Mating a Fisher 440 Amp to a 490-T Tuner

[larryderouin]

Joe; I typically jam a pair of 6" Needle nose pliers down in the guts, then clamp down on the other end, and twist the pliers from the can with the guts in the pliers. Then do the hair dryer thing with the tar. For a really tough one, I use a 4" deck screw and a pair of locking pliers.

Larry

 

[audmod01]

Larry;

There was something about this one that seemed to make it especially hard. I found it impossible to push anything into the guts. It was like dealing with a piece of wood. After I drilled a number of holes in it with a 1/4" drill bit I slowly dug out chunks of it with needle nose pliers until I got to the last inch or so and it finally came loose. By that time the trash can had lots of shredded material from the top two thirds already removed. I could not believe how hard this one's guts had become. If I had not put a hole in the side I could have still used it. I hope to have better luck with one of the two donor units I have remaining.

Thanks for the suggestions though.

Joe

 

[larryderouin]

Sounds like a dried out one I had in a 400 2 yrs ago. Even hit the tar end with a propane torch. Sucker wouldn't come out. Sometimes if you have spares it's just easier to use one of those as a replacement.

 

[audmod01]

Larry;

I just hope the two donor units I have are not dried out like that one. At least I know that once I get one filled with new parts that it will not be a problem waiting to happen. I have not even applied power to this unit yet and will not until all the potential failure electrolytics are replaced. That includes the ones in the 490-T tuner section too. It deserves good parts so that it will serve its purpose well.

If one of the donor units is also dried out, I plan to take it out to the barn and use my drill press to drill out the inside until I can get all the old part remnants out.

One thing I noticed about this particular design is the use of the 7247 driver/phase inverter tubes. They are described in tube manuals as being a combination of one 12AX7 triode driver stage with a 12AT7 triode phase inverter stage. The phase inverter stage has a lower impedance output and may deliver slightly better results than a 12AX7 would do under similar circumstances.

Joe

 

[larryderouin]

Fisher in '64 changed over to 12ax7's in the Driver/P.I. tube in those amps. If you look at the schematics from the C.E. IV thru the C.E.VII there really isn't much change between them until the C.E. VII when they went to 12ax7's.

 

[audmod01]

Larry;

Did you notice any sonic differences between the models that used the 7247 tubes versus the 12AX7 tubes? It might be hard to say, since often there were changes to speakers as model years moved along.

Joe

 

[larryderouin]

No. Even on the same speakers they sound pretty much the same. I really couldn't tell any difference.

 

[audmod01]

Larry;

OK, thanks for that input on the different driver/inverter tubes. It may be that any significant benefits from using the 7247 tubes would only show up in the more powerful amps with the 7591 tubes or EL34/6CA7 types.

Today I decided to tackle the two remaining donor electrolytic cans that I had on hand. I did manage to get the folded over aluminum rim ground down significantly using the electric grinder in the barn. Then I brought them back into the study to finish up with a hand file. Once I managed to get the first one's twist-lock tab ring removed I applied heat using the hair drier and it came out pretty easily. I turned to the second one and tried the same process. The heat from the hair drier was not successful in getting the internal part out. I decided that rather than trying to drill out another one I would first try higher heat. I took it into the kitchen and turned on one burner of the gas cook-top on the range. I held it over the flame using my siezers (or hemostats) until it was really hot. I made sure I held it such that if it were to pop it would not strike my face. I finally took it back to the study and it was so hot that I had to double the thickness of my leather glove to keep from burning my hand. This time it came out rather cleanly and suddenly when I tugged on a couple of the solder tabs.

So now I have two donor cans to use and their insulating wafers plus the contacts from them and more contacts from the OEM fisher electrolytic and its insulating wafers that I can use in constructing replacements. I stopped for the day and will work on putting together a rebuilt electrolytic tomorrow.

