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Fisher 700T--First Solid State Restore

Discussion in 'Fisher' started by Dave451, Nov 21, 2018.

  1. Dave451

    Dave451 AK Subscriber Subscriber

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    IMG_4995.JPG I bought a pair of speakers from a fellow on BT and he threw in a 'partly working' 700T as a gift. I had not had any experience with early Fisher solid state gear, so was quite interested to pop the covers and see what it was all about, particularly the sound after restoring. This one was dusty and had been dropped on the upper left hand corner of the (very robust but now quite bent) front panel and had a piece of old masking tape on the back labeled "bad RT channel." I figured there would be some work and parts involved but, hey, it was free and gave me a chance to compare the late 60's initial Fisher SS gear to the tube gear I have restored.

    I did some research and there's not a whole lot out there on the early Fisher solid state gear (at least compared to the classic tube amps and receivers), but I found a couple of good AK Fisher Forum threads that were very helpful and a couple of other on-line threads as well. I found a very good scan of the Service Manual, but have never been able to find an owner's manual anywhere on line (the 600T OM is available and appears close, feature-wise).

    The 700T was the top of the line receiver of all Fisher types (tube and SS) in the 1968 time frame, offered alongside the 200T, 220T, 500T, 550T and tube models 400, 500C, and 800C as they phased out. The 700T is the highest power rated solid state receiver offered in this lineup at 50W RMS per channel (see attached description and specs from the 1968 Fisher catalogue--interesting comparison with the tube receivers). The 220T and 550T were noteworthy for having 'microcircuit' components (IC's).

    The 700T was priced at a level that would be thousands of today's dollars and is a very well built piece of gear. All the transistors are silicon, including the drivers and outputs and the design appears quite robust. The tuner is a "Super Synchrode" FET type.

    Initial examination showed:
    -Socketed transistors! Most transistors in the unit are mounted in their own socket (except for the outputs).
    -Roomy and pretty robust-looking PC boards for the main circuits, as well as some free-mounted transistors mounted to tie strips on the chassis.
    -LOTS of small electrolytic capacitors in the signal path and otherwise, including quite a few 1 uF, 0.5 uF, and 4 uF (more on these later!).

    Initial power-up revealed several problems, particularly:
    -No shorted power supply caps (dim bulb tester stayed dim when bringing it up on the Variac). Fuses all intact (including the output channel fuses)
    -Very weak audio output on the left (not the right!) channel
    -Stereo beacon light not working on even strong, stereo stations
    -No FM stereo output in FM stereo mode--all mono all the time
    -No audio in "FM Stereo" mode, but audio in "FM Auto" and "FM Mono" modes.
    -The usual scratchy pots and switches to deal with.

    All of this was dealt with over a few hours work on the bench, and I'll describe what I found and how I resolved it in subsequent posts just for the record.

    I neglected to take 'before' pictures, but a couple of cleaned-up 'after' shorts are shown below. More to come!
    Dave

    From the 1968 Fisher Catalog:

    View attachment 1336432

    My 700T in the 'after' state after some the restore (including straightening the bend front panel corner):



    IMG_4988.JPG

    IMG_4990.JPG


    IMG_4981.JPG
    View attachment 1336444
     
    Last edited: Nov 21, 2018
    lokerola and JeffPro like this.

     

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  2. Dave451

    Dave451 AK Subscriber Subscriber

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    I realize now the description and specs are not very readable here, so will try to upload them in a different format in subsequent posts. Sorry about that!
     
  3. lokerola

    lokerola AK Subscriber Subscriber

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    Looking forward to hearing the updates on the restore. It sounds like a very well built piece of gear.
     
  4. fred soop

    fred soop Super Member

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    Socketed transistors: Very common in the early days. The transistors were treated like tubes. Circuits were designed using one transistor per stage, setting the bias to the middle of the linear range, and it was done. They had not yet learned that transistors could be combined into pairs in tight feedback loops in a single stage. Additionally, transistors were shipped individually in boxes that were miniature versions of tube boxes, similar to the 6CW4 Nuvistor. An outer 4 side cardboard sleeve with a styrofoam insert for the transistor.

    Bump those coupling caps to 10µF or even 22µF if you replace them. They also did not know back in those days that electrolytic capacitors should be around 10 times the value necessary for the low frequency cutoff in order to bring distortion down to the noise floor. Any frequency shaping capacitor (filters, tone controls, etc.) should be polypropylene or at least polystyrene.

    Did you replace the knob covers or were they already in that good a condition?
     
