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SANSUI AU 7900, HELP!

jFF_Bulbo

jFF_Bulbo

Active Member
Hello AK'ers.
I´m working on a AU-7900.
before the recep: No short on DBT, no relay click
After recap: No short on DBT, no relay click

I recaped F2596 board, all 'litycs are new.
All output transistors are new's, On Semi 93g/94g installed.
Transistors TR601, 602, 603, 605 & 605 are new. Relay also new.
All VD1212 diodes replaced with 2x4148
Diff transistors A798 replaced with KSA992....

Well... there is no relay click. on the collectos of TR03 i have 44volts (conected to DBT...) is should be 24v.
The voltage on differencial is about 44 volts (shold be 0.6v according SM)
The bias work well a few seconds... it is about 17mv (with DBT) but then it downs to zero, both channels.....

I don´t know what can be......
i post some pics, maybe someone can see something i missed. Thank you for your help.


tOOUnen.jpg


jQN80vc.jpg


R8TYtyG.jpg
 
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jFF_Bulbo said:
All output transistors are new's, On Semi 93g/94g installed.
Click to expand...
Why would you replace the output transistors?
The MJ's are not good substitutes, although they will work and cause no damage they are less than half the bandwidth of the originals and will not give the same performance. If you have the originals and they test OK, put them back in.

Always solve the faults before carrying out any restoration work, you now have a whole lot more possibilities for things which may be wrong.

Can you measure the DC offset at a position before the relay?

Also, if there is no short on the DBT, take it off the DBT as the measurements you make on the DBT are meaningless because of the current and voltage limiting.
 
your base and collector seem swapped .
most likely fuse resistors as mentioned earlier .perhaps R41
jFF_Bulbo said:
Well... there is no relay click. on the collectos of TR03 i have 44volts (conected to DBT...) is should be 24v.
The voltage on differencial is about 44 volts (shold be 0.6v according SM)
Click to expand...
 
Last edited:
Thinking about this, as the other guys here are saying you might want to look at the fusible resistors, I do recall replacing fusible resistors in this model, Ive done a few 7900's and they have a few gotcha's like that.
 
jFF_Bulbo said:
Hello AK'ers.
I´m working on a AU-7900.
before the recep: No short on DBT, no relay click
After recap: No short on DBT, no relay click

I recaped F2596 board, all 'litycs are new.
All output transistors are new's, On Semi 93g/94g installed.
Transistors TR601, 602, 603, 605 & 605 are new. Relay also new.
All VD1212 diodes replaced with 2x4148
Diff transistors A798 replaced with KSA992....

Well... there is no relay click. on the collectos of TR03 i have 44volts (conected to DBT...) is should be 24v.
The voltage on differencial is about 44 volts (shold be 0.6v according SM)
The bias work well a few seconds... it is about 17mv (with DBT) but then it downs to zero, both channels.....

I don´t know what can be......
i post some pics, maybe someone can see something i missed. Thank you for your help.


tOOUnen.jpg


jQN80vc.jpg


R8TYtyG.jpg
Click to expand...
Put the differential A-798's back in or connect the ksa992s like this.20200620_075321.jpg
 
Willy6 said:
I changed the 798s out on mine that way and ended up putting them back in because it wasn't working right.
Click to expand...
Yes because those package transistors are carefully matched and thermally coupled, so it pays to not replace them unless you really have to.
 
kevzep said:
Why would you replace the output transistors?
The MJ's are not good substitutes, although they will work and cause no damage they are less than half the bandwidth of the originals and will not give the same performance. If you have the originals and they test OK, put them back in.

Always solve the faults before carrying out any restoration work, you now have a whole lot more possibilities for things which may be wrong.

Can you measure the DC offset at a position before the relay?

Also, if there is no short on the DBT, take it off the DBT as the measurements you make on the DBT are meaningless because of the current and voltage limiting.
Click to expand...
hello.

the unit has 4 original d388/b541 and 4 fakes, on semi 93/94g used to replace them.

a didn´t explain thist... but after recap, relay work fine, dc offset about a few mV, bias on one side perfect on 18mV (ksa992 matched already installed at this point) bias on the other side was zero. BUUUUUUT when i tried to check the zero bias, i accidentally left down the board...... about 3-5 cms.... i dont know if it touched the main caps... or something else... but after that little fall the relay didn´t work anymore...

I´M ASHAMED FOR THAT...

