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400 Eye-tube NO STEREO BAR

larryderouin

larryderouin

I'm VERTICAL and Breathing...most of the time.
Subscriber
On the 400 I've been working on for Doug (rogerfederer), I have a problem with the eye-tube. Closure is extremely weak. Serial is 48751F

EM-84A that came with it is very faded and shows less than 45 on the emissions tester (40 is in the replace area). MPX-65 has 3 very good reading 12ax7's with fairly matched sections (by gm.) Emissions on all 3 is in the MID GOOD band. (about 75 on scale on both the EICO 625 and the Philco 9100.)

MONO closure is about 5/16" from the end on both sides. And that is with a very strong signal on air signal or from the SG-165. I also checked it for separation and get a little over -30db on both channels. Audibly the separation is very good on FM.

When shifting to STEREO the eye-tube pulls back to NO BAND AT ALL or the smallest bare line on the edges.

I put a Brand new NOS MAZDA EM-87 in it and the response is practically the same. I changed the 470K resistor on the socket to a 1megohm and no change. For shits and giggles I lifted the 1.8megohm resistor and the MONO Response is now about 1/3 to 1/2 a bar. I tried a 1.0 megohm resistor and got a miniscule increase in MONO RESPONSE but the STEREO response is not more than the edge lit. I then tried a 470K and just a little more. I don't really want to go lower as the underlying problem may be alignment as my test gear isn't calibrated. I did set another tuner next to it and could match station for station just on the oscillators nulling each other out.

Then I put a brand new 6FU6 EI Brand tube, along with the circuit changes for it, and response was worse, so the EM-87 and it's changes were put back in.

I rechecked and tweaked the 10.7 with the SG-165 and VTVM to get max voltage on each stage per manual, and checked the dial alignment and peaked that also (using the procedure Dave outlined for 400's vs zeroing the indicator pin at zero.

This tuner on a "T" dipole hung on the wall picks up most every station in the DC and Baltimore area with no problems other than the attendant overlaps from being a wide band tuner and the stations being right on top of each other. All stations are from 10 to 55 miles away. and all sound very good even in stereo, but the eye-tube only responds to MONO and only with the strongest signals. NO RESPONSE in STEREO.

I swapped another MPX-65 into it and MONO response stayed the same, but STEREO response barely made 1/4" bar's. I pulled it back out and put the original back in as I couldn't see keeping it in there for a bare minimum response. Plus I didn't know how far out of alignment this MPX was.

So the question is, is there a problem in the tuner on this 400, or is it mostly all in the MPX. Both have had C214, c221, c222, R222, r223 replaced. Also C210. R222 & R223 were changed from 22K to 16K. Mono De-Emphasis resistor (R-28) was changed from 47K to 41.2K.

I'm thinking that the multiplex is out of sorts and I'll get it aligned and then swap it in to the 400. I'll need to check some resistors in the main chassis also. Specifically, R21,22, 23, and C33 & C34.

Doug has the 400 now and seems to be very happy with it. I don't know how much FM he listens to but the way this 400 is acting I'm not happy with it in it's present condition with the eye-tube response.
 
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Larry -- Either there is something wrong with the eye tube circuit, or there is something wrong with BOTH the indicator output of the FM tuner, and the MPX sub-chassis. Logic would indicate that there is something wrong with the indicator circuit.

If the brightness of the indicator is good, then it would tend to indicate that the control amplifier portion of the indicator tube is having issues. That's easy enough to check as there are only two components and the B+ voltage that can affect that tube. The B+ is the same B+ that supplies the IF strip, so if that's good, then that can be eliminated. The 470K plate load resistor for the control amplifier stage is located within the rubber boot of the indicator socket, so check its value next. Finally, there is the grid return resistor for the control amplifier stage R32. Check that it is reasonably accurate as well.

If all checks out, then try this: Use any 4.5 - 6.0 volt DC source. This can be a bench supply or even three or four 1.5 volt batteries in series. Operate the unit in any FM position. Ground the positive terminal of the DC volt source to the chassis, and touch the negative terminal to the junction of R22 and R23. If the indicator tube is an EM84A or equivalent, and the indicator circuit is operating properly, then the eye tube should show good closure with the negative bias applied to the tube as directed. This way, you will know if you have indicator problems, or tuner/MPX problems.

Let us know!

Dave
 
Dave;

I'm gonna have to get the 400 back from Doug. I'll see if I can get it this week and check it out. I had changed out the 470K resistor for a new one. Then when I put in the EM-87 I changed that to a 1MEGOHM, Both were checked for value before install. Right now there is the 1megohm resistor in the boot, with the EM-87. I have the 1.8megohm taped to the side of my resistor box, so it can go back in if needed. Right now that one is filled by a 470K.
 
