Radio Newbie in need of some guidance for an 84 - Printable Version
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RE: Radio Newbie in need of some guidance for an 84 - morzh - 01-11-2015
One way is having another radio with the same tubes...or buying a good known one. Short of that.....do you have a fellow collector you know nearby! Or a radioclub?
RE: Radio Newbie in need of some guidance for an 84 - rciancia - 01-11-2015
This is a really strange problem. The 10UF E.L Cap continues to get VERY HOT (cannot touch it hot) at power on and eventually blows. It takes about 15 seconds to blow, even with a new 80Rectifier tube. Since the symptom is blowing E.L. Cap and no Audio output, I am going to start poking around the 42Tube(Output). My challenge with checking voltages is I do not know what they should be at the plate for the 77's and 42.
RE: Radio Newbie in need of some guidance for an 84 - Steve Davis - 01-11-2015
It sounds very much like the polarity is reversed on the capacitor.
RE: Radio Newbie in need of some guidance for an 84 - rciancia - 01-11-2015
I have the NEGATIVE side of the 10uf going to the Center Tap and the positive side going to the Speaker Wire and Pin 1 from the Rectifier Tube.
I have the NEGATIVE side of the 4uf connected to the 10uf Negative and to the same Center Tap connection. The positive side goes to the speaker other various places around the chassis.
I view the Negative Side with the one with the Grey Stripe and this goes to the center tap for both caps
RE: Radio Newbie in need of some guidance for an 84 - sam - 01-11-2015
hi rciancia
I don`t know if you have schematic but here is from n/a
http://www.nostalgiaair.org/PagesByModel/875/M0013875.pdf
sounds like a dead short
you said you rewired, recheck your work maybe misplaced wire
sam
RE: Radio Newbie in need of some guidance for an 84 - rciancia - 01-11-2015
I do have the same schematic, thank you for posting it here again. I have gone over it already several times, but will do so again. I purchased another of the same radio ( for parts) at a great price and when it arrives will do a stare and compare as well to see if I did something wrong.
RE: Radio Newbie in need of some guidance for an 84 - morzh - 01-11-2015
1. Could it be your "new" 80 tube is also bad?
2. What kind of cap is it? Can you how a photo?
3. A you sure what you call centertap is indeed the centertap? Was it messed with? Let's put it in a simple form: when a good rect tube connected propely to a transformer and a properly rated cap the cap should not blow.
RE: Radio Newbie in need of some guidance for an 84 - rciancia - 01-11-2015
I just discovered that I mixed up the 490K and 240K resistor (#22 and 23 on the schematic).. had them reversed. Could that be the cause ? Here is a photo of the caps I am using and of my center tap (Yellow and green)
https://pbs.twimg.com/media/B7Hc-84IEAAYXSx.jpg
https://pbs.twimg.com/media/B7HdEYqIgAAp9oV.jpg
I am also ashamed to say I had a 500ohm resistor instead of a 500K. Newbie mistake.
Will replace them tomorrow. Do you think this could be the cause ?????
This picture explains where the error was..
https://pbs.twimg.com/media/B7HgNUfIcAAxTE-.jpg
RE: Radio Newbie in need of some guidance for an 84 - jerryhawthorne - 01-11-2015
If the 500K you referenced with the mix up between #23 and #24 above was the 500 ohm you mentioned at the end of your post, I doubt that will solve the smoking cap problem but will no doubt cause problems as we get by the major hurtle. As for "measuring voltages" no, not at this point. No power to the set until the magic smoke problem is solved. For that you need to measure resistances. For example from the + wire on the 10mfd to centertap. You don't need to have the cap in for this, in fact, better not to. We are looking for a short on the B+ line.
As Morzh said, there is not much involved at this point. New good caps blow like that generally due to wrong polarity, bad rectifier or miss wire getting a lot of A/C on them. Let us know what you get for resistance on the above test and we will then try something else.
