202-R Arrived

Discussion in 'Fisher' started by audmod01, Jun 27, 2017.

  1. audmod01

    audmod01 Well-Known Member

    Messages:
    754
    Location:
    Tioga, TX
    OK, results: I do now see a definite peak in the 10.7mHz signal by adjusting the primary of the Mixer output IF coil. The secondary does also. I do also see that among the many stations that I pick up on the air, more of the stations now show close to level 2 on the signal meter (full limiting at that level). I picked up some more selectivity, but not any more indicated gain at the level meter. I checked the parts list and the two level meters do not have the same part number. They differ by one digit. Externally I only see that one has its needle pointing to the right (FM meter) and the other points to the left (AM meter). In other words both meter's pointers point toward the center of the dial. I suspect that both have the same movement and internal resistance etc. and so one more check I am going to make is to temporarily disconnect the AM detector feed to the AM meter and lift the lead going to the FM meter and run a clip lead from the meter out connection point to the AM meter input to compare and see if the detected level of the FM is higher on that meter.

    Another thing I might do is to use my scope at the IF frequency and input a low level 10.7mHz signal to the mixer grid again and look at gain stage to stage. There must be at leas one or more stages that are not providing gain for whatever reason unless the level meter is lying to me.

    Every IF stage up to the FM level meter shows a peak at the FM level meter which appears to be a sharp peak the best I can tell so far. I will try again tomorrow to see if I notice anything else.

    Joe
     
  2. audmod01

    audmod01 Well-Known Member

    Messages:
    754
    Location:
    Tioga, TX
    Dave I failed to spot your post until after I posted my after lunch results. I may go that route and use a shield. I do have some floating about here somewhere. That would also tell me if the method I used injecting at the grid was detuning the circuitry. However the part replacement did provide a peak in the Mixer plate circuit that was not there before. I think I will try that shield method tomorrow to see if the tuning point changes. That will be done with the bottom shield back in place, which of course is the way it should ultimately be.

    Unless the FM Level meter is lying to me, I believe there is still a fault in the IF circuit. The meter is fed by a 24pFd coupling cap from the secondary of the IF transformer from the previous stage, so the fault still has to lie somewhere in that chain. I am only seeing about 0.045 to 0.047VDC at the feed to the meter. The AM meter shows considerably more deflection (almost to full scale on many stations). I need to see what its highest levels reach in real DC read on my Fluke 8600A meter.

    Joe
     
  3. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    8,676
    Location:
    Ball Ground, GA
    Joe -- Does the muting function work correctly? It achieves its reference negative bias at the same point that the meter is driven from. V-6 must be good as well, since it is the diode that creates the negative voltage that appears at the secondary of Z4. I suppose that even Z5 could potentially have some effect on the meter reading, since it being tuned to resonance (or not) could affect current flow through V-6 -- but since V-6 is a limiter following V-5 which is also a limiter, that's a long shot. The meter is driven from the voltage divider made up of R78 and R79.

    Dave
     
  4. audmod01

    audmod01 Well-Known Member

    Messages:
    754
    Location:
    Tioga, TX
    Dave;

    Yes, the muting function appears to work normally. I need to recheck the voltage at the Grid of V-6 with no signal input to see if it reaches the specified 3V. I wrote those numbers down before, but have not rechecked since I changed the cap in the front end.

    Joe
     
  5. audmod01

    audmod01 Well-Known Member

    Messages:
    754
    Location:
    Tioga, TX
    Dave;
    I rechecked the voltage at pin 1 of V6 after making sure there was no signal in, FM antenna switch set to Local. I am getting only -.15VDC at pin 1 of V6. That looks to be way off. That was measured with the probe tip of my old Heathkit IG18 VTVM. The Fluke 8600A gives erratic readings there and only has two wire leads, not a coaxial shielded probe like the VTVM. I trust the VTVM much more in this case.
    Joe
     
