Replaced R306 with another 1k ohm reisistor but learned something new and a possible fault. ~tube socket carbon flash or carbon trace?
on my diagram, i disconnected the red wire from T4 2nd IF.
resistance from blue to red was correct.
resistance from red to chassis was 250kohms
resistance from blue to chassis was 250kohms
with the 7h7 V3 removed, the resistances stayed the same.
i moved over to T3 1st IF circuit since its smilar to compare.
i disconnected the red wire from T3 1st IF.
resistance from blue to red was correct.
resistance from red to chassis was in the 3 to 5 Meg ohms
resistance from blue to chassis was 3 to 5 meg ohms
with the 7h7 V2 removed, the resistances stayed high.
I re-terminated all my wires and added another R306 1kohm resistor
I reinstalled V2 7h7 tube
I kept V3 tube socket empty...
I had my volt meter red lead attached to v3 7h7 pin 2... volt meter set to AC volts.
I powered up and heard the audible crinkle of R306 beginning to pass too much current.
I also saw smoke coming out of v3 pin 2 coming out the bottom and out the top.
I had no voltage reading on my meter.
we have (terry and I ) that i have a bad tube socket. something called a Carbon trace or something like that.
Looking now for a replacement Loktal socket or the option of disecting this socket, cleaning it up and re-installing the old pin. dono really yet which way i will go.
(This post was last modified: 04-28-2017, 09:07 AM by jcassity.)
opps.. a tad late
i removed center loktal retainer, lifted out teh insulator, extracted the pin.
I physically saw the spark gap cut into the plastic socket and along the edge on the underside of the insulator.
i sanded down the gap, sanded the insulator.
I sprayed the interior of the socket with varish made for wires & transformers.
I sprayed the center perimeter of the pin with insulating varnish as well.
Problem is now gone on this particular pin.
turned up power again, radio plays on am bands only,, it has Broadcast / AM / Phono only.
I hear static on SW and such,, will figure that out later.
My big problem now is with the updates to this schematics power supply,, the called out R102 was added to allow for a PM speaker.,, its directly off pin 8 of my rectifier tube.
This is the same reisistor that kept burning up on us in the 70's & 80's.,,and was the reason i am working on it now also.
I installed a 300ohm resistor that was about 3/8'' diameter and about 1'' long,, that one instantly got hot and i powered down.
I put 3) of the same physical size but 1k ohm parallel resistors in its place of the same size thinking i would be able to manage the heat (all in at 336 ohms),, no go.,, still got dangerously hot.
I installed this big a** 2'' long and prob 7/16'' diameter 500 ohm Green hollow resistor in in place of prior failures for R102. THis one hangs in there for a good long while but.... it got so hot the solder connection melted letting the resistor fall a bit.
so,, i am still stuck.
ideas?????
should i check my V8 socket to see if i have resistance to ground on one of them? the rectifier tube is not a loktal.
(This post was last modified: 04-28-2017, 05:45 PM by jcassity.)
carbon tracks leave a burn of carbon into the socket,,,,just like in a distributor cap,,,,WON'T go away,,,,gets worse,,,thats all ,,,I can say,,,,CHEERS
It is not surprising that the 300 ohm resistor gets hot. It replaces the field coil of the original speaker. If you look at the original schematic, there was a voltage drop of 75 V across the coil. If you consider the same drop across the 300 ohm resistor, it dissipates over 18 watts under normal operation. This is only an approximation because the actual voltage across the resistor is a rectified sine wave vs. pure DC, but it does give some idea of why the resistor gets so hot.
I would use two 600 ohm 20 Watt resistors in parallel, or use a 300 ohm 50 watt chassis mount heat sinked resistor that looks like this:
yeah, i was searching tonight, found those and others. was looking for a heat sync type.
I asked earlier what the wattage was on this 300ohm resistor and it was not a big deal a few posts ago.
I think i overlooked the idea that no matter what, the wattage was much higher and stands to reason by memory its always been a problem child when free floating as it was from the factory.
I just made a heat sync for a 500ohm resistor and it worked somewhat ok,, but still gets too hot for my liking.
on the loktal socket, take it easy guys, i did not see the post warning, i went out and fixed it.. let me focus on getting a running chassis and put my loktals on order. I wasnt contradicting anyone or doing what i want ignoring your adivce. I was just trying to keep progress going.
i know now i can solve the 300ohm resistor, now i need to figure out why i dont have short wave bands,, speaker has faint noise, thats all.
Believe me, jcassity, we weren't trying to gang up on you. We were just trying to give you fair and valid warning about a problem that will likely return.
"You don't have to agree with me. You have every right to be wrong if you want to be."
- Rev. Lowell Mason, RIP
this kinda works ok,, still too much heat.
This is plumbing tubing slipped up inside a 500ohm resistor then custom formed on the ends to allow heat out.
it sits atop an aluminum wrap round shield to help wick heat to the air and to the chassis.
will have to resort to ordering something that is designed for 50watts or more.
I will likely get my new part and mount it to an aluminum fin block and then the aluminum block will mount in the same location as my fabricated 300ohm design shown in the picture.
Whatever i end up doing, this resistor is certainly not going inside the bottom of my chassis. it will be exterior mounted.
Radio works,, except for shortwave,, looking into that now,, just getting a hummmm at my temporary speaker.
(This post was last modified: 04-29-2017, 01:58 PM by jcassity.)
on thing i forgot to point out and not many people know this,, its an avionics trick so to speak.
if you put a white dab of paint on your components, when they fail the failed part will visually tell you so.
the white paint goes to dark brown.
just a trick for anyone interested. its nice to start looking around for your indicator marks and "see" which part failed during your visual inpection.
yet im not an expert nor trying or wanting to make mistakes , this is my second radio repair attempt. let me know if this is a bad idea for tube radios or not.
(This post was last modified: 04-29-2017, 02:19 PM by jcassity.)
I think you still have something wrong in your radio as there are lots of similar radios out there that use a resistor or two in the power supply and they are rarely more than a couple watt units and they work just fine. You can be absolutely sure that Philco did not install a 20 watt resistor in this radio originally. Have you done a current measurement through this resistor to see what the total set current is? If not you should as something does not seem right to me.
