Pioneer SX-838 No FM reception ?

[Mark Blom]

I have been going through many threads looking for information on this. I found one but it never said what the fix was.
I have a Pioneer SX-838 that had a few issues I worked most of them out except one.
The FM has static with the FM muting off but I get nothing at all in the way of stations.
I'm tempted to just recap and change the transistors on the tuner board but my worry is that it maybe a IC chip is bad. This board is a pain to work on because it has to be removed to get to the bottom so I don't want to change stuff and not fix the problem.
AM works fine, I have cleaned all the switches and those seem fine now also.
The FM sounds like its just off of a station but I get nothing trying many antennas
Has anyone had this before or have ideas on what voltages I could check to narrow down the problem ?

 

[Skywatcher]

I have been going through many threads looking for information on this. I found one but it never said what the fix was.
I have a Pioneer SX-838 that had a few issues I worked most of them out except one.
The FM has static with the FM muting off but I get nothing at all in the way of stations.
I'm tempted to just recap and change the transistors on the tuner board but my worry is that it maybe a IC chip is bad. This board is a pain to work on because it has to be removed to get to the bottom so I don't want to change stuff and not fix the problem.
AM works fine, I have cleaned all the switches and those seem fine now also.
The FM sounds like its just off of a station but I get nothing trying many antennas
Has anyone had this before or have ideas on what voltages I could check to narrow down the problem ?

Voltages should be in the schematic to check...

 

[Mark Blom]

Yes it does I will check them today.

 

[Mark Blom]

So I found something strange last night checking voltages on the tuner board. Most voltages checked close to what they should be but that beside the point. there are only some voltages listed on the schematic. Ok Anyway on q1 on the tuner board the first 3SK45B when I tested the voltage when I touched my lead to the Drain all of the sudden the fm started working I thought at first maybe something was loose like a bad solder joint but no. It only worked when my lead was laying over the T1 which is the FM antenna coil so I think the RF was going through my lead and making it work. It was rather intermittent on getting it to do it but worked. So still not sure if its the 3SK or something like a CAP in between but the problem is in that area for sure.

 

[fromnowon]

Interesting!
The Pioneer forum on here is GREAT, I just wonder if you'd get more input over there.

 

[Skywatcher]

So I found something strange last night checking voltages on the tuner board. Most voltages checked close to what they should be but that beside the point. there are only some voltages listed on the schematic. Ok Anyway on q1 on the tuner board the first 3SK45B when I tested the voltage when I touched my lead to the Drain all of the sudden the fm started working I thought at first maybe something was loose like a bad solder joint but no. It only worked when my lead was laying over the T1 which is the FM antenna coil so I think the RF was going through my lead and making it work. It was rather intermittent on getting it to do it but worked. So still not sure if its the 3SK or something like a CAP in between but the problem is in that area for sure.

Well it looks like you narrowed it down. Could be a resistor has drifted high enough to where the FET isn't being turned on. Or the FET no longer closes one or both of the gates. It's an N-channel dual gate FET. The NTE sub could be 222 or 454, either should work. Here is the datasheet for the NTE454 https://www.nteinc.com/specs/400to499/pdf/nte454.pdf
The NTE 222 https://weisd.com/uploads/product/sheet_pdf/21307/NTE222.pdf
The Hitachi 3SK45B https://pdf1.alldatasheet.com/datasheet-pdf/view/130317/HITACHI/3SK45.html
Or try and find an original somewhere. Be VERY careful with an RF FET as the slightest static discharge can pop them. The new one should come with a "shorting" ring on all the leads that you leave on until after it's installed, then remove it.