A Songbird of a Different Color

OK, so when is an 800C receiver not an 800C receiver? When it's an 1800 receiver of course, and this bird (serial no 52406 T) just landed on my door step. It is unique with the obvious two-tone face plate, and dual primary winding power transformer that allows for either 120 or 240 volt operation.

Interestingly, this one has a standard Fisher 120 volt (American) power cord that appears to be factory wired in place, and also includes the two standard two-prong accessory outlets on the rear panel as well -- but all references to required operating voltage are absent. Against this however, the silkscreened rating for the fuse on the chassis is given as 1.5A slo-blo, which is basically 1/2 that of the 3.2A slo-blo fuse ratings for standard 120 volt 800C receivers. So, it's a unit that is factory wired for 120 volt operation, and supplied with a conventional power plug set, but has no indication as to what voltage it operates from, and indicates that it requires only a 1.5A fuse. Clearly, the fuse size indication is wrong for 120 volt operation, with a standard 3A slo-blo unit required for that voltage configuration.

And get this: This unit also includes the standard AC line bypass cap connected between one side of the AC line and the chassis, rated for the same voltage as the caps used in an 800C for that purpose, but are made with a ceramic body in place of the typical plastic ones commonly seen. So boy oh boy -- if this baby is wired for 240 volt operation and the plug is inserted so as to have the bypass cap on the hot side of the line, and you get between the chassis and earth ground, then you've just been tasered!!

Beyond these differences, the only other ones I've seen so far include:

1. The use of a bigger AM antenna assembly than those I'm accustomed to seeing on 800C receivers,

2. The negative bias voltage supplied to the output stages has been increased by about 1 volt over that in 800C receivers, and

3. The grid circuit of the MPX relay driver tube is slightly altered.

Otherwise, this is a typical 800C wrapped up in a purty wrapper!

A full recap is on tap plus the addition of a current limiter, and possibly some other work as well, all followed up with a full round of testing to ensure compliance with published specifications. Normally, such a project is pretty routine stuff here on the Fisher Forum, and I normally wouldn't have posted about such a project. But this unit is such a rare bird and so unique in appearance, that I thought folks might be interested in what Fisher did to turn an 800C into an 1800.

Before getting started however, this one's got to take a side trip for repairs, that would almost surely not be corrected by simply recapping the unit -- a scenario I find to occur more often than not anymore.

In performing an initial assessment on the unit, the AM section appears to operate well, but it quickly became apparent that the Stereo Beacon indicator is dead, as is the entire FM tuner section as well -- I mean, no noise from that sucker at all. The former will be easy to chase down, but the latter is showing itself to be the result of multiple issues.

Early diagnosis showed the last limiter stage tube socket was so dirty that the heater was not even lit up in that tube. Some significant coaxing caused it to light, which then brought on the normal FM rushing sound -- albeit somewhat subdued -- but that's all it brought. There was still not a station to be found. Further testing then showed that the second FM IF transformer has major issues (passing only a fraction of the signal it is supposed to causing the weak rushing sound), but even that still should have let me hear my own lab station. But still nothing. The last check then showed no IF signal at all coming out of the RF front end assembly, so that will be the next stop when I pick this back up.

If anything however, this shows why it is always best to fix all the problems noted in an initial assessment first, and then do the recap last. With this unit, the initial assessment still isn't even finished, as the DC output from the selenium rectifier in the DC heater/bias supply is so low, that I've held off even installing the output tubes yet. All testing so far has therefore been done with a scope and signal tracer. Once the current issues noted are resolved, then the assessment can move on to whatever else it might turn up (if anything) until it is finished. Had the recap been done first and then these problems noted (since its a virtual certainty that none of the issues so far are recap related), then you're left stuck trying to figure out what you did wrong, when maybe you didn't do anything wrong at all -- the problems were there to begin with. Or, if you do a good initial assessment, but still dive headlong into doing the recap first hoping it will fix all the ills noted, then you might find out you just recapped an expensive doorstop if the problems aren't resolved by the recap, and they in fact turn into a major $$ pit to resolve. Fixing as you go along and recapping last will produce the least surprises, and get the most done for the least cost.

