500T found

[ehennebury]

So... I've had a 500T come into my possession some time ago, and never really got around to doing anything with it. Today I was cleaning out the garage, and there it was. I decided to hook up a set of speakers and see if it was working. I found that the one of the channels was pretty much out. You can still hear it, but it's very soft.

That's when I hit the web and found this forum. I'm a complete newbie when it comes to this kind of thing, but i instantly became very interested. I have pretty good mechanical aptitude, and basic soldering skills.

My question is this... am I way to noob to attempt repairing/recapping this unit with the help of this forum? What is the tolerance for really basic lines of questioning (potentially REALLY basic)? I'm excited by reading the rebuild post from user jdsalinger, but I'd likely need a lot of help. Thoughts?

 

[notdigital]

You hit the right place!! Folk here are pretty laid back as well as knowledgeable of Mr. Fisher's stuff. If you ain't afraid of getting your hands a little dirty, you'll be in for a nice ride. The only caveat is that you DO NOT run ahead of those trying to help. Impatience is what usually runs people into trouble. A slow slog will get you to exactly where you want to be. Also, no question is too basic and, as the saying goes, the worst question is the one that isn't asked.

No pressure, but $25 bucks will get you access to Digital Docs where you'll be able to get a copy of both the Service and Operating Manual for your unit. These documents are probably available elsewhere gratis but I'd like to think that for the coin asked, you're still getting a super bargain + !!

Welcome aboard and good luck!!!

PS: It's a good idea to edit your profile to include your location. You never know: a member might be right next door!!!!

PSS: The fact that you're getting something out of both channels is a very good thing. Before things get serious, check out the Cleaning Sticky at the top of the forum. A good bath sometimes brightens up the most cranky controls.

 

[ehennebury]

Great! Thanks for the encouragement. Lets start with the cleaning then...

 

[larryderouin]

Couple of threads you need to read....
1.) The Idiot's Guide to Using De-Oxit (Revisited)

The names of the various DeOxit formulations have changed over the years. Today the 3 that you are or will be interested in are in this ORDER 1.) De--Oxit DN5 (This is the Cleaner/deGreaser). 2.) FADERLUBE (This is the lubrication formula as the D-5(DN5) will DeGrease the pot or switch so that there is NONE LEFT.
3.) De-Oxit GOLD (G-5) Protectant (protects gold plated mainly but also protects other metal formulas.

At minimum Get DN5 (D-5) and F-5. These will generally take care of 99.9% of cleaning and lubrication problems.

Spray generously inside ALL of the pots or switches. Don't try to get it in the switch or pot by the shaft or front of the switch. It has to get in via the slot like opening (on a pot) on the side of the housing where the legs come out. On a switch you will generally see an opening on the side and see the slide switch move back and forth ....spray there. Move the operating handle of the switch or pot 4-5 times to distribute the DeOxit. Let sit for 30 minutes. Then re-spray all again and cycle them 40-50 times. The wait time loosens the crud on the surface of the contact, and the cycling of the pot or switch scrub's the contact area. The contact area will be a fine line of contact material. After cycling all of the pots and switches, spray with faderlube generously and cycle 10 times to distribute the lubricant. DO NOT USE DeOXIT D-5 WITHOUT a follow up of F-5. Lack of lubricant can and will accelerate oxidation and corrosion due to the increase in friction between the parts. If nothing else, a good electrical grade lubricant (CRC 2-26) @ home Depot in the electrical Dept will suffice in place of the Faderlube.

As the contacts inside the pots and switches are predominantly carbon and plated steel, the GOLD is really not needed here. GOLD PLATED contacts should have an application of G-5 to protect them from friction and environmental contaminant's.

Let dry an hour or so and fire it up. Test all functions for operation, and lack of extraneous noise. Sometimes a 2nd or 3rd go round is needed depending on how much contamination and the length of time since last cleaning.

Larry

 

[rufleruf]

I'll be following with interest - I have two of these awaiting service, and not too far down the list

Be aware there are two service manuals for two serial number ranges of 500T I think starting at 10001 & 20001. Not sure what the differences are, but seems like a good idea to reference the right one.

By way of encouragement, I have a buddy with a 500T in his office and it sounds great!

 

[larryderouin]

The 500-T is the little Brother to the 700-T which was the TOTL receiver for 1967. x00 model # Solid State receivers are generally FM ONLY, whereas X50 receivers are AM-FM.

