Rotel RX-875 tuner issue

[Raccoon1400]

I'm working on a Rotel RX-875 with a non functioning tuner. It just scans for stations and finds nothing on FM and AM. I found that the supply voltage for the AM tuning chip, LA1245 was low, only about 2V, schematic says it should be 12-13V. I pulled the chip and it went up to 6. Replaced a cap and transistor (Q113, C171) and it is still around 6V with the chip pulled. The voltage entering the board is correct. I put the chip back in and it went down to 2V again.

Any ideas?

 

[Raccoon1400]

Any ideas? Looks like something else is dragging the voltage down.

 

[audiotemp]

Do you have a decent schematic? I looked at the one from the bwgroupsupport.com site but the pdf is so bad I can't read it.

 

[Raccoon1400]

Do you have a decent schematic? I looked at the one from the bwgroupsupport.com site but the pdf is so bad I can't read it.

Hifiengine has it.

 

[audiotemp]

Have you considered the possibility that the board is OK and the supply is weak? Do you mean that the voltage at the board edge is good when there is no load, or good under load and there is a drop by the time it gets to the chip? If the former, are you able to measure the current from the supply entering the board to see if that is excessive? If the voltage is indeed good at the board edge even under load, between where and where is the excessive drop?

edit: spelling

 

[Raccoon1400]

Have you considered the posibility that the board is OK and the supply is weak? Do you mean that the voltage at the board edge is good when there is no load, or good under load and there is a drop by the time it gets to the chip? If the former, are you able to measure the current from the supply entering the board to see if that is excessive? If the voltage is indeed good at the board edge even under load, between where and where is the excessive drop?

The voltage drops after Q113. It is about 14V entering the board.

 

[audiotemp]

There's no magic here, Rac.

The hfe schematic has impossible to read designations and pin numbers as well but what I am guessing is Q113 feeds both the LA1235 (FM) and LA1245 (AM), each through a separate 47 Ohm resistor. (The path to the LA1235 includes also a diode following the resistor.) You can read the current through each of those paths by measuring the voltage across each of those resistors. There is one other path off the collector through a blocking diode which shouldn't be able to sink any current but... All of the above is subject to whether I can read the lousy pdf.

 

[Raccoon1400]

There's no magic here, Rac.

The hfe schematic has impossible to read designations and pin numbers as well but what I am guessing is Q113 feeds both the LA1235 (FM) and LA1245 (AM), each through a separate 47 Ohm resistor. (The path to the LA1235 includes also a diode following the resistor.) You can read the current through each of those paths by measuring the voltage across each of those resistors. There is one other path off the collector through a blocking diode which shouldn't be able to sink any current but... All of the above is subject to whether I can read the lousy pdf.

I also noticed that the voltage on pin 14 of IC 103, LA3381 was about 2V instead of 4.5.