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Fun with MR71 detector: Replacing those Internal Capacitors!!

Apfelmus said:
Hi Pio, Dave,

many thanks for the link and further insight. I start seeing clearer now. So the sheet of mica I'm seeing is not the capacitor but just an isolation. And there are only 2 caps in the base, not 3. Why is the ceramic disc type cap not an NP0? There is a comparable NP0 connected to the terminals...

Drilling out the rivet, braking terminal 3, I have to think about it. And there is the initial thread using Deoxit + compressed air which sounds promising. If it doesn't work I can still do one of the others.

cheers
Martin
Click to expand...
The deposited silver areas on opposing sides of the mica sheet form the two shunt capacitors, with contact fingers from the terminal posts contacting them. The deterioration of the silver deposits is the problem.
The DeOxit treatment is likely, at best, temporary if it works, and a waste of time in the long run of permanence.
 
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Pio1980 said:
The DeOxit treatment is likely, at best, temporary if it works
Click to expand...
I agree, if you intend restoring the micas with DeOxit.

But I understood the original poster essentially proposed a method of disabling the micas, using DeOxit and then blowing out all the silver with compressed air. That seems easier and less invasive than drilling out the rivet and/or braking terminals.

*IF* it works... has anbyody tried out?

cheers
Martin
 
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I measured T6 again and again, always OK. Both filters could be aligned sharply on 10,7MHz with ample room to both sides. Exactly as for T5. The mica capacitors both measured around 35pF, the coil 0,7Ohm. Bandwith of all filters (of T5&T6) was around 350-400kHz.

Perhaps my fiddeling with the filters made them having good contact again, so for not risking to break up something that works I decided to put everything back. Unfortunately no significant change in measures... the primary of T6 is not aligning as sharply as does T5. Again I tried supplementing small caps (33p) between terminals 3&4 or 3&6, but except for some dampening I couldn't see significant influence on the resonance frequency. Strange.

At 100uV at the antenna, 1kHz Sine modulated with 75kHz (100%), I get 9 VSS at T5 (D4/R42) and only 2VSS on TP2. That might be normal given T6 is loaded with 10k instead of 47k for T5. Can anyone verify these measures on another MR71? When aligned, I could reach distortion level of 0,2% (100uV, 1kHz@100%, 100MHz RF), raising to 0,8 or 0,3% at each end of the scale.

cheers
Martin
 
Hi Martin -- I don't have an MR71, but I do have an MR65 -- of which I believe the MR71 was the final iteration of. The MR71 is somewhat more refined in some ways, but the basic RF, IF and dual discriminator circuits are quite similar -- certainly enough for the purposes of this conversation I would think anyway.

My 65 is in my system at the moment supplying holiday tunes, so I can't get to it for any lab measurements for now, but I'm wondering if you could clarify some of the data you've posted:

You are indicating that you are getting 9 volts at the junction of D4 and R42, and 2 volts at TP2. Now normally these points should/would represent 0 volts in a properly tuned/aligned unit, so are the voltages you're indicating the total range of voltage available as the secondary slug is adjusted to either side of optimum? Or is it from shifting the IF frequency in some manner from ideal? Point being, what are the conditions that are generating the voltages you're indicating at those points because normally, with a 100% FM modulation signal modulating the carrier, the voltage at these points would be 0 -- or am I completely missing something here? (Always possible!)

One of the biggest differences between your 71 and my 65 is that in my unit, the output of the first discriminator transformer drives the muting, AFC, and center of channel tuning meter circuits, with only the audio taken from the second discriminator -- whereas in the 71, the first discriminator transformer appears to only support the muting function, with the second discriminator transformer supporting AFC, center of channel, and of course audio output.

As a result of my unit's older heritage, one of the things I could do easily enough is access the equivalent of your TP2 at the MPX output jack on the back of the MR65. If it is of any significant help, I can say that with the tuner tuned to a strong station and the center of channel meter centered, the DC appearing at the MPX Output jack on my tuner is (of course) virtually 0.00 vdc, and will produce about +0.4 to about -0.6 vdc when tuned to either side of most station carriers. Even though the swing is not centered, it did produce basically equal excursions on the center of channel tuning meter -- remembering that that meter is driven off the first discriminator transformer in my unit, where as the MPX Output jack of course comes from the second transformer. I don't know if that helps any, but there it is........

