FM-200-B Refresh

This is one of those projects that stole a verse from the songbook that my 800C receiver came from. You know, the pristine looking unit cleverly marketed to represent something it wasn't? In the old days, it might have been advertised by saying "It was only listened to on Sundays by a little old lady......." The bottom line is you purchased something based on an understanding represented, only to find out that's not quite the case.

Currently, the item in question is a Fisher FM-200-B, that was supposedly refurbished, and ready to go for another 50 years. Refurbished mind you by a pretty well known name in the business. And, the unit did in fact work well enough by casual observation. But a closer examination showed it more likely to be good for a number of months rather than years.

A look-see showed that the work done included:

1. Replacing ONE of the two AC bypass caps.

2. Replacing 8 low voltage electrolytic caps.

3. Soldering in new fuse lamps to illuminate the dial glass.

4. Adding a large value (160 uF) cap to the power supply to augment filtering.

Other than this work performed, the unit was bone stock. The replacement caps were all installed using various versions of the cut and tack approach.

What remained unaddressed was:

1. Nine tubes that either registered as weak, shorted, or dead.

2. The original selenium rectifier, which was still in place.

3. The original can cap, which remained in place.

4. Fourteen original film caps which remained in place.

This, in a unit that has not had a lot of use since it was purchased and refurbished -- but how much longer would it have operated?

To a lot of Fisher owners, it isn't enough that a piece simply appears to operate correctly. It's equally important to know that a unit truly has been fully refreshed, operates according to specifications, and is in fact ready to last another 50+ years. The misrepresentation of this piece isn't the subject of this thread, but my observation is that this practice seems to be more and more prevalent these days. Getting on with the project then, a few pics will show what was, and what's been done so far as the project unfolds.

1. Original underside shot. It will be seen that initial work has already begun in the power supply area, with some of the first connections already removed from the selenium rectifier pack to effect its removal. The extra non-stock power supply capacitor is obvious, and with a little more looking, so are all the original film caps that were never replaced all throughout the unit.

2. Pigtails abound in this unit where the low voltage electrolytic caps were replaced. Some are awfully close to other wiring that they should not come in contact with.

3. Fisher always soldered a terminal of their filter caps to the chassis, and did doubly so with their tuners, always using plenty of solder. From a performance standpoint, that's a good thing, as weird things happen in RF circuits if power supply impedance is not appropriately low.

In this case however, so much solder was used that there was no way any conventional soldering gun was going to make for a clean removal of the old can, if for any removal at all. Therefore, out came the torch. The old can usually doesn't come out looking too good from such extractions, but after all, it IS the old can. It's still in plenty good shape for re-stuffing if the mood strikes. The other removed power supply components are presented as well.

4. The refurbished power supply area. Both AC line caps are now replaced, as is the filter can and selenium rectifier pack -- and the large extra filter cap has been removed as well. Note too that the appropriate dropping resistance has been added between the new silicon bridge and the stock B+ distribution circuit to maintain the original B+ levels produced by the selenium rectifier pack. With the new bridge and can cap in place, power supply noise is basically nonexistent -- and much lower than with the old can cap and added capacitance. Note that the torch was also used to solder in the new can cap (using a shield this time!), giving a nice smooth factory look about the chassis solder joints.

5. Top side, the new can is identical in size to the old can, and other than the label on the new can, you'd never know it was replaced.

As work progresses, I'll update accordingly. This project is working in and around my already in play X-202B project. But with it waiting for a few more parts, and fresh tubes having arrived for the 200-B, it got some time on the bench to keep it on track as well.

Dave

This will no doubt prove to be another very interesting thread!

Thanks for the information and the pictures.

Always a great learning experience are your threads, Dave!

dcgillespie said:

...[T]he pristine looking unit cleverly marketed to represent something it wasn't? ... Refurbished mind you by a pretty well known name in the business.

The misrepresentation of this piece isn't the subject of this thread, but my observation is that this practice seems to be more and more prevalent these days.

