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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    Fred, thanks for the 600T info. I'd been scouting the forum but had not found the definitive description of the 600T woes. From what I read, germanium transistors are in general robust in terms of response to abuse and load EXCEPT for having a narrow temperature range and maybe sensitive to moisture as well. I also saw that, as you said, the 700T was apparently the all-silicon replacement for the ill-fated 600T.

    EDIT: I subsequently located and enjoyed reading the informative threads on the 600T (including Fred's 600T resto thread), two of which had significant info on the 700T as well, shown here for cross-reference:

    http://audiokarma.org/forums/index.php?threads/fisher-600t-any-info.276367/page-2

    http://www.audiokarma.org/forums/index.php?threads/fisher-600-t-restorations.582113/

    Dave
     
    Last edited: Nov 27, 2018
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  2. Dave451

    Dave451 AK Subscriber Subscriber

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    Reading Fred and Larry's dialogue on the 600T and 700T, I note the discussion on bass and 'thump', which are both present with my 700T. I get a thump at power up, but not when shutting down (like my Adcom GFA-5500 amp did). The volume of the thump is not excessive, so I haven't felt motivated to try and address it, but intended to point it out on this thread and ask for any guidance on it, as I am now.

    The bass output of the 700T is certainly robust, as Larry L. notes, particularly in my Klipsch KG4's (good response down to 38 Hz). Fred notes that the 700T is flat in frequency response and that just higher levels of LF output, compounded by speaker placement, may contribute. This may well be the case with my 700T. With certain input sources/quality, I run without the Loudness control engaged, even at lower volumes and cut back on the bass setting.

    Dave
     
  3. fred soop

    fred soop Super Member

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    The turn on thump on the Heathkit AR-15 is about 5 V, then goes into a very low frequency decaying oscillation for a time. Simulation also shows that applying or removing a large low frequency signal causes some artifacts that take time to settle. As for low frequency available power on the 700-T, if you overdrive it, you will likely get a waveform that looks like someone took a bite out of it at around 120 degrees (just after the positive peak). This does not occur on the negative peak. Simulation follows the calculations of around 14 W maximum undistorted at 20 Hz.
     
  4. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Mine also has a slight thump, as does my 250-TX, 500-T, 550-T. It's very slight as it takes a few seconds for them to power everything up and start playing music. (Almost like a a protection relay circuit, but with a ramp up vs. a click.) I notice it more on my KG-2's, Heresy's, and my Sansui speakers ( all 94db or higher sensitivity). I Chalk it up to them all being cap coupled, as my Sansui 2000's all do it too.
     
  5. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Dave451. Just a heads up. One of the things that can cause an imbalance on the 700-T are the Tone control PEC's (PC50B187-25). The 10K resistor btwn Pins 1 and 7 has been shown to go out of spec. This isn't just a 700-T problem. It's shown up on 500-T's, and 550-T's. I had to build both tone boards for my 550-T to replace the PEC's. 1 in the 500-T and one in the 700-T. In all 4 the 10K had gone ballistic (>200K). I would be suspect of any PEC with the above #.
    Larry.
     
  6. Dave451

    Dave451 AK Subscriber Subscriber

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    Thanks, Fred and Larry. I'm tempted to hook my digital scope up and having a look at the nature of the start up 'thump.' In any case, it's very mild and, Larry, like yours mine takes a couple seconds to start playing music on FM after turn on. As far as the imbalance, the re-capping solved that. I checked the PEC's in my troubleshooting and didn't find any obvious issues, but something to look out for. I'll take another look at that section when I go in again.
     

     

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  7. Dave451

    Dave451 AK Subscriber Subscriber

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    Back with an update:

    MPX DECODER

    Recall that, once working, the FM tuner had no audio in "FM Stereo" mode and the Stereo Beacon light wasn't working. Visual check of the MPX decoder (located on the top right of the chassis) showed the usual leaky small axial caps and clear indication that the wiring had been disconnected and the board lifted in the past, but it was not clear what, if any, work was done. Scope checks showed no 38 kHz activity on the board, even with stereo input through the antenna terminals and 10% 19 kHz pilot present in the signal. The SB lamp worked if terminal 4F was shorted to ground, so the lamp worked but was not being triggered by the MPX unit.