Dave
 
Hi Dave,

my deepest apologies... I didn't realize I used the german term "VSS" instead of "Vpp"... (Volt-Spitze-Spitze). The DC voltages in my MR71 are indeed around 0 Volts at both points (D4/R42 and TP2).

It would be very helpful if you could do measures on TP2. Can you measure distortion? I'm curious if your MR65 gives the same distortion on all parts of the scale or if it does, as mine, has a sweet spot where it has been aligned (like at 100MHz, middle of the scale).

Coming back to the second discriminator around T6: the alignment of T6-primary feels like dampened by something. But even a thorough check of the components around the primary of T6 and also the on T5 did not reveal any offending part. Actually, a small offset of around 0,5V on D4/R42 (i.e. from the first discriminator that drives the muting circuit) is lowering distortion on TP2. On your MR65 such an alignment would lead to a bad center-indication, I guess.

I also checked the muting circuit. It should be unable to do anything offending if muting is set to "out". I guess the muting circuit is amplifying and rectifying the AF from the first discriminator and delivers a voltage that lowers amplification of V8, thus lowering AF output on the second discriminator. I tried checking the circuit but I couldn't get any amount of signal on the grid of V10A, although there is plenty on the other side of C42. Tuning into strong stations made the voltage at the anode of V10B lower to -2V, but way off the -9,2V as stated in the circuit diagram. I checked V10, its 130% good on all 3 systems. V7 and V8 checked out 100%. Voltages are OK, a bit to the low side (like 105V instead of 112).

If your MR65 gives comparable results, mine might just be fine and its me who has a problem... :-/ (like beeing too picky)

cheers
Martin
 
Hi Martin -- A couple of points for you to consider:

1. In your tuner, the muting amplifier is in deed driven off of T5, and while there is certainly audio present at the output of T5, that's not what the muting circuit is triggered by as if it were, then the muting circuit would operate in reverse, muting the strong station signals!

The output signal from T5 is first passed through an RF filter (so no local RF influence will trigger the muting circuit), but then passes through C42 to the grid of V10A, which is shunted to ground by L14/C7. These three components effectively form a high pass filter, so that the only significant signal that can reach the grid of V10A from T5 is noise above the bandwidth of a stereo composite signal. Inter-station noise has components that reach high enough beyond normal FM composite signals to effectively trigger the muting circuit. In that way, the circuit only reacts to appropriate noise, and not detected audio or stereo sub-channel information from a strong one.

As a result, the voltage at the plate of V10B should in fact go down when tuned to a strong station as in that scenario, there would be little if any noise to amplify, and therefore little signal to send to V10B for rectification. That then minimizes the negative voltage sent to grid No 3 of V8, allowing it to reach full limiting for maximum audio quality at the output of T6. McIntosh used discriminators for detectors instead of Ratio Detectors, so maximum limiting is important since discriminator detectors will happily detect AM as well FM signals! In any event, it sounds like your muting circuit is operating as circuit theory says it should.

2. I should be able to get to my tuner after the holidays for more detailed measurements. I'm old school, so I have HP 339A Distortion Test sets on my benches, so I'll be happy to make some distortion measurements when time becomes available.

In making your measurements, are you directly coupling the output of your generator to the antenna input of your 71? Or are you broadcasting it into the tuner via an antenna?

Dave
 
Hi Dave,

thanks for these very interesting insights. I might do some experiments, like modulating tones higher than 20kHz to see if they go anywhere. Since the Foster-Seeley also detects AM: is the "interstation noise" basically FM or AM? Or both?

BTW, this is a variant of Foster-Seeley without the choke but with a second coupling capacitor. Do you know why they did this?

