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Fisher TA-600 Restoration

Ron -- Based on your comments, it sounds like something is clearly wrong -- either that or your MPX decoder is quite different than what I'm used to. The output of a MPX decoder is a line level signal, and as such, should overload the Tape Head input. That is doesn't appear to be doing so is troublesome. At the very least, there should be so much signal there that you should barely be able to advance the volume control when using the Tape Head inputs. Is this in fact the case?

Dave
 
Dave

It's true. The output of this solid state MPX decoder did not overload the Tape Head input at all. I was easily able to advance the volume control, no overloading.

Hmmm...after reading your last post, it seems like none of my input jacks are letting much signal pass through. I really should lug the beast back down to the basement workbench, I guess, and also carry the Technics down and try it on phono. And while there, do some tests with signal tracer and scope. Hoo boy.
 
Check all of the adjustment pots on the rear. They may be turned down or just dirty.
 
Ron -- Virtually all MPX units are of a unity gain design. So are you saying that there is an actual loss of signal when using the adapter? As in notably so? What happens if you plug the MPX output straight into the Tape Head input. Will it overload it then?

Also note that the tuner section alignment is fairly critical for good MPX performance, while not nearly as important in retrieving a decent mono signal right off the ratio detector. Still, tuner alignment aside, if there is a significant loss of signal through the MPX adapter, then that would be a strong indicator that there is something wrong with the adapter itself.

Dave
 
Dave

This is what I have been saying all along.

In review:

eBay MPX in TA-600 MPX IN = loss of signal, no bass, flat, crappy
eBay MPX in TA-600 AUX IN = loss of signal, no bass, flat, crappy
eBay MPX in TA-600 TAPE HEAD IN = somewhat better than those above but still results in a loss of signal, no bass, flat, crappy <--- NO OVERLOADING
HP laptop audio in TA-600 AUX IN = low signal, no bass, flat, crappy
Fisher 432 TAPE OUT to TA-600 AUX IN = decent (but not great) sound with some loss, less bass than when played straight through the 432

Does the eBay MPX decode a stereo signal from the TA-600's MPX OUT? YES! Clearly stereo, its green stereo indicator lights up, I can definitely hear the difference between left and right audio but the sound quality leaves a lot to be desired compared to mono FM played straight through the TA-600. (And this was before I aligned the TA-600.)
 
What does it sound like if you connect the MPX output straight to the MPX in? You'll only get one channel and it will probably have a bit too much treble but the volume should be effectively the same as it is when set to normal FM tuner. If its significantly lower, something is funny.

Off the top of my head I don't remember which way is what on those pots. Give them a turn, it should be real obvious.
 
ok so as a quick sanity check, I hooked up my own TA-600 with an MPX unit. Initially it sounded crappy, but it was just that decoder. With the Fisher 500B decoder, the only real difference between "tuner" and MPX is that I get seperation. Almost identical volume, tone, etc. The Madison-Fielding unit I tried initially was low and lacking in bass. I'll have to do some work on that to see if its just crap, bad parts, or needing more voltage.
 
Okay. I'm at my basement workbench and have just ran some tests.

First, Gadget, I tried your quick check of running a cable from the MPX OUT jack to the left channel MPX IN jack. The sound did indeed have a bit too much treble...but it was the same amplitude as listening to straight FM mono without using the MPX jacks.

My oscilloscope is old and does not have any useful calibration settings - I can adjust gain, sweep speed, attenuation and "range" but I cannot set actual voltages with the knobs. So anyway...I took a measurement of the MPX output and took a mental note of the image.

Next, I tried the eBay MPX decoder again...and I owe you guys an apology...I misspoke when I said "tape head" inputs. I should have typed "tape monitor" inputs. :eek::oops::dunno: Again you were right, Dave; an input into TAPE HEAD does result in too loud, distorted sound.

So anyway...

I plugged the output of the eBay MPX decoder into the MPX IN of the 600. Result - sound was definitely lower, bass lacking, flat, lousy.

I then unplugged one channel from MPX IN and connected that to my scope.

The waveform was clearly lower!

Further tests indicated the following:

MPX to AUX IN - sound was low, bass lacking, flat, lousy.

MPX to TAPE MONITOR - sound was low, bass lacking, flat, lousy.

MPX to TAPE HEAD - way too high, distorted, until I turned the input adjustment of the eBay MPX decoder waaay down until I reached a point where it actually sounded pretty good!

I'm now convinced the TA-600 is likely OK, and this MPX decoder is definitely crap. :thumbsdown: Sure, it decodes stereo audio. But if its output is below line level, what good is it?

Now I'm going back upstairs and bring my Technics SL-D3 TT down, hook it up, and try it out. Will report back soon...
 
Results of TT test:

PHONO IN - Audio is low; requires cranking the volume up to 1/2 to obtain normal listening level.

TAPE HEAD - (I tried this just for fun) - Sounds good, normal audio without having to "crank" it up.

Technics SL-D3, Stanton 680EL cartridge.
 
OK, this is crazy...I just moved the phono plugs back to the PHONO inputs and, while it now seems *just slightly* lower than the TAPE HEAD inputs, it sounds OK now. :idea: :crazy: :dunno:
 
might just be dirty. Those sockets ain't much honestly. Overall gain from phono to tape head is virtually identical, just the EQ curve is slightly off. I modified mine to be a second phono input but functionally those curves are not horribly different.
 
I will clean all of the input jacks with DeoxIT tomorrow. I left the unit at the workbench to make that easier to do.

I might even remove the back cover and see if I can carefully, ever so slightly, tighten up the center terminals of each of the jacks with needle nose pliers.
 
Input jacks cleaned & tightened, adjustment pots in back all cleaned with DeoxIT followed by FaderLube. As far as I am concerned this one is finished, time to move on to my Fisher 400. Thanks everyone for the assistance, comments, etc.!
 
Glad I found this thread. I have a TA600 that a friend of mine went through the amp section by replacing caps and I have new output tubes.

so now I want to go through the tuner section.
 
Hi everybody! I'm a relative newcomer here, and the very proud recipient of this special unit. I have recently restored another TA-600, but jumped at the chance to acquire this one when I saw it on the auction site. All I needed to do with this one was replace a malfunctioning GZ34 rectifier, and it is playing beautifully. I'm looking forward to many years of listening pleasure with this one.

I'm in deep gratitude to Ron, Dave Gillespie, and many others who have contributed to this and related threads on the TA-600. I had a nice unmolested unit prior to Ron's that I thoroughly went through using all the guidance that was given here. Here is a summary of what I did with that one:
  • Replaced all electrolytic and film capacitors. Original “can” capacitors left in place for aesthetics; these were bypassed under the chassis with all new replacement capacitors.
  • Replaced all power (1W and above) resistors measuring out-of-specification. This includes the main power supply dropping resistor, which was replaced with a higher-wattage rating for increased reliability. Optimized lead dress to minimize (imperceptible) hum and noise.
  • Checked voltages at all tube pins for compliance.
  • Cleaned all potentiometers with Deoxit and tube sockets with QD Electronic Cleaner.
  • Added a thermistor to the AC power supply input to control current in-rush to protect the main power switch.
  • Removed permanent low-frequency filters at the phono input to improve audio fidelity. These filters were originally installed to prevent rumble or feedback when used in a console installation, and are unnecessary.
  • Added an adjustable bias circuit to ensure proper output push-pull amplifier operation and sonic fidelity. Circuit was wired under the chassis out of sight.
  • Deep cleaned the chassis, knobs, and front faceplate. Carefully cleaned and dusted the dial glass front and back.
I guess I now have the Fisher "bug"!
 
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