Zenith Color Roundie Alignment with B&K 415

[bhegges]

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:

• Grounded the 3 test points
• Nothing is connected to the tuner and set it in-between channel 3 & 4.
• Feeding signal from the B&K 415 via the direct probe to test point G
• I have tried with and without the resistor/capacitor assembly as per the Zenith directions
• Note this is the only connection from the B&K 415 as I have highlighted in yellow below
• I tested the B&K 415 cable end to end and it measures approx. 10K resistance on both the ground and signal clips
• Oscilloscope is connected to test point C1

The last pic is measuring at test point C1 while operation normally with a cross hatch video signal.


B&K 415 Setup





Zenith CM-106 Directions




Scope:

[old_tv_nut]

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.

[bhegges]

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.

• Next step try connecting the B&K 415 output labeled "To Scope Horz Input" to the Oscilloscope "CH1 or X"
• I assume I can connect the scope probe to the B&K 415 cable as shown below, note this measures as a simple straight through cable
• I currently have just a single 10x scope probe, model 4904-2RA, specs here: https://store-9x7452d.mybigcommerce....ta%20Sheet.pdf
• I assume I need a second scope probe to connect "CH2 or Y" to the TV chassis test point C1
• Looking at the B&K 415 connection diagram it also shows connecting the "To Scope Vert Input", when would this be used, I would think this connects to the scope "CH2 or Y" input, this is confusing, don't I need to be measuring TV chassis signals?

[bhegges]

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.

[zeno]

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
LFOD !

[old_tv_nut]

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.

[old_tv_nut]

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.

[Electronic M]

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.

[bhegges]

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?

[old_tv_nut]

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.

[jr_tech]

Is the timebase (sweep) knob on the scope turned to “x y” ?

jr

[jr_tech]

Originally Posted by bhegges

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.

He is using an older scope (453?), which uses the trigger input for external horizontal input. Your newer scope uses channel 1&2 for x-y display.

jr

[bhegges]

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:







On to the alignment, step 1 - 41.25 Trap Alignment:

• Set tuner in-between two channels, I also have the antenna disconnected
• Ground test points B, F, E
• Connect from the "Direct" output to test point C, note the direct cable has an in-line 10K resistor
• Connect from the "RF-IF-Video" output to test point G and set the function to IF and the probe to 75 ohm, also note I am not using the capacitor/resistor network as described in the Zenith service manual
• Connect from the "To Scope Horz Input" and "To Scope Vert Input" to the X and Y (CH1 & CH2) on the scope and set the Sec/Div to XY mode
• Alignment instruction state to center frequency at 43.5 mc which is not easy to see, with all of the markers shown in the service manual turned on I set the center frequency where the full wave form was visible
• Alignment instruction state to set the sweep at 6 mc, there are no markings for this, the manual states the IF range is less than 1 MHz to over 10 Mhz), I left the sweep width at max, note from about 3/4 to max there is little affect, below 3/4 and I start to loose the waveform
• Alignment instruction state to adjust the bottom slug on L4 to minimize the 41.25 mc response and then to adjust the top slug on L4 to further minimize the 41.25 mc response

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.

[jr_tech]

Try switching ch 1 to ac and adjust position and sensitivity to get a better centering of the relevant portion of the display.

Don’t know, never used a B&K sweep generator.

jr

[old_tv_nut]

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.