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 Post subject: B+ partially missing Zenith 8S463
PostPosted: Nov Mon 27, 2017 10:25 pm 
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I've been working on a Zenith 8S463and ran into an issue that is likely caused by something simple but evades me. The issue is that while the other electrolytics work fine the one that I believe is mentioned as C24 has no B+ at all. I noticed that the caps on either side of the field coil connect to a multi-section Candohm resistor. I measured the sections and it seemed ok.
The only other thing is that someone in the past had soldered in a 110 volt line and plug for a phono motor, a RCA jack and some other jack that I was unable to identify but connected between the Television ( Aux input ) and the radio/TV setting switch on the back. I removed all of that and I somehow wonder that perhaps one of these additions somehow tied into a part of the set's B+, which would make no sense to me but who knows. Below is a snap shot of the problem area...

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 Post subject: Re: B+ partially missing Zenith 8S463
PostPosted: Nov Mon 27, 2017 10:36 pm 
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Are we to assume that the speaker field coil is connected and measures approximately 1250 Ohms?

Curtis Eickerman

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 Post subject: Re: B+ partially missing Zenith 8S463
PostPosted: Nov Mon 27, 2017 10:37 pm 
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Sounds like you've got B+ before the field coil, but not after. Assuming that you're measuring with respect to chassis (and not the - end of these capacitors), then the problem has to be either an open field coil, or a short on the B+ branches going out from C24. You can check the field coil resistance easy enough.

If the field coil is OK, and you really read near zero volts on C24, that would indicate a very low resistance short, so you can proceed with just an ohmmeter measuring from the + end of C24 to chassis. Start unhooking things from the + end of C24, and figure out which branch of the circuit has the low resistance short to ground.

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 Post subject: Re: B+ partially missing Zenith 8S463
PostPosted: Nov Mon 27, 2017 10:45 pm 
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I think we need to see more psu circuit, I see c24 connected to a wire going to a cathode and that field coil is in a back bias system. I doubt there will be any voltage across the cap if the back bias is open.

Marc


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 Post subject: Re: B+ partially missing Zenith 8S463
PostPosted: Nov Mon 27, 2017 11:42 pm 
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Here is more (but not all) of the schematic.


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 Post subject: Re: B+ partially missing Zenith 8S463
PostPosted: Nov Mon 27, 2017 11:52 pm 
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The field coil tests at around 1150 so thus probably good enough. I'm slowly working my way through leads. Not really finding anything out of the ordinary so far


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 Post subject: Re: B+ partially missing Zenith 8S463
PostPosted: Nov Tue 28, 2017 12:04 am 
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The 6V6, pin #3, has a cap to ground. If shorted B+ would be very low. If actually zero volts and the field coil is good one of those resistors shown opposite the field may be open?

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 Post subject: Re: B+ partially missing Zenith 8S463
PostPosted: Nov Tue 28, 2017 12:27 am 
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Much as I suspected. C22 & C23 are on a floating CT. CT to chassis should be the the g1 bias of the OP tube, chassis positive, relative to CT.

I think what I would like to see are voltages:

B+ reading CT to chassis; Should be the grid bias of OP tube.
CT to cathode 6X5; plus cathode to chassis (pin8)
Pin 4 6F6 (screen) relative to chassis;
Voltage pin 5 6F6 relative to chassis;

If there is B+ voltage on pin 6 there is a problem with the wiring of that cap or its a dry joint B+ side: No Voltage, but voltage (may be 300v) at Cathode 6X5, wiring error of the field is actually open misswired or a dry joint. I would expect a fully shorted cap to drag the whole B+ down. Some do part fail & conduct hyperbolically as the voltage rises. If the back bias goes open, most of the grids & cathodes will approach B+ voltage.

Does this still have Wax Paper caps? The plate bypass is the most common fail: But get rid of all of them. That one is preferably no lower than 630V preferably more.

Marc


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 Post subject: Re: B+ partially missing Zenith 8S463
PostPosted: Nov Tue 28, 2017 12:47 am 
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Nostalgia Schematic http://www.nostalgiaair.org/pagesbymodel/455/M0025455.pdf

Bob do some Ohm Reading for us. Speaker plugged into the chassis. radio unplugged from the wall.

#1 Read Ohms from pin8 of the 6X5 socket to Chassis ground should read about 40,000 ohms. Yes the meter will jump around for a while. Connect the red lead to pin8 of 6X5 and black lead of meter to chassis with clip leads and wait until the meter settles down.