  6. audmod01

    audmod01 Well-Known Member

    Messages:
    754
    Location:
    Tioga, TX
    OK, I just found my first true bad FM IF Primary, Z2 at V3. There was absolutely no peak from end to end of tuning range of the ferrite of the primary. The secondary does peak. After adding a 33pFd to the primary it does peak, so does the secondary and the stage does produce gain. Moving to the next stage V4, and Z3 the primary peaks and so does the secondary and the scope shows good gain in that stage looking at the plate with the o-scope. In the V5 stage both primary and secondary appear to peak OK, but the stage actually shows gain loss looking at its plate with the o-scope and the same is true of Z4 secondary. The part number is ZZ50210-4 which I think uses a 24pFd tuning cap on both primary and secondary. I need to look at the other voltages at that stage too.
    Joe

    Joe
     
  7. audmod01

    audmod01 Well-Known Member

    Messages:
    754
    Location:
    Tioga, TX
    V5 has +170VDC on its plate and +46VDC on its screen grid. It appears that for some reason this tube is not conducting much. I must check further. There is definite loss of signal level in this stage.
    Joe
     
  8. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    8,676
    Location:
    Ball Ground, GA
    Now you're getting somewhere!

    Remember that V-5 is a limiter, and it sounds like it's limiting on you. Back your input signal down so it won't limit and then you likely won't see the loss. The stage sounds like its operating properly at this point.

    Dave
     
  9. audmod01

    audmod01 Well-Known Member

    Messages:
    754
    Location:
    Tioga, TX
    I looked at V5 again and although the voltage is still running about +172VDC the signal there is now higher than the stage before it. I think I was dealing with an intermittent connection of my o-scope probe. I went ahead and pulled the tube and tightened the tube sockets. I looked at pin 1 to ground to see if there was a high resistance to ground, but there was not. That was when I decided to tighten the socket contacts anyway. The signal level at the level meter still will not go any higher than 2 although the tuner sounds good and seems to work well.

    It may be time again to swap meters in the circuit and see if the AM meter reads different than the FM meter after moving its connections. If it does, then the meter is at fault. This in spite of an ohm meter test previously.

    Joe
     
  10. audmod01

    audmod01 Well-Known Member

    Messages:
    754
    Location:
    Tioga, TX
    One of the things I did previously was to check the resistor voltage divider network for the meter circuit. The resistors are 10% tolerance and they are in tolerance for the circuit unless I missed something somewhere. I need to let it rest and get some rest myself. Perhaps I will find something the next time I look at it with a fresh mind.

    Joe
     
  11. audmod01

    audmod01 Well-Known Member

    Messages:
    754
    Location:
    Tioga, TX
    I started looking at the screen grid resistors of the FM IF stages. I found several that had gone up in value. These are all carbon composition types rather than the often seen Fisher reddish-brown body high accuracy resistors seen in many of their audio products. I replaced those resistors and continued looking all the way to the end of the FM IF strip. I discovered an error in both the parts list and the schematic. R113 is not a 1K ohm resistor, it is a 27K ohm resistor. There is R114 which connects to R113 and is a 15K ohm resistor. The other end of R114 connects to the B+ 1K ohm isolation resistor R115. So R114 and R113 establish at voltage divider to provide about +90VDC to the screen grid of V8.

    Changing some of the screen grid resistors for V3, V4 and V5 made a marginal improvement in overall detected level at the FM Level meter. At this point I plan to go back and inject the 10.7mHz signal at the mixer by using the tube shield method and begin looking at the output side of V6.

    There is an AGC bus established by the diode action of the grid circuit of V5 that feeds back to the grid of V3 and also the grid of the FM RF Amp cascode stage to control overall gain in those circuits. My Heathkit post-sweep marker generator also has the ability to provide fixed bias on AGC lines, so I might look at changing the AGC bias and see what that does to overall gain and the level meter indication.

    Joe
     
  12. audmod01

    audmod01 Well-Known Member

    Messages:
    754
    Location:
    Tioga, TX
    I made an interesting discovery after changing the screen grid resistors at V4 and V5. I replaced R48 a 47K 1/2 watt resistor with a new carbon film type 1/2 watt resistor. At V5 I did not have an 82K ohm resistor for R63. R63 had gone up in value to about 87.6K ohms, not a tremendous increase but still an increase. R59 a 47K ohm resistor in series with R63 forms a voltage divider to establish the desired +40VDC at the screen grid of V5. I bridged R63 with a 1.2M 1/2 watt resistor to bring its value down closer to 82K ohms. After making those changes, from a cold start I am seeing about a 2.5 level on the signal level meter when tuned to a station. Then as the tuner continues to warm up the level keeps dropping until it is only about 1.8 on the level meter.

