Sansui au 777 Protection mode

[animefreeks]

Hello

I have a sansui au 777 that always on protection mode even with out input, no speakers connected on it, when I power on the Amp protection mode triggers after a few seconds, Some capacitor was already changed a few years back, the Amp is seldom used but when I tried to power it on again protection mode triggers. Any Ideas or information about this issue?

Thanks for the information

 

[animefreeks]

Will try to clean the Amplifier this weekend hopes it would help on my issue

 

[animefreeks]

I have cleaned some of the dirt on the Amp, it worked for a few minutes then protection mode started going back on. Do you recommend recapping the amp??

Does any body has a list of capacitor for Au-777?

Thank you

 

[animefreeks]

Currently restoring my Sansui AU 777 that keeps on enabling the protection mode
I am in the process on changing the the big capacitors on my unit

Schematic Original Replacement
C002 4000uF 80V Nichicon GU series 4700 100V
C004 1000uF 35V Nichicon GU series 3300 35V
C005 500uF 50V Nichicon GU series 1000 80V
C825 1000uF 63V Nichicon GU series 1000 200V
C826 1000uF 63V Nichicon GU series 1000 200V
C819 2000uF 35V Nichicon GU series 4700 35V
C820 2000uF 35V Nichicon GU series 4700 35V


I was able to change the C819 and C820 as the items already arrived, I powered the unit to try it, protection mode was is disabled, but when I put some test music even on low volume the protection mode triggers again after 10-15mins, have not powered the unit again, will wait for the other capacitors to arrive.

I also checked the DC offset on the unit with out load
L Channel 1.8~ -0.5 mV
R Channel 1.0 ~ 0.1 mV

 

[animefreeks]

Hello AK members

Any comments, suggestions or violent reactions with the above replacements are welcome.


Thank you

 

[rasenthiran]

Hi There,

The late response is due to limited members who own this unit. Its a very nice amplifier. I hope Tom will come in soon,he will help you.Sorry i cant assist on the technical part.

 

[Hyperion]

I have no idea why your posts gone unacknowledged until now, as you originally posted some time ago, it is a shame when that happens - we aim to help everyone.

However,
Firstly, you should not be changing components randomly to resolve the protection issue, as you may introduce extra faults which will complicate repairs a great deal.

The readings you have posted for DC offset (in post #4) appear to have been made at the speaker terminals? - the readings are so low that it suggests you have made the readings while 'in protection' - protection mode disconnects the speaker terminals - so you are reading nothing if you do this. You must obtain/work from the schematic diagram, locate and measure the DC offset at the output of the amplifier but BEFORE the protection relay contacts.

When you have found these points - you can at least attempt to adjust the DC offset with the adjustment pots - locate these by the use of the schematic/service manual. Do not adjust any of the pots without positively identifying the adjustment pot and what it does, otherwise you may also introduce other faults.

 

[animefreeks]

Hello Hyperion thank you for the concern, I tried to send the unit to a service outlet here in our area (Manila, Philippines), but due to its old age they decline to fix it, they say that the parts are no longer available, that is the main reason i am taking this track

Will try to measure again the DC offset while the protection is not engaged and post it here soon.

 

[Hyperion]

OK, quite a lot else you should be checking, like checking that ALL the power supply voltages are in spec'... and check for any blown fuses - again you will need the schematic and SM to help you locate/check this - do you have these?

 

[skippy124]

Hi,

Agree with Hyperion, you need to find the cause of the fault before replacing a bunch of components

The 777 is a cap coupled amp, so measuring the DC offset at the speaker terminals is not going to tell you too much, it can be several volts of there is no speakers connected

Re the protection circuit, this operates via an SCR that disables the voltage (B6) to the second transistor in the main amplifier on each channel. The SCR is turned on if there is abnormal voltage on the emitter resistor of the lower output transistor on each channel. It may be that the SCR has gone intermittent, or the SCR adjustment pot is faulty, or the bias is high on one or both channels

if you are comfortable working on live gear then you can measure the voltage across R853 and R854, or measure the voltage to ground on pin 11 and pin 12 on the 2SF-656 power supply board and report back. Be very careful not to let the meter probes slip!! If you have normal meter probes (not "mini-grabbers), then insulate the shaft of the probe tip - just leave the very tip showing.

Cheers

John

 

[Hyperion]

Thanks John - while I was up on my soapbox I forgot this is cap coupled - and no relay

 

[animefreeks]

@Hyperion yup all fuse are good

@skippy124 noted will try to get the details in the morning and will post the values, will refer to the sm i got from here for the resistors.


Thank you all for your help in advance, glad that your helping

 

[stereofun]

John (skippy123) is right on ! - If you don't feel comfortable measuring across the emitter resistors R853,854, or as John refers too - you only have "normal" probes - you can alternatively get the bias reading by removing the fuses F002/002 (between the two vertical driver boards) and then measure across the fuse holders - but measure for mA !! (the service manual available at hifiengine.com have descriptions on this) We also need the DC voltage on the pins as John described from the supply board 2SF-656. The SCR pot - VR901 - also located on that board could certainly be the culprit - these pots are failure prone, and if stored / operated in a humid climate for decades can become inoperative or intermittent. Here you should clean the pot by spraying a tiny amount of de-oxit onto it, and then move the pots several times from stop to stop. However, before you do that we want to make sure you leave the pot "in place" where you found it in case it was set correctly, and not the culprit. So setting you DMM to OHM, take a reading between one of the outer legs and the center. Note the resistance. After cleaning, set the pot to the same resistance by measuring on the SAME two legs.

