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Zenith Color Roundie Alignment with B&K 415
Looking for some help and hope maybe this thread will help others in the future. I have repaired a Zenith 25MC33 chassis to working condition. I would like to go through the exercise of alignment, or at lease the motions to see where it is at. I understand care is needed and there is risk of making things worse. With that out of the way I have the SAMS folder and the Zenith CM-106 service manual, also the B&K 415 instruction Manual.
I am trying to start with IF and Trap alignment but immediately am stuck. Probably a simple setup mistake but so far I get no measurable signal on the oscilloscope. Making changes to the B&K 415 has no affect. Per the instructions here is what I have done:
The last pic is measuring at test point C1 while operation normally with a cross hatch video signal. B&K 415 Setup https://i.imgur.com/PhKqyaV.png https://i.imgur.com/satrn4B.jpg https://i.imgur.com/M0HJv6t.jpg https://i.imgur.com/m2GLw8L.jpg Zenith CM-106 Directions https://i.imgur.com/nloPks0.jpg https://i.imgur.com/WeqeMqk.jpg https://i.imgur.com/8IxiLHV.jpg Scope: https://i.imgur.com/1EAmtII.jpg |
Hard to tell, but maybe you have the scope sweep set way too fast? The sweep generator should be sweeping about 60 times per second, so the scope sweep should be around 1 or 2 msec/div. Best is not to use the internal horizontal sweep of the scope, but set up for X-Y operation, with the "too scope horizontal input" of the sweep generator for X.
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old_tv_nut, thanks for the quick reply and suggestion. This leads to more questions. My apologies for the very basic questions, I am in completely uncharted territory.
https://i.imgur.com/PhKqyaV.png https://i.imgur.com/OukZvrw.png |
Watching Shango work on TV alignment with a B&K 415 and it looks like he has the "To Scope Horz Input" connected to the A Trigger scope input via a simple dual banana to bnc connector. Video here: https://www.youtube.com/watch?v=CmEyYAjjsn0&t=992s. Here is maybe a connector adaptor I could use: https://www.pasternack.com/banana-ja...-pe9011-p.aspx
I'm still not sure when I would then also use the "To Scope Vert Input" coming out of the B&K 415. |
First what ever you do is FLAG you alignment tool with a piece
of tape so you can go back to the start !!!! Write down the turns CW or CCW as you go. It shouldnt take much, like <1/2 turn. Traps can be adj with an OTA signal ( 41.25, 4.5 etc) Now the dirty truth. I was in the TV biz from 1970 til the end of CRT's I never did an alignment. Only one tech I knew in the area did one just to say he did. I may have seen ten sets ever that needed it. A few had a damaged transformer & got eye balled. A few more got tweaked but not by a tech. A few others were real Emersons built in the mid - late 60's. Total toilet sets ! good luck Zeno:smoke: LFOD ! |
You do not need any scope probes, but you do need cables with connectors that fit the B&K
The way the B&K works is as shown in its diagram. Instead of connecting a scope probe to the TV detector test point, you connect the B&K direct probe to the TV detector point. The B&K then adds the frequency markers to that waveform and outputs the combination of sweep waveform and markers to the B&K connector labeled "To Scope vertical input." Connect this to the scope Y input Connect the B&K connector labeled "To Scope Horizontal Input" to the scope X input. The alignment procedure you posted assumes that you have separate sweep and marker generators, but with the B&K they are combined internally and you do not need the resistor and capacitor assembly shown in figure 6, and do not need to connect the scope directly to the TV. In one sentence, the TV is connected only to the B&K, and the scope also is connected only to the B&K. |
Another tip: Set both the X and Y scope inputs to DC. If set to AC, the waveform will likely be distorted because it is only at 60 Hz or so, and you will royally screw up the alignment trying to compensate for the phony waveform.
