Fisher 500B Re-Hab

[Dave451]

ALIGNMENT FISHER 500B

I dis-assembled the dial cord mechanism and removed the tuning wheel. The spindle seemed a bit dirty (where the dial cord loops around it), so I buffed it with a Dremel buffer head just enough to clean it up a bit. Re-strung the dial cord and now it worked smoothly from peg to peg during tuning. The cord still 'walked' across the spindle as you turned the tuning dial, but no longer wrapped up on itself at the ends of the dial travel.

I used my SG-165 FM multiplex generator with unmodulated 10.7 mHz crystal-controlled output to align the IF per the "Alignment Instructions" in the 500B Service Manual. I pushed down the top half of the V1 6AQ8 tube shield and clipped the output of the Sencore matching pad to the glass tip of the tube and grounded the black lead on the chassis. This provided plenty of signal for the alignment and I could keep the negative voltage readings below -5VDC at TP-2 by reducing the generator output.

The coils were a bit sticky, but moved OK without excessive force being necessary to peak the stages. There is only one slug in the 500B Z4 transformer, so no confusion really between "top" and "bottom." The best news was that my "transplant Z5" transformer worked very well in Step 2 and 3. One thing to note: I was not able to keep the max negative voltage below 20 V in step 2, even without signal into the mixer. So, I peaked TP-3 negative voltage by adjusting Z5 and the final voltage was -35 VDC. No trouble at all zero-ing in step 3 with the new Z5 transformer after configuring the dual 47K resistor divider net and clipping it in.

Steps 4 and 5 went very well (adjusting dial accuracy using RF output modulated at 22.5 kHz with 400 Hz audio, per the procedure). The tuner was indicating broadcast signals about 1 mHz higher than actual. Using RF output from the SG-165 matched with broadcast signals, the signal was easily walked down by tuning L5 in the front end unit and tweaking L4 and L1 in the front-end unit 'silver box.' I tuned to the other end of the band, and no adjustment was needed on the high end at all, so I didn't tweak C20 and C10 in step 5. Middle frequencies were spot on the dial as well.

The tuning eye closes completely on strong stereo signals from broadcast or the SG-165. Weaker stereo signals open it up. It closes up pretty far on mono, but leaves plenty of room for tuning to peak signal. I'm presuming this is OK.

MPX DECODER

I tested stereo with the SG-165 and was getting plenty of separation and the audio from broadcast signals sounded fine on the bench speakers, so I did not mess with it pending a better listening test. The tubes (2 x 12AT7 and 1 x 12AX7) were all good, so none were changed. Separation pot is close to the middle (maybe 11:30).

The MPX decoder is, of course, not the MPX-65 in the Fisher 400. It says "WX" on the sub-chassis and appears to be the same unit used in the KM-60, FM-100B, and FM-1000. The service manual does not have an WX alignment procedure, but I presume it is the same as that for the WX unit in the the FM-1000 manual, which is the same as the one in the Fisher 400 manual, correct?

POWER TEST
Tested output to clipping with 1 kHz, single channel driven and got 23 WRMS per channel. Set the phase inverter for lowest 2nd order harmonic for the time being. Power sound correct?

Dave

 

[dcgillespie]

The MPX-65 and WX decoder chassis both align the same way, so the instructions are the same. The power output seems to be about 4 or 5 watts low based on your voltages given, and assuming that the output transformer primary impedance is about 6500 Ohms. What you're getting is more on the order of what I'm used to seeing in late model 400 receivers.

Dave

 

[Dave451]

Thanks, Dave. I thought it was a little light as well (very close to the output of my late-model 400). Thanks for confirming alignment procedure on the WX decoder. Things are sounding very well with the 500B, but she needs some physical clean up as well. I'll play with tubes and such on the power and see if that makes a difference.
Dave

 

[Dave451]

Dave, I increased the bias on the tubes from 30 mA quiescent to 33 mA per tube, and this jumped the power to 27 WPC RMS, single channel driven. That seems more like it. Checked the turns ratio on OP transformers and they are indeed about 6600 ohms. Give that Fisher probably ran the tubes hotter (35-40 mA per tube, maybe?), this sounds reasonable for 33 mA quiescent cathode current, right?
Dave

 

[larryderouin]

Multiply your quiesent ma by the plate voltage. That gives you the high end of the wattage of each tube. Subtract 3ma for screen load and you have your actual wattage. 80% of the Maximum wattage is the High end SOA of the wattage for NEW STOCK tubes. 15.2w is 80% of 19W. 13.3w is 70% So anywhere in between is good.

If your plate voltage is 412V and you've got them biased to 33ma 412.0 x 0.033=13.5w subtract 3ma 412.0 x 0.030=12.36Watts per tube. Take them up to 37ma 412. x 0.037 = 15.244W Subtract 3ma 412.0 x .034= 14.008W per tube. You can go higher to 39ma which after the screen load will give you 36ma or 14.832 or about 78% of max dissipation.
Substitute your actual voltages for the examples.
This is a more accurate reading of the tube output rather than using bias voltage @ pin 6. Using the ma's to equal out, the pin 6 voltages will all be different. Don't worry about it unless it's under -10v(-11 is my personal low voltage).