In my measurements I do connect the generator directly to the 75Ohm Antenna input of the MR71 (a BNC connector) via a coaxial cable. The generator has 50Ohm + an adapter 50-75Ohms. Surprisingly, with this setup and without any antenna the tuner also receives a bunch of local radio-stations. Some stations come in really strong (level meter goes up to 8) like SWR or AFN on 102.3MHz - American Forces Network. My home is in line-of-sight to the emitter which is on a tower distant by 1-2km. Is this effect due to the bottom cover removed? Should I shunt the 300Ohms antenna socket?

cheers
Martin
 
Hi,

here are some readings off the muting circuit. I injected an FM signal in the antenna input with modulation frequency above 20kHz. Below 20kHz there is indeed no signal in this circuit.
Seems to work as it shoud...
  • grid of V10A: maximum at 165kHz, probably governed by the LC-filter C7 and L14. Vpp is 200mV with 75kHz deviation.
  • anode of V10A: 4Vpp but only 20DCV (instead of 85). Have to check that again... the voltage comes from the MPX circuit, what the heck is that for? Higher muting levels if MPX is detected?
  • anode of V10B: 4Vpp.
  • on the muting switch -7V with 165kHz Audio at 75kHz deviation and 50uV.
cheers
Martin
 
Hi all,

I made yet another IF alignment, that gave very good results at the end. By now I have tried the following methods for alignment:
  1. injecting 10,7MHz at grid of mixer -> aligning by observing TP1 with wobbler/scope or network analyzer.
  2. Injecting 100MHz with wobbler -> aligning by observing TP1 with scope (network analyzer not possible due to frequency change)
  3. Injecting 100MHz with sawtooth AF 200Hz modulated FM at 500kHz deviation -> alignment as for 2. (Essentially the same as wobbling)
  4. Injecting 10,7MHz at grid of mixer -> aligning stage by stage observing output of each transformer with network analyzer via 100kOhm+10:1 probe.
  5. Injecting 100MHz with triangle AF 200Hz modulated FM at 500kHz deviation -> aligning stage by stage (as in 4.) with spectrum analyzer.
When using methods 2, 3 and 5 it is necessary to check IF frequency regularly as it depends on the receivers tuning. The drawback of methods 1, 2 and 3 is that bad staggering of transformers can be difficult to track down.

I got the best results with methods 4 and 5, with 5 being the winner. It requires a good RF generator capable of doing large deviations like 300kHz or more to each side. The spectrum analyzer allows looking at the shape without the need to synchronize with the RF generator.

At the end I could get good sensitivity well below 2uV (spec is 2,5uV) with good separation to strong stations, as I have plenty due to line of sight to the antennas on topt of the tower of Stuttgart.

As for distortion it is also better than before. I get 0,1% in the middle of the scale, still depending on tuning , i.e. rising to 0,3 or 0,6% at the ends.

I'm currently checking the supply, to see if improved filtering might influence distortion. For now it does not seem to, although I still have all original capacitors and the selenium rectiviers in use. Supplying additional 100uF across the first capacitor does not change anything distortion-wise but leads to significantly higher peak currents in the rectifiers.

So far, cheers
Martin
 
Hi Martin -- Interesting that you are approaching the alignment using a transformer by transformer approach. My own experience is that this can introduce errors in the alignment process, as while the 100K resistor effectively isolates the load of the monitoring equipment, it doesn't eliminate the the capacitance that virtually any external connection introduces into the circuit, which only needs to be miniscule to upset the apple cart. In the alignment of some tuners and receivers, monitoring at some IF mid point is often done similar to what you are doing -- however -- the isolation resistor used (typically 100K) is typically already included in the build, so that the capacitance created by it's connection into the circuit has been accounted for. When monitoring equipment is then connected to the open end of the resistor, alignment error by their connection is eliminated, or reduced to insignificance.

When the GE/Zenith format for multiplexing was approved, it required a wider bandwidth through the IF section than was previously provided, to allow for good separation at the output of the stereo detector. As a result, IF transformers were redesigned (the discriminator transformer requiring the greatest attention) and as a side benefit, the old stagger tuning method for aligning the IF strip was largely abandoned (because of their wider bandwidth). As a result, manufacturer instructions for IF strip alignment were then basically reduced to simply peaking the primary and secondary slugs of virtually all the transformers except for the primary of the discriminator transformer. With the better units, the alignment is still often broken into two groups (up to the 1st limiter, and from the limiter forward), but again, the isolation resistor to allow for such mid point monitoring is often included in the build of the unit to prevent the potential problems mentioned above. It would be interesting if you can determine how much improvement your transformer by transformer approach has produced over that of the typical instructions given.