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I changed the scope connectors on the front panel of my B&K over to BNC connectors (it literally was unsolder the center pin, unbolt old connector, bolt in new connector, and resolder center wire) so I can connect it to the scope with simple common cables with BNC at both ends...Works like a charm with the scope in XY mode. Have only tried one alignment so far.
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Thanks everyone for the help so far. I now have the scope connected in X Y mode direct to the B&K 415 and get a signal, see the below image. Further I have the direct probe connecting the B&K 415 to the Video detector (test point C1). The issue now is I get no feedback from the TV, whether connected to the TV or not I get the same curve on the scope.
Do I still use the Zenith service manual directions? I did try to connect the RF-IF-Video output and feed it into test point G as per the service manual but no change. Could something be wrong with the B&K 415? https://i.imgur.com/nLC9zRm.jpg |
Very curious.
That does not look like an X-Y mode display. What it looks like is possibly the signal that should be going to the X axis (the left to right sweep) is showing on the Y axis,while the scope is still using its internal x axis (time) sweep. Also, do you have adjustable AGC bias connected from the B&K to the appropriate point in the receiver? The Zenith manual will tell you where to connect it. |
Is the timebase (sweep) knob on the scope turned to “x y” ?
:scratch2: jr |
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jr |
The problems were operator error, Jr was correct, I had never noticed the the very last time division setting was for X Y. With the scope maybe(?) now setup correct I am ready to go.
Note the first section of the scope output seems to be dead, regardless of the scope setting or the sweep settings. The first pic is while measuring the TV, the second pic is just from the B&K 415 and TV not connected: https://i.imgur.com/XTh4xs9.jpg https://i.imgur.com/PnxzJn0.jpg https://i.imgur.com/yy9CJ2t.jpg On to the alignment, step 1 - 41.25 Trap Alignment:
Turns out I don't have the the right adjustment stick, mine is either worn or undersized, a 3/32 allen wrench fits perfect which is just under 0.1", I have order a plastic adjustment stick and will report back soon. Edit: adjustments sticks arrived, no adjustment was needed for L4. https://i.imgur.com/Ubaqqkc.png https://i.imgur.com/8IxiLHV.jpg https://i.imgur.com/9qGj6qi.jpg https://i.imgur.com/GcXAxKO.jpg https://i.imgur.com/Ng5Ef1M.jpg |
Try switching ch 1 to ac and adjust position and sensitivity to get a better centering of the relevant portion of the display. :scratch2:
Don’t know, never used a B&K sweep generator. jr |
Looks like you've got it!
The offset you show with a question mark is the pulse that you saw before the sawtooth when you were accidentally not in XY mode. If needing the horizontal position set to max left is annoying, you can try switching to AC coupling on channel 1 (X). You can watch to see if the waveform shape changes. If it does, it's still OK to use it even if the *shape* doesn't look exactly like the Zenith manual. The important thing is the relative amplitude (vertical) at each marker, which will be good as long as channel 2 (Y) is DC coupled. The Zenith manual should tell you what percent of peak each marker frequency should be. |
You can also try adjusting the center frequency, as long as that doesn't clip off one end or the other (left or right) of the waveform.
