-soldering the pins on the two hybrid ics and also the two regulator transistors that sit between them.
Hope that this helps
John.
I had similar problems with my Technics SU-A808, intermittend and stop working on high volume. On another forum I read most problems are caused by capacitors. I replaced C601 and C602 both 16V10u and both in the control circuit for the Fan. Until now: no problems anymore! Even a cheap work around to try out.
My Class AA SU-V85A once had intermitent relay problem. I checked almost every part for error but no clue.
One day I looked inside and I noticed the transformer solder joint looked a little different from others. So I resolder the joint and the problem disappeared, Then I resolder all the solder joints from the transformer, rectifiers, caps, up to the amp board, it works fine up to now.
you can try this, resolder all the joints from transformer up to the amp board.
One day I looked inside and I noticed the transformer solder joint looked a little different from others. So I resolder the joint and the problem disappeared, Then I resolder all the solder joints from the transformer, rectifiers, caps, up to the amp board, it works fine up to now.
you can try this, resolder all the joints from transformer up to the amp board.
Tovasky01
please describe how intermittent no sound is your amp.
is it both channels or one channel at a time,
it suddenly no sound and resumed after half a second or how long for it to resume.
how long is the intervals.
from the web I learned dry joints always happen.
did you re-solder all the joints on the board or
just re-solder from the trans. to the rectifiers?
the intermittent is probably caused by bad joints, mine is, yours may be.
I have one SU A800 in my shelf, which use same power chip as yours.
those IC power chips are highly possible to have bad joints on their legs due to the heat from the ICs while operating. re-solder the legs is needed.
please describe how intermittent no sound is your amp.
is it both channels or one channel at a time,
it suddenly no sound and resumed after half a second or how long for it to resume.
how long is the intervals.
from the web I learned dry joints always happen.
did you re-solder all the joints on the board or
just re-solder from the trans. to the rectifiers?
the intermittent is probably caused by bad joints, mine is, yours may be.
I have one SU A800 in my shelf, which use same power chip as yours.
those IC power chips are highly possible to have bad joints on their legs due to the heat from the ICs while operating. re-solder the legs is needed.
Hi Alexchoi, thanks for your response. I didn't resolder all joints only the onces on/with the trafo. I will resolder the joints on the main board as well. When intermitting, the relays are clicking for both channels. Usual it is only 1 or 2 times of clicking, but this morning it was more than 2 times. I switched off and turned on, now it is working again. But I definitely need to get at the bottom of this, so I will follow up your advise and keep you posted.
lets try to locate the fault
first we want to know what trigger the relay,
normally its DC. sometimes over current.
can you arrange your DVM, one lead clam to the output point.
(before the relay, at the joint of the emitter resistors, or solder a wire in series with a resistor 15r to 30r to the DVM. if no resistor in series, take care not to short the wire to ground.)
the other lead clam to the speakers output ground.
doing this is to see if there were DC at the output when the relay clicks open.
and see how high is the DC.
at the mean time I will search about the fault.
first we want to know what trigger the relay,
normally its DC. sometimes over current.
can you arrange your DVM, one lead clam to the output point.
(before the relay, at the joint of the emitter resistors, or solder a wire in series with a resistor 15r to 30r to the DVM. if no resistor in series, take care not to short the wire to ground.)
the other lead clam to the speakers output ground.
doing this is to see if there were DC at the output when the relay clicks open.
and see how high is the DC.
at the mean time I will search about the fault.
I just read the schematic and found there are at least 8 relays in it,
four for the input select,
one for speakers output (suspect)
one for headphone
one for tape select
one for the secondary AC supply ( for amps ). (suspected for dodgy contacts)
its good and its bad...
the good is there could be other relay problems that cause the protect..
the bad is they are hard to clean, they are prone to have dodgy contacts.
notice if other relay clicks too.
****
check the output one channel at a time,
unless you have two DVM
four for the input select,
one for speakers output (suspect)
one for headphone
one for tape select
one for the secondary AC supply ( for amps ). (suspected for dodgy contacts)
its good and its bad...
the good is there could be other relay problems that cause the protect..
the bad is they are hard to clean, they are prone to have dodgy contacts.
notice if other relay clicks too.
****
check the output one channel at a time,
unless you have two DVM
Last edited:
I didn't do any measurements yet with a DVM. With the resoldering I replaced the 2 SDT-SH-112DM, I had free samples and just wanted to try out (you never know). I'm also thinking about the cause and I remember I connected my PC by the sound card with the Amplifier and noticed due to the fact my PC was not connected with PE there was a voltage I was able to feel.....but I didn't do any measurements. I'm not sure if this might have caused these problems. Now the amplifier is only connected with a tuner and tape deck, these even don't have a PE. I will let you know if I have any updates.