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If memory serves from my own MPX adapter from a 500B, there is a small detector diode in the MPX unit that drives the eye tube output. It should tie into what feeds the grid of the middle tube, and there are a couple caps, a resistor, and the diode that make up the pilot detector circuit. If any of that is faulty, it won't work as expected. Not sure if your MPX chassis is the same as mine, but I believe they all worked in roughly the same manner.

Of course you'd want to make sure its not just a wanky connection or a problem in the selector. It switches the grid connection on the eye tube from the ALC line to the pilot detector on one of those contacts.
 
a report from the field: the 400 is in the spot where i've had a sansui 2000a and a pioneer sx626. the FM reception for the 400 is not as good as either the sansui or the pioneer. not a great spot for FM reception, so a decent test. i've tweaked the dipole antenna and now the fuzz on my favorite jazz station has decreased a lot. i understand the 400 is older and was built when the FM dial was not so crowded.

by the way, larry did an amazing job on this one!
 
Fisher tuners are by and large very good. my TA-600 pre-dates your 400, and it has little trouble pulling in stations. Its mono only, but mated with the stereo decoder out of a 500B it works extremely well.
 
Unfortunately we both live in areas where FM reception is far from Optimal. I've got a TV/FM antenna on the roof hooked up with 300 ohm all the way down so I can check it on that. Don't know why I didn't before. Been using "T" dipole mainly for convenience to the bench in the house. I've got the Executive on it and the 800C pulls stations in nicely. But the highest reading on it is about a 3 on the scale. Which is about average for any receiver I have including the aligned shop SX-939. The 400 picks up about 90% of all stations listed for baltimore as long as there isn't a completing station on the same freq within 50 miles. Sometimes it'll flip between the two. This I've found is normal for my particular area. Lot's of microwave and UHF towers to interfere too.

Gadget; I believe the 500B has the WX MPX on it. where the 400 has the mpx-65. Same as the 500C and 800C.

I'll get the control section straightened out when I pick it up tomorrow. Then when I get the MPX unit (spare) back from Dave, I'll swap them. Then it should be good.

Larry
 
Possible. I dont recall it having any particular markings on it to say what it is. I got it for a really good price when Rich was parting out the remains of a 500B that he turned into a power amp, and I stuffed it into the case of an old geiger counter that I bought as a parts unit to replace the broken meter movement in my own geiger counter.
 
dcgillespie said:
Larry -- Either there is something wrong with the eye tube circuit, or there is something wrong with BOTH the indicator output of the FM tuner, and the MPX sub-chassis. Logic would indicate that there is something wrong with the indicator circuit.

If the brightness of the indicator is good, then it would tend to indicate that the control amplifier portion of the indicator tube is having issues. That's easy enough to check as there are only two components and the B+ voltage that can affect that tube. The B+ is the same B+ that supplies the IF strip, so if that's good, then that can be eliminated. The 470K plate load resistor for the control amplifier stage is located within the rubber boot of the indicator socket, so check its value next. Finally, there is the grid return resistor for the control amplifier stage R32. Check that it is reasonably accurate as well.

If all checks out, then try this: Use any 4.5 - 6.0 volt DC source. This can be a bench supply or even three or four 1.5 volt batteries in series. Operate the unit in any FM position. Ground the positive terminal of the DC volt source to the chassis, and touch the negative terminal to the junction of R22 and R23. If the indicator tube is an EM84A or equivalent, and the indicator circuit is operating properly, then the eye tube should show good closure with the negative bias applied to the tube as directed. This way, you will know if you have indicator problems, or tuner/MPX problems.

Let us know!

Dave
Click to expand...

B+ Voltage is good. No problems there.
Tube is Brand New Mazda EM-87 with the mods as noted in your MPX tuner thread. 1megohm replacing the plate load 470K, and I' ve got a 470K in place of the 1.8m as a 1meg did not change anything on tube.

Plate load resistor is 1Mohm (.995kohm). NEW
Grid return resistor is new (R32) 6.8K 1/4w as is R31.
R22 & R23 were checked (3.34M & 4.78M)
C212 in mpx and R207 were checked and replaced.

NO CHANGE IN either MONO OR STEREO OPERATION.
Hooked up a 6volt battery (lantern type) to test leads. Took a votlage reading @ junction of R22 & R23.
-1.198vdc in MONO Eyetube closure of 3/16" per side
-0.780 TO -0.788VDC IN STEREO No Closure of Eyetube

Injecting a Negative 6v to the junction of R23 and R22 produced a closure with a 1/8" GAP!
 
OK -- If eyetube brightness is good, and you got good response to the 6 volt battery, that would indicate that the eye tube is good, and there are problems with the indicator signals.

If I'm understanding you correctly, you would not want to go lower than 1.8M for that (not shown) resistor at point D on the schematic. Going lower only hurts your cause in this case, acting to reduce eye tube sensitivity in the stereo position.

Assuming that all tubes, voltages, etc., are good, then first check the peaking of the secondary of Z4 the limiter coil. This will be the bottom slug as you found out and warned the group about. Peak it for maximum eye tube deflection.