I need to get a 500-T to complete the 550-T and 700-T receivers I have now. They both sound bigger than their wattage rating and have very nice sounding tone. There is one problem with them that seems to be aging of the parts is the PEC's (Packaged Electronic component) for the Bass/Treble. When it does go out you have a large balance imbalance. See JD Salinger's 500-T rebuild thread about this problem. Seems his 500-T and my 700-T had the same exact problem at the same time and replacing the PEC's solved the problem. The PEC # is PC50B187-25. The most common failure in the PEC is the 10K resistor between pins 1 and 7. Building a NEW PEC from discrete parts on a small bread board is a worthwhile effort, as it solves a potential problem and tightens up the tone control response if you use all 1% parts or 5% parts. The origianls were 10%.

Use ECQ-W film caps to replace all the 1uf 70V electrolytics. It makes a very nice change for the better in tonal quality. Any electrolytic below 2.2uf should be replaced with a film (preferably stacked film) unless size of the new part is excessive or available space is not enough to fit in the film. Generally these will fit any board on the 700-T so it'll fit the 500-T. Above 2.2uf and the film caps get too big to fit in the board space available.

 

[ehennebury]

The local shack had D5 but not F5, so I ordered online. Serial number of the unit is 15208B. Thanks for the tip on the manual.

First I'd like to see what the cleaning does. It's pretty dirty under the cover, although fairly clean looking under the bottom cover. I literally found this receiver at the side of the road. Knowing absolutely nothing about anything, I picked it up because I liked the look of it (figured maybe it would do in the garage). By all appearances, nobody has been inside since it was built. I'm hoping I can get it back in working order with a good cleaning. Then I can begin recapping and any other upgrades that might be recommended.

One point of interest... I was reading buffdriver's 500T service thread earlier where a combo zener/diode was distorted/melted. Mine actually has two components in place of this one. I wonder if they replaced the two components with the combined single component in later builds? I loaded pictures below for anybody who might be interested. One of buffrider's distorted zener/diode here:
http://i1127.photobucket.com/albums/l621/ehennebury/IMG_4175.jpg

and one of my separate components here:
http://i1127.photobucket.com/albums/l621/ehennebury/IMG_0153.jpg

D5 and F5 arrive next Wednesday. I'm looking forward to getting started. I fear an addiction in its infancy here!

 

[larryderouin]

Looks like someone replaced the diode set with a couple of standard diodes or maybe a regular rectifier and a zener. Lousy job of dressing it out too. . If you read the thread accompanying buffriders picture (I'm pretty sure it was that thread) I explained that the covering was the only thing that "melted" as it was an early variant of heatshrink tubing installed to keep the parts together. This is common with Early FISHER amp boards of the ERA. The outer covering is heated up by the diodes and it softens, shrinks and conforms to the shapes of the individual components. I've got a 1/2 dozen various FISHER S.S. receivers (160-T, 175-T, 250-T, 400-T, 500-TX, 550-T & 700T ) and the outer cover on ALL of these diode bundles are distorted. Nothing wrong with them. This is normal appearance. The later combined component was the sam parts in the same wrapper, it's just that FISHER gave it an internal part number. And then deleted the standard information so to keep unauthorized fingers out of the gear and make it harder to make up a parts list. You'll notice that on the Solid State stuff there are NO PARTS LISTs on most any of them. This in conjunction with FISHER's proprietary part numbers kept FISHER's Service Dept's open longer than most Co's. .

If you have a board with the original normal looking diode pack, you can actually take off the covering and look for diode id numbers on the individual parts. Then either leave them open or tape them up, or encase in a larger piece of heatshrink and heat it so it shrinks and conforms to the shape of the pack.

replacement parts for the Zener/ diode pack
CR851,CR854------>>512-1n5402 (QTY 4)
CR852,CR853------>>512-1n4736ATR (6.8V 1W Zener Diode) (QTY 4)

This will be enough to do both boards. The part Numbers are from Mouser. The Economy shipping will be more at $4.99 than the parts will be. Go figure. Make sure if/when you make the new one' up, that the striped ends are together.

 

[ehennebury]

The DeoxIT D5 and F5 arrived today (early). I went ahead and cleaned up the pots and switches along with areas of contact as indicated in Larry's earlier post. Apart from the pots and switches feeling a WHOLE lot better, the performance of the unit remains unchanged. In my original post, I had indicated that I was getting something out of both channels. Upon revisiting the unit, this is not accurate. The Left channel really has no output at all. The right sounds pretty fantastic. The bass, treble, and balance pots seem to work in as much as they affect the sound coming out of the right channel. Putting the balance all the way left results in pretty much silence though (I can still hear faintly from the right channel). I'm not sure that it would impact diagnosis at all, but the headphone jack yielded the same result... no left channel.

So what might the logical next step be? Could a bad capacitor cause this? Do I just start recapping, or is there other diagnosis to be done first?

Thanks for any advice. I look forward to hearing it.

