Fisher 500, how much can they be improved

[dcgillespie]

It surely sounds like the op-amp is not operating. Make sure pin 8 is grounded, and that the .1 uF cap appearing after the isolation diode is mounted close to terminals 4 and 8 of the chip. If so, then likely the op-amp is bad, or has been damaged.

The two 47K resistors create a voltage divider whose output at their junction point is equal to 1/2 of the supply voltage appearing after the isolation diode. Accounting for the load of your meter, it sounds like all is good with that. The output of the voltage divider is then used to bias the output of the op-amps at 1/2 of the supply voltage (by way of the 1 MΩ resistors), which is not happening: pins 1 and 7 will normally show a voltage of about 1/2 the supply voltage, and apparently they don't.

If all is good with your wiring and connections, then the chip is suspect. As suggested, this should be connected into the circuit by way of a socket, so that no direct soldering is made to its terminals. It also makes for easy change out if that becomes necessary.

I hope this helps!

Dave

 

[thornev]

Pin 8 is definitely grounded. I checked all grounds for continuity to each other. Pin 4... the 47K is connected directly to pin 4 and the .1 cap (post edited), non-grounded side, is connected to the same lead of the 47K that is connected to pin 4. Is that correct?

Last night I ordered 2 new chips and 2 sockets for them per Grind's suggestion and now yours. Here we go... take 3. Oh damn, I need to order more of the other components. Thanks, Guys. Onward ! Thorne

 

[thornev]

I have designed Unity Gain take 3. See attached image. I'm pretty sure I have it all connected correctly.

 

[Sam08861]

Looks like you're almost there and just need to insert the IC when it arrives. One thing I've read is that some of these chips are susceptible to static and suggestions to use a wrist ground strap when removing from the anti static bag they come in and for handling.

 

[thornev]

Thanks, Sam. I've got 2 of the chips. Will put it in last after soldering all the connections. Don't want to take any chances of melting anything in the chip.

 

[thornev]

The chip is in, but there's weirdness. I tested voltages all around with a multimeter and they all look good except for the voltages coming out the two .22uF 50V caps. I get 0 volts out of both of them. It doesn't matter which direction they are inserted, right? I get correct voltages (i.e. 1/2 the voltage going into the board through the diode) going into the caps, but 0 coming out of BOTH of them ??? Maybe I burned them when soldering? Guess there's a "take 4" coming. Sigh.

Now I know that capacitors will get charged and will get uncharged, but in the case of this board I don't know what makes them get charged. I sat and waited, turned the meter to the milliVolts setting and I'd get between + and - 1mV DC. Does that seem OK? Or do really need to reverse the caps? Thorne

AHA - I tried a different ground (before it was just the chassis. Now it's an internal terminal board-connected to ground) and get much better results: Around 30mV AC. Shouldn't it be DC at that point?

 

[thornev]

I connected all the wires and while the sound is OK, loud bass notes cause distortion and the stereo separation is noticeably narrower. Unless anyone has any suggestions or comments, I think I'm done with this mod as I just can't seem to get it to work properly.

UPDATE: Mono signals are fine. It's the stereo signals that have distortion. And even then, distortion occurs only when the stereo signal is not tuned in exactly. It's as if the MPX-65 is being affected somehow. Maybe this mod makes the receiver more sensitive to FM signals that are not perfectly tuned in?

I put a sine wave into an o-squill-o-scope on both headphone channels, tested the board at various points including the headphone signals, and the waves looked very good.

I'm wondering if it's the combination of Unity Gain + RIAA mods since they are both next to each other. I don't know that I want to test my supposition by removing the RIAA mod though. I'll sit with this for a while, see if I can figure it out. Would appreciate any feedback (but no "you're overdoing it with mods", please). Thanks, Thorne

 

[audmod01]

Thornev;

Have you tried a signal from the phono input to see if it has the same distortion that you are hearing from the FM Stereo function? There could easily be an issue with the output of the MPX demodulator. If you get good sound from a phono input or from one of the auxiliary inputs on both channels, then the issue is due to something in the RF front end, IF circuits, Detector or MPX Demodulator portions of the unit. Try one of those other input sources first to determine if the distortion you hear is always there or not.

I find that when I am doing mods that it is best to do one mod at a time and recheck operation before doing another mod. You might reconnect the circuitry as it was originally to see if the same distortion is there without the unity-gain mod. If it is not then concentrate again on the unity gain mod. If the same distortion is there without the unity gain mod., then it is time to trouble shoot the output from the MPX demodulator. It is possible for distortion of the MPX output to be due to need for FM IF and Ratio Detector alignment. The MPX demodulator is sensitive to the alignment of the front end, IF circuits and detector.

Joe

 

[thornev]

Well, after listening and adjusting the antenna to get best reception, the distortion apparently had more to do with the lack of a strong signal than the receiver. In other words, the distortion is no longer a problem. But what is at issue now is the loss of stereo separation. I don't seem to have the instruments panned hard right and left that I had before. It's obvious in headphones. I'm going to keep the mod in for a few days and see what I observe. It does seem like the overall sound is punchier with the Unity Gain mod and even cleaner when I have a strong FM signal.