All the following tests leave the red lead on pin8 of the 6X5 socket.

#2 Plus end of C23 ohms should read zero
#3 ohms to speaker socket One of them should read zero ohms
#4 one of the other speaker socket lugs should read 1150 ohms
#5 Plus end of C24 should read 1150 ohms
#6 pin4 of the 6F6 tube should read 1150 ohms
#7 pin3 of the 6F6 tube should read 1600 ohms

report your readings back here so we can figure out what you have wired wrong.

John k9uwa

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 Post subject: Re: B+ partially missing Zenith 8S463
PostPosted: Nov Tue 28, 2017 1:27 am 
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Arg. So a bit more info... I replaced all of the caps yesterday thus they're all brand new. So I did some measuring and here is what I found:

for 6F6:

pin 1: 0
pin 2: zero
pin 3: 1.3
pin 4: -44 ( supposed to be 220~!)
pin 5- nadda

k9uwa,
Here is what I got as far as readings:
pin 8 of 6X5 to chassis: 1200k

Pos of C23: 0.3 ohms ( basically zero)

One speaker lead: 0
Other one: 1200
Pin 4 of 6F6: 1226
pin 3 of 6F6: 1600


Just because a pic is a 1000 words I took some of the chassis:
Image

Tube socket to the right is the 6F6 tube
Image


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 Post subject: Re: B+ partially missing Zenith 8S463
PostPosted: Nov Tue 28, 2017 1:44 am 
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bobwilson1977 wrote:
Arg. So a bit more info... I replaced all of the caps yesterday thus they're all brand new. So I did some measuring and here is what I found:

for 6F6:

pin 1: 0
pin 2: zero
pin 3: 1.3
pin 4: -44 ( supposed to be 220~!)
pin 5- nadda

k9uwa,
Here is what I got as far as readings:
pin 8 of 6X5 to chassis: 1200k

Pos of C23: 0.3 ohms ( basically zero)

One speaker lead: 0
Other one: 1200
Pin 4 of 6F6: 1226
pin 3 of 6F6: 1600


Bob check ohms from pin3 to chassis and pin4 to chassis on the 6F6 socket. The other ohm readings you gave are correct. You have to have some short to chassis someplace to make that voltage all disappear from pin3 and 4 of the 6F6. Another possibilty that C24 litic the top one in your picture is electrically shorted or internally built backwards? Perhaps you have another litic 22mfd or 10 mfd whatever you have with a 450V rating? remove one end or the other of your C24 top one in your pictures and gator clip lead a different one into position. Then fire it up and read voltages again. An internal short in the 6F6 tube might cause it?
John k9uwa

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 Post subject: Re: B+ partially missing Zenith 8S463
PostPosted: Nov Tue 28, 2017 2:24 am 
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Expect cap C13 from pin #3 (plate) to ground is shorted. (very common) If not something else on this pin is shorted to ground.

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 Post subject: Re: B+ partially missing Zenith 8S463
PostPosted: Nov Tue 28, 2017 3:45 am 
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I'm going to have to delve into this further. I didn't find that C13 was shorted. Obviously a short somewhere in there...
I too thought maybe the lytic in question might be defective but it too tests fine.
I just know this is something stupid simple that I'm overlooking


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 Post subject: Re: B+ partially missing Zenith 8S463
PostPosted: Nov Tue 28, 2017 4:22 am 
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bobwilson1977 wrote:
I'm going to have to delve into this further. I didn't find that C13 was shorted. Obviously a short somewhere in there...
I too thought maybe the lytic in question might be defective but it too tests fine.
I just know this is something stupid simple that I'm overlooking


Start taking wires, parts and tubes away until the short disappears. That B+ from plus end of C24 and pin4 6f6 also goes other places in the circuitry.
John k9uwa

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 Post subject: Re: B+ partially missing Zenith 8S463
PostPosted: Nov Tue 28, 2017 5:23 am 
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If this is one of the models that has the plate lead to one of the IF or RF tubes inside a spring metal shield, the blue rubber insulated wire inside the shield is notorious for creating a dead short from B+ to chassis. That's the first place to look............I have had many of them with that problem, sometimes a small 1K 1/2 watt resistor associated with the B+ feed to the IF transformer has been so hot the resistance is down to almost zero as well, creating a nearly dead short from B+ to chassis.