  5. mrphilco

    mrphilco AK Subscriber Subscriber

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    :lurk:

    I have a 700-T which I restored. Great receiver. Good luck with yours!
     
  6. Dave451

    Dave451 AK Subscriber Subscriber

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    Thanks, guys! Fred, yes I have seen some ham radio gear from this period into the early 70's with transistor sockets. It is very interesting to see how the front end and IF are designed, with one transistor where a tube used to be. Also interesting are the IF and MPX cans that are identical (more or less) in size and function to those from tube gear, before they started to miniaturize those in later SS gear. I was fascinated also by the fact that two of the receivers had IC's (not this one; the 200T and 550T), which I didn't realize were in use in the late 60's; thought they were a '70s thing.'

    As you will see, I did not bump up the values of the couplers and was unaware of the '10x' dynamic you mentioned (I also had a bunch of 1 uF, 0.47, and 4.7 uF caps on hand...:)). More on that later....

    Oh, and yes, the knobs are original and much brighter than you would expect for the amount of storage time. I just washed them with soap and water. Little tarnish on a couple and I had to glue the cap back on the tuning knob--came off after washing.
     
    Last edited: Nov 22, 2018

     

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  7. Dave451

    Dave451 AK Subscriber Subscriber

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    Mrphilco, will be interested to compare and contrast our approaches at restoring this receiver. I will say at this point that there was not one bad transistor in the unit and that surprised me a bit given the audio issues at the beginning. Almost all the issues were due to bad capacitors.
     
  8. knockbill

    knockbill Addicted Member

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    I have a 220T, I found at a hamfest for a couple bucks,,, recapped and new diodes in PS and rebuilt the pre amp board with help here(Fred and Larry),,, really sounds good for an old, low powered receiver.,...
     
  9. mrphilco

    mrphilco AK Subscriber Subscriber

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    Dave, I rebuilt mine nearly four years ago. Mine had bad audio output transistors when purchased. It has new ECG 162s (ECG, NOT NTE) in it now.

    Fred - I wish I had known that about changing the coupling caps to 10 or 22 uF when I rebuilt mine. Well, it's done and still working after nearly four years, so I can't complain.
     
  10. fred soop

    fred soop Super Member

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    In normal operation, you probably won't notice. It's interesting that we can now build nearly perfect amplifiers, then connect them to speakers with 10% distortion and -10dB frequency response, then load those into a room with very bad acoustics. It should also be noted that the biggest source of electrolytic capacitor induced distortion in the 700-T (and others) is that huge output coupling cap. My Heathkit AR-15 has the same problem and since the power transformer does not have a center tap, there is no way to rebuild or modify it other than to convert the output into a full bridge circuit which would float the speaker terminals at half the supply voltage. Probably not the best thing to do.
     
    Last edited: Nov 22, 2018
  11. bhamham

    bhamham AK Subscriber Subscriber

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    Great write-up and introduction to the 700-T, Dave. I've never worked on a SS unit but always been curious. I'll be interested to see your restoration progress.
     

     

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  12. fred soop

    fred soop Super Member

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    Here are some power calculations of the 700-T. The power supply voltage is 63 V, single supply. This is equivalent to ±31.5 V. Allowing for minimum voltage drop across the driver and output transistors, we have approximately 30 V peak available from the transistors.

    Between the transistors and the speaker, we have the 0.82 Ω emitter resistor, a 0.15 Ω resistor in the speaker return for feedback, and the 1200 µF output coupling capacitor. The capacitive reactance at 1 kHz calculates to 0.13 Ω. Assuming the load is 8 Ω, adding the resistance gives 9.10 Ω. 30 V across 9.10 Ω provides a peak current of 3.30 A. Peak power to the load is then I(SQ)R = 87.03 W. Continuous power is half of peak power by definition, so the continuous power would be 43.5 W assuming no voltage drop in the power supply under load. This is in close agreement with published specs.

    But, lets look at 20 Hz. The capacitive reactance of that damn coupling capacitor is now 6.63 Ω and the total resistance of that, the emitter resistor, the speaker return resistor, and the 8 Ω load is 15.6 Ω. Peak current is now 1.92 A and peak power at the load is 29.5 W. Continuous power is only a measly 14.75 W. The 4 Ω load case is even worse at 13.4 W continuous.

    It should be noted that the Heathkit AR-15, more or less similar circuit, uses a 4000 µF coupling capacitor. The reactance at 20 Hz is 1.99 Ω. Using that size capacitor in the 700-T with no other changes would get us to a reasonable 30 W at 20 Hz. Redesigning the circuit to get feedback without the speaker return resistor and changing the bias to use 0.33 Ω emitter resistors would get us to 33.8 W.

