Resistor not the same as schematic

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Its a Sansui 5050 receiver. The power amp works, the radio tuning meter
moves, so it seems that works. But the preamp board is dead. Not a
single transistor or any other component on that board has voltage.
There is 44vdc on a wire from the power supply. It goes to a resistor.
No voltage on other side of this resistor. Multimeter on ohm scale
confirms resistor is open.  

The problem is that resistor is red red brown @ 5% (gold). Thats 220
ohms. Yet an online schematic shows R601 as 100K.  

OK, I know that resistor is dead, but how can the actual color code be
so different from schematic? The resistor is not burnt, but may have
gotten warm. Yet the color code is plain to see, except the gold band is
kind of greenish.....

My plan is to clip in a resistor, but where do I start with such a
conflict of values....

Yes, I triple checked this is R601. Unless the schematic is labeled
wrong????

This appears to be the only voltage source to this board.....

Any ideas or suggestions?

Thanks


Re: Resistor not the same as schematic
On 11/29/19 6:13 AM, snipped-for-privacy@myshop.com wrote:
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Think about that for a minute. How is any of the 44 volts
supposed to get though a 100K resistor and be able to do
anything?

Replace the 220 ohm resistor with another 220 ohm.


--  
"I am a river to my people."
Jeff-1.0
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Re: Resistor not the same as schematic
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Better first find out why the other one blew.  It is likely that
a decoupling cap (tantalum?) downstream has failed.

Re: Resistor not the same as schematic
On Friday, November 29, 2019 at 7:13:38 AM UTC-5, snipped-for-privacy@myshop.com wrote:
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Schematic errors are very common.  We expect them and are never surprised by them.

100K won't deliver enough current to run a pre-amp board.  If it is the B+ feed to the preamp, then it's going to be a much lower value resistor.  Does the schematic show any voltages on the transistors that the 44V source feeds?  

Re: Resistor not the same as schematic
On 11/29/19 6:36 AM, John-Del wrote:
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The most it could deliver is 440 micro amps.

--  
"I am a river to my people."
Jeff-1.0
We've slightly trimmed the long signature. Click to see the full one.
Re: Resistor not the same as schematic
Looking at a schematic, there is an R601 on the F2646 Tone board (The  
one with the tone control pots) shown as a 220 ohm, 1/4 watt fuse  
resistor.  There is a 100K R601 shown on the F2648 board schematic, but  
not the parts list, that I could find.

    So, have a look at the manual and make sure you are looking at the  
diagram for the correct board, as there are (at least) TWO R601's in  
this unit.  Or to quote a Canadian friend: "Clear as Mud, Eh?"

Regards,
Tim Schwartz
Bristol Electronics




On 11/29/2019 7:13 AM, snipped-for-privacy@myshop.com wrote:
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Re: Resistor not the same as schematic
The print I just got from hifiengine says it is 220 ohms.  

There are errors all over the place, I have seen a lie, which represents a  
wire or PC foil as having 12 volts on one side and zero on the other.  

You have to use your common sense. Look at the thing, how the hell could th
at be a 100K ? And where did you get that print ?  

Sometimes it is bad if you don't catch it. I was fixing a Phase Linear 400-
2 and the print had the emitter resistors as 0.22, but they were 0.33s. If  
I were to have put in 0.22s you know what would have happened ? Do you know
 how critical such resistors are ?  

And then it would be a good idea to find out if something else blew that re
sistor, but the first step is to change it. Then you jump it out and get yo
ur voltage readings (compare channel to channel) and you could conceivably  
find some smoke to follow. And feel around, see if anything is hot.  

Maybe even just jump it out first. Check for proper operation and any heat  
buildup. Ten when you put a resistor actually in there then watch the volta
ge drop across it, then ohm's law tells you if it is pulling a reasonable a
mount of current.  

That resistor is not there as a fuse, it is there to isolate that circuit f
rom power supply fluctuations.  

Being a 220 ohm it is not as likely as resistors over 100K to open on their
 own, but it does happen. But it says half watt. That circuit should only p
ull a few milliwatts.  


Re: Resistor not the same as schematic
snipped-for-privacy@gmail.com says...
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Companies are always making minor production changes that do not show up  
in the schematics.  Going from .22 to .33 emitter resistors may not make  
a noticable change to most, but maybe someone with a very good ear could  
tell a difference.  

I have seen service bullitens that come out and they tell to change a  
part to another value for various reasons.  A 10 or 20 % change may not  
mean much to many, but going 10 or more times is often very noticable.

It would be very difficult to have 12 volts on one end of a wire and  
zero on the other that is the size of a PC board trace.  

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