Dave

Click to expand...

Clippy strikes again!!!


As to the new rectifier, I don't see it in the pic. Is it tucked underneath the added terminal strip?? :scratch2:

Having recently finished up my own tuner, I too am watching with great interest!!! :thmbsp:

That tuner must have been used a lot.
Tuners don't usually push tubes hard, do they?

Were the weak tubes stock tubes or replacements, making it look like a tube raid was done as part of the refurb?

Nicely done so far Dave.

Dave, I'm guessing you are refering to one of those mini butane type torches? Never thought of that.

Forgot to add... I always worry about too much heat to the can since you can't really put a heat sink on a can's body. Should I forget about that?

audiodon said:

That tuner must have been used a lot.
Tuners don't usually push tubes hard, do they?

Were the weak tubes stock tubes or replacements, making it look like a tube raid was done as part of the refurb?

Click to expand...

Don, those were the tubes that were in it when I got it, also the same tubes that were still in it when it came back from up north to be properly redone. Seems as that that restore was "lacking" as well. I bought replacement tubes as requested and tried to go with the Euro theme tubes that would be period correct, Telefunks and a couple Bugle Boys.etc.

No doubt the good tubes were scavenged before I got it somewhere down the line. At least this time I know it will be done right!

Hifi -- A mini butane torch it is. I (obviously) didn't use a heat shield when removing the old can, and the phenolic insulator shows it. I didn't care too much about it however as it basically had two open sections in it -- no doubt the reason that the extra filter cap was added to cover up that fact. With the installation of the new cap, I used a heat shield (simple piece of aluminum) strategically placed to prevent the heat from hitting the terminal area. It works perfectly, and makes for an original factory installation look. No heat sinking is required, as the chassis itself is the primary sink. Remember too, the original can was installed with some device of similar heat capability to make the nice clean chassis solder joints. The torch is really the only way to go. Just be careful to remove all extraneous parts from the area, and be careful with the torch. The flame can do a number on whatever it comes close to if handled only casually.

Not -- Clippy in fact strikes again! The new bridge rectifier is made up of Ultra Fast discrete diodes located on the back side of the new T-strip mounted on the rear of the chassis. I felt that approach made for a very neat and clean look, with secure connections made in the traditional fashion.

Don -- Tuners can in fact be hard on tubes -- most particularly the Front End RF tubes, and the first couple of IF amplifier stage tubes. Because of the high frequencies involved, these circuits operate at low impedances, which translates into fairly high current flow. That's why you'll find that these tubes run noticably hot, and will definitely not last as long as say a 12AX7 will in a typical line stage or phono preamp position.

Dave

It is always a huge treat to see the level of skill and craftsmanship in these threads. My "bookmarks" have almost become unmanageable!! lol

One thing I try and practice when using the mini torch is covering all areas adjacent to the lug I'm unsoldering with aluminum foil. It seems like cheap insurance, and so far it's been successful.

Audio and MPX Circuits

In the underside pic of the unit as originally received, you can see the two output coupling caps used to block the DC at the output of the cathode follower outputs from the output jacks in the lower left corner of the pic. Not the easiest caps to get at and remove, particularly if you remove their compete leads rather than just snip them out. But, they should certainly be replaced in any full refurbishment.

Ditto for the MPX subchassis. The 1 uF composite signal electrolytic coupling cap always gets the proper attention to be replaced (as it did from the previous tech), but the pilot signal coupling cap and audio output coupling caps rarely get any mention, let alone replacement. But these caps are of the same kind that get loudly decried to be replaced in the general audio circuits, and with good reason. They don't often develop any leakage (although they can), but you can bet they will eventually become intermittent. Therefore, these caps too should be replaced with any full refurbishment effort. Also not the easiest to get to to replace with a neat, finished appearance, but well worth the effort in terms of long term dependability.

