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 Post subject: Zenith 1204 Chassis - YAQ - Yet Another Question
PostPosted: Sep Tue 03, 2019 6:48 am 
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Hello, I'm working on a Zenith 1204 taken out of a 12s267. This radio produces a steady, loud buzz on all bands and at all volumes when turned on. Reading here on ARF would indicate that the candohm is likely to blame. I think the candohm wasn't taken completely out of the circuit. I'm rebuilding it at this time.
As work progresses, I'm testing the passives in and around the candohm. I can't identify the big 12K ohm resistor on the terminal strip in the upper right corner. This size of resistor isn't listed in the parts list, I'm guessing it's 3W. It is also connected to a floating soldered "terminal" off of R2 (100k) on pin 3 of 1st RF 6J5G. Any idea why this might be there and if there is a corresponding component that it may be substitute for?

Image
1204 as found.

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Last edited by sergneri on Dec Thu 05, 2019 10:53 pm, edited 1 time in total.

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 Post subject: Re: Zenith 1204 Chassis
PostPosted: Sep Tue 03, 2019 12:06 pm 
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Assuming you meant 6J5G 1st Audio amp and not RF amp.

The 12 K resistor looks like it might be a replacement for the R17 10K 2 watt original component value. It is part of the B+ supply for that tube.

You are right about the candohm, it appears to have some replacement resistors hung across some of its lugs and should be removed from the circuit.

Attachment:
Zen.jpg
Zen.jpg [ 176.84 KiB | Viewed 1043 times ]

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 Post subject: Re: Zenith 1204 Chassis
PostPosted: Sep Tue 03, 2019 12:58 pm 
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Location: Mountains of Mourne. Ireland.
Original operating instructions→ here 13.6MB

Zenith chassis 1204 (notated schematic).
http://antiqueradios.com/forums/viewtop ... 5#p2940105

Alignment...
viewtopic.php?p=3034920#p3034920

------------

Head scratchin' time - - - schematic errors... :shock:
viewtopic.php?t=157389


Greg.


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 Post subject: Re: Zenith 1204 Chassis
PostPosted: Sep Tue 03, 2019 1:10 pm 
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Minister of Information - Greg, Comes through once again! :D

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 Post subject: Re: Zenith 1204 Chassis
PostPosted: Sep Tue 03, 2019 3:02 pm 
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A problem with the Candohm shouldn't result in a loud buzz at all volume levels. Half of the Candohm (the 11K ohm
resistor segments) is a simple voltage divider used to knock B+ voltage in half to feed the screens of the RF and IF tubes;
the other half is a separate voltage divider that sets the negative DC voltage offset used for audio tube biasing purposes.

I've never seen a Candohm with a burned-out segment "regenerate" but I suppose that it could happen- or even worse, short to the
steel wrapper through the insulation paper at the burnt spot. Cutting the original failed part out of the circuit is a good idea,
anyway...just in case.

Because of the negative DC bias supply, the filter electrolytics are wired in a manner that can result in hum if they are
incorrectly wired. The negative terminals of electrolytic caps C21, C25, C26, and C27 all connect together and go to the power
transformer's high voltage winding center tap, NOT chassis ground. The positive terminal of C21 connects to chassis ground- many
people find this to be counter-intuitive when they first come across this circuit.


Last edited by lorenz200w on Sep Wed 04, 2019 2:05 am, edited 1 time in total.

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 Post subject: Re: Zenith 1204 Chassis
PostPosted: Sep Tue 03, 2019 11:31 pm 
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Thanks for the information, it is very useful.
Re: the candohm, I will press on with its replacement and will use a 10K for R17, I didn't see that on the parts list, thanks for pointing it out.
Re the electrolytics - the negative poles do go to the a single terminal then to the PT, I hope it is the right return, this includes all the 10 MFD caps (19,20,21) and one goes positive to ground. C27 from the candohm and goes negative to the HT.
There is a non-Zenith can in use here for (I think by process of elimination) C25 and C26, which is also grounded to the HT, but I can't get any readings from them on my Fluke 179 unless I go across the caps, like diamond to square, nothing if I go diamond to ground. I'm waiting for the replacements to come in, I have some at 400V on hand but I read that isn't a proper substitution for a 450V. The previous hand in here used 600V brown El Menco's to replace what they chose to replace.
Its a bit of a puzzle but with eternal vigilance, we will persevere.