    Using a twin power supply (transformer would need to have a center tap) and eliminating the capacitor would get us to 44.6 W at any frequency.
     
  13. Dave451

    Dave451 AK Subscriber Subscriber

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    Fred,
    Very interesting information! For reference, I after the restoration work I tested the power amp at 1kHz, single channel at a time, to clipping and got 38W RMS per side after carefully setting the center voltage and bias on each side to spec. This may have been due to power supply limitations, as I made some component substitutions during the work, but they would not have compromised the supply, I think. A bit more on this later.



    RE-CAP PREAMP AND AUDIO CONTROL AMP BOARDS

    I first wanted to run down the source of the weak channel and, long story short, ended up replacing all the small axial electrolytic caps on both the PREAMP and the AUDIO CONTROL AMP boards, which resolved the problem. EVERY ONE of these small axials (installed vertically with a plastic sheath over them) was leaking and tested bad in one way or another when removed. In the preamp, I replaced two 4 uF 70V caps with 4.7 uF 50V and two 0.5 uF 70V with 0.47 uF 50V. In the control amp I replaced all 10 of the 1.0 uF 70V caps with 1.0 uF 50 V.

    A note on the axial caps: throughout the receiver these small axials were all rated at 70VDC. Looking at the circuit, this voltage rating looks very conservative and I had good 50V Nichicon's handy and used them for replacing the small axial electrolytics throughout. No issues seen so far with the 50V units. I suspect the 70V rating was used because of component availability/economy throughout the receiver and I couldn't see a place where a 50V replacement wouldn't serve. Please flag any places you see where this might be an issue.

    A further note: I did not change any of the 160 uF 6V radial lead caps on the preamp board. I pulled and tested some of these types and found none of them bad in the receiver, so I generally left this type in place (see photo below).

    One interesting feature of units with transistor sockets is that you can swap the transistors between channels to see if they are the cause of problems in the circuit. It turned out that, as I mentioned, all the transistors were good in this 700T, but this was a helpful technique for a quick check between channels. The Service Manual cautions, though, that you have to be sure the power supply caps are all drained before you remove and replace transistors. Shorting the supply to drain the caps led to grief elsewhere in the restore process--more to come on that.

    Here is a photo of the re-capped pre-amp board with the round, gray 'encapsulated' electrolytics I'm referring to that I left in place (the discoloration on the one is from a quality control sticker I removed):
    IMG_4972-1.JPG

    Here is a view of the re-capped audio control amp board:

    IMG_4974.JPG

    Here is an example of the small, failed axial electrolytics scattered through the receiver. Note the electrolyte leaking out on the postive end:

    IMG_4913.JPG
     
    Last edited: Nov 26, 2018
  14. Dave451

    Dave451 AK Subscriber Subscriber

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    A further note on the pre-amp board: All the inputs, including AUX, Tuner, phono, and tape go through the pre-amp. There is no separate "phono preamp" circuit in this amplifier. Equalization circuits for phono and tape are switched in and out, depending on the input, by the Selector switch. The amp has jumpers in place at RCA jacks where "REVERB" can be connected in and out--shades of the old "Space Expander" circuit from the tube gear, I imagine!
     
  15. fred soop

    fred soop Super Member

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    Vert close to the calculated 43.5 W. Calculating in reverse, this would indicate a 10 V drop in the power supply under load. With both channels driven, that would be a 4.36 A peak load or 3.08 A RMS.
     
  16. fred soop

    fred soop Super Member

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    Another problem on all of the units after the 600-T. Attenuate the high level signals and reamplify. The Johnson noise from the attenuation resistors now gets amplified by about 40 dB. Bad design. It was all part of the overall parts count / cost reduction after the 600-T power amplifier fiasco. Fortunately, this problem CAN be fixed with preamp redesign at reasonable cost.
     

     

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  17. Dave451

    Dave451 AK Subscriber Subscriber

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    Interesting, Fred. Can you say more about the 600T situation? Too costly and not enough sales?
     
  18. fred soop

    fred soop Super Member

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    CALCULATION ERROR in post #15. When calculating the voltage across all of the load (speaker, emitter resistor, speaker return resistor, coupling capacitor) I used the RMS value of the current and should have used the peak value. So the power supply voltage drop calculation would be 2 V per side or 4 V total. This assumes that the posted value of 63 V is not greater due to higher line voltage. This indicates a solid power supply.
     