So, all the old film coupling caps have been replaced in both of these areas now. With that and the power supply circuits finished, all that remains then are about a half dozen caps in the Auto Stereo and Microtune control circuits. A complete alignment check will top off the project after all the work is complete.

But, with the parts in now for the buffer circuit of the X-202B project, it will move back to center stage for the moment to keep it on track. A few pics show the progress:

1. The finished audio circuits.

2. The MPX circuits showing the 3 caps to be replaced.

3. The new pilot signal coupling cap.

4. The new L & R audio output coupling caps.

5. The finished MPX section.

Back when there's more to report.

Dave

Control Circuits

With the buffer amp for the X-202B well under way now, I thought I'd go ahead and finish out the FM-200B to at least get one of these projects all but wrapped up before Christmas. The underside of the FM-200B is all done now, with just a few details left to call the project a wrap: A couple of tubes are still coming to finish out a complete set of strong tubes, after which the alignment will be checked A-Z, and the overall performance checked against specifications. In the mean time, I can look at the festoon dial lamp sockets, so that modern fuse lamps can be easily installed and removed, without having to solder and unsolder them in the process. Otherwise, this puppy's a wrap, so I can go full steam ahead on the X-202B -- well, as fast as full steam ahead is with Christmas just five days away.......

Some pics:

1. The Auto FM Stereo matrix control circuits.

2. The Muting control circuits. The small black electrolytic at the bottom left of the pic had been previously replaced with a non-polarized electrolytic cap, where a polarized cap is required. Since non-polarized electrolytic caps don't work very well with a DC bias across them, this caused the final (muting) limiter to operate rather strangely when set for high muting thresholds. A proper polarized electrolytic has now been installed.

3. The Microtune control circuits.

4. The finished underside view. All original low and high voltage electrolytic caps, as well as all original film caps have now been replaced. The selenium rectifier has been replaced with silicon, and a proper dropping resistor has been added to compensate for the low drop of the silicon devices. Other than the change to silicon, the unit is now completely stock in configuration.

Underneath then, this unit is ready for at least another 50+ years of service.

Dave

JBL GUY said:

This will no doubt prove to be another very interesting thread!

Thanks for the information and the pictures.

Click to expand...

It sure is. It points out how humble my own skills are.

cc43fan said:

It is always a huge treat to see the level of skill and craftsmanship in these threads. My "bookmarks" have almost become unmanageable!! lol

One thing I try and practice when using the mini torch is covering all areas adjacent to the lug I'm unsoldering with aluminum foil. It seems like cheap insurance, and so far it's been successful.

Click to expand...

Absolutely. Thanks Dave, for sharing your skills in this thread, it's much appreciated.

Thanks for all the kind words! I'm hardly trying to put my skills on a pedestal, as that's not my style, and the fact that there's sooo much talent here at AK that we're ALL learning something from somebody else all the time!

I do try to go into a unit however, and perform my work while keeping the underside looking as true to the original build as possible -- which is to say that other than the obvious appearance of the modern replacement components used, the installation of them is entirely in keeping with the original build quality of the unit. In other words, the work in fact should NOT draw attention to itself! With Fisher equipment, working to achieve a high level of execution is just one more part of the effort to help preserve the qualities that went into making up what Fisher equipment represents.

The FM-200B is likely one of the very best FM Stereo MPX tuners ever produced back in the heyday of the Stereo/Hifi craze -- virtually as good performance wise and immanently more practical than the top of the line, but studio monitoring oriented FM-1000. It, as well as all of Fisher's products stand proud representing an era gone by, and deserve the finest effort we can give them to help preserve their mark on the history of high quality vacuum tube sound reproduction.

Dave

I sent mine to TN and had Terry do his magic. Been singing ever since.


Good luck with yours!



Barney

Cant wait to get this home after Christmas is over and hooked back up to the X202B Dave went threw in a earlier thread, Thanks Dave!

Joining in as I have one of these waiting for some work. Couple of pics to share. It's a nice clean one but i wish it had the wood case.
John