Hi Marc,

I don't know if this is good or bad but I think I just found something. I was browsing the web and looked at a website that said you could test if the transformer was ok by taking out the rectifier tube and seeing if it still hummed, or smelt. Well, I thought I'd try it. I took out the rectifier tube, plugged it in (after cleaning the circuit, getting rid of more dust and reheating a few dodgy looking solder points with my iron) and turned it on. No hum, no crackle. The rest of the tubes heaters lit up slightly. NO SMELL! I know the transformer wouldn't have load then, but is it a possibility this could all be caused by the rectifier tube, or am I out of my mind?

Tim.

That is basically along the lines of what I was saying but I was going to take out all of the valves.

I would check the voltages on the HT secondary to make sure that they are roughly the same.

Amend my statement re the line, its now seven and one is another Midwest. A Radiola coming in, has a speaker weighing in at 10Kg (I have had one at 6Kg). It looks like I graduated with the course & can now concentrate on clearing the line.

Salt mines...... It is now fairly clear that there is a short circuit on the HT.

This could be a wiring mistake when you re-capped, a reversed C14, or 17 or a solder dag between two pins. eg. pins 3 & 4 of the rectifier.
Too low a voltage caps on the HT punching through?

I would start by going with my initial gut feeling any way. I would think R11 & 12 are damaged. R11 is wire wound and 33 Ohm. R12 is not clear on the circuit I have it appears to be 270 Ohm (stated before) and at 1 Watt is the more likely to cook.

Pinouts of the valves are clockwise with the pins facing you and not in the radio.

Keep fingers off of the probes of the ohmeter (contact with one only is forgivable, only if you finger does not touch another pin). IC clips are good.

Initial check. There should be no resistance between:

6V4 pin 3 and the heaters 4&5
6AQ5 pin 2 " 3&4
6BD7 pin 3 " 4&5
6N8 " " " Not a reliable valve
6AN7 " " "

This is a quick check, ideally one needs a valve tester.

I would not bother turning the set on again until you have, had a really good look at it, or get someone with a bit more experience to trawl over it with you.

Marc

Hi Marc,

I did the test you specified for the tubes. Here's the results. I take it that for 6N8 and 6AN7 you do that same test as for 6BD7 (e.g. pin 3 + 4, pin 3 + 5). Anyway, 6V4, 6AQ5, 6BD7, 6AN7 showed no resistance. I guess you know 6N8 pretty well. I did the same test on it as 6BD7 and when connected from pin 3 + pin 4 (heater) it showed 3.9ohms of resistance. But when I connected pin 3 + pin 5 it showed no resistance. Internal short?

Tim.

Hi Marc,

Sorry for all the messages but I think I've just found the short. Now that I've had time to sit down and have a good hard look at the circuit, and do all the tests with the ohm meter that you have previously mentioned, I did tests on the speaker transformer. There are two primary cables coming out of the speaker transformer being a red one and a blue one. I connected each on up to the multimeter and the other probe to the chassis, or ground. Blue to chassis showed 2.85Kohm while Red to chassis showed 10.4ohm. Then I did the tests you previously outlined by measuring pin 3 of the rectifier socket to ground, this showed 10.6ohm. I then did pin 5 to ground and it showed zero resistance. What to do now? I note that the rectifier valve socket is loose on one side (two solder tags holding it on to the chassis, one has fallen off), but other than that, is it just the speaker transformer? It was the original source of the smell as well.

Tim.

There seems to be confusion here.

In order to test the speaker transformer primary for grounding it must be disconnected from the circuit.

The pins indicated as heaters will have continuity between themselves, but not with the pins indicated as cathodes. The valves should not be in the set to do those checks, on the valves themselves.

The type setting went AWOL when posted. Read cathodes of the last three as pin 3 and heaters 4&5

One does not use an ohmeter normally to test HT to ground. If its an anologue meter the polarities reverse and the electrolytics will be charged by the meter.


Having said that 10.4 sounds like a possible wiring mistake on the Centre tap, if the reading has any relevance ?

The CT should go to ground via R12 and R1. If HT is going to ground via R11 or 12, they will cook & so will the transformer.

Marc

The tubes were tested while they were out. I tested the tubes, not the sockets (was I suppose to test the sockets instead?). I redid the test of the speaker transformer with the previously mentioned blue and red wires disconnected and found zero resistance in each socket.

What is the centre tap may I ask, this is really my first experience with tube electronics. Is it the centre circle in the middle of the tube socket? What is HT as well (I'm guessing it's the main transformer circuit). Anyway, if you guide me through that, I'll check those resistors.

Thanks,
Tim.

I found something that doesn't look right. The big filter capacitor. I replaced this, into the same position the old one was put. Is this big capacitor suppose to go to ground? The negative side of it is going to a terminal lug that has nothing else connected to it, just the neg end of the cap. Is this right?

Tim.

Ok .... no play with sockets.