Pics include:

1. A songbird of a different color for sure! And what a neat and unique way for Fisher to start the transition from their vacuum tube gear, into solid state. All the brights appear to be original except for one.

2. The antenna assembly appears bigger than I am used to seeing. The power transformer is physically identical, but includes dual primary windings to allow dual voltage operation. I've never seen a tan colored can insulating cover on the top side of the chassis, either.

3. Underneath, it looks like almost any other 800C you might have seen. Almost.

4. Except, look at those very Dynaco like aqua colored caps in the MPX unit. They were standard (stock) on almost all of the small Dynaco amplifiers, but I've never seen them in a Fisher before....... This unit uses the newer encapsulated diode ring detectors.

5. Nor have I ever seen flame-proof power supply dropping resistors in a Fisher before. Must be something to do with international standards.

6. The AC bypass caps are basically double the physical size of those used in a standard 800C, but rated at the same voltage. That selenium rectifier has got to go. And look at the different value bias dropping resistors: 8.2K and 27K, versus 5.6K and 15K. The new combination adds about an extra volt of negative bias to the output tubes.

And so a new project begins. More, as there is more to tell!

Dave

Wow. That two-tone faceplate is killer. Gives the fisher a whole different look. It moves it into the "scientific" realm of the Audio Research stuff. I like that look!

Hey, Doc, Could you look at the last page of my "Doing my fusistor mod" thread and give me a verdict of which bias setting I should use, or if all of those results of a resistor swap are unacceptable? Thanks!

Interesting!

Thanks for the pictures.

Looking forward to this one!

Wow! That is a very neat piece of gear. I'll be watching this one. :yes:

Dave, I still have all the FM IF transformers from my 500B build. I don't know if they are the same or not, but if they will help you out, they are yours for the asking.

Man, looking at that thing again reminds me of how much I want one

What a perfect name for this thread! I will most certainly be watching to see all the details on you bringing that beauty to it's Fisher glory.

Love it - just gorgeous! :yes:

The owner should be very happy it is in YOUR capable hands with those odd anomalies and all.

The changes noted may just be due to parts availability - the 1800 was the last and only tube receiver from Fisher for a while. Since it was sold at military PXs, worldwide power was required. Was it marketed elsewhere outside the US?

Great post Dave, will be watching as things progress on this! Beautiful Fisher by the way. Al

Plowing Ahead

The FM section of this unit has really had some strange issues to address, but progress is being made:

1. The RF Front end had two issues causing zero output from that sub-chassis:

A. The 6GK5 RF amplifier tube is basically a trophy tube: It lights up all fine and good, but has a Gm of.......Zero. Not just bad or low reading. Zero. It's an original Fisher tube that clearly has had an internal connection go south....... The rest of the FM tuner tubes were either quite good, or serviceable enough to let the assessment continue.

When I saw this, I thought that nailed the RF front end problem, but no, it would still not receive any stations. More troubleshooting turned up:

B. The 1000 ohm B+ decoupling resistor inside the RF sub-chassis that supplies the local oscillator also read? Nothing. It's not burnt, it's not discolored, or physically damaged in any way. It just has no resistance value to it at all. I've never seen anything like it.

In any event, replacing that resistor with an identical type resistor (to eliminate the possibility of a different type component causing drift) brought the local oscillator back to life again.

So, with the front end repaired, a temporary 6GK5 installed in the RF amp position, and the tuner tube sockets all cleaned up, the FM section was able to receive stations again. But, it's still not out of the woods yet. There is little movement of the tuning meter, and the output is appropriately weak, too. Back to the FM IF transformer.

Njcanuck -- Thanks for the offer, but this transformer is Fisher part number ZZ50210-39B, which does not cross to any of the transformers used on the 500B. If anyone has such an animal to offer up, don't be shy about it!

In the mean time, I have removed the transformer and torn it down, and am in the process of possibly repairing it (one of the internal leads appears to be almost burnt, and potentially shorting to another lead it was laying against). There doesn't appear to be any of the mica disease, and the shunt caps appear to be stable and identical. So, maybe there's some life left in this little guy yet.

