Fisher 500-tx, recap?

Rick Stout @ www.stereomanuals.com
I've got to look at mine. the Tuner isn't working in AM OR FM on Regular mode (lot of static and a Zero Beat around 97mHz), but the presets and AutoScan are working fine.
Larry

I took off the wood case, and powered it up. Everything worked. WTF???:scratch2::scratch2: Double checked all hook ups. Then I looked at the case. The metal grill on the top had come loose, (front end) and dropped down on the wire connections at the high end of the dial scale. Something grounded.

Turns out the little staples FISHER used to secure the grill to the case had backed out and became lost allowing the grill to drop. Some 15 minute 2 part epoxy and some clamps to hold it together. Allowed an hour to dry. Re-installed in case and everything working like a charm! :D

Larry
 
So after poking around in the receiver some more, I have found a few more "hack jobs". First is I have a couple of what I think are ground wires that have been cut by the audio filter selector board. Next on the power amplifier module, they used 2 diodes in series each for C801 & C802. Same diodes as the one used to replace the zener on the power supply. I did find replacements for them, NTE116. Also one of the power transistors 1038-5 has been replaced with a motorola hep704. NTE cross reference number is NTE130. The kicker is it is reference as a npn, and 1038-5 form looking at the schematic is suppose to be pnp. When I reference 1038-5 I get NTE180 as a replacement. It's a little expensive at $10! But I guess I have to get one. One of the 1034-5 transistors was replace with a ge 7462. It's big and square looking. Found replacement NTE159. Found more resistors with crappy soldering. I'm including some pictures so I can get help with the 2 cut ground wires. These aren't the only problems I have found, but they're the only ones I am concerned with at the moment.

P.S. I circled where the wires are cut and where they go to ground.


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[/url] IMG_1184 by krisnalexdad, on Flickr[/IMG]


 
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You replaced a "zener" with a nte116??? The NTE 116 is equivalent to a 1n4004 (or so they say) which is a regular diode, not a zener. GET IT OUT OF THERE and replace it with the correct one. What # is the part. The correct ZENER is a 15V 3W. DON'T POWER IT UP with a 116 in there.

Get yourself a pile of 1n4004's for replacing regular diodes in here. It's the "de riguer" diode for 90% of the small stuff. Fairchild makes a very good one @ $.16 through mouser. You can get at least a dozen for what you paid for the NTE parts , and are a lot more dependable than NTE. We try not to use NTE here unless there are no other alternatives.
If you look at the schematic there is a .6v drop between sides of the diodes. Check for that drop. If it's ok, and they work, replace with 1 1n4004. If multiples of .6 use that many series'd.

As for the GE 7462. It's probably OK. It's a newer equivalent of what would have been in there. And one hell of a lot better than anything NTE has. If it works, Leave it.

The "2 ground wires" that go nowhere are shielding wires. Leave them wrapped around the others.

DID YOU TEST THE OUTPUT TRANSISTORS? IF NOT TEST THEM 1st BEFORE Opening your Wallet.

The "HEP704" is an old Motorola Replacement # for the 1039. I posed the same ? about a year ago for output replacements and Echowars provided these #'s ONSEMI MJ15003 an MJ15004. They are obsolete now. HOWEVER the 15003G and 15004G are available @ mouser for $3.50 ea.

Please don't go willy-nilly on replacements if it works. If it doesn't then work thru each section tracing signals back from the amp. BEFORE THAT, MAKE SURE THE VOLTAGES on the POWER SUPPLY ARE CORRECT! IF NOT FIX IT FIRST.

Larry
 
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I haven't done anything yet but look inside and a making parts lists. The nte116 I was referring to is for the diodes on the power amplifier board, I found 1N5929BG for the zener on the power supply. My main problem is I have static and what not in one channel. But that can be due to the leaky caps and the Hep704. My plan was to fix the power supply first and then fix the amplifier, but was trying to save money on shipping by ordering everything at once.
 
Ok. the way i was reading it was, it had been done.

Doubtful the HEP704 is causing the static in one channel. More likely caps or resistors in the tone amp, equalizer, or the driver section of the amp or even dirty contacts in the switches and pots. Is the static in ALL Functions (aux/Phono/FM/AM/tape monitor)? Have you cleaned the controls yet with de-Oxit? If not, clean them good. You may have to pull the boards out individually with the pots on to get them clean enough. Especially the rotary switches. Try pulling the jumpers from the back and see if you still get the static. Clean them and the jacks while they are out. If you still get static then it's in the amplifier. If not then it's before the jumpers. Which side is the static and noise. The HEP704 is on the RIGHT SIDE. Clean it before making the order. That way if it does clear up..................


If you have another amp, you can make a probe with a rca cable, and a .01cap attached to the center lead. Cut the plug off one end and separate te center and outer leads by about 2'. Insulate the outer lead and out an alligator clip on it. Attach this to chassis ground. Solder a .01 cap to the center ground (a orange drop will work). Heatshrink all but the tip of the probe end. Then probe each component, and switch contact, working back from the drivers (both sides of each component) to find where the static is coming from.

Seriously, clean everything before ordering the parts. You may find you don't need them all, except whats necessary to clean up the cheesy work. I'd leave the HEP704 and the GE7462 intact unless they prove to be the problem. of the two The GE is more likely than the HEP704. But of all the FISHER Transistor units I've had come thru I've only replaced one transistor in one unit. and that was on a tone board.

Larry
 
I haven't cleaned anything yet. The channel has a lot of static, a low hum and the volume is real low. I tested all the out transistors and they checked out fine. I do have a question about them. Should they have an insulator between them and the heatsink? Two of them do but the others don't. Also I just finished basic electronics, so figuring some of this out isn't the easiest for me. Thanks.
 
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In a word, Yes! Remember the casing is part of the circuit (BASE) on a TO-3 transistor. Order new insulators, and silicone heatsink grease. Don't get the Arctic Silver type, as it's electrically conductive.

Clean the Heatsink really good. You may find the insulator stuck to the heatsink with a clear heatsink glue used by some factories. Clean it all off and regrease both sides and sandwich it in between the transistor and the heatsink. MAKE SURE you line up the mounts and the transistor to isolate it from the heatsink. If you don't you'll get magic smoke released. And your wallet will not be happy.
 
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