    In spite of that the screen grid voltage at V5 remains steady at about +48VDC. At V4 the voltage at the screen grid is holding steady at +90VDC. The AGC voltage developed at the signal grid of V5 starts out a -2.90VDC and changes to -2.95VDC or so when warm. The voltage at V4 holds at +90VDC. I am trying to spot a location where the voltage keeps changing as the tuner warms up.

    Joe
     
  13. audmod01

    audmod01 Well-Known Member

    Messages:
    754
    Location:
    Tioga, TX
    Back after supper - tuner is fully warmed up and the level meter is showing only 1.8. My Kenwood KT-7500 which is sharing the same antenna through a two-way splitter is showing a steady signal on the same station from cold to hot of the 202-R. V3 plate is measuring +160VDC and the current through the tube is 2.85mA, screen grid is +144VDC. The screen is supposed to be at +100V. The changing gain and this stage showing high screen grid voltage may indicate it is either being throttled back by AGC or the tube may have issues. I plan to swap this tube with one of the other 6AU6 tubes from the limiters and see how it acts after that from cold start. Even though I ordered NOS tubes, that does not mean that they are perfect. I wound up with a mix of RCA and Sylvania tubes. I wish I had some NOS GE 6AU6 tubes for test purposes.

    Joe
     
  14. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    8,676
    Location:
    Ball Ground, GA
    I hate to throw another fly into the ointment Joe, but in my experience, it is quite common with Fisher vacuum tube tuners and receivers for the initial peak signal strength reading at a cold turn on to fall slightly after the set warms up. My 800C, FM-100B, FM-200B, and KM-60 all display this trait with their signal strength meters, while interestingly, I do not perceive a widening gap in the eye tube of my 202-T (when set for main carrier signal strength indications) as it warms up from a cold start. I had originally written the trait off to the heating of copper windings in the power transformer, IF transformers, and even the meter itself, but after seeing how the eye tube gap of my 202-T does not change as the set warms, I'm more beginning to believe it may be due to the delicate winding in the meter itself.

    As a perfect example, on my 800C, at cold turn on, strong stations have the meter's needle just (but consistently and decidedly) clearing past the "4" on the meter face, but after the set warms however, the meter needle for the same stations rests over the middle of the number "4", where it resides for the remainder of the time that the set is turned on. It's hardly a huge drop, but a noticeable drop none the less if you're looking for it.

    Your meter may be dropping more than this, but without a doubt, some of that drop is simply inherent. You might monitor the voltage to the meter itself to observe any change in it, versus a change in its indication as the set warms.

    Dave
     
  15. audmod01

    audmod01 Well-Known Member

    Messages:
    754
    Location:
    Tioga, TX
    Dave;
    Thanks for those comments. I think you are right because I did monitor the voltage to the meter and do not remember seeing it change more than a few milivolts - not enough to matter I think. I need to concentrate on the big issue which is the overall low level that it does not exceed on the level meter. Yesterday I tried connecting the FM level voltage to the AM meter, but did not see but the tiniest deflection there. Later I looked at the AM meter circuit closer and realized that I needed to disconnect both AM meter leads under the chassis and then connect it to the FM meter lead connection points using some clip leads. I had left one end of the AM meter connected to a 27K ohm resistor that is in its normal circuit, so the whole procedure was useless for evaluating the FM meter. The two meters connections to their respective circuits are quite different. I also tried swapping the various 6AU6 tubes about to V3, V4 and V5 and saw no change in the slow drop of the level meter reading. I looked closely at the screen grid voltages of V3, V4 and V5 from cold to hot several times and did not see any change in the voltages there, so that pretty well confirms what you are saying.