If the coil is peaked, and the alignment is good, then I would go back and check that both windings of each IF transformer in fact produce a definite peak when adjusted. Remember to use as small of a signal as possible (try to keep the set from limiting). If any of the transformer windings only produce a very broad change in signal when adjusted, then likely that IF transformer has gone south.

It is important to get the FM mono indication right first, as if the tuner portion is not providing a good signal to the MPX unit, it may not be able to trigger the eye tube correctly in stereo mode, either.

Let us know!

Dave
 
On the SG-165 it's recommended to set the microvolt output t X100 and 7 (1st and 2nd pots) which is as low as I can go and still get a clean signal. Any lower and the signal gets noisy/fuzzy/distortion. The transformers were checked and peaked for maximum - voltage (below -5 volts). I got on average not more than -1.2v from Z1 to Z4 lasst Saturday morning. I get conflicting info from the manuals. One says start at Z1 and work to Z4 or the last one, and another says work back from Z4 or 5 to Z1. I've done it both ways and get basically the same results. On the 48001 and up 400, do I work back from Z4 to Z1 and then go back to Z5? OR Start at Z1 and work UP to Z4, and then adjust Z5 after peaking the rest?

I put a new 1.8megoHm resistor back in. Immediately got a bit more on the MONO. Tuned to the Strongest signal I could get on the eyetube, with a "T" dipole and started with Z4 bottom. Surprisingly I ended up turning Z4 BOTTOM a little over 1/2 turn and got almost a full indication (1/4" gap in middle),with it in Mono. So I tweaked each transformer, Z4 to Z1, top then bottom using the eyetube as a reference vs. the voltage as outlined in the 400's manual. And it wasn't much(<1/8th turn on each) I could get each one to peak the eyetube with a sharp dropoff on the otherside. So i rocked each back and forth with the tweaker, until I had the peak centered. Then switched to STEREO. I now have on Strong Stereo signal about 1/2" gap in the middle (EM-87 tube) . I ran the separation test on the 400 with the SG-165 ad it's still 30+db. I DID NOT TOUCH the transformers on the MPX. On the voltages as a check, they averaged about -.01vdc to -.02 vdc LESS after peaking using the eyetube than by peaking by using the voltages as outlined in the manual.

With the SG-165 not being calibrated, I don't know if the 10.7 signal is exactly on (or it could be for all I know). But all the transformers are peaked to that.

IT sounds a lot better now although the stereo has a slight roughness to the hiss when it's operating in STEREO. I think the other MPX you're aligning will take the roughness out of the equation now.
 
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That's great news! It also shows how important it is that your test equipment be on the money, or it can actually hurt rather than help! Aligning all the IF transformers to your SG frequency will certainly catch any that are off the mark from the others, but ensuring absolute frequency accuracy is important as well. The transformers need to be peaked precisely at 10.7 mHz so that the necessary bandwidth is available on either side of the center frequency. This will then allow the transformers to properly handle wide band MPX stereo signals without degradation. Check to see if the SG-165's IF Marker frequency is generated by a quartz crystal. If so then you can be reasonably sure of its accuracy.

I sent you a PM about your MPX sub-chassis.

Dave
 
I've had plenty of bad rocks, so I wouldn't be absolutely sure that a crystal oscillator is still on frequency. If you have a frequency counter you can sync up to WWV using a signal generator tweaked to zero beat, you can use that to check the output frequency and get it plenty close enough for RF alignment purposes.
 
Interesting comment Gadget. My experience is that the frequency of a crystal hangs in to its last breath, but that the output slowly gets weaker and weaker, until it is no more. Have you observed differently?

Dave
 
I've got some old CB rigs that use crystals, either as a direct frequency oscillator or as part of a mixing matrix arrangement. Some of them have gone off enough to be a problem, some have just totally died. The late 1960s Browning uses direct frequency crystals in the 27 mhz range, and some of them have been a few hundred kc off, and a couple were just dead. The Tram runs a mixing arrangement, and some of those needed the padder caps changed to a larger value to drag them back on frequency. The Tram has trimmers, the Browning does not. I've had less issues with them in solid state gear, maybe its a heat-related failure in the tube equipment. Usually with SS stuff, if its off frequency a small tweak on the trimmer will get it where it needs to be.
 
An alternate method, if you have a frequency counter but no signal generator, is to use a station for initial tuning. The front end doesn't actually select a station, but remaps the band to different frequencies such that the desired station is at the intermediate frequency. The IF strip then provides most of the selectivity.

So, you can do the following:
Tune in a station for max on the tuning meter.
Connect the frequency counter at the same stage as the tuning meter.
If the reading is not the correct frequency, tune away from the station until the frequency is 10.7 MHz (or 455 kHz).
Without retuning the station, tune the IF stages ahead of the tuning meter for maximum reading.
The remaining stages can now be tuned for maximum.
 
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