5000 Bias Diode and Transistor - need Tech help!!

S

skippy124

Super Member
Hi,

I am currently repairing/restoring a 5000, and have read the sticky regarding the F1040 driver boards, and I would like to replace the LV-2 bias diodes as per the sticky.

The reason for this thread is twofold, the alternative bias diodes shown in the sticky - NTE-605a and STV-2h are not available locally, and from what I can gather are almost impossible to get hold of anywhere, so I need an alternative for these. The second reason for the thread is that the fault on the 5000 I have was left channel distortion, which tuned out to be a corroded base wire on the 2SA485 bias transistor, and I am having difficulty finding a suitable alternative for the 2SA485/2SC485 pair.

My thoughts are as follows, and this is what I would like some opinions of from the techs here,

1. I believe that it would be possible to use 2 1N4148 diodes in place of the old LV-2 diodes??

2. In looking for suitable replacements for the 2SA485/2SC485 bias transistors, I have found either 2SA1358/2SC3421, or potentially BD139/BD140 as replacements electrically, but case style differs. Basic specs as follows taken from the datasheetarchive site (only the PNP part is shown)

2SA485
AMH
Various
pnp transistors,transistors
AMH, 2SA485, Silicon, PNP, 800mW, 80V, 80V, 5V, 1.5A, 150

2SA485
Silicon PNP
N/A
pnp transistors,transistors
Category: PNP Transistor, Transistor
MHz: 20 MHz
Amps: 1.5A
Volts: 80V

2SA1358
AMA
Various
pnp transistors,transistors
AMA, 2SA1358, Silicon, PNP, 10W, 120V, 120V, 1A, 150

2SA1358
Silicon PNP
N/A
pnp transistors,transistors
Category: PNP Transistor, Transistor
MHz: 120 MHz
Amps: 1A
Volts: 120V

BD140.16
Si PNP Power BJT
Motorola
npn transistors,pnp transistors,transistors
V(BR)CEO (V)=80
V(BR)CBO (V)=100
I(C) Abs.(A) Collector Current=1.5
Absolute Max. Power Diss. (W)=8.0
I(CBO) Max. (A)=100n
h(FE) Min. Static Current Gain=120
h(FE) Max. Current gain.=250
@I(C) (A) (Test Condition)=150m
@V(CE) (V) (Test Condition)=2.0
f(T) Min. (Hz) Transition Freq=75M
Package=TO-126
Military=N

I beleive that the BD140 looks pretty close except for the higher Ft, will that be a problem??

If it is feasible to use either the 2SA1358/2SC3421, or the BD139/BD140, then this is the way I intend to do the physical arrangement for the bias diodes -

2 1N1418's soldered in series, some heatshrink and a BD140

5000biasdiodemod1.jpg


assembled together

5000biasdiodemod2.jpg


I would be greatful of comments/suggestions etc.... am I on the right track or am I "going up the garden path"??"

For reference the circuit for the F1040-1 is as follows (the F1040 I have has Tr805 and Tr806 as 2SC756) and apologies for the poor

5000ampdiagram.jpg


Thanks for looking and hope I can get some feedback/opinions/guidance.

Cheers

John
 
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If you can not get enough bias voltage swing, you need to increase number of diodes. You check bias voltage across out put emitter resistors of both channel. Good luck
 
birdyman said:
If you can not get enough bias voltage swing, you need to increase number of diodes. You check bias voltage across out put emitter resistors of both channel. Good luck
Click to expand...

Thanks for the advice, I am hoping that the two diodes will be enough. Interesting that the 5000 service manual does not include bias and centre voltage adjustment procedures, but has the full am/fm ajustemnt procedure. I have a 5000A manual, and this has the amplifier bias current adjustment procedures but not the centrs voltage procedure (and this must be an early 5000A manual because it has the early F1040 driver board). Bias current is set by measuring the total driver PCA current at the protection fuse, and it is 23ma.

The 5000A manual also has the protection circuit adjustment procedure, (not in the 5000 and 5000X manuals), and the 5000X service manual has both bias and centre adjustment procedures.

Got to love Sansui service manuals!!
 
skippy124 said:
Thanks Kev, I figured that this would be a worthwhile exercise and if it works satisfactorily it gives an option to do the mod using commonly available components.
Click to expand...

I had great success using a VBE multiplier on my G-7700 hybrid, it works really well.....

Here is a link to the diagram.http://www.audiokarma.org/forums/attachment.php?attachmentid=226429&d=1282273141

I could reduce the sensitivity of the adjustment by increasing the value of the resistors, but I didn't end up doing that because I have a steady hand.....:D
 
Well......

Completed the modification to the left channel tonight, and I'm pleased to report that so far it works very well!! Bias has been set at 23ma (measured at the protection fuse) as per the service manual, and is pretty much rock steady (and the adjustment point wasn't too far off the original setting). I am going to do some soak testing tomorrow (and maybe even some comparative temp measurements between the old and new transistors if I can find my temperature meter and thermocouples), but the finger test shows about the same for both sides.

I ended up doing it differently than the example in the first post, I noticed that one side of the bias diodes was connected to the base of the associated transistor via the PCB, so I decided to make use of this.