Joe - to address your comments... I do wait quite a while between mods so I can observe the difference a mod makes. But undoing mods is a lot of unsoldering and unwiring that I prefer not to do. I do understand your saying that it's one way to observe mod differences. I'll try the other inputs and see if I get the same results. I didn't think of that, thanks. Process of elimination. I had the MPX professionally adjusted so unless these mods require recalibrating the MPX and other aspects of the receiver such as transformer tuning - which I haven't tried - I should be OK with the MPX as is.

 

[thornev]

It's official - I have lost stereo. Not just with FM. Also with AUX IN. Even when the FM STEREO light goes on, the station sounds exactly the same as when I turn the SELECTOR to FM MONO. This loss happened when I installed the Unity Gain mod. What would cause that?

UPDATE: I thought I had lost stereo on all input devices, but no - I get good stereo with AUX. It's just FM that seems bad. Wonder if I somehow moved that blue stereo adjustment control somehow.

 

[larryderouin]

Look at the pot. The tab on the knob (blue plastic) should be approx 1/2 way between FULL CW and FULL CCW.

 

[thornev]

Larry...Dave adjusted it when he gave my MPX a pro setup. I'm a bit apprehensive about moving it, but then I guess I can't hurt anything. I've done it before and with good results using only my ears and headphones. Is there anything you know of I can do with an oscilloscope? I can input a sine wave or something?

 

[larryderouin]

If Dave adjusted it, LEAVE IT BE. The Separation is as good as it will get on his adjustment (way better than by ear!) You can inject a 2KHz sinewave and run it thru the FM section in MONO and see if it's good or crap. If it's good, Take a photo of the wave's (L & R ) and then switch to Stereo. Compare the Sinewaves. I'd probably swap the tubes in the multiplex with spares 1 by 1 and check the sinewave for changes (good or bad). My guess is the V100 or V101 lost a section or has a weak section. You do have it on an external antenna, right?? If you have an outside antenna, connect that and hook it to the "LOCAL" screws. Then listen again. You could be just experiencing a weak Stereo Signal abberation (Stereo requires more signal strength than Mono.)

 

[thornev]

Larry - V100 is a 12AT7 which was your recommendation in some post somewhere. I tried it but didn't notice much change. V101 and 102 are old RCA 12AX7's. That combo has been sounding good so I don't suspect them. Thorne

 

[thornev]

Well, I'll be damned. It was the 12AT7 in MPX position V100 that was causing the "mono takeover." That tube has been in there quite a while with no problems. But when I put a Sovtek 12AX7LPS in that position, I got my stereo back ! Go figure. Can a tube go bad like that that it loses stereo signals?

BUT... the distortion is back big time. Sigh.

But then after listening for 30 minutes, the distortion is gone. What's going on?! I guess it's the weather or something in the air that caused bad radio reception. I guess FM dipole antennas are useful only so far.

 

[thornev]

For the very first time since I've owned my highly-modded 500-C I really cranked the volume into my AR-2AX speakers and oddball subwoofer (I'm gonna guess at about 95dB). OH...MY....GAWD !!!! This radio sounds UN-BEE-LEAVE-UH-BILL !!! Every frequency just jumped right out of the speakers and clear as a bell. I know the 500-C is a great radio, but I had no idea how well it performs at louder than normal volumes. I am BLOWN AWAY !! And the volume knob was only on about 9:30. I would love to hear this thing when hooked into really large speakers and the volume at noon.

 

[RS Steve]

For the very first time since I've owned my highly-modded 500-C I really cranked the volume into my AR-2AX speakers and oddball subwoofer (I'm gonna guess at about 95dB). OH...MY....GAWD !!!! This radio sounds UN-BEE-LEAVE-UH-BILL !!! Every frequency just jumped right out of the speakers and clear as a bell. I know the 500-C is a great radio, but I had no idea how well it performs at louder than normal volumes. I am BLOWN AWAY !! And the volume knob was only on about 9:30. I would love to hear this thing when hooked into really large speakers and the volume at noon.

Hook the 500-C to a pair of Cornwall's and see what happens!

 

[thornev]

RS - You mean like these? https://www.klipsch.com/products/co...g0CFypgFuZmgxqszwjhr06XEDVwRfnXhoCwY0QAvD_BwE

$6K for a pair of speakers is way too rich for my blood. Although I could almost justify based on my love of music and that I listen every day.

I do have a pair of bi-amped Snell Type D towers in my living room but don't think they'd sound as good as the AR's.

 

[RS Steve]

You don't need brand new ones, they can be found used from 600.00 to 1,200.00. Just reluctantly sold mine, had someone over listening to equipment and they liked the Cornwall's so much they made an offer I couldn't refuse. They were the third set I have owned, most likely will get another set if any come up available for a good price. Efficient speakers really pair up well with tube equipment, AR's are not known to be efficient and usually do better with more wattage.

 

[thornev]

I sure did prove that AR's like wattage last night. If my room weren't so small (9'x9'), I'd crank them up even more. Maybe I'll try with ear protection. Oh that's just silly.