  19. fred soop

    fred soop Super Member

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    The 600-T situation has been documented in a number of threads here. That receiver was top of the line, spare no expense, but was also somewhat experimental, being one of the very first transistor receivers. The output stage was a series string of 4 germanium PNP transistors to handle the necessary voltage. The same circuit (with minor variations) was used in several consoles. It was very sensitive to load impedance and heat. If the unit was covered with magazines and newspapers, it would eventually overheat and blow up even at low volume levels. In addition, there were open frame pots used for bias that would get dirty and if they intermittently went open, the amplifier would blow up. The 600-T had 5 versions in 1965 and it was mentioned elsewhere on this forum that it nearly bankrupted the company. The consoles seemed to have done slightly better, probably due to a controlled situation for speaker loads and space in the cabinet.
     
  20. Dave451

    Dave451 AK Subscriber Subscriber

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    Further on the restore work for the 700T:

    REPLACING CHASSIS-MOUNTED ELECTROLYTICS (including the cans)

    A couple of the axial caps showed signs of leakage, so I replaced them all. I replaced the three 500 uF 35V axials with Sprague Atoms with the same ratings (C31, C21, C22). I replaced the 200 uF 35V C17 and C30 with paralleled Ilinois Capacitor 100 uF 50V radials (again, from on hand stock). There was a 10 uF 35V (C4) near Q5 that was leaking, so I replaced this with a 10 uF 35V IC radial.

    The two 25 uF 35V caps located near the Pre-Driver board were replaced with Sprague Atoms axials with the same values.

    The clamp-mounted cans generally looked good and ran cool, but C20 (1000 uF 80V) was leaking out the vent so I replaced it with a 1200 uF 80V Nichicon snap in. I replaced the 2500 uF 80V C19 with a 2700 uF 80V Nichicon snap in. These are modern capacitance values that are more economical than the originals and with solid state PS rectification, I figured the additional capacitance could only help without being an issue with the solid state rectifiers.

    I also replaced the two 1200 uF 80V C11 and C12 audio output couplers in the PA with 1200 uF 80V Nichicon "audio grade" units (see picture). I did not alter the values given their critical coupling role in the power amp to the speakers.

    When mounting the cans, the modern caps are, of course, smaller physically than the originals. When this is the case, I have a nice sheet of expanded TEFLON sheeting that I use to hold the smaller cans in the clamps when needed. The sheeting compresses nicely--a little but not too much--and has the benefit of being durable, essentially non-flammable and easily worked (cut with scissors). This doesn't look awful, holds the caps securely, and saves having to to replace the clamps. It's a bit of an expedient, but I do consider it to be a permanent solution.

    OTHER ELECTROLYTICS TO REPLACE

    I found a 4uF 70V C309 hiding on the IF board that I changed to a 4.7 uF 50V Nichicon. On the DRIVER boards, both of the 200 uF 35V (C853 on each board) were very leaky and were replaced with 220 uF 35V Nichicon axials. Confession: there is a 200 uF 35V axial located in a very difficult place behind the selector switch that checked good for ESR and was not leaking. I left this in place for the time being.

    I also re-capped the MPX board, but will leave the details for another post given the amount of attention it needed.

    POWER SUPPLY

    Another confession: when testing transistors, I used a shorting clip lead across C19 to fully drain the main PS caps before pulling and replacing the transistors. One time I got careless and left the short in place when re-powering. The slow blow main fuse blew fast, but not fast enough and I lost two of the 3A120 rectifiers in the main power supply bridge circuit. Normally, I would have left these in place, but now I needed to replace them. I used a 400 PIV 3A bridge rectifier unit (GBU4G) to replace the four 3A120's which, best I could find out, are 3 amp rated and probably around 400V diodes (couldn't find a data sheet for the 3A120). I did not heat sink the bridge and subsequent tests showed that it ran quite cool pretty much all the time.

    The bridge and up-rated electrolytic caps seem to work well and the power supply voltages all check good vs. the schematic at 117VAC line voltage.

    Here are some photos of the work:

    Leakage from C20 power supply cap:

    IMG_4958.JPG

    Re-Cap work around chassis, including power supply and new output coupling caps. I also replaced the 'death cap' with an X1Y2-rated safety cap (blue disc by AC outlets):

    IMG_4971.JPG

    Top view of newly-mounted can caps:

    IMG_4975.JPG


    Here's the 'pile-o-caps' that were replaced. I counted 36 electrolytics total. Replacement cost about $40 total. Dead diodes also in the pile:

    IMG_4996.JPG

    Here's a look at the 'audio grade' (per Mouser) Nichicon's used to replace the audio output couplers:

    IMG_4966.JPG
     

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