    I recapped the entire board, using Nichicon radials and a couple of axials. Research indicated that C419, shown as a 1 uF 25V tanalum cap, could be a source of problems, so this was included in the replacement (I had a moden 1 uF, 25V tantalum unit on hand). I replaced this with a tantalum unit because another poster on line said that an ordinary electrolytic might have too much natural leakage for this circuit. I read that others had just used a 1uF electrolytic and it apparently worked, but I had the tantalum unit so I put it in. Other notes on the recap:

    -I replaced the 0.22 uF 25V C422/423 tantalum output couplers with 0.22 uF 200V film caps. One of these units was actually a 0.56 uF electrolytic for reasons not clear (maybe an on-hand replacement from prior work?).
    -C413 in the Q403 circuit is shown on the schematic as 10 uF 35V, but there was a 1 uF 35 V cap installed instead, again for reasons unclear. It didn't look like a replacement had been installed by prior owner, but I replaced the cap with a 10 uF 35V Nichicon radial. Anyone seen this before in a 700T? I usually replace parts as I find them, but knowing there was prior work and the schematic and parts list clearly identified 10 uF, I changed it. Could this have been a factory change? Anyone else able to peek at their board and see what's originally installed? What would be the effect of a 10X change in the value of this cap one way or the other?
    -The axials were nearly all leaky and tested bad, particularly the 4 uF C424 mounted on the underside of the board.

    Checks of the diodes revealed that CR401, an AA113, was bad (open). This likely accounted for failure of the MPX. I replaced it with a NOS AA113 on hand. Not sure why this would fail, but it did. By its location in the circuit, a failure of this diode would ensure that the stereo functions would not work.

    After these repairs, the MPX decoder began working again. The Stereo Beacon lamp now activated, 38 kHz was present at TP 401 and audio returned in the "FM Stereo" mode on the Selector switch. Nevertheless, there was no L and R stereo present (just mono even in stereo modes), so I figured an alignment was needed. More about that in subsequent post.

    Here's the re-worked MPX board. You can see the (new) CR401 AA113 just above the lower left can (Z401). The 10 uF 35V C413 is visible right in the center of the board next to two other black diodes (CR403/CR404):

    IMG_4908.JPG

    PS NOTE: When replacing C424 under the board, I noted a cut trace in the circuit board near where this cap and a couple of resistors were mounted. It was very neatly done, but fearing some prior 'unauthorized' work, I checked the circuit board carefully against the schematic and all was correct, so I figured this was a factory mod made during manufacture.
     
    Last edited: Dec 3, 2018
  8. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Dave; The black box (CHOPPER), has diodes, resistors and the like. Here's the FISHER Schematic for making a replacement chopper if needed.
    700-t chopper.jpg

    And the Service Bulletin if you have a HUM
    700-T Service Bulletin.jpg
     
    Last edited: Dec 3, 2018
  9. Dave451

    Dave451 AK Subscriber Subscriber

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    Many thanks, Larry! You are practically reading my mind, as I do have a slight 60 Hz hum in my 700T and was going to get around to chasing it once the other heavy lifting is done. Can I use plain shielded audio cable in place of the graphite shielded cable specified by the Fisher bulletin? I have some pretty good stuff I can use.

    The 'chopper' appears to be working OK as, after alignment, I achieved good stereo operation and separation. There is one lingering issue with FM stereo I will outline in the next post dealing with the alignment.

    Both of these docs go into my 700T folder!

    Dave
     
  10. Dave451

    Dave451 AK Subscriber Subscriber

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    RECEIVER ALIGNMENT

    After all the re-capping and work on the MPX decoder, I performed an RF/IF/MPX alignment per the Service Manual instructions. I used my Sencore SG-165 for most of the work and also used my recently acquired Sound Technology ST-1000A generator for comparison. The SG-165 was more straightforward to work with when the 10.7 mHz sweep with markers was needed (no marker capability for the ST-1000A). The ST-1000A was a bit better for stereo multiplex output and separation adjustment.

    FRONT END ALIGNMENT
    Proceeded pretty much as per SM instructions. The front end, including dial calibration, were pretty much spot on and very little or no adjustments were needed.

    ONE QUESTION: On the last steps in the front end procedure (and in subsequent alignment instructions) we are asked to increase the RF output of the generator to 200 mV. Neither of my generators will generate 200 milli-volts of output. Do the instructions mean 200 micro-volts? Perhaps the Fisher Model 300 generator will output this much RF?