First I bent the legs of the new transistor to follow the PCB hole pattern and did some test fitting. Note the holes in the PCB are small and the BD139/140 legs are big so the holes need to be clean, and no solder on the pad side encroaching on the hole.

sansui5000BD139_140legsbent.jpg


1N4148 Diodes assembled to the transistor

sansui5000BD139_140withdiodes.jpg


Finished assemblies (white wire is on the BD139, anode of the first diode connected to the base, cathode of the second diode to the white wire, red wire is on the BD140, cathode of the first diode connected to the base, anode of the second diode connected to the red wire)

sansui5000BD139_140finished.jpg


and fitted into the Driver PCA

sansui5000BD139_140assembled.jpg


Once I have completed testing the right channel will be modified in the same way. At the moment the right channel bias is struggling to get above 19ma, it will be interesting to see if this is resolved after the mod is complete.

Now, A word of warning for people with the older F1040 PCA's (at least, and this may be the same for some of the F1040-1). As stated in the first post, the left channel driver fault on this 5000 was caused by the base leg corroding off the body of the transistor. When the 2SC485 was removed, there was extensive corrosion present on it's base leg as well

sansui5000oldbiastransistors.jpg


Note that the base leg has sleeving.... and this is the leg that has corroded. The unit is pretty clean inside, almost no signs of surface corrosion etc but it is interesting to note that there is a spot of corrosion on the chassis directly under the bias transistors (see the fitted into the driver PCA pic above). There is signs of lesser corrosion on the chassis under the right channel driver, and a peek at the upper bias transistor shows the same corrosion on the base leg. There is obviously some mechanism that is causing this, perhaps the sleeving traps moisture??. In my particular unit they were a time bomb waiting to go off, so if anyone is modding these in the future, or has the top off for any reason, have a look at these bias transistors for sleeving on the base leg and/or signs of corrosion......

If it is considered worthwhile, I'll post up the mod instructions and the warning re the corrosion in the 5000 sticky, or perhaps post a link to this thread???

Will report further progress tomorrow

Cheers

John
 
Last edited:
skippy124 said:
Thanks for the advice, I am hoping that the two diodes will be enough. Interesting that the 5000 service manual does not include bias and centre voltage adjustment procedures, but has the full am/fm ajustemnt procedure. I have a 5000A manual, and this has the amplifier bias current adjustment procedures but not the centrs voltage procedure (and this must be an early 5000A manual because it has the early F1040 driver board). Bias current is set by measuring the total driver PCA current at the protection fuse, and it is 23ma.

The 5000A manual also has the protection circuit adjustment procedure, (not in the 5000 and 5000X manuals), and the 5000X service manual has both bias and centre adjustment procedures.

Got to love Sansui service manuals!!
Click to expand...

Great Job and thanks for the details. This info belongs in the sticky as you suggested for sure, maybe add a post to that thread and link to this one.

Could you post the protection circuit adjustment procedure.....
 
goat67 said:
Great Job and thanks for the details. This info belongs in the sticky as you suggested for sure, maybe add a post to that thread and link to this one.

Could you post the protection circuit adjustment procedure.....
Click to expand...

Will do both once I have completed the testing

The protector circuit from the 5000A service manual is as follows (I haven't checked as yet if this is the same PCA in the 5000 an 5000X)

Sansui5000Aprotectoradjustment.jpg


Apologies for the poor copy.....

Cheers

John
 
Completed the other channel earlier today, and the lack of sufficient bias current adjustment in this channel is now fixed. There was some corrosion on the base of the 2SC485, but none the base of the 2SA485, it had corrosion on the emitter leg, so the theory of the sleeving contributing to the corrosion doesn't appear to be valid.

Before doing the right channel, I compared case temps of the BD139 in the left channel and the 2SC485 in the right, and they were within a couple of degrees of each other, am happy with that.

Also replaced to outputs in one side, used MJ15003's and they look a pretty good match to the original 2SD118

I will make a post in the 5000 sticky thread tomorrow showing the details of the modifications.

And thanks for the kind words!!

Cheers

John
 
It was great to hear this amp up and running tonight. Plenty of grunt and she sounds beautiful! I'm quite impressed with this beasty.
 
Nicely done. As far as the corrosion, who knows maybe the assembler put a leg of each transistor in her mouth as she was putting it together. Really had to know with out being there when it was built. Again, nice work getting it going and giving a path for others with the F1040 boards.:music:
 
Thanks John for including your work and ideas in the sticky note at the top of the forum list. Nice work!
Casey
 
Hi all,

I recently got a Sansui 5000a from a friend which initially needed only minor fixes: some dial lights were broken and the potentiometers needed some deoxit. According to my friend, besides some noise when turning the knobs the receiver should play perfectly. I did not verify this yet. Then I found out about the F-1040-1 board problems. I was hoping to upgrade the board with the very good and usefull information in this thread (thank you skippy124). When I opened the receiver up, I noticed something strange. The diodes above the transistors which cause the initial problems are not in any way thermally coupled to the transistors. They just float above them (all of them), leaving some space (a few mm) in between them, see this image:

DSC01256_small.JPG


Did anybody ever seen this before? I assume that this is not standard? Is this because somebody already messed in the receiver and removed the holders of these diodes? Or did they forgot them in the factory? I cannot understand that this receiver works for longer periods of time without the thermal feedback. Many thanks for your opinions in advance!

Best regards,
Krikke
 
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