WELL... i search for bias problem and found the VR01 was broken.... i changed it but the unit still in protection.

I work the amplifiers by placing them on their side, so I do not move them during their restoration
 
Willy6 said:
Put the differential A-798's back in or connect the ksa992s like this.View attachment 2345208
Click to expand...
i use KSA992 (mouser) with hfe about 400 and matched.....
the original A798 one seems good on transistor tester, hfe : 360/362, but the other ones had hfe: 359/216.
first time i turn on th eunit qith KSA's, relay worked, but id had an little accident (I released the circuit board from a tight 3-5 ms height on its side, maybe it touched the main capacitors, maybe not)
and after that relay stop workig... 44v measured on the relay coil, the same for tr03 collector (should be 24v)...
 
UPDATE 19-9-21
All the transistor was pulled out from the board, all tested OK (A917, C1951, A733, C945... ALL OK)
I reinstalled the previous D388/B541 output transistors.
i Changed the IC01&IC02 (3 times...) with new KSA992 matched, hfe 366/365 and 395/396.
TR03 (c1364) changed with C2383 and also with KSC1815.

DBT: OK
DC OFFSET about 30 volts. (yes.... volts :()
BIAS: FINE FOR A FEW SECONDS. it rises well and stable for seconds.... suddenly bias go down to zero.

it is blowing my mind.
 
I think you will have to check every transistor, not just the ones you replaced, but also the ones you didn't replace, just like the zener diodes.

A few months ago I recapped an Au-7900 and I really don't see anything abnormal in your photos.
 
I do not understand why you keep changing components which are not faulty.
Replacing a 2SA798 with 2 separate KSA992 is not a good substitute although it will work if you have no other option.
A handheld transistor tester does not give an accurate HFE reading, you need to do it properly with full operating voltage and a curve tracer.
The 2SA798 is thermally coupled together, this means the gain tracks perfectly as the heat changes. Two separate transistors cannot do this.
If the 2SA798 is not faulty, put it back in.

The MJ21193/94 as I have already said is not a good replacement for the original transistors, they have less than half the bandwidth (ft) then the originals, this does change the sound.
You want to use MJ15015G and MJ15016G, 6mhz and 18mhz respectively.
These will give closer performance to the original transistors.

The reason you see 44V on the relay coil is because the relay is not on, when the relay is turned on then there will be 24V on the coil.

You are seeing 30V on the output because there may be an imbalance in the power supply to the front end of the driver circuit which is too great to equalise with the DC offset adjustment.

You need to print the schematic, get yourself a highlighter, and verify all voltages are correct. When you find something is not right, then you can investigate that part of the circuit.

You need to stop pulling components and swapping things because eventually you are going to damage the circuit board traces and you are going to then have bigger problems.

Let us know how you get on.
 
kevzep said:
I do not understand why you keep changing components which are not faulty.
Replacing a 2SA798 with 2 separate KSA992 is not a good substitute although it will work if you have no other option.
A handheld transistor tester does not give an accurate HFE reading, you need to do it properly with full operating voltage and a curve tracer.
The 2SA798 is thermally coupled together, this means the gain tracks perfectly as the heat changes. Two separate transistors cannot do this.
If the 2SA798 is not faulty, put it back in.

The MJ21193/94 as I have already said is not a good replacement for the original transistors, they have less than half the bandwidth (ft) then the originals, this does change the sound.
You want to use MJ15015G and MJ15016G, 6mhz and 18mhz respectively.
These will give closer performance to the original transistors.

The reason you see 44V on the relay coil is because the relay is not on, when the relay is turned on then there will be 24V on the coil.

You are seeing 30V on the output because there may be an imbalance in the power supply to the front end of the driver circuit which is too great to equalise with the DC offset adjustment.

You need to print the schematic, get yourself a highlighter, and verify all voltages are correct. When you find something is not right, then you can investigate that part of the circuit.

You need to stop pulling components and swapping things because eventually you are going to damage the circuit board traces and you are going to then have bigger problems.

Let us know how you get on.
Click to expand...
Well. This voltages Taken with DBT.
I reinstalled the original A798 on one channel and the d388/b541 are reinstalled too.

Q9dZwwe.jpg

All is going good until R12 . 13v on schematics. 45v measured.
Will keep testing voltages.
 
Last edited:
I think you might have a faulty Zener, ZD02.
Looks like a 13V , I believe this is where your problem is.
 
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