Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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Old November 29th 19, 01:13 PM posted to sci.electronics.repair
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Default 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


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Old November 29th 19, 01:36 PM posted to sci.electronics.repair
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Default Resistor not the same as schematic

On Friday, November 29, 2019 at 7:13:38 AM UTC-5, wrote:
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


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?
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Old November 29th 19, 02:16 PM posted to sci.electronics.repair
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Default Resistor not the same as schematic

On 11/29/19 6:36 AM, John-Del wrote:
100K won't deliver enough current to run a pre-amp board.


The most it could deliver is 440 micro amps.

--
"I am a river to my people."
Jeff-1.0
WA6FWi
http:foxsmercantile.com
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Old November 29th 19, 02:37 PM posted to sci.electronics.repair
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Default 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, wrote:
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




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Old November 29th 19, 02:42 PM posted to sci.electronics.repair
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First recorded activity by DIYBanter: Jan 2011
Posts: 87
Default 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 that 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 resistor, but the first step is to change it. Then you jump it out and get your 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 voltage drop across it, then ohm's law tells you if it is pulling a reasonable amount of current.

That resistor is not there as a fuse, it is there to isolate that circuit from 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 pull a few milliwatts.

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Old November 29th 19, 09:07 PM posted to sci.electronics.repair
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Default Resistor not the same as schematic

On 29/11/2019 11:13 pm, wrote:
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


**R601 is a 220 Ohm 0.5 Watt resistor. That is what is indicated on the
schematic from Hi Fi Engine.

As others have stated, you need to use your noggin when fault-finding.
For a 100k, 0.5 Watt resistor to show signs of stress (not burning) it
would need to dissipate at least 0.5 Watt. That would suggest a Voltage
drop in the order of 250 VDC across the resistor. Do you see 250VDC
anywhere in that amp?

--
Trevor Wilson
www.rageaudio.com.au
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Old November 30th 19, 08:19 AM posted to sci.electronics.repair
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Default Resistor not the same as schematic

Another thing is they could have used a one ohm, or just nothing. But that resistor is actually not a fusible, it is a decoupling resistor. Look at test instruments, they will have 12V and 12VDCPL that only means there is a coil or resistor on there to isolate DCPL from ripple on the line, high or low frequency, DCPL means decoupled.

Most of them you can actually just jump out, but at 220 ohms it was probably meant to drop a few volts. But don't jump it out permanently.

Just jump it out and see if anything smokes or gets hot, if so that is usually the fault. If not then the resistor just went bad. It happens.


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