    I have a suspicion that I may need to concentrate on the V6 plate circuitry as you had mentioned before. If there is a problem there it may affect the development of adequate signal grid voltage which in turn indicates signal level. I did get some 8pFd disc ceramic COG/NPO capacitors. I took time out yesterday to make a special sleeve to slip over the Mixer/Oscillator tube in the front end. I ran into trouble using the tube shields I had due to the shield occasionally shorting out to the grounding ring at the base of the tube. I made a shield using some cardboard from a toilet roll and some copper tape that I have on hand. It is a snug fit and now I can slide it over the tube and it stays put. The copper gives a good conductive surface to a clip lead to feed the signal to the shield. I should be able to adjust Z5 while looking at the signal grid of V7 using my O-scope or look all the way to Test point 3 perhaps. If there is no peak with adjustment, then the 8pFd that resonates Z5 is likely open and I can tack a new part across it again and recheck for a peak.

    I have to take time out today for a check-up with my doctor. I am doing great and have no more pain or hiccoughs to deal with now for about 6 days, so I am thankful that the surgery was a success. Thanks to everyone here for their thoughts and prayers!

    Joe
     
    Last edited: Aug 16, 2017 at 3:31 PM
  16. audmod01

    audmod01 Well-Known Member

    Messages:
    754
    Location:
    Tioga, TX
    Today I did some further checks on the FM IF alignment because what I had done yesterday in the last 3 Limiter stages upset the Microtune operation. I had tried peaking the signal at the plates of the Limiters using my O-scope on X10 probe setting. It turned out that even this probe setting still added enough capacitance or loading to the circuit that it de-tuned the Limiter coils. This morning I went back and tack soldered some 470K ohm resistors at each plate circuit to reduce loading of the scope probe on the tuned circuit. When I looked at peak to peak 10.7mHz signal again I had to retune each one for a peak. I went back and checked the operation of the tuner against real radio stations and now the Microtune works correctly again. I have not made any adjustments at all to the Ratio Detector primary or secondary.

    One thing I noticed in the Limiter stage where the V7 6BN6 is used is that the 10.7mHz signal at its signal grid is quite strong coming from the secondary of the interstage transformer from the V6 stage. At the plate though the signal looks weak by comparison even if looked at without the 470K ohm isolation resistor. However, it is made up for in the last V8 6AU6 stage. I noticed that there is R84 and R103 which are 4.7K ohm resistors across the Limiter coil Z5 and Z6 except for the 18pFd DC coupling cap inside Z5. What this does for the resonant circuit in both these stages is to broaden the IF response. In the FM-1000 tuner the same stages use 2.2K ohm resistors in the same circuitry which would broaden the IF response more. Dave has mentioned that the later tuners used Ratio Detectors with a broader IF response to better handle the MPX subcarrier region for composite detection. This helps assure that the L-R information is not attenuated to any significant degree compared to the L+R audio below the 19kHz pilot tone.

    I am going to go back to checking the FM Level meter against the AM Level meter one more time. This time I will make sure I take the AM Level meter completely out of its circuit so the comparison will be accurate (provided that their internal sensitivity is equal. However, I simply do not see -3V at the signal grid of V6 anyway. That is likely still the reason I do not get higher readings on the FM Level meter. I notice that at the same location in the FM-1000 that the series resistor to the meter circuit is a 22K ohm resistor while the 202-R uses a 47K ohm resistor. Also in the FM-1000 there is a 500 ohm variable resistor across the FM Level meter, while in the 202-R there is a fixed 560 ohm resistor across its meter.

    Joe
     
  17. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    8,676
    Location:
    Ball Ground, GA
    Good progress, Joe. Another option to easily peak all of the limiter stages without reconnecting your indicator connections at each stage is to connect (preferably) an analog VTVM across the electrolytic cap in the secondary of the ratio detector transformer. Inject your signal directly at the first limiter stage, and tune all the limiter coils to the highest possible DC voltage indication on the VTVM -- but keep that indication below 5 vdc at all times, so as to keep the last stage out of actual limiting.

    Do the meters display the same DC resistance?

