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Resistor not the same as schematic

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

Reply to
tubeguy
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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 
WA6FWi 
http:foxsmercantile.com
Reply to
Fox's Mercantile

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?
Reply to
John-Del

The most it could deliver is 440 micro amps.

--
"I am a river to my people." 
Jeff-1.0 
WA6FWi 
http:foxsmercantile.com
Reply to
Fox's Mercantile

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

Reply to
Tim Schwartz

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.

Reply to
Jeff Urban

Better first find out why the other one blew. It is likely that a decoupling cap (tantalum?) downstream has failed.

Reply to
Rob

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.

Reply to
Ralph Mowery

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