Regards,
-Eric

 

[larryderouin]

Make sure all of the POWER SUPPLY voltages are present at both the power supply and their destination (need the schematic for this.) Fix the power supply 1st. Once fixed and properly outputting voltages, THEN start working on everything else.


Pull the output's on the affected side, ONE at a TIME. Clean off any old heatsink grease residue and then do a 6 way test on each one. Mark OK or NG and put it back in it's place BEFORE Removing the next one and testing it. If they all test good, then start pulling transistors on the amp board 1 by 1, test, mark and reinstall. Once you know which are bad We'll get into what to replace them with. Also test the emitter resistors for both sides. These are 1 ohm resistors R37,38,39,40. I believe they are 3w resistors. If not sure about the markings take pictures, and post.

Do 6 way test on all of the transistors on the AMP BOARD. Electrolytic caps can/will be bad, and not show any outward appearances that they are. Replace them with comparable value and voltage.

 

[jdsalinger]

I would say that it doesn't get much easier to work on an amp than this ( except for point to point ). Fisher put a lot of thought into the serviceability of this amp. It is a breeze.
Do you have an ohm meter and a soldering iron at minimum? Practice your soldering skills before you start working on recapping the 500T.
Also, get the service manual and study it, so you know where the signal path goes.

As for troubleshooting, Someone else may have a suggestion, but here is what I would do:

1) do a visual inspection of the left channel. Look for any burnt resistors or bulging electrolytic capacitors. Look carefully for any broken or cold solder joints. This will help you get familiar with your 500T.

2) If you have another receiver or amp, you can use it to figure out where your problem lies. Get some ohm meter probe clip leads and make a test interconnect cable that has RCA jack on one end to go into your good receiver and a clip on the other end for the 500T test points and signal ground. Plug the RCA jack into the your good receiver and then you can clip the ground lead to signal ground on the 500T and the positive clip into different spots in the LH channel of the 500T and quickly figure out where the signal is blocked. That will shorten your troubleshooting time. You should get a schematic first, so you know at what points to connect your test lead.

3) Once you determine where the signal is blocked, see if there is a transistor that you could swap from right to left to see if that solves the problem. This will be an easy check because the transistors on a 500T are all socketed. (That is a hold-over from the tube era that I wish would have continued.)

Be VERY careful if/when you swap them. Make sure that you let the amp discharge after powering the unit off and be careful not to short out the pins. Mark which socket each transistor goes, so they can be replaced where they came from.

The transistors with the heat sinks from your picture, are obsolete ( I think ), so hopefully they aren't the problem.

Swapping transistors will be an easier test to rule those out before you start changing capacitors. And IMHO, a transistor will be much more likely to cause NO sound than a capacitor that has gone bad. If a transistor is dead, there won't be any sound...a cap should still transmit some signal, I would think. But I am far less informed than Larry or Fred or the other legends of this board, so listen to them.


When you are finished with the restoration, you will want to re-center the voltage and bias ( I think ). The procedure is in the service manual.

I'll say it again, I LOVE my 500T. There is a hint of solid state sound, but it has the soundstage and detail of the tube amps I've heard. The SS sound is just more clean & cool, perhaps....not a bad thing.

 

[larryderouin]

One thing to add to that test probe. Install a .01uf 600V film cap between the clip lead and the wire. This will block any stray DC that may want to try and get by and do further damage.

 

[ehennebury]

So I've been a little busy with other projects. I managed to carve out a meager half hour to dedicate to the 500T tonight. Previously I had already tested the main transistors across the back of the unit (Q1 through Q4) and the heat sinked transistors on the driver boards as well as the 1 Ohm resistors Larry had mentioned with no issues found. Tonight I decided to make some test leads as was suggested and started probing around to see if I could find the signal block. That is when I discovered that I was now receiving sound from both channels! Clear good sound! The balance is off. The left channel is weak, so to balance, I have to turn the dial to about the 2.75 marker on the left. The only thing I can think i that there might have been a questionable socket connection that reconnected when I reseated it. Alternatively, I suppose it's possible that the DeoxIT corrected something over time.

So now instead of diagnosing a dead channel, I'll be refocusing my efforts on recapping. I wonder if it's possible that the weak channel is due to a bad cap or caps? Anyway, I have a ways to go before I even understand how to choose appropriate replacements based on the ratings of what's there (or even how to recognize electrolytic caps for that matter). Then I need to decide if I'm going to restuff the existing large caps or just solder the new ones in place. My time to dedicate to the project is pretty limited, but I will post if I have any questions, or if I have any relevant updates.

Thank you all so much for the help to date!

 

[jdsalinger]

Good news!