A cap coupled amp like this, have no adjustment for DC-offset as we know it from Direct coupled amps (with or without relays) - instead it has adjustment for "center voltage" or as the manual calls it - "main amp section output adjustment" - while the manual calls for an oscilloscope you can still get it done using just a DMM - we can cross that bridge later. However, with the amp on, speakers selected, idle signal and either speakers OR an 8-16 ohm load connected to the terminals you selected, you can measure the DC-offset, even if you cant adjust it. Here, in an ideal world, you should see 0mV, but will likely have something along 5-15mV. Much higher than that would indicate that your Output capacitors C825/826 are not doing their job of blocking DC, something that along the other ailments should be addressed as well.

 

[Hyperion]

Great ! even skippy124's younger twin brother is here skippy123 !

 

[stereofun]

Great ! even skippy124's younger twin brother is here skippy123 !

...... Please tell me I am not the first one to slip up here ......

 

[animefreeks]

@skippy124 I have measured the voltage DC on R853 AND R854 (Hoping my orientation i correct) DMM Negative is connected on the ground/chassis of the amp, Positive terminal is tested on the upper and lower terminal (Please refer to the image below).

(Facing the amp)
R853 Upper terminal is 80Vdc / Lower terminal is 53.4Vdc
R854 Upper terminal is 80Vdc / Lower terminal is 54.4Vdc

for the 2sf-656 board I am hoping I got the right one, there is no visible tags on the board unlike the other one's

12) 0V
11) 0V


NOTE: I did the measurements with protection mode engaged(I hope I did it right )

Thank you

 

[animefreeks]

Hello

additional information gathered this week, I found pin 11 and pin 12 after cleaning the back side of the board, Pin 11 0.0V Pin 12 0.0V as stated above, also I have checked SCR901 base on the SM it is equivalent to 2sf-656 but I am not familiar with the codes they use, but I checked the voltages on them

SCR901
K - 0.0V
G - 0.8V
A - 0.9V


I also found out that R906 is getting hot, its a 330kohm 3W resistor (i think its ceramic resistor), i accidentally touch it and its quite hot, I measure the voltage of that resistor

R906
47.6V <> 22.2V.



I also checked D904 and D903 if it will help

D904
-0.8V <> 47.5V

D903
-0.8V <> 47.6V


What do you think I should check on next.........


Thank you

 

[animefreeks]

Hello checking the diagram this morning I saw that pin 1&2 should have 33v and pin 3 should have 44v
Mine is
Pin1 -0.8v
Pin 2 -0.8v
Pin 3 47.3v


Hmmmmm......... Where should i look now heheheheheh anybody??????

 

[stereofun]

Sorry you're not getting much help - It has to do with the 777 being a relatively rare piece. In addition few of us have experience troubleshooting the protection circuit. Lets first get some things cleared up. Both Skippy 124 and I wants to know what bias you are running. Skippy asked you to measure voltage across the resistors not on each terminal to ground- I offered an alternative method of measuring mA across the fuse holder. This will tell us the bias or "idle current" presently running through the output transistors.

I looked at you uploaded pin out drawing, which you have removed ? - it was mirror wise or flipped 180 if I remember. No critique here, just letting you know that we want to help and I have been looking ;-) On that note keep in mind that when you measure on one side of the rectifiers D903/04 (pin 1,2) you have to set the meter for AC on the incoming side and DC on the out going.

Ok - lets first recap what is going on.The SCR901 also known as a thryristor engages when you power on the unit. That is, it receives enough voltage feed back from EDIT: E1/2, the feed back line that goes through D901/902 as to engage the gate, which open the SCR to conduct from kathode to anode. As I read the schematics this causes B6 to be shorted to ground, effectively shutting down the output section by removing voltage for the initial stages..

Before going any further I think you should measure the bias - see earlier mail where I guide you on how to measure across the fuse holders, and report back. Moving on from here, there is one thing you can try, but you have to do it while assuming some risk ! . Alternatively have the unit looking over by a professional !

Here we go:
The voltages you are reporting are on the high side: 80V on the rails (as coming off the emitter resistor terminals to ground) and 47V coming off the transformer right after D903/04 - here the numbers should be closer to 75/44 respectively. Did you buy the unit from another country ? should the voltages selectors be set differently ? - check and make sure. Now lets assume the SCR based on the variable resistor VR901 is receiving just a little too high voltage than set for, consequently triggering the gate on the SCR to open and short B6. Here is what you can try. With the VR901 pot facing you (adjustment screw toward you), measure the resistance of the two outer legs, now calculate 5% of that. Next measure between the first two legs, left to right - and now turn clockwise until you have added 5% resistance. Now restart the amp and see if it stays out of protection. This is just something to try but again, it is not without risk. You make the choice. Cheers -

 

[animefreeks]

Hello stereofun

Yup I removed the image i created due to that the pin numbering are wrong (Sorry) I cleaned the board (TR-90A) and refer to the SM for it and I saw the pin numbers

here are the pin numbers
12) 0V
11) 0V
10) 1.1V

3) 47.3V
2) -0.8V
1) -0.8V


9) 22.0V
8) 0.2V
7) 0.2V
6) 0.2V
5) 0.2V
4) 1.3V

My dad had this Amplifier since I was a kid, it was working well back then and put on storage, the power selector is on 220V, will try to get the details and report back as soon I have them, thank you for your help. will get back to you soon