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F-1303 pre-amp issue right channel - completely lost (including myself)

matjo

matjo

Member
Dear highly regarded Sansui-specialists.

First of all, I hope everybody is healthy and safe in this strange, strange corona-time.

After restoring a 555a with the help of Leestereo, Stereofun, Slimcity and The Fuxtor, I decided this time to do another project, a AU-101. Still not a professional electro-technical Engineer (not even on amateur level I suppose), but learning every time more and more (where this Forum is a huge base of knowledge).

After the recap, I am confronted with a problem: The right channel of the Phono-stage is not working. AUX is doing fine, so I guess it would be a problem on the F-1303 board, at the Phono-preamp.

I had no idea where to start. I double checked the Transistor-wiring (KSC1845FTA), I desoldered and desoldered every component. Checked the solder joints. Checked for broken leads (due to the amount of soldering I ruined a few places, but was able to fix them, see pictures attached).

Even bought another AU-101 to compare. started to measure the DC-voltage on the F-1303, starting on the other side of the board. And there my curiosity started: The results were very, very different, also compared to what is stated on the schematic in the user manual.

on original AU-101: voltage after R002: 21,91V (schematic states 22,5V) - I have 30V !
on original AU-101: voltage after R615 & R616: 10V (like on schematic) - I have on R616 29,9V, on R615 16V !
on original AU-101:voltage after R609 & R610: 1,77V - I have on R609 2V and on R610 8,6V !

If I test the resistors, they seems to be fine, simular to the values in the schematic.

I must say that I do not own a super-reliable device like a Fluke; I am stuck with a much cheaper multi-meter, an even cheaper capacitor/transistor ESR meter and a very old Hameg 20 Mhz Oscilloscope (HM-203-5).

Measured all the values of the replaced components; they where all good.

WHAT AM I NOT SEEING? Am I looking at the right place???

I have attached some pictures to be more clear; really hope somebody can tell me where I am making a mistake.....

IMG-2511.jpg IMG-2512.jpg IMG-2513.jpg IMG-2509.jpg IMG-2510.jpg




I have changed the following components:
C601-602 Radial Leaded Aluminium Electrolytic Capacitors - Radial Leaded 1uF 50V 20% Audio SILMIC II
TR601-604 & TR701-702 KSC 1845FTA Bipolar Transistors
C001 Snap In Aluminium Electrolytic Capacitors - Snap In 4700uF 63 Volts 20% Tol.
C605-606-609-610 Radial Leaded Aluminium Electrolytic Capacitors - Radial Leaded 10uF 25V 20% Audio SILMIC II
C003-004 Radial Leaded Aluminium Electrolytic Capacitors - Radial Leaded 50volts 470uF 16x35.5
C603-604-607-608 Film Capacitors 68pF 1000 Volts 5%
C611-612 Radial Leaded Aluminium Electrolytic Capacitors - Radial Leaded 4.7uF 25V 20% Audio SILMIC II
C613-614 Film Capacitors 100V 0.012uF 5% LS=5mm
C615-616 Film Capacitors 4700 pF 63 VDC 2.5%
C617 Film Capacitors 220 pF 100 VDC 2.5%
C618 Film Capacitors 250V 0.033uF 5% LS=5mm
C701-702 Film Capacitors 330 pF 100 VDC 2.5%
C703-704-715-716 Film Capacitors .022uF 63 Volts 5%
C703-704 Film Capacitors 0.022 uF 100 VDC 5%
C713-714 Film Capacitors 2200 pF 63 VDC 2.5%
C713-714 Film Capacitors 2200pF 63 Volts 10%
C717-718 Film Capacitors 22000pF 100V 1% Orange Drop
C719-720 Film Capacitors 630VDC 0.22uF 10% MPP L/S=7.5mm
 
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matjo said:
WHAT AM I NOT SEEING? Am I looking at the right place???
Click to expand...
I think you are almost certainly not looking in the right place, you are looking at a symptom - not a cause.

What voltage do you have across C001 (now 4700µF, 63V) ?
 
Last edited:
Dear John, thank you for your fast reply.

Always willing to learn, especially on Sansui's! Like I said, I have serious limited knowledge about these matters....sorry about that.

The voltage across C001 I measured is 49,3V. (DC). I attached a picture to show how I measured. The red wire is going from the main board (F1302-3) to the F1303 board.

Thanks already for your time and help.

Greetings from the Netherlands!

