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 Post subject: Re: Zenith 1204 Chassis
PostPosted: Nov Fri 15, 2019 8:46 am 
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So, B+ current =49 V/590 ohms= 83 mA. A bit low for this particular chassis, but in the ballpark. More
significantly, it does not exactly match the calculated B- current through the high voltage winding center tap reference
resistor to chassis ground.

The two values have to be the same (Kirchoff's Current Loop Law). A possible reason for the non-agreement
is that you have mis-identified/mis-measured some aspect of the resistor that ties the HV CT to chassis ground. In order to
have -35 V at the HV CT at 83 mA, the reference resistor (nominally 75 ohms) would have to be 422 ohms (35 V/ 0.083 A).

Since the reference resistance in the originally-built radio was via a Candohm, which has been totally replaced, I would
start looking for issues there: both resistance values and connectivity.


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 Post subject: Re: Zenith 1204 Chassis
PostPosted: Nov Fri 15, 2019 1:47 pm 
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Agree with Lorenz200w. A wiring error seems likely. Review the areas you worked on, or any areas in the set appear to have been serviced in the past.

In your photo, am having a hard time seeing the point where you grounded your replacement candohm. Is it grounded between the second 11K resistor(s) and the 75 ohm resistor?

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 Post subject: Re: Zenith 1204 Chassis
PostPosted: Nov Fri 15, 2019 7:06 pm 
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processhead wrote:
Agree with Lorenz200w. A wiring error seems likely. Review the areas you worked on, or any areas in the set appear to have been serviced in the past.

In your photo, am having a hard time seeing the point where you grounded your replacement candohm. Is it grounded between the second 11K resistor(s) and the 75 ohm resistor?


Errr - I was careful not to ground the wiring for the new candohm - I just checked the replacement wiring again and, using a continuity check, it isn't grounded.

Looking at the original candohm (which I kept) - there are 5 lugs, the third is grounded as part of the 75Ω hookup, which I'm missing in my replacement. In the first photo of the original wiring, that lug is behind the big blue 22uF cap. Fix this then re-check the voltages?

The only other candohm I've worked on was in a ZTO G500.

Thanks,
John

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 Post subject: Re: Zenith 1204 Chassis
PostPosted: Nov Fri 15, 2019 8:00 pm 
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Yes, it needs to be connected just like the schematic or you will have problems.

Attachment:
Candohm.jpg
Candohm.jpg [ 55.87 KiB | Viewed 252 times ]

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 Post subject: Re: Zenith 1204 Chassis
PostPosted: Nov Sat 16, 2019 12:48 am 
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I was pondering that exact segment yesterday, but when I checked it, I was checking C21 + to ground, - to CT, not the candohm too, but I wondered about it.
Thanks

Added:
In a post by Leigh, he referenced http://www.atwaterkent.info/Images/Zeni ... istors.pdf which lists the values of the Zenith candohms with their wattage.
The schematic I'm working from list 63-809 but the actual part I pulled is stamped 63-569, there is a difference for that resistor, the 569 is 180 Ω and the 809 is 78 Ω. I'm going to change up the 75 5W concrete I have for a 180 1W carbon and see how that works wired to C21 +, which also goes to ground.

Columns are model no, total r. segment r, wattage, tolerance, size and price. ($0.65!)


Added: The initial readings on the candohm after the change seem more reasonable: 235, 83, -3.2, +8, I'm going to test the rest of the values after dinner.


Attachments:
Zenith 1204 Candohm 63-809.JPG
Zenith 1204 Candohm 63-809.JPG [ 87.71 KiB | Viewed 245 times ]
Zenith 1204 Candohm 63-569.JPG
Zenith 1204 Candohm 63-569.JPG [ 50.75 KiB | Viewed 245 times ]

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 Post subject: Re: Zenith 1204 Chassis
PostPosted: Nov Sat 16, 2019 4:15 am 
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I would suggest staying with the values you have in your present replacement for the 809 candohm. When in doubt, it is advisable to stay with the documented component values in the schematic for the model.

You really don't know the background behind the old candohm in the radio and there is no certainty that it was original.

Have you grounded the candohm as shown it the schematic and rechecked voltages?

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 Post subject: Re: Zenith 1204 Chassis
PostPosted: Nov Sat 16, 2019 5:30 am 
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Replace your 78 Ω with 100 Ω
viewtopic.php?p=797932#p797932

Zenith chassis 1204 schematic with pinouts...
viewtopic.php?p=2833340#p2833340

Greg.


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 Post subject: Re: Zenith 1204 Chassis
PostPosted: Nov Sat 16, 2019 6:14 am 
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I did change the 180, putting the 75 Ω 5W back in line.

The candohm now shows: 230, 84, -1.6, 9.85
Drop across the spk. fld coil is 57 (287-230)
Samples of plate voltages are good, one problem with the oscillator, way low.
Heaters are all around 2.7VAC - across 2->7 = 5.7VAC
Its back to the wires again, folks!