    IF ALIGNMENT

    This alignment proceeded pretty much per SM instructions. A couple of the transformers required a bit of adjustment (perhaps as a result of prior tweaking in the unit). Again, the instructions required a generator output of "200 mV" to adjust the signal meter, but I was getting that level with max output from my generator, although far short of 200 mV. I was able to achieve good symmetry for the bandpass curves and to center them well with the markers (more or less matching the SM figures). I took extra care with the detector alignment (see picture) for best amplitude and linearity compared to Figure 3 in the SM.

    The adjustment of the FM Muting trimmer did not seem to provoke much of a change in response of the receiver. Ultimately, the muting adjustment worked fine with no noise between stations if their signal strength was lower than 1-2 on the signal meter.

    NOTE: I was puzzled that the MUTING control couldn't be turned off in the "FM Stereo" selection. With no user manual available, I took a look at the 600T manual and it notes that muting is always ON in the FM Stereo selector position. This appears to be the case with the 700T as well. Muting works fine with the switch on in the FM Auto and FM Mono positions.

    MPX DECODER

    I used the 'preferred alignment method' specified in the SM. I was able to get a good Lissajou figure by feeding output from the IF strip, including the 19 kHz pilot signal (audio output terminal 3K) to the horizontal input of my Tek 2445A and the vertical input from TP403 through a 1Meg resistor, as specified. In this stage, you have to be careful to get the right phase on the Lissajous or the L and R outputs will reverse (the Lissajous folds over) during adjustment. Either way can be made stable, so I checked after performing step 4 to be sure L and R were proper using modulated output from the generator.

    Ultimately, I was able to get good stereo separation (around 32 dB) on each channel from the other and the sound level difference was less than 0.2 dB between R and L pre-amp outputs; all well within the specs. Interestingly, the stereo separation trim pot is not located on the MPX board (what I thought was the separation control on the MPX board turned out to be the TRIGGER adjustment for the stereo beacon), but on the pre-amp board.

    When adjusting the TRIGGER control in step 10, best results were obtained from starting with the trimmer fully clockwise and adjusting counter-clockwise to get a trigger at the specified 0.8V at terminal 4F. If you go the other way, the lamp will light at 0.8V, but then stay lit even between stereo stations (at least this was my experience with this adjustment).

    After a couple passes at these alignments everything seems to be working very well. The tuner is incredibly sensitive, selectivity in crowded portions of the band is very good, muting works as it should and the stereo beacon is working very well. This tuner is one of the best I have seen in ANY solid state or tube unit!

    REMAINING ISSUE

    Despite the care in alignment and apparent proper tuner and MPX operation, I believe I'm hearing distortion in the FM mono and stereo audio on heavily modulated stations (not in other input sources, like AUX). A lot of the pop stations using strong audio compression with very little dynamic range in the audio. On the local true Hi-Fi college station (without heavy processing) this is far less evident unless they are playing metal or very heavy rock when the modulation is close to 100%. Classical and Jazz usually sound great.

    Ordinarily, I would chalk this up to improper detector alignment and/or operation, but I took great care with the alignment and the curve looks OK to me (does it to you?). I would appreciate any thoughts or suggestions on what to tweak or look for with respect to this issue! One mea culpa: I did not change out the 10 uF ratio detector electrolytic cap on the IF board, as it was the type I mentioned earlier that didn't seem to leak or fail and it checked good for ESR. Perhaps I should? Also, does the detector output curve below appear proper? I'm a little concerned that it seems to be a bit rounded at the bottom, but is quite linear and crosses right in the middle against the markers in the main part of the curve.

    Dave

    IMG_5014.JPG
     
  11. fred soop

    fred soop Super Member

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    C419 - Now you know why many of us here refuse to use tantalums. I had a new one in a 600-T and had the same problem. The NEW tantalum was a dead short. Cleaned out several parts drawers of tantalums and put them in the trash.

    Looked at several service manuals and none of the others mention raising the generator output to 200 mV or any other value. The manuals do mention changing generator output for specific readings within the unit.

    Muting - There are actually 2 separate muting circuits. One works with signal strength and blocks the signal in the IF amplifier. The other is only in the "FM STEREO" mode and will mute at the audio amplifier stage after the multiplex decoder. The circuit will always be muted unless the stereo beacon is on.
     