My money is on one of your tone control PEC resistor values has drifted. Get a schematic and measure the resistance on the legs that have resistors only.
If they have drifted high, then, you can solder a resistor in parallel (across the legs) to get the value back where it belongs. (parallel resistor will lower the resistance)

 

[larryderouin]

I concur with jd. This is a fairly common complaint on the 1967-69 series of early Solid State receivers with a particular PEC(Packaged Electronic Circuit). The PEC that you will look for is on the Tone control board (Fisher called it the Audio control amp), and is numbered PC50B187-25. It's a little bigger than a stick of gum (there are 2 of them), and has 7 leads off it into the board. The most common fault is the 10K resistor between pins 1 and 7, but also check the one between pins 3 and 5. You can bridge the pins with a paralleled resistor after metering the one's inside to get 10K, or build a new PEC from scratch on a perfboard and install. Your choice. If you can't figure out how to calculate parallel resistance, just post your results on the board and we'll give you a hand, figuratively.

FIX the problems with it 1st, then do a recap. Unless a cap is known to be bad, (it's physically puked or it meters bad) usually only the sound quality is degraded somewhat. Recapping will bring back the original sound quality of the 500-T, and it's quite good. They sound very much like their tube counterparts from the era. Plus they are pretty ballsy. The 500-T is rated at about 40-45watts rms IIRC (it might be a little more). Some strategic resistor replacement will probably be mandated also, although I haven't looked at the schematic and board layouts, I'd think it would be similar to the 700-T in that regard with all of the power supply point to point. The caps being replaced will be smaller by 1/3 to 1/2 for the same value and voltage. Don't let that goad you into replacing with same physical size parts. ALL OF the silver can caps can be opened up and stuffed with new replacements, and installed in the original location. A lot of the point to point caps will be placed between 2 terminal strips. Try and not use radials here. You can but you'll end up adding wire to lengthen the leads to get them to fit. ONLY IF THERE IS NO AXIAL Available. replace all of the power supply caps, the DIODE Bridge (it'll be 4 separate diodes (use 1n5406 here), and the POWER RESISTORS. They take a beating from heat cycles, and in the larger scheme of things cheap.
DO THE POWER SUPPLY 1st. Making sure the power supply voltages are up and correct is one of the 1st things done to ensure it will run right. None of the other circuits like voltages that are off either high or low.

Then recap the main amplifier, and output's (replace the emitter resistors for the output transistors). TEST The 500-T after EACH BOARD. If you run into problems, you know it's in the board or area you were just working on. Fix and move to next section back, rinse and repeat, until you get to the inputs. Then you'll be done and have a 500-T that should be good for another 35-50 years, and your grand-kids can fuss and fume over the archaic design and assembly.

INSIDE ONE's TOO!

 

[jdsalinger]

The only thing I can think i that there might have been a questionable socket connection that reconnected when I reseated it. Alternatively, I suppose it's possible that the DeoxIT corrected something over time.

I ended up replacing the input jacks on mine. Not fun, but worth it. The original Fisher input jacks are not modern standard spacing and mine had some serious oxidation. It's peace of mind to know that the continuity is good. It's also possible that you have a broken solder joint at the input jack and wiggling it.

With all due respect.....as much as I would like to watch and support a 500T restoration......it sounds like you may want to take it slowly with the recap. Decide if you want to invest in it by sending it to an electronics technician. If not, set it aside and take your time reading as much as you can about the 500T restoration (or any vintage solid state restoration). We can help you with the cap list but practice on something that isn't a classic (DAMHIK).

 

[fred soop]

.....
The 500-T is rated at about 40-45watts rms IIRC (it might be a little more).
.....
You can but you'll end up adding wire to lengthen the leads to get them to fit. ONLY IF THERE IS NO AXIAL Available. replace all of the power supply caps, the DIODE Bridge (it'll be 4 separate diodes (use 1n5406 here), and the POWER RESISTORS. They take a beating from heat cycles, and in the
.....

Based on the 55V power supply, the output power would be closer to 32 watts rms. 40 watts would require slightly better than 60 V from the supply.

The 1N5406 is a 3 amp diode. That's a bit light. At 32 watts, peak current will be 2.8 amps per channel (2 A average) and you also need to add all the current draw outside the power amp itself. I'd use 10A04, 10A05, 10A06 diodes. 10 amps is probably more than is needed but the 6A06 doesn't seem to be stocked by Mouser or DigiKey.

 

[jdsalinger]

I was listening to my 500-T on Fisher XP-10 speakers tonight (source: Pandora standard through a smart TV). It was superb. The XP-10's always sounded boxy with pronounced mid-range that I couldn't fix, but they sound SO right with the 500-T. Makes me want to start collecting more before the word gets out.