Thanks a lot for the help fellows, it is appreciated. The forum needs a beer buying system.

Cheers,
John

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 Post subject: Re: Zenith 1204 Chassis
PostPosted: Nov Sat 16, 2019 1:02 pm 
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If the oscillator B+ is low or missing, check that node. There are a couple of resistors that may have drifted high or completely open. Of course a broken wire or bad connections in that node could also affect B+

Attachment:
Osc.jpg
Osc.jpg [ 248.92 KiB | Viewed 219 times ]

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 Post subject: Re: Zenith 1204 Chassis
PostPosted: Nov Sat 16, 2019 3:54 pm 
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The oscillator coil that feeds the oscillator tube plate is shunted by resistor R6. Both R6 and the oscillator coil winding
would have to be open in order to have no plate voltage at the tube- assuming that the B+ feed resistor R7 is intact. It seems
more likely that R7 would be open or miswired, a double failure (coil + R6) seeming to be a bit improbable (but not impossible).
Should be straightforward to trace between the tube plate and the B+ rail.

If the plate voltage is low (but not zero volts) my suspicion is that R6 is intact but the coil winding has
opened up. Easy to confirm with an ohmmeter across R6- if the coil is OK, the reading should be a few ohms or less. If it reads the approximate value of the resistor (3.3K?) then the coil winding is open.


Last edited by lorenz200w on Nov Sat 16, 2019 4:00 pm, edited 1 time in total.

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 Post subject: Re: Zenith 1204 Chassis
PostPosted: Nov Sat 16, 2019 3:57 pm 
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lorenz200w wrote:
The oscillator coil that feeds the oscillator tube plate is shunted by resistor R6. Both R6 and the oscillator coil winding
would have to be open in order to have no plate voltage at the tube- assuming that the B+ feed resistor R7 is intact. It seems
more likely that R7 would be open or miswired, a double failure (coil + R6) seeming to be a bit improbable (but not impossible).
Should be straightforward to trace between the tube plate and the B+ rail.


Agreed. Good observation.

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 Post subject: Re: Zenith 1204 Chassis
PostPosted: Nov Thu 21, 2019 6:49 am 
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R6 shows 0.9 Ω across, I replaced it last month, values check.

I've studied the wiring for many hours and have answered a few outstanding questions, R9 by the IF tube on the schem was wrong resistance, that's corrected to 1K, R3 to the 6V6G was also wrong and corrected, it was difficult to map those to the schematic for me.

So, with the candohm grounded, the above corrections, and some other clean up, I get a buzz which starts out high pitched as the set warms up and drops to a steady low buzz when it is warmed up. Placing my probe on pin3 of the IF Amp stops the buzz. Various tones can be achieved when testing the e-cap voltages and the candohm. I'm thinking I'm still messing up the e-cap wiring, that isn't clear to me yet.

6K7G RF Amp - pin 2 - 2.7 VAC, pin 3 -> 229VDC, pin 4 82, pin 7 2.9VAC - all close to spec.
6J5 Osc pin 3 is now at -5.6, improved but off by 50%.
6L7G Det is low on pin 5 @ -0.5VDC should be -10.

Candohm, 1st postition = 236, next 11k Ω = 100, next = -0.89, last = 10.84
Drop across Sp. Fld Coil 290 - 234 = 56
Center Tap = -0.8

I'm digging in to find where I've screwed up the e-caps to get that buzz. With eternal vigilance we will prevail.
Cheers,
John

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 Post subject: Re: Zenith 1204 Chassis
PostPosted: Nov Thu 21, 2019 12:47 pm 
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sergneri wrote:
I did change the 180, putting the 75 Ω 5W back in line.

The candohm now shows: 230, 84, -1.6, 9.85
Drop across the spk. fld coil is 57 (287-230)
Samples of plate voltages are good, one problem with the oscillator, way low.
Heaters are all around 2.7VAC - across 2->7 = 5.7VAC
Its back to the wires again, folks!

Thanks a lot for the help fellows, it is appreciated. The forum needs a beer buying system.

Cheers,
John


What is the B+ voltage on the oscillator plate now? Did you get that fixed?

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 Post subject: Re: Zenith 1204 Chassis
PostPosted: Nov Thu 21, 2019 10:16 pm 
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Paul - the value -5 posted above is for OSC PIN 5, not 3. PIN 3 on the OSC is still at -1VDC vs 121 VDC per the specs, and it is negative back to the Band selector and R7. I must have the B- hooked up here (?). Still looking for the fault. thx John

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Last edited by sergneri on Nov Fri 22, 2019 12:24 am, edited 1 time in total.