One is supposed to be checking the wires on the transformer primary to see that they are not contacting the metal frame.
That's why I want them floating. Might be an idea to also check for conductivity between those wires & the speaker ones. Speaker ones do not need to be disconnected. Just use a speaker terminal (failure there is rare but it's a Kriesler)

HT is B+ Which is the 243V DC feeding the plates, and bites misplaced fingers.

You will need to get "au fait" with circuits, you are actually diving in the deep end with this one. Do you have a circuit for this? Even that you may not fully understand it.

When you went looking at transformers, you may have noted a reference to say, 230 - 0 - 230 V. That is a transformer with 460 Volts across the out side, but half in the middle. So if you reference to the centre, it will have 230V either side.

To get full wave rectification the zero,or middle (Centre tap) is taken to ground and the two outsides to a diode plate (6V4 is a double diode). So in this case the outsides go to pins 1&7 respectively. The B+ is taken from the Cathode (pin3).

In the case of the 11-71, this is " back biased". The middle or centre tap does not go directly to ground, it is -12V above it as it grounds via the series circuit of R12 (270R?) & R11(33R).

I would expect the CT to appear at the end of R12, at the same point would be an R3B (470K YVY) which would go to a mid point between the coupling capacitor (.01) from 6BD7 and the 47K (YVO) grid stopper that will go to either pin 1or7 of the 6AQ5.

Also originating from that end of R12 shoud be the negative of C17 30μF 350V, the positive end will track back to pin 3 of the rectifier.

R11 should go to ground from the opposite end of R12. There may be a 1Meg ( Bk Br G ) also at this junction (going to the 6N8).

C14 the other HV electrolytic 16μF (use 22 uF) Comes off of the two parallel 6.8K resistors (blu Gry R) that end up feeding to the screen of 6AQ5 (pin6) and other points. Positive on the line to pin6 negative to ground.

This is why I recommend taking several photo's of the pan, prior to attack. One can use it to help reverse the senior moments.

Marc

Hi Marc,

I do have a schematic and overview page on the model (thanks Gfr53), but I really have a hard time reading schematics. It's got the parts list on it though, so that will help me order the resistor replacements.

As you said on the centre tap, the Kriesler is 12V. If those resistors burn out, does that mean it will ground on the chassis at 12V? I'm going to replace all the resistors to be sure and I might redo all the solder points.

What do you think, am I heading in the right direction?
Anyway I'll follow your instructions throughout the day and have a look.

Thanks,
Tim.

If those resistors burn they act as a very slow fuse and gradually disconnect the earth return. The transformer may burn long before this.


If you feel the need to replace resistors use 1Watt ones as, unless you get specifically High voltage ones, many of the modern ones will not handle the voltage. If the resistor is not more than 10% out, naught will be achieved. The exception is R11 I would use a I/2 Watt as there is little voltage & it can be the fuse.

By ohms law..... Its the current going through the sum of R11 & R12 that gives the voltage drop. ie if those add up to 303R, then around 40 mA would be going through them.

I think a little more study on the circuit around the power supply & the output tube.

Marc

I was re-soldering all the points that the cables of the transformer attached to and I found a point that doesn't look right, in the way that it's not suppose to be connected to there. The blue cable coming out of the transformer goes straight to a solder tag on the terminal strip that on it's other lugs holds the 2 resistors, the filter cap and a green and yellow cable (that share a lug). The blue cable's lug has nothing else connected to it. When I was re-soldering it, the plastic (i think it's plastic - I hope it's plastic) that the terminal strip is made of started burning a bit and produced the smell and a bit of smoke. A very familiar smell. It's the smell that appears when the radio is on. Is this blue cable in the right place, or is it directly putting power towards the terminal lug and heating it up, hence producing the little bit of smoke and the smell?

Tim.

As stated the only things that should share the pin with the CT are the negative of C17, R12, an R3B from the 6AQ5. The blue wire may go to a terminal near the 6AQ5 to pick up the 470K Grid leak (R3B).

6AQ5 will not be happy without grid bias.

If this is not the case, then there is a major transposition of wires in this area.

It is easy to find out if the wire is the CT of the transformer HT secondary.
The resistance between it and pins 1& 7 of the rectifier should be about even and between it & ground the sum total of R11 + R12.

Marc

Sorry for the bit of quietness for the past few days but I hadn't been able to login, but now I can. If I took a photo of the area, could you help me through it? I've discovered that the negative of the filter cap goes to a spare lug with nothing else on it as well. I'd say this is the problem area where something is seriously wrong.

Thanks,
Tim.

Tim,

If you need a photo embedded in your post you can e-mail it to me and I will add it for you. If it is a big photo I can link to it from a smaller one that fits in your post.


Brad.

Would agree, serious error.

This is why before photo's can be handy.

Unfortunately the AWA I now have on the bench, came with several wires with no owners & labels like "paper capacitor" on them?

Well I can eliminate the ones that the wire cannot reach, as a start.

Marc