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 Trapped in cyberspace

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marcc

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PostSubject: Trapped in cyberspace   Tue Oct 12, 2010 4:45 pm

Gearhead222: Re circuit for intermittent radio. My last comment before, was check AGC.

I did not get a chance to get circuit before I was presented with "error 404". If you can organize, with the moderators, we may be able to exchange info direct?

Marc
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gearhead222



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PostSubject: Re: Trapped in cyberspace   Tue Oct 12, 2010 8:26 pm

Dear Marcc-Thanx for hanging with me! Here's the Talisman 308-U schematic from Phil Nelsons website (thanx Phil!):

http://www.antiqueradio.org/art/tesla308uschematic.jpg



While measuring the dropping resistor, I noticed that the 700 ohm section measures about 250 ohms and the 200 ohm section measures 200 ohms, so I ordered a 700 ohm 8 W and a 200 ohm 5 watt resistor as replacements from Mouser. While in error (low volume) mode, the plate voltages for the 2 UCH21 tubes were 94 instead of 145 VDC and the UBL21 audio tube was 110 VDC instead of 176 VDC, although I don't understand how a lower dropping resistor value would cause the plate voltages to drop. Still, this explains the low volume!!I suspect this dropping resistor
also since the radio's performance has been related to heat-the hotter the radio got the more stable the volume became.
Anyway, will keep you all posted on this radio, as it has been the radio from HELL! Take care all:)-Gearhead
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marcc

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PostSubject: Re: Trapped in cyberspace   Wed Oct 13, 2010 3:28 am

Must be the season for that sort of burnout. I have a Philips (transformer type) on the bench that gave me a surprise. I was replacing the rotten rubber, that should have been done by the character that changed the caps. The 2500R back bias resistor had shed its jacket underneath and half of it was shorted out. More than half of the resistors have failed. Apparently its volume is down a bit and I know why.

I have not looked at the circuit in detail. I note delayed AGC. Check R15 & R18 for some reason I see a lot of them in that position fail?

The failure of C46 would send positive voltage into the AGC, which the UCH21's would not appreciate on their heptode control grids.

Z1 & Z2 appear to be globes, by the symbol? If so these would likely act as Barretters in an overload situation, cutting the current as they heated. That's probably why, no fuse.

I would suggest in overload C20 in particular may get damaged.

I would make sure that the 220V / 120V plug / link is in the right way? There is the posibility that at some stage the 120V was in and 220V was applied.

Marc
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gearhead222



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PostSubject: Re: Trapped in cyberspace   Wed Oct 13, 2010 12:25 pm

Dear Marcc-Appreciate the quick response. FWIW, Z1 and Z2 are the 6.3 volt Pilot/ dial light bulbs. The 120/220 volt plug has always been in the correct position and there's no way fro me to apply 220 VAC anyway here in the states:). ALL of the caps and resistors were replaced and the radio was still exhibiting the same sporadic volume. The speaker ohmed out at 4.2 ohms and the audio xformer ohmed out at 392 and 0.9 ohms. My Lockhart Elmer told me how the windings can short out on these dropping resistors, thereby lowering the resistance.Thank GOD, MOUSER was open yesterday and I was able to order a 200 ohm @ 5 W/460 Volts resistor and a 700 ohms @ 8 W/ 250 volts resistor! Will unwind the 200 ohm coil in the dropping resistor and just install both new resistors and energize with my Variac. This radio should be like new by the time I repair her!Wink-Gearhead
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marcc

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PostSubject: Re: Trapped in cyberspace   Wed Oct 13, 2010 12:47 pm

If you replace everything there is a better than even chance you will get it.

As said, it looks like the globes can act as baretters as they are in series with the load. That also raises the interesting point of their current rating. if they were replaced with a lower current draw globe than spec. they would cut the voltage.

See what happens when you sort R1 & R2 as they are both in the heater circuits at 120V.

Marc
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PostSubject: Re: Trapped in cyberspace   Wed Oct 13, 2010 5:41 pm

Dear Marcc-Did a neat job installing the replacement 200 and 700 ohm resistors and NO CHANGE! Arrggggggghhhhhhhh!!!!!! Is it possible that the speaker is ohming out good but is bad? That's my only other idea right now! Going to the gym to try to forget this!Sad-Gearhead
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marcc

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PostSubject: Re: Trapped in cyberspace   Wed Oct 13, 2010 7:30 pm

Facinating little swine. I have had radio's that have taken weeks to sort out. You do need to walk away sort out another & get back to them. It is clearly something obscure although the voltage falling has to be something dragging it down. As said new parts can fail. and the most likely are those with B+ on them.

I would actually wonder if one of the RF stages is actually oscillating generating enough voltage to shut down the AGC. You may have to put a signal into the audio & see if it changes there. Also look at the audio transformer, preferrably with an insulation tester with both OPT primary leads lifted. This to see if it has conduction to ground.

