Hello, im currently troubleshooting a Sansui AU-5500 amplifier, it does not output anything, not even noise. I have already made a couple of checks:
-The protection relay RL01 (pag 18 of pdf) does not click and does not enable output;
-Inputted a sinusoid through the tuner input and checked with an oscilloscope the signals right before the protection relay: output is present on both channels, and responds correctly to front panel controls;
-Measured voltage between the two ouputs, it hovers around 20 mV; Measured between both channels outputs and chassis, both measure around 20 mV;
-Checked voltage on R79 which is in series with the relay coil, it measures 0 V meaning the relay is not passing current: checked resistance in circuit of the coil, it measures ~600 ohms. Unless the voltage is taking a strange path the coil does not look like its open, and im guessing it means that the relay is not being told to turn on;
Checked in circuit transistors and diodes listed in section 5.2-2.1 of the manual in the vicinity of the relay with diode mode while in circuit: junctions respond correctly in forward bias with readings of~600 mV except for the weird D15 SV-02 diode which reads higher than the others. In reverse bias they all hover around 1.7 V;
Im currently at a loss on what i should be trying next: i could start pulling out components off and testing them, but the board they are on is huge and seems nested in the amp and a chore to extract. Unfortunately i'm not knowledgeable enough to understand how the protection circuit operates and what voltages and currents should be present on each component, so i cant narrow it down any further without tearing it apart. Any ideas on what could be wrong? Thanks in advance
EDIT: Located faulty TS-01 thermistor reading way too low when cold. Now needing to source a replacement
-The protection relay RL01 (pag 18 of pdf) does not click and does not enable output;
-Inputted a sinusoid through the tuner input and checked with an oscilloscope the signals right before the protection relay: output is present on both channels, and responds correctly to front panel controls;
-Measured voltage between the two ouputs, it hovers around 20 mV; Measured between both channels outputs and chassis, both measure around 20 mV;
-Checked voltage on R79 which is in series with the relay coil, it measures 0 V meaning the relay is not passing current: checked resistance in circuit of the coil, it measures ~600 ohms. Unless the voltage is taking a strange path the coil does not look like its open, and im guessing it means that the relay is not being told to turn on;
Checked in circuit transistors and diodes listed in section 5.2-2.1 of the manual in the vicinity of the relay with diode mode while in circuit: junctions respond correctly in forward bias with readings of~600 mV except for the weird D15 SV-02 diode which reads higher than the others. In reverse bias they all hover around 1.7 V;
Im currently at a loss on what i should be trying next: i could start pulling out components off and testing them, but the board they are on is huge and seems nested in the amp and a chore to extract. Unfortunately i'm not knowledgeable enough to understand how the protection circuit operates and what voltages and currents should be present on each component, so i cant narrow it down any further without tearing it apart. Any ideas on what could be wrong? Thanks in advance
EDIT: Located faulty TS-01 thermistor reading way too low when cold. Now needing to source a replacement
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Confirmed, cathode of D21 sees ~32.3 V, same value for the cathode of D11 at the relay coil terminal
You already confirmed no output offset on power amp outputs.
Any offset of about .5V or more will turn on TR33 or TR31 and discharge C23.
Any offset of about .5V or more will turn on TR33 or TR31 and discharge C23.
Orientation of D13 is wrong in schematic - confirmed by board layout.
Voltage on C23 must be more than 1V for TR29 to conduct.
Just replace C23 for a try.
Voltage on C23 must be more than 1V for TR29 to conduct.
Just replace C23 for a try.
I pulled out C23 which is a 470 uF 6.3 V electrolytic cap and tested it with my multimeter (645 uF) and a really cheap component tester (ESR 0.07 ohm, Vloss 2.2%, 554.8 uF) for these results.
Unfortunately i do not have a replacement 470 uF electrolytic cap, i have 220 uF and 1000 uF scavenged off random boards
Unfortunately i do not have a replacement 470 uF electrolytic cap, i have 220 uF and 1000 uF scavenged off random boards
Use one of these for a try, re-check offset fault condition.