Joe

 

[mrphilco]

I've only rebuilt two FP electrolytic cans in this manner so far, but I have not had to pry around the bottom seam where the insulating wafers are (knocking on wood).

And I have a cheap Harbor Freight heat gun that only gets used for unpotting capacitor cans like this as well as the older cylindrical and rectangular units in vintage radios. I only use the heat gun occasionally and sparingly, considering the source from whence it came...so far so good (again, knocking wood).

 

[audmod01]

Today I took on the task of building a new C93 electrolytic. This is the final one for the 440-A Amp/Pwr Supply. As I suspected, I had to resort to using the gas cooktop to get the old capacitor really hot in order to get the internal parts out easily. Once that was done, the rest went smoothly. I am getting better at doing this the third time around. The most difficult part of the rebuilding of these is getting the bottom aluminum rim folded over the wafers that hold the solder lugs.

I also replaced the .01 AC spike suppressing capacitors on the AC input and the power transformer HV secondary winding to ground. I used Z5U safety parts for both.

The two aluminum cans at the back of the chassis are now about the same size even though the internal parts of the one closest to the power transformer are higher voltage rated than those in the other can electrolytics here.

The next step will be to replace the output tube signal grid coupling capacitors to help insure immunity to red-plating. After that is done I can move on to the 490-T tuner chassis and get it in good shape again.

Joe

 

[audmod01]

Larry;

After supper today, I took time to change the coupling caps in the 440-A. I used your idea of wrapping the new capacitor leads around a jeweler's screwdriver shank. That worked very nicely and they slide over the original component lead easily. It only takes a few minutes to do these. I was reminded of the Sprague griplets that came out in the 1960s. It was a similar idea using wire with built-on rosin core solder that could hold two component leads together after crimping with needle nose pliers, then all one had to do was to add heat from a soldering iron or soldering gun and presto it provided the solder and made the connection.

I then debated about rebuilding the only electrolytic on the 490-T chassis. After looking at it there appears to be some heavy wrapping of component leads and wires on each solder lug of the capacitor. I decided to wait until tomorrow as I am not so eager to dig into another electrolytic this evening. Looks like I am going to have to make a work brace for the chassis to keep from damaging things topside while I work on the wiring below.

Joe

 

[audmod01]

The morning was spent mowing the yard. After lunch I began work on the 490-T tuner/preamp chassis. I started by drawing the components connected to C74 and the orientation of its three sections. The can electrolytic on this chassis was to be removed and rebuilt and reinstalled. When I was unsoldering the twist tabs I used solder wick wetted with rosin flux to help remove as much solder as possible. That allowed me to see how component leads were attached and allowed less heat to do a better job of disconnecting parts and straightening the twist lugs for removal. Once I had most all the solder removed it only took a few wiggles of the top of the can and it dropped out. This one was another can electrolytic that did not want to come out with the heat from the hair drier, so I used the gas range again. Then the internal parts of the capacitor came out easily. The tar liquified and I had to be careful about how to handle it. The can was very hot. Laid it in an empty cardboard box to avoid getting tar on things in the kitchen. My better half would not have been amused if I had!

Since I had the parts on hand it was a matter of positioning parts in the correct order, connecting to the solder lugs and adding a wire for common ground. I passed the common ground wire out through an unused position on the bottom wafer of the capacitor. Everything went together smoothly and the rebuilt can was remounted, twist tabs twisted to retain the can and then parts were reconnected and soldered. There was a 22K 1/2 watt resistor connected between the first two terminals and I measured it at 23.4K, so I put it back in circuit.

I thought I had an 8uF@50VDC for the ratio detector circuit, but I did not. I do have a 10uF@50VDC and am wondering if it is OK to put this one in instead of the 8uF?

When I was preparing the chassis to work on it, I cut two boards to the same length as the top height of the front dial portion of the chassis. This allowed me to turn it over and work on it safely without damaging things topside. I held the two boards against the rear chassis flange with two spring loaded clamps. This worked well and was quite stable for any work I needed to do.