Marc



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PostSubject: Re: Trapped in cyberspace   Thu Oct 14, 2010 11:53 am

Dear Marcc-Just ordered a new audio xformer from Radio Daze, along with a new 8 ohm 4 " speaker to reach the $10.00 min. order. Will keep all of you updated!Smile-Gearhead
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PostSubject: Re: Trapped in cyberspace   Thu Oct 14, 2010 10:51 pm

I hope this was not a rash decision? I do a bit of electrical test work & as I have said several times, can justify a "Megger" / Insulation tester.

Highly useful for testing "earth leakage", non polarised caps in its operating range and cleaning metal hairs & burrs off of tuning gangs.

Marc
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PostSubject: Re: Trapped in cyberspace   Fri Oct 15, 2010 6:14 am

Dear Marcc-Considering all the work that I have performed on this radio, I don't really have too many choices left!-Gearhead
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PostSubject: Re: Trapped in cyberspace   Fri Oct 15, 2010 6:51 pm

I would agree that you are running out of non replaced bits. The idea of insulation testing the transformer was to see if it was possibly OK or flashing over to its frame?

One of the obscure things that can happen, which I have seen more than one instance of , is that something on the secondary side of the output transformer goes open. The transformer then starts "ringing" and generates EHT. That then causes a flashover between the screen & plate pins if they are side by side (minatures) and could easily wipe out C41 by punching it through, or damage the insulation of the windings.

Marc
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PostSubject: Re: Trapped in cyberspace   Mon Oct 18, 2010 7:01 pm

Dear Marcc and Fellow Radioheads-This is only the second radio that has ever flummoxed me. I substituted a new audio transformer with the original loudspeaker and then subbed the new audio transformer with a new speaker-NO IMPROVEMENT! I honestly don't know what direction to go with this radio, other than out to the gun range:(-Gearhead


UPDATE-Although I don't want to go in there, I am wondering about replacing the 130 pF caps that are in parallel with each coil in the IF transformers. Has anyone ever operated on one of these Tesla IF Cans?
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marcc

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PostSubject: Re: Trapped in cyberspace   Mon Oct 18, 2010 7:44 pm

Only from what I read, the American Siver Mica's can be an issue.

Again anything Micamould is an issue.

I rarely change Mica's here. If you chart that course get as close as you can to the 130pF even if that involves a 100pf & 33pF in parallel. If you are going to dismantle the coils check for mechanical problems, as I have seen two, (in several hundred radio's) where there has been breakage. IF can caps disconnecting (going open), actually, in a superhet would throw that can off frequency: This would cause volume loss. Depending where it is, an open coil would kill it, as often they feed elements like plates.

Marc
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PostSubject: Re: Trapped in cyberspace   Mon Oct 18, 2010 9:31 pm

Dear Marcc-That's what's do wierd-If I had problems with the IF can, you'd think that I'd either have loud volume, no volume, or intermittent volume with static. All I have is a steady low volume ceiling now, no intermittent loud and sofr volume ceiling-strange!-Gearhead
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marcc

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PostSubject: Re: Trapped in cyberspace   Tue Oct 19, 2010 3:22 am

If its gone permanently soft, that is perhaps good. It would indicate that whatever it is has failed.

I would judge from the diagam that this has slug tuning in the IF's. You could actually try a realignment and that would tell you if the frequency has suddenly changed. It would be handy if you had one of those insulation testers like one of mine, which steps from 100V to 1000V in around four ranges. It will tell you if a cap has shorted.

The bits I would re check bearing in mind again new ones can fail c35 (easy as it could pull the oscillator off frequency) c21, c23, c46, c41, c42, R5 & c26, Back bias R6 & R7.

Note I would prefer to see Blockers C16 & C45 mains rated or very high voltage. Make sure that the antenna is not shorting to ground. I would suggest Neutral be made to go in via the globes and active the two resistors R1 & R2.

Note the caps across the IF coils do not have a lot of voltage across them. But they do on them.

Aslo recheck for bits of resisor & cap wires and stray bits of solder in places that they should not be.

Marc
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PostSubject: Re: Trapped in cyberspace   Tue Oct 19, 2010 6:37 pm

Dear Marcc-Thanx again for the detailed reply! Just finished testing some vintage tubes and restoring 2 radios, so I have not had a chance to tackle the Telsa again. I DO know that removing one of the pilot lights kills the filament string-wierd! I also know that the antenna is not grounded from the connection at the chassis. FWIW, I did replace all mentioned caps and resistors, save for C21/C22/C23 and C24, as they are in the IF cans. What do you mean by this? "Note I would prefer to see Blockers C16 & C45 mains rated or very high voltage. Make sure that the antenna is not shorting to ground. I would suggest Neutral be made to go in via the globes and active the two resistors R1 & R2"-that blew me outta the water!Wink. Am wondering if one or more of the IF can caps has gone bad and thus throwing the frequency off in the IF can-perhaps some CAREFUL tweaking of both IF cans will tell me? This is all new to me,although I have rebuilt IF cans before. Never taken apart these Tesla IF cans and don't want to do so unless absolutely necessary! Thanx again-Gearhead

UPDATE: Measured resistance on 1rst and 2nd IF and they measure 8.3/7.9 and 6.1/8.1 respectively. Unfortunately, the ferrite cores look pretty decrepit:( Very leary of tweaking them!
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marcc

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PostSubject: Re: Trapped in cyberspace   Thu Oct 21, 2010 5:46 am

If you pull the globes out they are in series with the load & will have the same effect as a switch or fuse.