Pics include:

1. The internals of the problematic IF transformer.

2. A closer inspection shows how the insulation has nearly been burned or melted away on one of the primary wires, which is where it was touching one of the secondary wires, leaving a mark at the point of contact.

3. This discoloration doesn't appear to be mica disease and does not extend up to where the actual cap plates are, so hopefully this is not an issue.

4. The rest of the FM section problems: The open 1K resistor, a very nice looking but completely dead 6GK5 tube, and an equally dead bulb from the Stereo Beacon indicator.

5. A NOS resistor of the same type installed where the open resistor was, allowing the local oscillator to operate again. You'd be surprised at some of the stuff I've collected over the years.......

I hope to get back to the IF transformer later on tonight to see if it can be saved, but at least life is starting to return to the FM section now. No doubt however, this section is going to need a good alignment when all is said and done.

Dave

FM Tuner Success

Whoa -- With the RF front end previously repaired, what a difference the repaired IF transformer has now made!

The problem was clearly the two internal wires partially shorting together, which ruined the performance of the transformer of course, and because of the two specific wires that were shorting, caused the burning of the one lead as well. I was able to cut out the burnt part, and extend the lead back down to the transformer terminal again using very fine magnet wire. The brown spot of concern on the base of the transformer as shown in the previous post turned out to be some of the glue used in the production of the transformer. So the repair is permanent and complete. Previously, the transformer housing was sensitive to the touch or even simple movement of the entire unit due to the way the two wires were lightly touching each other. Now the transformer and the unit itself are completely immune to any physical movement. Not only that, but the tuning slugs even remained peaked as well, so the repair really allowed this unit to dodge a bullet for sure. There's no telling how long this problem existed with this unit.

Pics include:

1. The location where the IF transformer lives, reinstalled again in this pic.

2. Underneath, reinstalled so you'd likely never know it was ever removed and torn down.

3. Well looky here. This baby is actually acting like a Fisher now. In my basement lab (which is underground), with a 2 foot piece of wire on the bench for an antenna, I can easily receive Atlanta stations from my location about 65 miles north of that city, and even trigger a steady Stereo Beacon indicator now.

4. The needle of this meter used to live below 0. Now, under the conditions described in #3 above, there is plenty of signal strength on numerous stations.

During the troubleshooting episodes, one other difference I noted between this unit and any version of the 800Cs, is that this unit includes an additional automatic volume control (AVC) feedback loop in the FM tuner section (for a total of two loops). The net effect of this difference won't make the 1800 any more sensitive to FM signals, but it will let it handle overload signals better. The other thing it does is cause the signal strength meter to read slightly lower for a given signal strength condition than an 800C receiver will sitting right beside it and connected to the same antenna.

I'm not sure why Fisher did this as I've never observed the 800C to suffer from any overload concerns, but the extra loop was installed none the less.

These repairs have produced an absolutely normal FM section now, and in the process, have shown that the automatic switching circuits operate normally as well, as even does the MPX decoder by all accounts. No performance measurements yet, as this is still just the initial assessment period of this project.

To finish the assessment will require the rebuilding of the DC heater/bias supply next, so that the output tubes can be safely installed. Currently, the unit is still operating on life support (from a variac with external power amps) so as not to cause any undo harm the the Westinghouse 7591s supplied.

Chores tomorrow, but should be able to get started on that portion of the work tomorrow evening with a little luck.

Dave

Dave; WRT the Aqua caps. I noticed that FISHER used them in all of the different model SS units I have. All of my SS units date from 67 and up so that would fit date wise with the 1800. Great job on the tuner transformer.

I've seen open but undamaged resistors in tuner sections before, usually in the IF strip though. Don't know what causes it, but its the only time I've ever seen it.

:thmbsp:

Decisions, and Finishing the Assessment

The only portion left of the 1800 to assess is the complete audio section. But with the DC heater supply only developing about -18 vdc, that had to be rebuilt before output tubes could be safely installed, and anyway, -18 vdc would take the rest of the year to heat the small signal tubes into any kind of operating condition.