Joe

 

[audmod01]

Dave responded to my question to him by PM about the value of the Ratio Detector electrolytic capacitor. I had a 10uF@50V on hand instead of the usual 8uF@50V used by Fisher. Changing to a 10uF@50V is OK to do, so I went ahead and changed the old part and installed a new 10uF@50V there.

Typically the 1uF electrolytic coupling capacitor from V100 to the MPX detector circuit goes open with age. I replaced it with a polypropylene 1uF@400VDC metalized film type capacitor. This one will likely never go bad for the life of the unit.

To improve the sensitivity of the MPX-65 to FM signals, I added a 47K 1/2 watt resistor across the R204 1M resistor in the 19kHz detector circuit of the first 1/2 of V100 which controls the amount of negative bias applied to the grid of the second 1/2 of V100.

I will be checking the eye tube for the "dancing" issue discussed by Dave Gillespie in another thread here on the Fisher Forum. If any changes are needed there I will do them then.

This unit has one original edge dial lamps and someone has substituted a second incandescent lamp that has attached wire leads and is soldered to the clips that would normally hold the edge dial lamp. I plan to get some LED replacements for both lamps.

Here is a picture showing the changes.

Joe

 

[audmod01]

Some final ohm meter tests were done to check for any possible B+ shorts or leakage, seeing none, I hooked my DC volt meter to the high B+ in the power supply, connected speakers, FM antenna and interconnect audio cable from the tuner/preamp chassis to the amp/pwr supply chassis and turned it on. Momentarily the B+ rose to hear 500V and began dropping as tubes warmed up. Here are the various B+ voltages I measured after some warm-up and with a signal received plus audio playing through the speakers:

At R126 & C103 +390-404VDC
At R126 & R123 +340VDC
At R123 & R110 +295VDC
At R127 & C98C +199VDC
At R127 & R124 +192VDC
At R124 & C93B +176VDC
Those measurements are a little high at the highest B+ but not out of line considering the AC input level of 122VAC. I might consider installing a CL80 input varistor on the AC input.

Overall listening evaluations:
The unit was tested with a considerable number of old tubes in it. All of the tubes in the tuner/preamp chassis are old tubes. The 440-A chassis has all new tubes which include a matched quad of new 6BQ5/EL84/7199 tubes. The cathode bias was easily set to +44VDC as recommended on the schematic. At first try the cathode bias was running +49VDC. No difference in audio quality was noticed after the bias was set. I have yet to try the phonograph input.

On AM, a suitable number of stations were picked up. The same was true on FM. On FM stereo there is the same descending beat notes heard in the audio of my 500-C unit until it is tuned in properly. As all of the 12AX7 tubes in the MPX section are old tubes I am not too concerned at present. I need to check all the tubes and place an order for new tubes and do a retest after I receive the new tubes and install them.

Initially there was some noise as the volume control, treble control and balance controls were rotated. There was noise also when switching the function switch. Application of Deoxit cured these noisy controls. No inordinate hum was noticed through the speakers.

Overall I am pleased with the results. I intend to make a few more modifications in the audio circuitry. Some RF and IF alignment might be needed after new tubes have been installed.

Joe

 

[dcgillespie]

Joe -- A couple of points for your consideration:

1. The resistor acts to limit the effectiveness of the noise detector circuit, allowing for a more dependable pilot signal to be applied to the 38 kHz sync stage and the stereo indicator output. As a result, whether it is the FM Automatic circuit of the 500/800C receivers, or the eye tube of the 400 receiver and console tuner/preamps, they will both operate more dependably as a result of installing the suggested modification. Therefore, by installing the 47K resistor, you have already installed the fix for the dancing eye tube. However......