 

[bhamham]

Larry... and, you keep saying you're no good at math!

Dave, also check the supply voltage to those preamp tubes should be around -25v

 

[larryderouin]

Number to numbers I can do. Put a letter in for one of the numbers and I vaporlock. I would literally get migraines in high school algebra class. Geometry would have been fun, but the sumbitches put letters in the equations and I would freeze up. You'd have to break out a Rosebud torch(used to break loose propellers from the shaft) to just warm me up.
I still get a Brain BSOD when someone puts an equation on a post with a letter in it.

 

[Dave451]

Thanks much, Larry. Looks like we're running a little cool still. The Tung Sol's are hot tubes, apparently. I am running the grid voltages around -18.5 to -18.7 to get to 33 mA quiescent cathode current. This has 'maxed out' my IBAM with the 22K resistors. For this quad of tubes, I'm having to crank the bias all the way "down" (most positive) and still need a little lower to get more than 33 mA. Recall that with the 10K resistors, I was cranked all the way the other way and still seeing something like 35/36 mA, so the resistor needs to be somewhat less than 22K for these tubes to have the right range of adjustment. I'm like Goldilocks again--looking for 'just right.'

'll fiddle with that and report after I try a second quad of Tung Sol 7591A's to see where they land. Recall from above that one tube was not matched properly in the quad. Rather than return the single tube, they want the whole quad back because they don't have a single close to the remaining three. That's why I'll need to try the other quad I have on hand. From what I've seen, they are going to be pretty close to the ones I'm returning. Hopefully they all match (another vendor).

Bruce, presume you mean the -25 supply voltage? It is indeed running right at -25VDC. That has the filaments where they need to be, I think. The tubes are wired in series-parallel, with each pair wired in series and the pairs in parallel to the -25V supply. I had to double take on that at first, since all the other gear I've worked on, Scott and Fisher, had the whole four wired in series and running around -46/48 VDC.

Dave

 

[Dave451]

Fiddled with the resistors in the IBAM unit and settled on 15K resistors. This gave me the ability to bias up to 40 mA per with brand new tubes. I put in the new quad of Tung Sol 7591A's and set the bias to 37 mA per tube. Everything set up easily and had little extra room on the high end of cathode current. Line voltage was 119.5 VAC after the CL-80, plate voltages 405 VDC, screens 378 VDC. With the cathodes set at 37 mA, this gives 34 mA net plate current. At 405 VDC on the plates, this gives quiescent idle dissipation of 13.8W, or about 73% of 19W max. We'll go with that for now as the output tubes burn in.

Grid voltages to achieve the 37 mA cathode current per tube were: V13 = -17.0, V14 = -17.6, V15 = -17.2, and V16 = -17.2 VDC.

Checked the power again at 1 kHz and single channel drive was L= 28 WRMS and R = 27 WRMS. Distortion was quite low on both channels by the look of the scope (FFT showing various harmonics) at clipping. Both channels driven gave right about 21 WRMS per channel (sound reasonable?).

Tuner is acting very well and this 500B is making even my bench speakers sound good. The electrical work is good for now, I think, so now we'll pull the Scott 340B off the shelf and hook up the 500B for a good listen. Still need to clean everything up physically, but want to be sure the sound and tuner remain good as everything plays in for a few hours.

Dave

 

[larryderouin]

Good Show Dave.

 

[bhamham]

Good Show Dave.

I'll second that - well done! A good reference thread for the 500b to go along with your excellent 400 thread.

 

[Dave451]

Thanks, Gents! As always, appreciate your help, counsel and comments. Learning a lot and enjoy restoring these fine old pieces. One of my old buddies saw my workbench with the 500B on it, covers removed, and the scope and all the old gear and said "man, you ARE a nerd!!" (fist bump). Guilty!
Dave

 

[dcgillespie]

Welcome to the club!

Great job!!

Dave

 

[larryderouin]

Go requisition a Pocket Protector. That and Birth Controll Glasses (aka Gilbert on Revenge of the Nerds) are the identifiers for other NERDS.

 

[Dave451]

Alas, my pocket protectors are all gone but my brothers tell me my reading glasses qualify as BCG's. Have to be period correct when working on the old gear, right?
Dave

 

[larryderouin]

NAH! Close enough is fine.

 

[Dave451]

Hey, here's a question on the 500B tuning eye. While listening and running things in, I note that on non-FM functions, the tuning eye on this receiver stays active. In fact, when listening on AUX function, the eye seems to read the strength of the station that was tuned prior to switching and even varies as the tuning knob is adjusted. Does this mean the FM tuner is still active (along with the eye tube) when you're not using the tuner?
Dave

 

[Sam08861]

Mine does the same thing. Unfortunately, I don't know if it's just the tuning eye or everything is powered up. Will have to take a peek at the mpx tubes if they're lit up when it gets dark here in nj.

 

[larryderouin]

Yes the tuner is active all the time. All your are doing with the function switch is changing the signal input to the amp.

 

[Dave451]

Yep. I was used to the 400, which de-activates the tuner and the eye, so the 500B was a little different. Thanks!