Pioneer SX-850 Low volume on FM only

[jeffpaletz]

I'm working on a Pioneer SX-850 that a friend bought from its original owner. It's in great condition. AM and phono sound great. FM has low volume though clean sounding with no distortion . FM reception is fine, with good signal strength and FM in stereo with the Stereo light lit up. If you turn the volume up all the way it's reasonably loud but nothing like AM or Phono. I've used the appropriate bottles of Deoxit to clean the pots and switches, including the FM de-emphsis switch on the back but the problem remains.

I'd appreciate suggestions for what to look for to fix this problem.

Thanks.

 

[markthefixer]

Take an RCA cable from the FM Det Out, to Adapt In Right Channel, Then compare the volume of the RIGHT channel (left will be dead when Adapter is ON with the lever switch DOWN) with adapter switch up and down.

C48 0.1uf 25v CSSA on the tuner board COULD be going bad and causing problems with the tuner audio output muting circuit (q8, q9, q10) which is mostly concerned with power up sequencing.

IIAC removing this cap, a troublemaking sanyo "capsistor" should cause the muting circuit to disengage, BUT this hasn't been tested, nor completely analyzed by myself.
edit - after analysis, removing cap will probably cause 100% full time muting, SHORTING that cap will allow full sound volume without one of the power on delays (the 7.5vac circuit has a small delay)

Other failures like a failing F5 low pass filter generally affect just one channel.

 

[jeffpaletz]

I get beautiful full FM volume when connecting FM det out to Adapter In right channel. I do see C48, a blue .1uf 25v capacitor on the tuner board. I don't know what you mean by IIAC. Do you think I should remove C48 and it that solves the problem, replace it? It would be easy to clip one lead and resolder it if that's not the problem component.

 

[rcs16]

I think he meant IIRC, sure change out all those old Sanyo ecaps with Nichicon UKL low leakage types or a film would even be better if it fits, try/search mouser for a 5mm lead spaced one. C47 use UKL, possible film or ukl for C48
Do not remove C48, measure it, change it if necessary.
You are going to have to trace/scope the signal gong into the PLL MPX decoder(U6) then at its o/p, through the LPF(F5) for each channel and then through the mute gates, Q8,9. The voltage at Q8,9 bases control if they are on or not. they are pnp so the +>0.6 to turn them on = muted.
the 7.5VAC at terminal 35 gets rectified to create a negative V, that and Q10 off to keep them off for a un-muted condition. check this muting action with your instruments.

Good luck Jeff
Rick

 

[markthefixer]

Well, you made me take the time to think it through.

"If I am correct", I THOUGHT doing your proposed lead clip will have one of two results: MUTE would come on hard and you would lose sound, or you would get full volume. That is IF the defective cap is acting as a resistor, and biasing q10 partially (in a grey area) ON.

Those CSSA (sky blue sanyo solid aluminum) Capacitors are guaranteed troublemakers They leak electricity until they finally turn into dead shorts. Get RID of them in ANY unit. This is a common symptom in these units ( 850, 950 ) that have a tuner output transistor muting circuit, we just haven't proved a test for it yet. The 1050 and 1250 have a sticky relay while the x80 series gave us the unobtanium pa1002 chip instead, for tuner output muting...

The operation of that cap in the circuit gets convoluted to explain, and as I think through it and the circuit, the more I think THIS :

INSTEAD OF CLIPPING THE CAP, SHORT IT for full sound volume.
But ONLY do the short at the cap's leads, maybe it can be rotated with enough lead length to get the leads to touch.
Poking around with +13v as a test is too risky for other parts of the circuit.

I now think in THIS circuit, when charged this cap's minus(-) lead is at +13 volts providing voltage to bias Q10 on.
Clipping it would shut down q10, and result in total sound loss (just like the power on mute, but the missing cap never charges and thus Q10 never conducts and releases the mute)

Of course the BEST way is to just replace this guaranteed troublemaker, and it's kin....