     

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  12. thornev

    thornev Well-Known Member

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    Dave451 - Is there any way to measure stereo separation using any of the RCA output jacks on the back of a Fisher? I'm curious to see what my FM stereo separation is in order to determine if an adjustment is required. I tried using a Fozgometer which is used to measure separation on phono cartridges, but I believe the signal strength is very different. The Fozgometer didn't work. If not RCA jacks, where in the circuitry did you measure separation? (I suppose the 500-C manual contains that info) Thorne
     
  13. Dave451

    Dave451 AK Subscriber Subscriber

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    Hey, Thorne. As you say, how to measure FM stereo separation is described in the alignment procedures for the MPX65 decoder included in the service manuals for the 400/500C/800C manuals, where you are instructed to measure the output voltages at the audio output lugs on the decoder itself. You might be able to use the "record output" jacks to do it, but I haven't tried it for Fisher gear.

    For the 700T, the procedure calls for you to measure the output from the pre-amplifier stage. You are taking (AC) voltage ratio of the signals for the driven channel over the un-driven channel and applying the formula dB Separation = 20 log [Vdriven / Vun-driven] (the formula for voltage ratios).

    The hard part is you need a calibrated source of FM multiplex signal output (including the 19 kHz stereo pilot signal) that can be audio modulated one channel at a time that has better stereo separation than what you will see in the receiver. This is where FM stereo generators like the Sencore SG-165 (or SG-80), Sound Technology ST1000A, or Fisher Model 300 generator come into play. The SG-165 modulates each channel at 400 Hz and the ST1000A at 1000 Hz. Some MPX alignments call for a variable pilot signal output as well to test for decoder stability at different pilot signal strengths (corresponding to various FM broadcast signal strengths). The SG-165 has two pilot levels (5% and 10%) and the ST1000A's is continuously variable to 15%.

    I have not been able to find any 'quick and dirty' way to do this. Some of the experienced hands will tell you that, lacking the equipment, have a look at the blue separation trimmer on the MPX-65 and put it in the middle (12 o'clock) position if it looks like someone's been twisting it and make sure the tubes are good. The thing is, the quality of stereo separation is more driven by the phase adjustment of the MPX decoder (where you use a Lissajous X-Y pattern on a scope) than the separation control (which is in the audio output circuit) per se.

    Also, the MPX decoder can be set up just fine, but if the IF section (particularly the detector/discriminator) are mis-aligned, you won't get any stereo either!

    Maybe some of the more experienced hands can offer tips, but this is the best I've been able to determine so far after a lot of research and a fair amount practice. Dave Gillespie, for example, will forget more than I'll ever know about this subject!
    Dave
     
  14. thornev

    thornev Well-Known Member

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    Thanks, Dave. I did try the RCA jacks...no dice. I have in the past, using headphones, adjusted the blue dial to adjust to get the best stereo separation. I was told that method is not good enough, but most people don't know that I have bionic ears. ;) (I can usually, as an ear-trained, long-time guitarist, tell you what the chords in a song are seconds after hearing them) But you're right that the blue dial can only get the device as good as the MPX unit allows. I was just curious since you seem so well-educated about it if you knew of an alternate method. I should know better that there are no short cuts when it comes to electronic circuits. Thorne
     
  15. Dave451

    Dave451 AK Subscriber Subscriber

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    Fred, yep, I've picked up on some of the consternation about tantalum caps. The 700T is the first Fisher gear I've come across with them in it. There are a lot of them in ham gear and, now that you mention it, I've replaced more than one defective tantalum cap in transceivers.

    You responded to my question on "200 mV generator output." Here's one of the references in the 700T manual I was referring to:

    upload_2018-12-4_10-1-21.png
     
  16. Dave451

    Dave451 AK Subscriber Subscriber

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    Well, no one opined on my detector curve, but the more I look at it, the more I am worried about non-linearity. The difference in amplitude and shape from one side of the other is worrisome, so I'm going to go in, change the ratio detector electrolytic, check the detector diodes and transformer and generally see what's what around the detector and the limiter stages.
     
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  17. Dave451

    Dave451 AK Subscriber Subscriber

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    Took a pause from the 700T to do some long-delayed maintenance on a Scott 350B tuner (with some key learnings on alignment of same) and to re-finish some 'barn-stored' Bozak 302A Century speakers (wonderful speakers, but tough to lug around!). Plan to get back to the 700T as per above after Christmas and to wish all the best for the holidays and new year!
    Dave
     
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  18. mrphilco

    mrphilco AK Subscriber Subscriber

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    I have the 302A Bozak Urbans in our living room, and 302A E-300s in my home office.

    Gotta love Bozaks.

    Merry Christmas Dave and all.

    Ron
     
  19. Dave451

    Dave451 AK Subscriber Subscriber

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    Happy New Year to all! After a couple of detours, I got back to the 700T in the past couple of days and have completed the alignment process, I think. If you're not much interested in the alignment process, skip over this post if you like.