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 Post subject: Re: Zenith 1204 Chassis
PostPosted: Sep Fri 27, 2019 4:43 am 
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Hello,
I've finished the replacement of all the resistors, the few paper caps left, replaced all the electrolitics and rebuilt the candohm. The radio came up on the variac to 115 VAC with no smoke or strange smells, pilot light is bright.
It is also silent now on BC with all tubes in, before it made a loud buzz when it warmed up on all bands.
There is no crackling from rotating the volume pot, a few crackles from shifting the tone settings, no stations or other static with a long wire attached to the A terminal.
Placing the DMM probe on any of the negative circuits gives a few pops and crackles, as you'd expect.
All the temperatures seem good under the chassis as measured on my cheapo IR thermometer.
Using a Fluke 178 to read voltages, I get the following:
5Y3 Pin 3 and 5 @ 326 AC, pins 7 and 8 @ 264 DC
Heaters on pin 2 are all at 2.7 VAC
6L7G Det/Conv shows pin 3 at 210 VDC, pin 4 at 83 VDC, pin 5 at -2.7, pin 7 at 3.02

Candohmn replacement shows 211 VDC at the 1st 11k terminal to the rear of the chassis, 86 VDC other side of the next 11k, -35 VDC at the 78ohm and -25 at the final terminal.
Any other measurements I should make, please advise. I have not tried the other bands yet.
Regarding the electrolytics, the wiring to the CT is correct, and C21 + is grounded to chassis.
If any of this looks odd, please let me know.
Thanks,
John

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 Post subject: Re: Zenith 1204 Chassis
PostPosted: Sep Fri 27, 2019 1:57 pm 
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The negative voltage measured at the transformer high voltage center tap node seems to me to be incorrect. It should be closer to -25V than -35V; however I'll admit that I never actually measured it on this radio. What is the actual (measured) value of the resistor that links it with chassis ground? If the resistor is high in value it will reduce the B+ voltage and increase the B- voltage. Also, what power rating did you use for this resistor? It should be rated at least for 2 watts.

This set is a bit odd in that it uses a center-tapped 6.3V secondary to power the heater bus, with the center tap grounded and the two
winding ends routed to the two heater pins respectively on each tube. (Usually there is no center tap and one of the secondary wires is
simply grounded to the chassis, with a single wire being chained to the various tubes; one heater pin being grounded at each tube socket). So, to get the actual value of what each tube is "seeing" heater-voltage-wise, you will need to put your AC voltmeter across pins 2 and 7.


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 Post subject: Re: Zenith 1204 Chassis
PostPosted: Sep Fri 27, 2019 6:29 pm 
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Hi - I reused the exiting 75ohm 3watt wire wound at that location, it tested fine. The others are 50 + 130ohm 3W wire wound and the 10 + 1k 5W ceramics make up the replacement candohm. The measurements were taken at the junctions from rear to front.
It is big and clunky but there is no heat problem and I may upgrade it in the future with a nicer looking setup.

The blue electros are 25mfd 25v for used for C19 -> 21.
I am unsure if I replaced C27 correctly, it was on the candohm as seen at the top pic, I moved it and wired it to what I think is a better setup.

Image

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 Post subject: Re: Zenith 1204 Chassis
PostPosted: Oct Thu 03, 2019 6:34 pm 
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I checked the 75Ω resistor and it was OK, but I replaced it with a 75Ω 5W and redid the wiring. Not much difference on the B-, still running around -40VDC.
The advice above, if I understand it correctly, would be to lower the resistance to decrease the B-, so I've ordered some 50Ω 5W to try that. Can someone confirm this is the right approach? I don't get this part of the puzzle, this is my first chassis to use a CT vs the chassis ground. Where does this B- originate? Seems like it magically appears at the candohm but I'm sure of one thing, that's not the case.
schematic -> https://bit.ly/30HrgH4

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 Post subject: Re: Zenith 1204 Chassis
PostPosted: Oct Thu 03, 2019 8:00 pm 
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I wouldn't try to "tune" the B- voltage by selecting resistor values that deviate from those shown on the schematic.
It is probably possible to do this but the most likely reason that the voltages are out of whack is that B+ bus current draw is
excessive for some reason, and tinkering with the resistances will only mask the real issue.