I was sorta thinking aloud on the neutral under the heading "what if". C16 & C45 are going to be in the position af having mains potential on them. If they fail short, you may go out at the same time as the globes. That's why the comment re "safety caps" or what I refer to as "Mains rated".

In a neutral earth return power system Neutral is normally at Earth potential. The negative rail is usually earthed & C45 goes directly to the negative rail. Pedantic perhaps?

The idea of tweaking the cans would save pulling them to bits to find out if they had wandered. One of the dangers of soldering coils is the risk of wires breaking or things falling off inside the can. Rare, but having seen it .......

I don't have voltages on the sheet I have. One would hope there is another sheet with them. A few European sets actually had all of the currents as well as the voltages. That takes the fun out of fault finding.

Using a high impedance meter to check the current grid voltages might be handy, particularly to see if any have gone positve.

Marc
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PostSubject: Re: Trapped in cyberspace   Thu Oct 21, 2010 1:00 pm

Thanx Marcc, but I'm completely clueless on your last response. I DO have a 20,000 ohms/volt Simpson VOM. Can you PM me with what voltages to check and where? FWIW, I did measure plate voltages with a Simpson 260 VOM on both UCH 21 tubes and at R8 and they are all low. At the plate of the first UCH21 tube I measured 80 volts instead of 145 volts, at the plate of the second UCH21 tube I measured 94 volts instead of 145 volts. At either side of R8 I measured 120 and 90 instead of 190 and 145 volts. Strangely enough, I measured 105 volts instead of 110 volts at the plate of the UBL21 audio tube. HTH and thanx again:)-Gearhead
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PostSubject: Re: Trapped in cyberspace   Fri Oct 22, 2010 5:19 am

The idea of checking the grids with a high impedance meter, like a DVM or VTVM, is that the meter acts like a resitor & draws current. The analogues, depending on their sensitivity will load the circuit down, dropping the voltage.

If the control grid on the tube goes positive it will not be appreciated by most tubes and will cause (generally) a maximum current to be drawn by it.

I have no voltage data on that radio. The tube data suggests around -2.0V on the grid of the heptodes -13.0V for the output tube.

Judging from those voltages something is either pulling the voltage down or R8 has cooked.

You are now in the position where every capacitor that has b+ on it inluding C26, that is going to ground (B-) is suspect. also C43 (would like a mains type there as well).
C19 or 20 may have let go from being underated or old.

Pentagrids are notorious for going short. UY 1N may have failed. I would, check / have checked, the tubes for internal shorts; Heater to cathode in particular, before proceeding.

Marc


Marc
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PostSubject: Re: Trapped in cyberspace   Fri Oct 22, 2010 8:51 am

Marcc-Thanx for the quick reply. I cannot overemphasize that EVERY capacitor and EVERY resistor has been replaced. The radio has been optested with NOS and tested tubes-no change.I tried subbing the audio transformer and speaker-again, no change! I also cleaned all tube pins and sockets with Deoxit, checked the volume pot, doubled checked all connection and resoldered many. Lubed the tuning cap and sprayed it's contacts with Deoxit. This is not an easy fix! Pulling my hair out on this one!-Gearhead
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PostSubject: Re: Trapped in cyberspace   Fri Oct 22, 2010 9:26 am

We do occasionally get the set from hell! I had one recently where there was only data on the set that it had been based on. Did I have fun getting a signal through it.

The IF frequency (turned out) was not the same, this was a factory custom with no circuit.
One IF was a bandpass filter (savagely sharp).

The fact that parts have been changed is no garantee that there will not be a failure. I have restored many sets and have had the odd one that will not survive burn test. Old components can fail and stastistically there will always be the odd feral in the new components that will let go.

Unfortunately I am still of the opinion that something has let go and is now drawing lots of current.

Using modern 1/2 & 1/4 Watt resistors on B+, can end badly. New electrolyitics can let go if they have been left sitting for a long time. They can also let go if they are too low in voltage for the application.

This may not be what you wanted to read but I have seen a lot of radio's in 40+ years of fixing them.

If you have other tubes, now is the time to retry them. Also try an ohmmeter between the cathodes & heater pins (no fingers on probes) to see if there is conduction. Should not be any.

Marc
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PostSubject: Re: Trapped in cyberspace   Fri Oct 22, 2010 1:11 pm

Dear Marcc-I bought a brand new and $ 32 mF x 2 450 VDC rated electrolytic from AskJanfirst and am wondering if this went bad. We all know how electrolytics can dry up with age and introduce a hum. How can new electrolytics go bad and lower B+ but still filter hum? Am about ready to replace this filter can with 2 33 mF x 150 VDC electrolytics, just for sh**s and grins. Again, I already tried subbing NOS and tested tubes, but no luck:(-Gearhead-PS-Would sincerely appreciate any specific info on which pins to check for internal shorts on the tubes:)
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