I guess you inspected the solder joints.
I guess you inspected the solder joints.
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Installed 220 uF cap, same condition, voltage on C23 250 mV and very slowly climbing
I cleaned the dust off the underside of the board (it was caked) and gave it a quick look, nothing looked clearly out of place. Ill give it another check with magnification and a strong light.
Edit: scrubbed the protection circuit part with IPA 99% and a toothbrush, the board is really caked with flux and grime, and looks like there are maybe micro solder balls left around (or maybe they are pieces of crusty flux). Other than that in the protection circuit area doesnt look like there are any cracks
Imgur: The magic of the Internet
I cleaned the dust off the underside of the board (it was caked) and gave it a quick look, nothing looked clearly out of place. Ill give it another check with magnification and a strong light.
Edit: scrubbed the protection circuit part with IPA 99% and a toothbrush, the board is really caked with flux and grime, and looks like there are maybe micro solder balls left around (or maybe they are pieces of crusty flux). Other than that in the protection circuit area doesnt look like there are any cracks
Imgur: The magic of the Internet
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I have also found this thread Sansui AU-5500 in auto-protect | Audiokarma Home Audio Stereo Discussion Forums
on a different forum which has a very similar problem, but apparently the thread didnt go anywhere..
on a different forum which has a very similar problem, but apparently the thread didnt go anywhere..
I figured it out, after taking more voltage tests around the circuit and recreating the circuit in a simulator and tinkering with it i tracked it down to a faulty NTC thermistor, TS-01 that having a really low resistance when cold was diverting the current from the transistors and to ground. I checked the resistance on the node between R99 and TS-01 and measured ~450 ohms. I then tried to lift one leg of the thermistor off the board to disconnect it and the resistance to ground jumped to ~22k ohms. Major symptom of this fault is the voltage of node between R99 and TS-01 being nearly 0 V.
Now i need a new thermistor. What could be a suitable replacement?
Now i need a new thermistor. What could be a suitable replacement?
Next step I would have recommended to have a look at the charging resistors
including NTC. I do not think it is essential to have it unless you want to do a
party and use 4 ohms speakers.
including NTC. I do not think it is essential to have it unless you want to do a
party and use 4 ohms speakers.
You are probably right but i still prefer replacing it because while i'm planning to use it mainly with ~64 ohms headphones it is going to sit in a very cramped and very hot room, so i would be more at ease i still have overtemperature protection.
This TS3-85 pdf, TS3-85 description, TS3-85 datasheets, TS3-85 view ::: ALLDATASHEET ::: should be the datasheet of the thermistor. Apparently its a very specific NTC with a very sharp transition around 85°C. Similar model replacements are available at around €20 on ebay, but i would prefer to pass on that. Has this kind of NTC gone the way of the dodo just like tunnel diodes?
Currently im cleaning everything, output is a bit distorted and unbalanced on both channels but fiddling with the balance control i managed to get a pretty good output, i think its just the pots being a bit scratchy, nothing a good squirt of contact cleaner shouldnt fix. Tomorrow im going to buy some, i already have a can but i have lost the tube and without it i cant reach inside the amp to squirt the cleaner inside the pots.
This TS3-85 pdf, TS3-85 description, TS3-85 datasheets, TS3-85 view ::: ALLDATASHEET ::: should be the datasheet of the thermistor. Apparently its a very specific NTC with a very sharp transition around 85°C. Similar model replacements are available at around €20 on ebay, but i would prefer to pass on that. Has this kind of NTC gone the way of the dodo just like tunnel diodes?
Currently im cleaning everything, output is a bit distorted and unbalanced on both channels but fiddling with the balance control i managed to get a pretty good output, i think its just the pots being a bit scratchy, nothing a good squirt of contact cleaner shouldnt fix. Tomorrow im going to buy some, i already have a can but i have lost the tube and without it i cant reach inside the amp to squirt the cleaner inside the pots.