IMG-2514.jpg
 
matjo said:
The voltage across C001 I measured is 49,3V. (DC).
Click to expand...
Well, that partly explains the issue, that voltage should be 44V according to the schematic, so that voltage is starting off just over 5V too high, it might be as expected, but it depends on the answers to the next two questions.

Next questions - what is the mains voltage of the unit set to, (220V? or 240V?) and what is the mains voltage in your area? (if you know it)
 
The mains voltage of the amplifier is set to 220 V. The mains voltage as I read of the socket where the amp is plugged into measures 236 V (We have 230V at 50 Hz as a standard the Netherlands).

I did give it a thought in the beginning....could the 5V difference at C001 make such a difference that I would measure 29,9V after the R616? I thought that that was not possible, so I excluded the 5V difference as a problem... curious if it will now shows that I was wrong in my way of thinking.
 
by the way, I also did recap the main board. Did not mention it since I thought it had nothing to do with the issue.......
 
With 236V as the local (measured) mains voltage, and the Netherlands standard as 230V, I would set the mains voltage to 240V if I were you. ;)
 
We achieved some improvements, but still some readings are way out of spec.

I changed the mains voltage to 240V.
Now the voltage over C001 is 45,2V (was 49,3V)
After R001: 33V (was 35,8V)
After R002: 27,4V (was 30V)
After R610 it is now 8,56V (was 8,6V), still way too high compared to the "not touched" AU-101 (1,77V).
After R616 it is now 27,4V (was 29,9V), still way too high compared to the "not touched" AU-101 (10V).
 
I am gradually coming to the conclusion that some or all of your transistors are installed incorrectly and therefore not drawing current - hence the high and weird voltages everywhere.
 
On both channels the voltage went down a bit,

On the left channel:
After R609 it is now 1,93V (was 2V), higher compared to the "not touched" AU-101 (1,77V).
After R615 it is now 14,64V (was 16V), still higher compared to the "not touched" AU-101 (10V).

The R001, according to the color code has a value of 1,5 kOhm
With the multimeter it shows 1,54kOhm
 
Double check the installation of your transistors TR601-TR604 - as Rob says at least TR604 looks like it is installed with B & E reversed.
 
I agree with Hyperion and others. You probably have either TR602 or TR604 installed incorrectly. The best way to verify is to identify which way it mounts by verifying what other components are connected to the base, then collector, then emitter. Compare to the schematic connections rather than any marking s on the board itself or the board pictorial. Don't trust the marking on the board because they can be incorrect. (either TR602, or TR604 being installed backwards can cause that very high voltage reading after R616. Or any broken foil connections repating to TR602 or TR604 could also cause similar problems.
 
Robisme said:
Is tr604 in backwards?

Rob
Click to expand...


Hyperion said:
I am gradually coming to the conclusion that some or all of your transistors are installed incorrectly and therefore not drawing current - hence the high and weird voltages everywhere.
Click to expand...

Dear Rob & John. How stupid I feel right now...... I was so convinced that I installed the transistors were installed correctly (spending really hours on determine the leads on the board and on the resistor) that I never triple-checked it.

So you were right! The TR604 was installed the wrong way around. Now I have the following readings:

Right channel
TR616: 11,4V
TR610: 1,74V

Left channel:
TR615: 10,92V
TR609: 1,75V

I will check later if I will have sound on the right channel again. I will let you know the results.

A great great great thank you for the support, time and patience......

And yes....it is now time to start to make fun of the Dutch. Especially when you realize that I made exactly the same mistake on my first project, the AU-555a.
 
Tom B said:
I agree with Hyperion and others. You probably have either TR602 or TR604 installed incorrectly. The best way to verify is to identify which way it mounts by verifying what other components are connected to the base, then collector, then emitter. Compare to the schematic connections rather than any marking s on the board itself or the board pictorial. Don't trust the marking on the board because they can be incorrect. (either TR602, or TR604 being installed backwards can cause that very high voltage reading after R616. Or any broken foil connections repating to TR602 or TR604 could also cause similar problems.
Click to expand...
Also thank you Tom, you were right like the others.......
 
I was going to show a picture of the 2SC871, but it appears you and the others have determined that was indeed the problem. For posterity's sake showing 871's pinout of BCE:

871.jpg
 
UPDATE: Everything is working! Thanks to everybody here on this forum, especially for this project: Hyperion, Tom B, Robisme and lbcgav.

Next project: A recap of another AU-101, restaurantion of a pair of SP-200 speakers (any opinion?), upgrading a Pioneer PL-514X and finding the mistake I made with a AU-505 (mot probably the same, so must not be too hard).

Stay healthy and take care everybody!!!
 
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