    Recall that while the tuner was sensitive and selective, there seemed to be some distortion on highly-modulated FM signals and the detector sweep curve didn't look quite proper when centered at 10.7 mHz. I was worried that I had not changed out the ratio detector cap (C318) out of laziness and hope and needed to do that. Bottom line is that the cap was bad and I had also misaligned the IF and detector stages. After all this was corrected, things worked fine!

    So I bit the bullet and unhooked the IF board on the side toward the middle of the receiver and was able to roll it over and replace the RD cap (10 uF 35V) with a new Nichicon (had to be very careful with the small diode connected on that side). The cap was failing (leaking at the + terminal) and replacing it seemed to reduce distortion a little, but it was still present.

    IF BOARD AND WITH NEW RATIO DETECTOR CAP

    IMG_5101.JPG IMG_5102.JPG

    Then, I started working with the SG-165 to check things out. The best sweep output trace I could get on the detector still looked funny with a rounded bottom and lack of 200 kHz bandwidth (same as in prior post), even though it crossed at 10.7 mHz.

    IMG_5103.JPG

    I decided to take a page out of the Fisher 400 alignment procedure and first tune the ratio detector transformer secondary (bottom slug) with a 10.7 mHz no-sweep signal for most voltage across the RD cap C318 I had just replaced. Then, I measured the DC voltage at the audio output of the detector with a 10.7 mHz tuner input (again no sweep) and tuned the primary (top) slug for a zero (balanced) voltage. What I found out was that I had tuned the detector primary to the wrong place on the coil. I remembered that there was a 'false tune' you could get in other Fisher receivers I had aligned (suspecting this was what caused me to do the voltage alignment).
    These adjustments put me in the right place on the coil and the detector curve looked much better, BUT was not centered at 10.7 mHz. In this shot, you can see the detector response looks OK, but it crosses between the 10.7 mHz marker pip (the biggest you can see on the trace) and the 10.6 mHz pip.

    IMG_5109.JPG

    I figured at this point that I needed to check my prior IF alignment and, long story short, the pass band was not centered at 10.7 mHz. I re-aligned the IF strip to get the IF pass band centered at 10.7 mHz, as you can see in this trace after the re-alignment, taken after the TP-302 alignment step. It looks pretty well like "Figure 2" on the alignment procedure at this point.

    IMG_5117.JPG

    Re-sweeping the detector now showed the proper shape crossing right at 10.7 mHz with a better 200 kHz bandwidth spread between the 10.6 and 10.8 mHz markers.

    IMG_5119.JPG

    Now the tuner sounds excellent with no audible distortion. I had previously aligned the MPX decoder after several repairs, so I rechecked the stereo separation and it (of course) had also improved. I am getting 37 dB with left modulated and 35 dB with right modulated, measured per the procedure with SG-165 input to the antenna terminals, 10% pilot signal, and 100% (75 kHz) modulation.

    Sorry if this was too much detail on the alignment and my uneven progress on it, but wanted to include it for any of those few who may be interested in alignment of this receiver.
    Dave
     
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  20. Dave451

    Dave451 AK Subscriber Subscriber

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    IMG_4988.JPG So, to wrap up, a couple of other things I did:

    1. Replaced the left dial festoon lamp with a 'cut down' LED bulb left over from my Fisher 1800 work (the lamps are shorter in the 700T)
    2. Replaced the rotten polyurethane light control foam on the dial with new weather-stripping tape.

    I power tested the power amplifier and, with one channel tested at a time, got 38WPC RMS at 1 kHz before any sign of clipping.

    I have not yet:
    1. Added a CL-80 or CL-90 to get rid of the turn-on 'thump' in the speakers
    2. Re-routed the audio input leads to get rid of a low hum that might be present, per the bulletin provided by Larry D above. I think a bit of 60 hZ is there, but not very noticeable. If it starts to bug me, I'll do this mod (thanks for providing, Larry).
    3. Done anything about the worn-off gold lettering on the speaker switch and tuning knob on the front panel.

    I also plan to do some distortion and frequency response testing for this 700T to compare with the tube units and I'll pass that along in this thread

    So, for about $40 in parts and some bench work on a 'free' receiver, I've got a very good sounding receiver with an outstanding FM tuner. I'm going to put it in service as the workbench receiver through a couple of book shelf Paradigm speakers for now.

    Dave
     
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