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 Post subject: Re: Zenith 1204 Chassis
PostPosted: Oct Fri 04, 2019 4:43 am 
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A few more voltage readings, all to chassis ground:

5Y4G - Pin 3 & 5 = 331 VAC - Pin 7 & 8 = 272 VDC (spec 318)

6K7G - RF - Pin 3 = 216 VDC (spec 237) - Pin 4 = 87 (spec 83)
6K7G - IF - Pin 3 = 218 VDC (spec 243) - Pin 4 = 87 (spec 83) - Pin 5 = -1.6 (spec -10)
6V6G - 3rd Audio - Pin 3 = 200 VDC (spec 231)
6V6G - PP Output - Pin 3 = 200 VDC (spec 231)
6J5G - Pin 5:
    Osc = -36 (spec -10.5)
    1st Audio = -35 (spec -0.5)
    2nd Audio = -39 (spec - 2)

Across Pin 2 & 7 = 3.0 VAC on various tubes (spec 3.2).

I'm still tracing the wiring against the schematic, no suspects so far. Rechecked the tubes to see if I'd blown anything, all tested good on the old Jackson 715.

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 Post subject: Re: Zenith 1204 Chassis
PostPosted: Oct Sat 12, 2019 10:24 pm 
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I'm following along a "how to" by John Goller (K9uwa) on the Shutterdial disassembly and bumped into one question. He suggests, when the dial is removed, to free the brass pulley on the tuner and test it for free movement (it is OK), check the stopping tab (OK) and then remove the gears and clean the tuner:
"Spray with your tuner cleaner the wipers and bearings in the tuner and flip it to make sure it all operates nice and easy right now. "

I was thinking of removing the grounding straps and doing a full on immersion cleaning of the tuner with the gears on. I don't see anything wrong with the gear setup so think I'd like leave it alone.

Would that be a good or bad idea?
Edit: I've decided to remove the gears per John's instructions and remove the grounding straps and tuner for cleaning. Any comments still welcome.

Image

Image

*Unsure why the pictures aren't appearing ...


Attachments:
S_Dial_T_Cap.jpg
S_Dial_T_Cap.jpg [ 299.74 KiB | Viewed 647 times ]

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 Post subject: Re: Zenith 1204 Chassis
PostPosted: Nov Thu 14, 2019 5:50 am 
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Another update to this lonely thread ...

Cleaned, lubed, and replaced the tuner gang and shutter dial. Also had a good chance to clean the tone control switch and replaced the resistor and cap there. I replaced the tube cap connector to the RF can, it was frayed badly. The other cap connectors in the tuner gang are good. Removing the grounding straps was easy, getting them bent back into place a bit of a hassle, but they are all secure. The motorized tuning works fine.

Went back to testing voltages, still no buzz or static from the speaker, all quiet when changing bands or rotating the volume pot. I get a small pop from the tone control switch which shows the amp is working as it varies with volume. While testing, I was getting some good buzzes from various pins as I probed.

There is still a high negative B- to the center tap running at -45 VDC and no clue why it is so high. I've been going over the wiring and can't pinpoint the problem, yet.

I was wondering if the high voltage winding were drawing too much via the CT, maybe the power transformer was flaky, so I dug up some old threads on ARF on how to test them.
Between pins 3 and 5 on the 5Y4G, there is 227.5 Ω, pin 3 to the CT is 120 Ω and pin 5 to the CT is 108 Ω. So, it seems to be roughly ½ the total split between pins 3, 5 and the CT, which if what I read it correct, should be right. So, the high voltage CT is OK, I can't tell yet if there is a winding short on one side or not.

Should that eliminate the CT from pulling too much B-?
Back to examining the wiring against the schematic - which burns me out in about 60 minutes.

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 Post subject: Re: Zenith 1204 Chassis
PostPosted: Nov Thu 14, 2019 6:43 am 
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One more observation for tonight - the schematic shows the speaker field coil at 670 Ω. The plug for the speaker, transformer etc has 4 pins, if we're looking from the chassis right-side up, the pair closer together are on top, the pair further apart are on the bottom. The top pair, closer together, are the speaker field coil (?), this pair shows 560 Ω, the bottom pair, which I think is the speaker transformer shows about 200 Ω. Readings were taken on the pins with the plug removed from the socket. Does that 100Ω deficit mean a problem at the speaker field coil? I have no idea what the value for the transformer might be.