But before doing that however, decisions had to be made as to whether any modifications would be installed. As a basic premiss, because the 1800 is such a rare bird (pun intended), the overall goal is to keep the unit as stock as realistically possible. But in today's audio environment, certain modifications are deems warranted, even with such a rare bird as the 1800. Therefore, along with the recap, some modifications will be installed, but only those that are the most practical and least intrusive into the original design. The total extent of the modifications then include:

1. Installing matched 10 ohm 1/4 watt cathode resistors to allow for the sampling of cathode current, and providing fusing action.

2. Install 100 ohm Screen Stability resistors to prevent any tendency towards output tube arcing with the installation of modern power supply capacitors.

3. Provide a single bias adjustment to allow for adjusting output stage quiescent current. This will continue to require the use of matched output tubes, but this always provides optimum performance anyway, and keeps the unit consistent with the original design in that regard. The adjustment then will allow for a wide range of matched quads to be used, which is more in keeping with the reality of vacuum tube production/availability today.

4. The Tape Monitor/Aux input bleed circuits will be removed. With a greater variety of input sources available today than ever before, having two completely separate high level inputs available on the 1800 just makes sense.

5. The improper channel identity at the headphone jack will be corrected, allowing headphones to be used, with the front panel controls operating correctly as represented.

With the decisions regarding modifications nailed down, the DC heater/bias supply has been completely rebuilt now, with an adjustable bias circuit installed, and individual cathode resistors installed as well. This will now allow the initial assessment to be completed, to check for any potential problem areas within the audio portion of the unit. The screen stability resistors won't be required until the power supply recap is performed.

Pics of the work include:

1. With the old can cap, its retaining clip, and the old selenium rectifier removed, there is plenty of space for modern components, and a small board to contain the adjustable bias circuits. The neat thing about this bias circuit, is that with the control full down, it provides the exact same bias voltage to the output tubes as the original design did. Therefore, the stock setting can easily be maintained if desired. Also, protection is built into the design, so that if the wiper of the control ever lifts from its carbon track, the unit also returns to a stock bias setting. The control is shown at the setting that produces an average current draw of 33 ma per tube with the particular tubes used, operating from a 121.5 vac line. Overall, the rebuild of the supply produces a neat, clean appearance, with easy access to the bias control when the underside is accessed.

2. Under the board are two 105 C Nichicon caps replacing the old can cap, which are mounted to the chassis with silicone adhesive. A piece of fiber board is also mounted with silicone to the tops of these caps. The circuit board is then attached to the fiber board with small wire straps. I have used this approach many times before, and it works very well indeed. Also, with the lug on the T-strip freed up that was originally used for the bias supply wire, that lug can has now been used to install a CL-80 without any "air" connections.

3. The original 15 ohm dropping resistor has been increased to 25 ohms, which together with the installation of the CL-80, provides an ideal -24.0 vdc to the small signal tube heater strings when operating the unit from a 121 - 122 vac line voltage.

4. Cathode current sensing resistors have been installed as well. With the bias control at the full down position, the four output tubes supplied idled at 39 ma, 36 ma, 31 ma, and 28 ma. The quad could be broken up into two decently matched pairs, but represents poor matching as a quad.

With the unit safe to operate now completely under its own steam, the initial assessment will be finished tomorrow, after which, with everything then known about the unit and the way forward with it, a parts order can be placed to finish out the work.

Dave

dcgillespie said:

The neat thing about this bias circuit, is that with the control full down, it provides the exact same bias voltage to the output tubes as the original design did. Therefore, the stock setting can easily be maintained if desired. Also, protection is built into the design, so that if the wiper of the control ever lifts from its carbon track, the unit also returns to a stock bias setting. The control is shown at the setting that produces an average current draw of 33 ma per tube with the particular tubes used, operating from a 121.5 vac line.

Click to expand...

I love it!

Dave, what are the 2 heathkit analyzers behind the receiver?