smoke escaping from resistors - Printable Version
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RE: smoke escaping from resistors - morzh - 03-18-2012
Arran
I thought you suggested the smoking occurs due to the field coil being disconnected.
If it is connected, anything can be the reason, and I thought of RF 78 tube shorts.
Part #19 short (if the field coil IS connected) would require also some short in the Det. oscillator tube, but even this will not reault in #7 smoking as it is serialized with some 50Kohms resistance and the drop on it (again, if #19 is at fault) would be so small (0.5% of the total voltage), it is far from causing anything like 200 Ohms resistor (also probably 1/2W) to smoke.
If #61/62 do smoke, this indeed may be (potentially) due to the field coil missing; #7 - unlikely.
I would say, if #7 smokes with the field coil removed, it will smoke with the coil in place too.
I would look for shorts in wiring - the schematic does not show any path that can do it.
RE: smoke escaping from resistors - raydog - 03-18-2012
OK finally an update. Had a chace to power up with the field coil plugged in, no more smoke. It appears that this was causing the load on #7. However no sound or hum. Now that I can power up and leave the power on, I will check voltages and it looks like I have a tube or two that may be questionable. Now all I need is some time.
RE: smoke escaping from resistors - Arran - 03-19-2012
(03-18-2012, 11:20 AM)morzh Wrote: Arran
I thought you suggested the smoking occurs due to the field coil being disconnected.
If it is connected, anything can be the reason, and I thought of RF 78 tube shorts.
Part #19 short (if the field coil IS connected) would require also some short in the Det. oscillator tube, but even this will not reault in #7 smoking as it is serialized with some 50Kohms resistance and the drop on it (again, if #19 is at fault) would be so small (0.5% of the total voltage), it is far from causing anything like 200 Ohms resistor (also probably 1/2W) to smoke.
If #61/62 do smoke, this indeed may be (potentially) due to the field coil missing; #7 - unlikely.
I would say, if #7 smokes with the field coil removed, it will smoke with the coil in place too.
I would look for shorts in wiring - the schematic does not show any path that can do it.
Well then you explain it? You are shooting down one of my theories, which is little more then a guess anyhow, but you haven't come up with any other explaination that hasn't been though of, as to why part #7, the 200 ohm resistor, is burning up. I did also mention that there may be a wiring error or a stray blob of solder etc. that could cause it. Part number #19 is directly in the path of the B+ voltage along with part #21, part #22, and part #14, there is no other path for the B+ voltage to get to part #7 as part #7 is in the cathode circuit of the 78 and 6A7 tubes.
With no load on the B+ line in that branch of the B+ circuit the voltage will be high and behave like the resistors #21 and #22 weren't even there and possibly cause the dielectric of part #19 to break down and short out. Again short of a wiring error there is no other direct path to that 200 ohm resistor. Another possibility is that the field coil being missing, somehow caused the cathode current to increase, but that doesn't make much sense since the only way that could happen is if the plate and screen voltages were there and the grid bias voltages were not. But when it comes to antique radios, the age of the components and tampering by former service persons means they sometimes do whacky things that are not easy to explain.
Regards
Arran
RE: smoke escaping from resistors - morzh - 03-19-2012
(03-19-2012, 12:29 AM)Arran Wrote:(03-18-2012, 11:20 AM)morzh Wrote: Arran
I thought you suggested the smoking occurs due to the field coil being disconnected.
If it is connected, anything can be the reason, and I thought of RF 78 tube shorts.
Part #19 short (if the field coil IS connected) would require also some short in the Det. oscillator tube, but even this will not reault in #7 smoking as it is serialized with some 50Kohms resistance and the drop on it (again, if #19 is at fault) would be so small (0.5% of the total voltage), it is far from causing anything like 200 Ohms resistor (also probably 1/2W) to smoke.
If #61/62 do smoke, this indeed may be (potentially) due to the field coil missing; #7 - unlikely.
I would say, if #7 smokes with the field coil removed, it will smoke with the coil in place too.
I would look for shorts in wiring - the schematic does not show any path that can do it.
Well then you explain it? You are shooting down one of my theories, which is little more then a guess anyhow, but you haven't come up with any other explaination that hasn't been though of, as to why part #7, the 200 ohm resistor, is burning up. I did also mention that there may be a wiring error or a stray blob of solder etc. that could cause it. Part number #19 is directly in the path of the B+ voltage along with part #21, part #22, and part #14, there is no other path for the B+ voltage to get to part #7 as part #7 is in the cathode circuit of the 78 and 6A7 tubes.
With no load on the B+ line in that branch of the B+ circuit the voltage will be high and behave like the resistors #21 and #22 weren't even there and possibly cause the dielectric of part #19 to break down and short out. Again short of a wiring error there is no other direct path to that 200 ohm resistor. Another possibility is that the field coil being missing, somehow caused the cathode current to increase, but that doesn't make much sense since the only way that could happen is if the plate and screen voltages were there and the grid bias voltages were not. But when it comes to antique radios, the age of the components and tampering by former service persons means they sometimes do whacky things that are not easy to explain.
Regards
Arran
Arran,
I was saying something before (mentioning shorts in wiring) that completely agrees with your notion of wiring error: it is, like you said, either wiring (error/short due to old wiring) or solder blob.
But if you look at the sch, if the wiring is intact, you yourself can see that there's no way in heck the absence of the field coil can create the smoking of #7, unless there are pre-existing conditions (just mentioned by you and me). A tube short is possible also, but first there have to be other shorts, so a shorted tube can create this if the plate power is absent.
I mean no disrespect.
Mike.
RE: smoke escaping from resistors - raydog - 03-20-2012
What about the antenna tranformer not being plugged into the chassis? Would that solve the mystery with #7 burning?
RE: smoke escaping from resistors - morzh - 03-20-2012
(03-20-2012, 01:06 PM)raydog Wrote: What about the antenna tranformer not being plugged into the chassis? Would that solve the mystery with #7 burning?
By the Antenna transformer do you mean #3? (it is easier if you refer by position numbers).
No, that won't burn anything.
1. Here's the thing: all the tubes seem to connect through what essentially is a piece of wire (coils) for the DC current to the Plate voltage. So, yes, make the bias right and your resistor will smoke.
For this one to smoke quickly you will need probably several watts (don't know the rating), with 1 Watt being dissipated at about 70mA of current.
The problem is, you've disconnected the field coil. Without this coil there is (if we assume everything is intact, no shorts etc) NO power whatsoever to burn anything near #7.
This is why you can do whatever with your grids' biases - it won't hurt.
2. Now, if it smokes, there are probably shorts.
3. The single DC path to the RF 78 tube with the Field Coil disconnected is #55 resistor (32Kohms). If we are to assume it is intact, and IS 32Kohms, there is not enough current to burn anything, as it is about 10-12mA tops without any loads.
If however it is burned in the "low-resistance" mode, there may be more.
But: you have to have the tube IN the socket and it has to be shorted itself.
With the tube pulled out this path does not exist either.
4. You need to take your meter, disconnect the parts that smoke and start measuring, seeing where the voltage comes from.
Does it still smoke without RF 78 tube in place? WHat are the plate voltages? etc etc.
The usual approach to smoking situation is: follow the current path. Find the power source.
And, of course - be careful and safe.