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 Post subject: Re: Zenith 1204 Chassis
PostPosted: Nov Thu 14, 2019 9:10 am 
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How much voltage is being dropped across the speaker field coil?


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 Post subject: Re: Zenith 1204 Chassis
PostPosted: Nov Thu 14, 2019 1:03 pm 
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Have you confirmed that your speaker is the original p/n for your model of set? OEM should have a p/n 49-185
A substitute may have been made at some time and was deemed "close enough".

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 Post subject: Re: Zenith 1204 Chassis
PostPosted: Nov Thu 14, 2019 2:51 pm 
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lorenz200w wrote:
How much voltage is being dropped across the speaker field coil?

Here's why that info is of interest here:

You have previously reported (earlier in the thread) that the voltage at the HV winding center tap ("B minus") is -35 VDC. Also,
that the 75 ohm resistor linking the HV CT with chassis ground is indeed 75 ohms.

Given those two pieces of data, we can use Ohm's Law to compute the B minus current (which will be identical to the B+ current): it is
35V/75 Ohms = 0.466 Amps. This is far above the normal B+ current draw of the radio, which should be around 110 mA.

This assumes that the HV CT really goes to ground via a 75 ohm resistor- if there is a miswire and the CT really goes to another resistor,
or the reference resistor value really isn't 75 ohms, all bets are off.

As a sanity check, measure the voltage being dropped across the speaker field coil while the set is in operation.
If it is around 60V, the B+ current would compute to be 108 mA (again, by Ohm's Law for a 560 ohm field coil resistance). This
figure doesn't agree with the above computed value for B+/B-...conclusion: your radio has a miswire or incorrect B- reference
resistor value.

On the other hand, if the voltage being dropped across the field coil is much higher than 60V, the HV CT reference resistor
and its connections are probably OK and you have a severe current drain on the B+ bus somewhere in the radio. If this is
the case, you will need to methodically remove fanout connections from the B+ distribution node and taking measurements
until you find the branch that is causing the overload.

If the B- current draw is really nearly half an amp as computed, your rectifier and possibly the power transformer won't last long.


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 Post subject: Re: Zenith 1204 Chassis
PostPosted: Nov Thu 14, 2019 9:45 pm 
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processhead wrote:
Have you confirmed that your speaker is the original p/n for your model of set? OEM should have a p/n 49-185
A substitute may have been made at some time and was deemed "close enough".



49-185AB - 670 ohms - the white "stuff" on the transformer is a hard, powdery substance, almost like alkaline cell corrosion - this is the original condition.

Lorenz - that's quite a help, I'll dig into it and sort it out - again, thanks for the pointers.


Attachments:
12S267 Speaker.jpg
12S267 Speaker.jpg [ 685.51 KiB | Viewed 388 times ]

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 Post subject: Re: Zenith 1204 Chassis
PostPosted: Nov Fri 15, 2019 7:40 am 
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lorenz200w wrote:
lorenz200w wrote:
How much voltage is being dropped across the speaker field coil?

Here's why that info is of interest here:

You have previously reported (earlier in the thread) that the voltage at the HV winding center tap ("B minus") is -35 VDC. Also,
that the 75 ohm resistor linking the HV CT with chassis ground is indeed 75 ohms.

Given those two pieces of data, we can use Ohm's Law to compute the B minus current (which will be identical to the B+ current): it is
35V/75 Ohms = 0.466 Amps. This is far above the normal B+ current draw of the radio, which should be around 110 mA.

This assumes that the HV CT really goes to ground via a 75 ohm resistor- if there is a miswire and the CT really goes to another resistor,
or the reference resistor value really isn't 75 ohms, all bets are off.

As a sanity check, measure the voltage being dropped across the speaker field coil while the set is in operation.
If it is around 60V, the B+ current would compute to be 108 mA (again, by Ohm's Law for a 560 ohm field coil resistance). This
figure doesn't agree with the above computed value for B+/B-...conclusion: your radio has a miswire or incorrect B- reference
resistor value.


FYI
Reading from pin to ground on either side of the coil:
238 - 189 = 49 VDC across the field coil.
FC resistance measurements tonight with the speaker unplugged = 590 Ω

Still looking for the fault and haven't found it yet.
Cheers,
John

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