2. In developing the fix to address the dancing eye tube and overly high FM Automatic stereo switching threshold, I found that the optimum value for the resistor was dependent on the number of IF stages that a unit contained. That is, the more effective the limiting is that can take place in the ratio detector, the less the effectiveness of the noise detector circuit needs to be limited in the pilot amplifier stage as a result. Therefore, more IF stages equals a higher value "fix" resistor, while less IF stages equals the need for a lower value. The 47K value I offered was based on the effective limiting that typically occurs in a 400 receiver's IF strip. With your current project however, there are only 3 stages in the IF strip, so you may find that an even lower value is required for the fix resistor, since the effective limiting will be reduced even more.

Understand, that the noise that the noise detector circuit responds to has components that can reach to at least 100 kHz. Therefore, while you may not perceive a huge difference in audio noise on a given station that would be implied by the performance difference between the 3 tuber IF design of the 490-T and the 6 tuber design of your new 202-R tuner, the noise detector certainly knows the difference and so the need to reduce its effectiveness as the FM limiting is reduced.

I hope this helps!

Dave

 

[audmod01]

Dave;

Thanks for those comments. I seem to remember you once mentioned using a 39K resistor across R204 in the MPX circuit for some lesser number of IF stages. I might try that at some point. I need to put together an order for new tubes in the tuner chassis so that I can make sure of adequate gain from each tube. I have not yet checked them and many are original tubes from the Fisher and from some other units I have lying about. Once I get known good tubes in it I can revisit the various issues and check alignment etc.

Joe

 

[dcgillespie]

Great job Joe! I use the 590-T tuner/preamp (basically identical to the 490-T) I mentioned earlier on in this thread on a daily basis, and thoroughly enjoy its performance as a lab rat. I hope a fresh set of tubes in the MPX chassis resolves your issues there.

I just did a couple of quick tests on my 590-T, and a 33K resistor in the fix position really cleaned up the eye tube dancing that appeared on all but on station. That may not be the optimum value for your unit, but it will no doubt likely be found somewhere in the 30K decade of resistance.

Remember too that the fix values I suggested in all instances is based on a properly aligned and operating tuner section.

Dave

 

[audmod01]

Dave;

Thanks again, I understand about alignment. Once I get a new set of tubes in it I can also look for possible silver mica disease, make sure all tube sockets are clean (haven't done that yet) and see how it does again, then check other possibilities if needed.

Joe

 

[audmod01]

I did some more listening tests today, tuning in a number of FM stations in stereo. It turns out that the only station I am having trouble with is one that is badly interfered with by an alternate channel frequency due to a strong local station. Most of the issue seems related to RF/IF intermodulation distortion. All other stations that come in strong enough to produce stereo sound OK.

To improve the low end some, I am thinking about adding a 22uF@10VDC electrolytic across the 1K cathode resistor of V10 for each channel. Also change C47 and 48 to .047uF capacitors. It is not shown on the schematic, but the volume control used in this 490-T chassis has a loudness tap and there are some added components. I think 22K ohm resistors and .0022 caps in series with the resistor. The schematic leaves these out for some reason. It appears that loudness compensation was on permanently because there is no separate control to turn the loudness compensation off. On the other hand, the lack of bypassing on the cathodes of V10 and small value coupling capacitor in the plate circuit of V10 prevents very much low frequency amplification in those stages. The preceding stages for handling phonograph input may need attention too. There is a limit of course on how much improvement may be obtained. The output transformers are not very large, and low iron weight does have its limiting effect on the low end. Fisher likely was keeping low frequency output rolled off to prevent acoustic feedback from speakers being in the same cabinet as the record changer. In my case this unit will be used with bookshelf speakers placed elsewhere in the room.

Was the phonograph input on this unit designed for a ceramic high output cartridge or for a magnetic low voltage output? It looks like it might be for a magnetic cartridge.

Joe

 

[larryderouin]

Joe; It was / IS Magnetic. I took out the 22k/.0022uf network on my CE VII and the loudness turned on full all the time. Much better low end although at very low volume the bass tends to overshadow the rest of the frequencies. Turning down the bass a couple of notches helps this.

Larry