 

[jeffpaletz]

Thanks for taking the time to look this over. RCS16 I don't have the equipment or skills to do what you suggest. Mark, I shorted the capacitor using a metal screw driver tip and nothing changed. I ordered some new 0.1 electrolytic capacitors so I'll replace it when they come. I only saw one of the Sanyo caps on the tuner board. I'll report back after I get the capacitor installed.

The capacitors I ordered are Elna brand. Are those good quality?

 

[rcs16]

When you say you are working on them, I assume that you might have at least a DMM, so you could test the DC op of the mute ckt. To trace the signals through, you need a scope, which I guess you are saying that you do not own one. Maybe one day you could get one. Not sure how you would set offset,bias etc with out a DMM.
Elna makes lots of different ecap's but I am sure that they should be fine, for this purpose. Lets hope they fix the problem.

 

[jeffpaletz]

Yes I have a DMM and know how to use it to measure a variety of things including setting Offset and Bias. I don't own a scope. Most of my experience is in rebuilding Zenith radios from the late 1930s which involves replacing the old wax capacitors and resistors that are too far out of spec. I've also rebuilt a couple of Juke Box amps from 1955. I also repair turntables and have replaced bad transistors on my 1990s era pinball machines. In the 1970s I built some Heathkits, including a TV. I'm good at following directions but don't have the knowledge on how things work like you, Mark and some others here have.

As you may recall, you helped me determine which components to change in the low filter circuit on my SX-1980 to match the circuit in my SX-1050 because I needed the low filter to start at 30hz instead of 15hz. That's worked out very well.

 

[rcs16]

Yes I remember Jeff, I am not losing it yet
You can test out the mute circuit at least.
Select FM, you do not have to tune to a valid station, when you toogle FM mute switch what do you hear, you should hear signal un-muted and no signal when muted.
You can do some measurements to check the mute ckt operation.
1) Measure DCV at terminal 33(Mute_In) with respect to ground terminal 20(E), toggle FM mute on/off
2) Measure DCV at junction of R61(2k2) and D5, it should be a negative DC value
3) Measure the base of Q8 or Q9. For mute it should be 0.6V, for un-mute it should be a -ve DCV
4) measure voltages on Q10, e,b,c in both muting conditions,
Lets see how it is working.

Rick

 

[jeffpaletz]

I installed the replacement 0.1mf 25 volt electrolytic capacitor this morning and it solved the problem. FM is as loud as the other sources. Thanks rcs16 and Mark for your help.

There are two of the old Sanyo 0.1mf caps on the tuner board. Replacing the one that Mark indicated on the left edge of the tuner board fixed the problem. Replacing the other one which is in the middle of the tuner board did not affect the operation of the AM or FM tuner. I'm including these details in case anyone else uses this thread.

 

[rcs16]

Way to go Jeff. Certainly would not hurt to change out all the old ecaps now that one has failed. Changing ecaps on the tuner should not take it out of alignment.

 

[jeffpaletz]

I replaced the two Sanyo caps on the tuner board, both were 0.1 electrolytics. Is that what you mean by Ecaps? I have to say I was laying in bed this morning so happy that changing that one capacitor fixed the problem and amazed that Mark knows these receivers so well that upon being presented with a problem he can say "Change out C48" and that fixes the problem.

 

[rcs16]

There are more ecaps (electrolytic capacitors) in the tuner are there not?
Yep, he knows his stuff for sure, years of experience counts and everyone is lucky that he shares it with the members. No other forum has this valuable asset.

 

[jeffpaletz]

Got it. I don't know Ecaps was shorthand for electrolytic caps.

 

[wsramahi]

Hello, i have the exact same issue with my pioneer sx-3700, i can hear the fm when max the volume with little hum, can you help me locating the bad capacitor please?

 

[EastPoint]

Hi, this thread is about an SX-850. Yours is not that one. Start a new thread.