As for upping the size of the first filter cap in a cap input filter design, larger caps do draw greater peak current levels during the charging period, but they also discharge less during off-peak periods as well, all else being equal. As a result, these two events tend to off-set each other, so that while greater peak current is in fact drawn, it is for a shorter period of time, leaving the net change in ripple current largely unchanged. The biggest concern with an increased cap value is during the startup period (or hot switching events if applicable), where some sort of current limiting is desirable if the cap value has been increased significantly.

Dave
 
Hi Dave,

interesting what you say about the transformer by transformer approach. I did not have any experience with FM tuners before I had to repair mine. Perhaps I should have left the IF stages alone after changing the oscillator tube and just do the oscillator alignment. But as some say, the better is the enemy of the good. My latest alignment gave really good results, also for the MPX portion. I thouroughly checked the audio paths but could not find any noticeable difference between both channels, so I did not do any re-capping there (supposing two caps do not turn bad the same way in both channels). The way the components are soldered into the lugs is so beautiful that I'm reluctant to unsolder anything. And, in general, all components I actually checked were good. Except for the panel light bulbs where the metal caps are getting loose.

I did add some capacitors in the power supply. Not the first one that I prefer to leave alone to not put additional stress on the selenium rectifiers. BTW, the 95V line (connected to C60D) showed large voltage drops (down to 80V) when I turn on the pilot tone on the MPX generator. Perhaps this is due to the 38kHz oscillator and pilot lamp drawing more current.

There is an MR78 sitting in the basement, not working. When I get to it I will be a little more cautions before touching the alignments. And make measurements so I can tell if my actions improve anything.

cheers
Martin
 
Followup: the MR71 is now happily sitting in the rack and playing very well, even with a small stub en-lieu of an antenna...

An issue that I discoverd only once everything was put back is the startup from cold. When switching on the tuner is misaligned by 2-300kHz, jumping back to alignment after a minute or so. Seems to be a part that is heating up quickly. When I correct the tuning before the "jump" the station is OK, and misaligned after the "jump". So I guess its caused by the oscillator. I had to change the oscillator tube, but its not a new one. The original tube had one broken heater, the other half was at 70%. The tube I put in also has around 70% on both systems and may have an issue during initial heatup.

Since the tuner is working well after the "jump" I'll leave it for the time being...

cheers & merry christmas & happy new year
Martin
 
Hi Martin -- If you're referring to the dual triode 12AT7 in the front end of the MR-71, neither section of this tube is performing as the oscillator function, but has one section functioning as part of the cascode RF Amplifier input stage (in conjunction with the 6DS4 Nuvistor), while the other section is the Mixer stage. The Oscillator function is performed by V3, a 6BN4A, which is a single section tube.

As for the drift displayed, are your comments made with the AFC control fully advanced, or is it not advanced at all, or at some part way position? If the control is not advanced, then a properly tuned receiver at shut down will initially display drift when restarted later after a complete cool down. I'll check how long it takes mine to re-stabilize at the optimum tuning position again, but with the AFC fully advanced, my MR-65 stabilizes to an optimum setting after just 30" from turn on, which includes the tubes warming up as well. You can use this as a guide to check AFC performance of your MR-71, since it is all but identical to that in my MR-65.

Dave
 
Hi Dave,

oh yes, you're right. I didn't look well enough at the schematics... it was the 2nd system of the 12AT7 that was out, i.e. the mixer. The tuner was totally deaf then, so for some reason I could only think of the oscillator. The fault was visible by the naked eye once I took the black shield off the tube.

For the drift: the initial "jump" appears to have self-healed... :-/ or perhaps it doesn't like dry weather. For the measurements AFC was always off (i.e. fully ccw). Now, when I switch it on, after initial warmup the tuning indicator is at the right edge of the black field, reaching dead centre after 1m30s... With AFC fully on it would be considerably faster.

Since you're so proficient with the circuitry: what is this LDR stuff (144-010) that is connected to the MPX lamp doing? Why did they need such a setup?

cheers
Martin
 
Hi Martin -- The LDR was a rather ingenious way that Mac added a nice feature to the MR-71, that did not exist on previous models of this linage.