Jerry
RE: Radio Newbie in need of some guidance for an 84 - morzh - 01-12-2015
(01-11-2015, 10:39 PM)rciancia Wrote: I am also ashamed to say I had a 500ohm resistor instead of a 500K. Newbie mistake.This is not likely to cause smoking either.
Will replace them tomorrow. Do you think this could be the cause ?????
This picture explains where the error was..
https://pbs.twimg.com/media/B7HgNUfIcAAxTE-.jpg
(01-11-2015, 10:39 PM)rciancia Wrote: I just discovered that I mixed up the 490K and 240K resistor (#22 and 23 on the schematic).. had them reversed. Could that be the cause ? Here is a photo of the caps I am using and of my center tap (Yellow and green)
No this will not cause any smoking.
Now. Back to smoking.
I thought you said you used 630V caps for the lytics.
Your pic shows 450V caps.
If your tubes do not conduct and the voltage stays at the 500V level, that can blow a cap....in theory.
Have you measured the voltage across the cap when it was heating?
RE: Radio Newbie in need of some guidance for an 84 - codefox1 - 01-12-2015
Don't think anyone asked, but where did you get your replacement electrolytic capacitors. I haven't seen recent manufacture 8 mfd types, and decades old production even if never used is very likely to be bad even if reforming is attempted. Anyway 10 mfd 450 volt electrolytics are plentiful, inexpensive, and perfectly OK for this set.
RE: Radio Newbie in need of some guidance for an 84 - rciancia - 01-12-2015
Hi, I originally started with 10uf 650V and they blew as well, so on to the 450v.
I have not measured voltage across the cap as I don't want to power it on again until I find the source.
I did remove the two resistors( 250k,500ohm) from the circuit before I measured and have not replaced them yet
As suggested, I measured resistance in the following places and here are results.
Filament of 80 (6&7)--->CenterTap of 9&10 = 0
Center Tap of 9&10 (-
---> Chassis = 307kPlate of 80 9&10 --->Chassis = 521 and 506k
Center Tap of (3&4) (5) --->Chassis = 0
Filament of 80 (6&7)----Chassis = 0 ( 10uf EL cap attaches here)
Filament (3 & 4) ---Chassis = 3 & 6k
Speaker
Green to White = 0.441k
Green to Stripe = 1.600k
White to Stripe = 1.160k
Power Transformer Data Below
https://pbs.twimg.com/media/B7JWb9wIMAADTRY.jpg:large
RE: Radio Newbie in need of some guidance for an 84 - Eliot Ness - 01-12-2015
(01-12-2015, 07:43 AM)rciancia Wrote: As suggested, I measured resistance in the following places and here are results.
Filament of 80 (6&7)--->CenterTap of 9&10 = 0
Center Tap of 9&10 (-
Plate of 80 9&10 --->Chassis = 521 and 506k
Center Tap of (3&4) (5) --->Chassis = 0
Filament of 80 (6&7)----Chassis = 0 ( 10uf EL cap attaches here)
Filament (3 & 4) ---Chassis = 3 & 6k
Center tap 8 should read ~ 325 ohms to gnd (through resistor 30). But the bigger issue is 0 ohms from the filament taps (6&7) to #8. Unless I'm overlooking something you need to figure out what is shorted here. I'd start by taking good notes/pictures and then disconnecting things to see what is shorted or mis-wired.
Do this with the 80 removed and I would probable disconnect the power transformer also so I could check for shorts. I suspect something has been mis-wired, but you'll have to double check all wiring against the schematic to verify.
RE: Radio Newbie in need of some guidance for an 84 - rciancia - 01-12-2015
I will unsolder the filament leads from the 80 tube socket and see if i get a reading to the center tap. That should tell me if the socket is somehow grounded on those leads.. will report back
RE: Radio Newbie in need of some guidance for an 84 - rciancia - 01-12-2015
Unsoldered 6&7 from Filament Socket and the Center Tap(-
When placing an Ohm meter on 6 & 8 - 0
When placing an Ohm meter on 7 & 8 - 0
When placing an Ohm meter on 6 & 7 - 0.4-0.5
PRAYING I do not have a bad transformer. thoughts ?