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Continued 41.25 Trap Alignment:
https://i.imgur.com/SnXTXZZ.jpg |
L1, L2, L3 alignment:
The direction call to adjust for minimum response as you adjust each coil and there may be some interaction, I was not sure on how to do this as it relates to the scope response, I just tried to get the response cure to match the picture, zooming in on the scope was some help. SAMS directs making these adjustments and using a VTVM to indicate. I tried this too but not easy to adjust to the minimum, note I cheated and used test point C2 (accessible on the chassis) which is in front of R49, 330 ohm, where SAMS has test point E on pin 7 of the sound & sync amp tube. Adjustment to L1, L2, and L3 got tricky real quick and trying to keep track of the adjustments in case I need to go back went out the window. I did find adjustments in the subsequent step helps with getting a matching response curve. I likely will repeat all of these steps as I zero in on the alignment. https://i.imgur.com/59uepBS.jpg https://i.imgur.com/mxldVuy.jpg https://i.imgur.com/DDaQ2PS.png https://i.imgur.com/3UgU1G9.jpg https://i.imgur.com/94pxjLE.png Starting point with the same 7 markers turned on as indicated in Fig 12, note Zenith uses a 41.75 while the B&K 415 has a 41.67, close enough :) https://i.imgur.com/PWBjuFZ.jpg |
Tuner converter plate coil & IF Transformers T1, T2, T3 alignment:
I will follow up with some pics of where I land for the final IF & Trap alignment. https://i.imgur.com/BE7CErg.jpg https://i.imgur.com/grhL3GV.jpg |
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2) What do you mean by "fall in the correct place?" If the procedure says min or max at the marker, it means only that - the vertical position of the marker should be min or max. If the curve ends up looking wrong *at the end after all adjustments have been made*, there is something wrong with your setup or you made a mistake earlier in the procedure. It is possible the curve won't look right when you are only part way done. |
The 75 ohms is the setting on the B&K 415 Signal cable, you can select 75 or 300
https://i.imgur.com/kOLPwDR.png "fall in the correct place?" maybe not the best way to describe but as I adjust a coil the response curve on the scope moves right or left which has the effect of moving the marker up or down
I agree on needing to bring it all together, I will follow up with where I am at in a bit. |
OK here is where I am at:
Scope measuring on test point C1, all 7 markers are turned on, note the B&K 415 has a 41.67 marker vs a 41.75 marker which is what Zenith references. Directions are to make adjustments to the 41.25, 47.25, and 39.75 traps (L1, L2, L3) for minimum responses, accuracy of trap attenuation and marker locations. https://i.imgur.com/1s8rpFt.jpg https://i.imgur.com/sVXHXKY.jpg Scope measuring on test point C2, same 7 markers are turned on. Goal is to make adjustments to the converter plate coil and 1st IF coil (T1) to match the response and also to adjust the 2nd and 3rd IF coils (T2, T3) for maximum response and to approximate the response curve. https://i.imgur.com/wpi2kKC.jpg https://i.imgur.com/LSjbWCQ.jpg |
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The spec for 41.25 attenuation at C1 is 24 dB or greater, which means it's Ok if it's higher on the scope (closer to the top, which equals zero signal) than shown in the illustration.
At C2: a 1 dB reduction in signal from the peak is the same as a 20% [correction - 10%] reduction. So the distance shown as 1 dB in the illustration should be 20% [correction - 10%] of the max height. (the sketch looks poorly drawn - more like 10%.) [Correction - the sketch looks correct]You are getting over 30% attenuation at the markers, so this indicates the curve is too narrow. |
Please see correction to previous post - 1 dB should have been 10%, not 20%
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https://i.imgur.com/K0QSL1t.jpg |
I have tried to get the 43 to 44 mc at 1db (10%) of the 42.75 and 45 mc markers. I got it down to 20% but this comes at a cost of moving other markers out of proportion. Thoughts on which markers are most important to have correct? Note currently I am trying to get the curve shape to match and to keep the 42.75 and 45 mc parallel.
https://i.imgur.com/eAZK1Ah.jpg https://i.imgur.com/EYaZH8i.jpg |
Curious that the drawings show 42.75 and 45 equal for both test points, but your measurements don't. Not sure what to think about that.
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OK, I went back through the whole thread from the beginning, and I see you are not using the RC network from figure 6, and have the sweep generator output termination set to 75 ohms.
Try this - keep the sweep generator output termination set to 75 ohms, but add onto it the RC network from Figure 6. My guess is that the 75 ohm impedance is loading whatever grid circuit you connect to and that is causing the incorrect responses. |
I think I read too fast earlier and mistook the RC network in figure 6 for an external marker adding arrangement - apologies!