Beginning with the MR-71, the LDR (light dependent resistor) module was included to provide an Automatic Stereo/Mono switching feature -- a nice feature because you don't want to listen to a mono FM station with the tuner in Stereo MPX mode. Remember that back at the time the MR-71 was produced, there were still quite a few mono FM stations broadcasting in major markets. Listening to them with the MPX circuits active adds unnecessary noise to the signal that wasn't original part of the transmitted signal, so the lowest noise FM mono reception is always produced with the tuner set for FM Mono operation. But during that day, that meant forever having to switch the tuner back and forth between Mono and MPX Stereo modes depending on which type of signal was being received. The new feature eliminated that necessity.

It works because the within the LDR/Lamp package, the LDR resistance is low (on the order of about 700Ω) when it is exposed to light. When automatic switching is selected then, the lamp within the LDR package is connected in parallel with the front panel MPX lamp. When an FM Stereo MPX signal is received (indicated by the presence of a 19 KHz Pilot signal), the front panel MPX lamp lights -- and now, so does the lamp within the LDR package. With that lamp lit, the LDR resistance goes low, allowing the 38 kHz oscillator stage to operate. When it does, the missing 38 KHz sub-channel carrier is generated, and detection of the 38 KHz suppressed carrier double side band stereo information sub-channel takes place, so that stereo sound is produced from a Stereo MPX broadcast. If the pilot signal is not present (it is not part of a conventional FM Mono transmission), then the front panel MPX and LDR lamps are extinguished, and the LDR resistance then goes high. This effectively kills the 38 KHz oscillator (and its noise), so that mono stations are then received with the lowest possible noise.

I hope this helps!

Dave
 
dcgillespie said:
I hope this helps!
Click to expand...

Indeed, it does!

I wasn't aware that automatic stereo switching has not been available on other tuners of that era. Its indeed not a good idea to leave the 38kHz oscillator running without a pilot tone.

Why was it necessary for McIntosh to use an LDR as a potential-free switching device? I imagine switching the oscillator on and off would otherwise have required an additional tube.

cheers
Martin
 
It was likely the most noiseless way to accomplish the required switching, without the need for any mechanical contacts in the circuit.

Dave
 
Oh yes, thats a good reason. I think the MR78 uses a relay, I vaguely remember a clicking sound when tuning into a station. Or was it the muting relay?

BTW, the MR71 did it again: on startup from cold, AFC off, after 15-20s initial warmup it comes up with an offset of approximately 1 step to the right on the 0 to 100 scale of the chosen station. After 45s this offset suddenly disappears and the tuner jumps back to the right station. From there on, warmup was nominal, progressively reaching perfect tuning after 1m30.

One step on the decimal is an offset of about 200kHz... To clean the chassis I took out all the valves before it went back in the rack. Maybe I introduced a marginal contact issue. Or the oscillator valve has a problem. Once back from winter vacation I will check out.

cheers
Martin
 
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Apfelmus said:
Oh yes, thats a good reason. I think the MR78 uses a relay, I vaguely remember a clicking sound when tuning into a station. Or was it the muting relay?

BTW, the MR71 did it again: on startup from cold, AFC off, after 15-20s initial warmup it comes up with an offset of approximately 1 step to the right on the 0 to 100 scale of the chosen station. After 45s this offset suddenly disappears and the tuner jumps back to the right station. From there on, warmup was nominal, progressively reaching perfect tuning after 1m30.

One step on the decimal is an offset of about 200kHz... To clean the chassis I took out all the valves before it went back in the rack. Maybe I introduced a marginal contact issue. Or the oscillator valve has a problem. Once back from winter vacation I will check out.

cheers
Martin
Click to expand...
I expect if one can has the silver disease, the others are also suspect.
Like a piano that won't hold tuning, this may turn into an endless project.
 
Pio1980 said:
I expect if one can has the silver disease, the others are also suspect...
Click to expand...
Yes, of course, the cans CAN produce these jumps, and it may certainly be heat related, too. But to what I read so far this happens at random, not in such a narrow timeframe after cold start.

If it was the cans, I think it should take considerably longer to react. The jump (if it happens) occurs consistently a minute or less after cold start. I think that means two things:
- there must be a steep temperature gradient to make it happen consistently in such a short timeframe
- so it must be a component heating up very quickly - rather by itself and not by radiation or convection.
I can only think of valves and eventually a power resistors that may allow for such a behaviour.

What do you think of my reasoning?

cheers
Martin
 
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