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 Post subject: Re: B+ partially missing Zenith 8S463
PostPosted: Nov Tue 28, 2017 6:06 am 
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The resistance to ground is close to that of the field. So we could have a B+ short load side. I would however be tempted to remove 6X5 & re-measure ohms to ensure no heater cathode issue (which they are notorious for) Ohms with Ecaps connected can be wrong, but. pull the OP tube as well. If its not there is cannot short.

Any sort of debris, oversized solder blobs and strands of wire sticking out can cause shorts. I do not like 450v caps (too many failures).

Systematic removal of B+ wires & Ecaps along the string is probably called for if its not obvious. I have seen the tops of top trimmer IF cans staved, in & short to the primary. That method will get it.

Marc


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 Post subject: Re: B+ partially missing Zenith 8S463
PostPosted: Nov Tue 28, 2017 5:41 pm 
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Quote:
If this is one of the models that has the plate lead to one of the IF or RF tubes inside a spring metal shield, the blue rubber insulated wire inside the shield is notorious for creating a dead short from B+ to chassis.


It totally is one of those models because I specifically recall seeing a blue rubber coated wire going through a spring type thing into one of the RF cans. I try not to disturb that kind of insulation if it looks intact but perhaps that's worth a look.
Quote:
The resistance to ground is close to that of the field. So we could have a B+ short load side. I would however be tempted to remove 6X5 & re-measure ohms to ensure no heater cathode issue (which they are notorious for) Ohms with Ecaps connected can be wrong, but. pull the OP tube as well. If its not there is cannot short.

Any sort of debris, oversized solder blobs and strands of wire sticking out can cause shorts. I do not like 450v caps (too many failures).

Systematic removal of B+ wires & Ecaps along the string is probably called for if its not obvious. I have seen the tops of top trimmer IF cans staved, in & short to the primary. That method will get it.


Thanks for the tips. I did try another 6X5 yesterday as the one in it tested rather weak. But your idea of removing it as well as the 6F6 seems like a good idea. I'm not around the set today but I'll look into some more "experimentation" tonight.


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 Post subject: Re: B+ partially missing Zenith 8S463
PostPosted: Nov Wed 29, 2017 12:02 am 
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I have circled a couple of things.

A: I do not like wire sticking out like that: Not the best look & in some places can result in shorting.

B: This is the sort of terminal (node) where solder can run downwards & cause a short. Other than solder wick, a soldering iron, applied appropriately, will remove excess solder.

C: Rubber wire doing what rubber wire of that era does, with age: This will eventually crumble and I have seen several shorts in shielded wire, where rubber covered wire is the core. Often you can re-core it.

B+ dead shorts, are in my book the easiest to find, and uncontrolled powering to find it a recipe for disaster. Do trace out the direct B+ rail to all its nodes before all else, even put a dob of paint (marker) on the nodes its on. You may even see the short as you do (wiring error). That will then show where the B+ needs to be disconnected to narrow down the area. With a dead short (as noted in a recent post) I will often use a battery & flying lead lamp. You can start at the mixer end and disconnect the rail until the lamp extinguishes. Which would put the fault closer to the rectifier. In a lot of cases the main take off point for B+ is the screen of the OP tube, or the speaker socket: A good point to start disconnecting.

Once I have sorted it, the double check is to power B+ only from the bench power supply. That will catch out Ecaps (if connected) that like to leak badly at a specific voltage & that can be confirmed with the reformer. Good methodology will get it.

Marc


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 Post subject: Re: B+ partially missing Zenith 8S463
PostPosted: Dec Fri 01, 2017 11:03 pm 
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Ok guys,
So I've made progress but still have some issues. As some of you pointed out the spring enclosed wires were a problem. That was the cause of the short. But now I have another issue and its odd. So when I turn the set on I will get full B+ everywhere. But after the set has been on for say- a minute or so I will get a drop of around 15-20 volts on B+ and its chassis-wide, as in I can then measure AC off the power transformer and it too will drop, but with the speaker disconnected there is no drop from the transformer. I thought maybe there is some odd internal short in the field coil but even with another speaker I found at the museum that is good... same problem. I am sort of scratching my head over this one


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 Post subject: Re: B+ partially missing Zenith 8S463
PostPosted: Dec Fri 01, 2017 11:36 pm 
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That sounds pretty normal to me. When the tubes warm up, the set starts drawing B+ current, which causes the B+ to drop a little. 20 volts would be very typical -- could even be more.

When you disconnect the speaker, it has the effect of completely disconnecting all B+ power from the entire set, because the B+ flows through the field coil. As a result, your B+ voltage comes back up, because no B+ current is being drawn.

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