Fisher RS-851 ... No FM, bad amp.

[eccerr0r]

Well, I bought a Fisher RS-851 as a gamble as I want to replace my dying and annoying Sony stereo that I probably will toss (though it still works, it has mechanical failures: all rubber parts need to be replaced and all the switches need cleaning). I lost the "bet", the Fisher is broken - but no matter, I wonder how bad it is to fix it. I pretty much knew this thing was bad, but did not know how bad.

Oddly enough there are two things that I can tell are wrong with it. It powers up at least, but FM reception is nonexistent - all I hear is static. Trying to receive AM, I can hear the local stations but the sound is terrible - really scratchy. Feeding in some line-in signals produces the same scratchy sound. The graphic equalizer however seems to sound like it's working but hard to gauge without without some real music samples.

I am sort of mad because of the multiple failures, I wouldn't be so upset if it had a single failure like if the FM worked and amp didn't or the other way around - but perhaps both issues are one and the same? I hope?

Anyone with a schematic?

[eccerr0r]

Re: Fisher RS-851 ... No FM, bad amp.

It's a bad sign when Google starts pointing to BCN back to my original post
Anyway this looks like an Sanyo LA1266 (RF/IF mixer) / LA3400 (Detector) / TC9172P (DPLL?) digital tuning system. Now I wonder how the digital PLL is fed back in, all I'm familiar with are varactor diodes used to change capacitance to do tuning, however, PLL you can get the exact frequency you want to mix into IF and the varactor isn't really needed except perhaps for the initial antenna tank circuit tuning...

Ah analog. I've been doing too much digital *sigh*

[rievax_60]

Re: Fisher RS-851 ... No FM, bad amp.

Try to find the VT rail. It applies tuning voltage to the center pin of the vaicap diodes via resistors, usually 100k. If the voltage on the VT rail increases with increase in tuned frequency, the PLL is working.

[eccerr0r]

Re: Fisher RS-851 ... No FM, bad amp.

Now the hunt for the varactor diode(s)... Hmm...

I do see some output on the DPLL in AM but I only have a slow analog scope (20MHz). I'm not sure if it's fast enough to catch intermediate frequency info and/or

I did notice that the detector output (LA3400) is pure noise, with a small external amp (and a 0.01uF cap to DC-decouple) it sounds just like the noise coming out of the loudspeakers, so yes, the FM radio problem is the RF/IF/PLL/tuning stages Of course that doesn't narrow it down much... The same detector output in AM radio mode sounds like it's working though and does "match" the speaker output. Unfortunately the AM detector and FM detector are unique circuits...

Need to do some more research on analog tuning PLL circuits, all I've ever looked at is digital PLL clock frequency synthesis. I do wonder how well analog phase detectors work with a sinusoidal VCO versus a square wave digital signal with its harmonics... Might work just fine perhaps? Hmm...

[eccerr0r]

Re: Fisher RS-851 ... No FM, bad amp.

The radio board...

[eccerr0r]

Re: Fisher RS-851 ... No FM, bad amp.

Hmm. I think the FM VCO is borked. But now I'm not exactly sure why or how it's generated. Probing around the TC9172P, which is most of the PLL guts minus the VCO, I see a related frequency coming in when it's in AM mode on the AM input, but no signal on the FM input. I do wonder if it should be the IF or the actual FM frequency that should be fed in or not, but in either case, it looks kind of dead.

I'll need to take a back shot of the board to figure out how this is connected... and where the VCO really is... Could it be discrete? Not sure...

[eccerr0r]

Re: Fisher RS-851 ... No FM, bad amp.

I found the tuning voltage finally, with the help of a similar RS-911 schematic. This tuning voltage works as expected in AM mode as I can see the voltage go up and down as I tune different stations. When in FM mode, the tuning voltage is stuck at 12+ volts.

I still think the VCO is borked but I can't imagine how it could have broke. That, and how the VCO actually works... it looks like a tank circuit hooked up to the base of a common collector transistor amp, but doesn't totally look like a Colpitts or Hartley oscillator. Hmm. (and this circuit is underneath the metal shield...)

[eccerr0r]

Re: Fisher RS-851 ... No FM, bad amp.

I pulled up the tuner board and suddenly it started working! I don't know if it was perhaps a bad connection or a bad solder joint somewhere. The tuning voltage now works as expected and no longer stuck at 12V. The amp output is still crappy. Oddly enough through headphones it sounds OK... this is a key point because I think the headphones are hooked up to the speaker output with a resistor network.

I fed the receiver's line output into my computer speakers and it looks like the tuner and mux are working well - so now I have to consider that the amplifier chip is perhaps fried. It does boggle why both channels are not sounding well. One thing I did notice, though the 30W amp uses a STK-4151-II chip, I see the voltage rail at 48V (!) ... The spec sheet says max voltage is 42.5V. I wonder if this is why it fried - it could very well be. I did notice a few STK-4151-V's and even STK-4191 pulls for sale, I wonder if I can swap these in, perhaps the 4191 will survive the higher voltages...

The PSU for the amp is just a transformer and rectifier - no regulation as far as I can tell so I can't adjust it - and probably doesn't need it if the amplifier was designed to be supply voltage independent up until the signal has to hit the rails...

[eccerr0r]

Re: Fisher RS-851 ... No FM, bad amp.

Err...it was already a STK4191-II, so now really unsure why it broke. Oh well. Ordered a STK-4191-V that has a slightly better input stage and lower THD. Hope this fixes this thing other than that FM which I think I narrowed down to a flaky inductor, I can now get it to stop and start working by jiggling it a bit. May need to reflow it to see if it helps, but the solder looks OK...

[eccerr0r]

Re: Fisher RS-851 ... No FM, bad amp.

Well, problem solved. My Fisher is now working pretty much perfectly now. I replaced the amp module with the new module. While removing the module I noticed one pin was broken (it was an easy desolder with a vacuum pump, but the pin apparently broke before I removed the module.) The pin was #13... which may explain why the amp was not working properly.
Now 50W per channel is a lot more than any of my speakers can handle...

[joshnz]

Re: Fisher RS-851 ... No FM, bad amp.

very nice