    Dave
     
  18. audmod01

    audmod01 Well-Known Member

    Messages:
    754
    Location:
    Tioga, TX
    Dave;
    Thanks for that suggestion about peaking the Limiter coils. I will recheck, but I think I have it OK now. It won't hurt to learn to do it two different ways.

    I did some more investigation of the FM Level meter voltage source after peaking all the Limiter coils and now I notice that if I actually measure at the junction of C82 and R78 I am seeing about -7 to -8 volts now although the Level meter still only reads about 2.3 when cold and 1.8 when warm. Dave's observation about the meter slowly changing in performance as the set warms up is confirmed. I did swap the AM Level meter into the FM Level circuit and it reads exactly the same as the FM Level meter. So the internal resistance of each meter is identical for all intents and purposes. The FM Level meter is M766-136 and the AM Level meter is M766-137. The only real difference in them is that one has its movement pointing to the center from the left and the other points to center from the right. In fact the polarities of the two meters are swapped in physical position. It appears that the only true difference in them is the label inside with the numerical markings! One is reversed compared to the other such that when the AM meter is installed its polarity connections are the physical reverse mirror image of the FM meter.

    The FM-1000 tuner uses a M700-119 FM Level meter and of course it has no AM Level meter. I suspect that it is actually the same as the meter movements used in the 202-R and a number of other Fisher tuners and receivers. I am going to measure the actual internal resistance with my Fluke 8600A meter and report the results. An interesting feature of the FM-1000 is that the resistance used to feed the negative voltage developed by the IF stage it is connected to is only 22K ohms. In the 202-R the resistance used is 47K ohms. The 202-R FM Level meter has a 560 ohm shunt resistor across it. In the FM-1000 there is an adjustable 500 ohm variable resistor to set the indicated level so that it reads a certain amount with a given level of RF signal input. The FM-1000 also has external level metering connections on the back which allows for the connection of an external signal level metering circuit using a 2000 ohm/200uh meter. Those connections have a strap across them from the factory which establishes the ground for the internal M700-119 meter. If an external meter is attached, then the link is swung to one side and the connections made from the external meter to the terminals on the back of the tuner.

    With the levels I am seeing now from the grid detection circuit of V6 I believe I could duplicate the action of the FM-1000 tuner's FM Level meter by changing the value of R78 to a 22K ohm and using a variable 500 ohm control to shunt the FM Level meter in the 202-R.

    Based on the degree of Limiting action in the 202-R and the lack of any noise in the received signals on most all stations, I believe the 202-R is performing as it should. The visual indication of the FM Level meter is just off due to the circuit design and drifting of parts values. I am also considering changing the 202-R meter shunt resistor from 560 ohms to 1.2K ohms just to see what level it displays then.

    Joe
     
  19. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    8,676
    Location:
    Ball Ground, GA
    The signal strength meter on the 800C reads notoriously low when set to AM, and based on the the performance of my own unit as well as others I have serviced (which react identically), it is clearly a calibration issue. The same meter reads fine in the R200 tuner which uses a similar circuit except for two stages of IF amplification in the AM section. I've always contended that somebody forgot to re-calibrate the meter circuit when they dropped the one stage of IF for the 800C configuration. Poor calibration is always a possibility. It would be great if others with a 202-R could chime in and let you know how the meter in their unit reacts.........

    Dave
     
  20. audmod01

    audmod01 Well-Known Member

    Messages:
    754
    Location:
    Tioga, TX
    Dave;
    It is interesting to see if the same exact meter is being used in the two applications with different results. It has to be calibration.

    In the 202-R the FM Level meter is labeled from 0 - 5 at the highest. I changed R78 to 33K ohms and the R79 shunt resistor to 620 ohms. With those values the FM Level meter now reads at 4 or just under from cold to warm on stations with strongest signal levels. Signals from stations that are marginal with some noise present range in the 1.0 to 2.0 range with signals lower than 1 being noisy although some are still listenable. Muting and Microtune both work normally.

    I have not yet measured the meter movement resistances. I am taking a rest from the work, having started early this morning and only stopping briefly for lunch.

    Joe
     

Share This Page