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 Post subject: Re: Zenith 1204 Chassis
PostPosted: Nov Thu 21, 2019 11:23 pm 
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sergneri wrote:
Paul - PIN 3 on the OSC is still at -1VDC vs 121 VDC per the specs, and it is negative to the Band selector and R7. I must have the B- hooked up here, no? That's what I'll try to find, thx John


The B+ supply wiring for the 6J5 oscillator tube also provides B+ to the RF Amp, Mixer, and IF amp tubes. Do you have B+ plate voltage at these tubes?
If so, I would look for a bad connection near the 6J5 since they all basically share the same circuit until branching near the individual tubes.

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 Post subject: Re: Zenith 1204 Chassis
PostPosted: Nov Fri 22, 2019 6:05 am 
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Seems I had mis-wired B+ and B- at R7 . I suspected this since yesterday but finally ruled out most other issues.

Using a continuity check based on your observations of the B+ circuit, went from a known good 219VDC on plate/pin 3 of IF AMP/6K7G thru 'C' 2nd IF TF to R9 (1K) on a terminal strip below the band switch back wafer. A lead from here went to the pin below R7 (22K 2W) on that wafer (wrong). At R7, I'd wired in the B-, coming from R2.

I checked them over and switched them on the wafer and bingo, I see 119 VDC on PIN 3 of the OSC and no humming.
When probing pin 5 on the OSC, I get a station and -0.005 VDC, remove the probe and the station fades out but there is still background static. Same results when repeated.

The candohm voltages are a bit more moderate now, 218, 57, -1.17, and 9.94 (chassis front to back).

A little break tonight and then back at it tomorrow to figure out why I'm not oscillating except when probing pin 5, see why that voltage is off.

Thanks again!
John

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 Post subject: Re: Zenith 1204 Chassis
PostPosted: Nov Fri 22, 2019 2:13 pm 
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sergneri wrote:

The candohm voltages are a bit more moderate now, 218, 57, -1.17, and 9.94 (chassis front to back).

A little break tonight and then back at it tomorrow to figure out why I'm not oscillating except when probing pin 5, see why that voltage is off.

Thanks again!
John


57 volts way too low for screen voltages. Should be 100 volts.
John k9uwa

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 Post subject: Re: Zenith 1204 Chassis
PostPosted: Nov Sat 23, 2019 2:28 am 
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Thanks for the pointer John (and for the shutter dial tutorial!)

I was listening to local stations for about 20 minutes and then it died - running at 62 VDC on that circuit for the screens. I rechecked the circuit connections and replaced one lead of the old wire from the candohm to the IF 6K7G and re-flowed the candohm connections but I can't find a vampire solder connection or other problem on that line. It isn't a complicated circuit so I'll take any other hints you folks might have to get the voltage up to spec.

Nice to be able to travel the dial however, along with our usual Bay Area stations, I did find a crackle-fest on the low BC, a few hints of reception on the SW bands. That wood tuning knob is sweet, what a nice feel. The AGC must be working, using the motor tuner silences the radio, back on when stopped. I would still be cursing over the bench if you folks hadn't helped!

Cheers,
John

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 Post subject: Re: Zenith 1204 Chassis
PostPosted: Dec Wed 04, 2019 7:45 am 
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It's me again :D

In another thread, I was able to get the tuner gang fixed which removed the crackling on the low end of the dial.
After that, performed an alignment and, amazingly, the radio was still aligned in the BC band after all the changes I made.
While aligning, I noticed a 100% increase in signal strength while keeping my fingers on either side of the cap wire from the 6L7G detector. I checked the ARF threads for info on this and read where incorrect or low voltages could be a cause. I started another inventory of pin voltages and found this puzzle on the circuit going to pin 3 plates on the 6J5GTs 2nd Audio and Phase converter:
Voltage off the candohm is 224 V.
This goes to R17 (10K Ω) where it is dropped to 180 V.
Then, from a terminal strip, it goes to two R2 (100K Ω) resistors I've marked #1 and #2 below.
#1 reads 180 V on one side and 19V on the other, #2 give me 180 on one side and 27V on the other - both go to pin 3 of their respective tubes which are spec'd at 74 and 76 V.
I lifted those resistors (both 1W) and checked them, they test at 99K Ω each.
I resoldered the terminals and tried to find a wiring fault but don't see one.
If you look at the R2 marked #3 by the 1st AF tube, it too is 100K Ω/1W and gives me 180 V on one side and 64 V on the other, which is what I'd expect on #1 and #2.
1 and 2 are in circuit with C24 which is "downstream" from #3, but I can't see that causing these to register such different voltages. C24 is a 4uF 450V Ecap.
The radio plays fine, good volume and is picking up the BC stations I'd expect. All measurements were taken with a Fluke 178.
Any ideas why these three 100K Ω resistors are showing such differences?
Thanks,
John

Attachment:
12014 Grid Circuit.JPG
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 Post subject: Re: Zenith 1204 Chassis - YAQ - Yet Another Question
PostPosted: Dec Fri 06, 2019 5:19 am 
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I replaced the 100K Ω 1W with 100K Ω ½ W, both #1 and #2 - no difference in testing 180 VDC before the resistor, 24 or 29 after.

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