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Oh! Revelation! You are using the RF cable, 75/300 ohm switchable, which should only be used for antenna input. Use a plain unterminated cable as shown in the Zenith illustrations, with the RC network of figure 6.
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https://i.imgur.com/UyxlmII.jpg https://i.imgur.com/TgALh9P.jpg The Zenith service manual directions for the IF alignment steps in question state to connect the sweep and marker signals to the converter control grid (Test Point A on the tuner). No reference is made to any coupling circuit, which matches your note to use a plain unterminated cable. Zenith info below: https://i.imgur.com/53rbg0u.jpg https://i.imgur.com/I6vNFkP.jpg |
In that case, the 75 ohm setting may work when used with the Zenith RC circuit added. The 75 ohm setting may actually just be a direct feed through.
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https://i.imgur.com/0Uc9DjM.jpg Zenith Service Manual Sweep and Marker generator circuit drawn below. https://i.imgur.com/lYEkMiq.jpg The directions note to use the above circuit on the first step which is 41.25 Trap Alignment, it does not mention it's use for the subsequent steps, either you don't need it or it is assumed you use for the remainder of the steps. Below is all that is said for connecting to the tuner. If the circuit is to be used the B&K 415 circuit is close, if you are not to use this circuit than the B&K 415 supplied cables are lacking as it did not come with a BNC pass through cable, which I hope means you don't need it. https://i.imgur.com/AeKfeYk.jpg |
OK, the 75 Ohm setting of the B&K termination is closest to the Zenith drawing, and has a capacitor of reasonable size in series.
At this point, I'm not sure if the difference is enough to cause a problem, but the best way to find out is to use a direct cable with the Zenith RC network and see. Another thought: is it obvious which of the B&K clip leads is ground and which is the 750 pF? Could they be reversed? |
Take a look, below are 3 scope images using different cables to feed signal into the tuner converter control grid (Test Point A). Note I had to turn down the vertical scope by one setting for the direct connection cable as the signal came in stronger. I tried to overlay all three images in photoshop, each was slightly different in horizontal or vertical size but I would have to argue the curve and marker locations and marker attenuations all look very close.
Maybe antidotal but while waiting for the set to warm up I was watching some content via the converter box and the picture to me looked better, so at least I am not making it worse. https://i.imgur.com/5qVsJsw.jpg |
I may try to further tweak the IF alignment but its maybe about as good I can do. Would there be any components that are suspect? See below for all of my in-circuit measurements, I did replace the 2 film capacitors also using NOS tubes that test very good, I guess I could swap just to see what happens.
https://i.imgur.com/h1jsX5m.png https://i.imgur.com/DQ0OJaE.png |
I don't have enough direct experience with this particular chassis to point at a particular component and say "look at that." The curves are close to
the manual except more peaked in the center, so actually may be within normal range, especially since you see differences depending on the cable. One experiment you could try: Since A9, A10, and A11 have their tuned frequency specified on the schematic, simply tune each one back and forth a bit to see if each is really producing maximum response at the specified frequency. If they are, I would say there's no more to be done. |
Here is an interesting finding, I swapped out the three IF tubes with spare tubes. No real change for the first and second IF tubes but the third IF tube (6EJ7) did yield a much different response curve, see below. Note the tubes were tested with a B&K 747 tube tester where good starts at 65 on a 0 - 120 scale.
Sylvania 6EJ7 testing at 77, this is the tube I have been using https://i.imgur.com/TkCw8UD.jpg Alternate tube, Zenith 6EJ7 testing at 80, note I still need let this tube run for a good amount of time, but interesting how much the tube affects the response. Also note about 1 turn on the alignment transformer and I can get the curve about correct but not as good as the first tube. https://i.imgur.com/oWMIjpN.jpg |
A thought: while each tube type tests differently the good 6EH7 tubes I have all test around 90 to 100 while the 6EJ7 tubes I have only test around 75 to 80. Again different tube type but maybe the 6EH7 tubes I have are a bit weaker.
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