Hi Marcc,

The tone control on this set is on the feedback comming off the speaker winding and going to the cathode of the 6BD7 first audio valve.

If you can email me at gfr53dodo.com.au, I'll reply with a schematic.

What are the chances of a primary to secondry short?

I agree with you that the burn looks like a soldering iron was placed too close.

Cheers, Graham...

If you get a severe burn there may be a chance of primary to secondary, I have only ever seen that once. Not withstanding the fact that I have found that, Krieslers are the worst offenders at taking out speaker transformers, in the several hundred Radio's that I have repaired.

If this one was on my bench I have the means of testing the transformers for insulation failure. One of the tricks for not burning power transformers was to place different wattage globes in series with the mains transformer primary.

I tend to use a variac, after I check it out . That enables you to monitor the HT and spot that you have a problem, before too much power goes into anything.

I will make contact.

Marc

OK I have a circuit.
If the wires from the output of the speaker short that, I speculate should not cause that problem. There will be no sound from the speaker, if it shorts

There are two 6.8K resistors coming off of the 6V4 cathode... are they cooked?

Is R11 33 Ohm, and / or R12 270 Ohm cooked?
C17 30mfd should not go to earth directly.
check c14 has its negative end to chassis.

Use an ohm meter on say 1K range (analogue) on the 6AQ5 (without having fingers on both probes) With valve removed; check for continuity between pin2 and pins 3&4 (heaters)

On the rest pin3 and 4&5 (heaters) Pin numbering, clockwise with pins facing you.


Feedback result.

How can I tell that the resistors are cooked? I didn't replace the resistors, only the capacitors but I will if I have to. I'm presuming you use a multimeter to do this?

I'll have a look when I have some free time after school and get back to you.

No the feeling I am muddling through an assignment for a Cert IV course.

You can use a multimeter but unless you are au fait with where they are, you cannot measure in circuit. Albeit there are devices that apparently can.

Cooked resistors tend to come in ranges like BBQ sausages, ranging from uncooked to varing degrees of brown to black then charcoal. some even split up the middle longways (had one of those a few weeks ago).

The brown coated metals used a lot in back bias, often develop a wierd colour band, viz brown white brown. The white is powder, it's cooked.

Who said radio can't have its moments.

Marc

I haven't been able to do anything recently because exams have just started, but I'll try and have a look when I've got some time to spare.

-Tim

Look for shorts everywhere you have worked around & or might have disturbed .
There are a few tags on some of those sockest that ( due to the camera ) I cannot tell if they are shorting to the chassis or not - some look awefully close - but the camera can be decieving.
While your at it - throw away the 2 core mains cable - and wire a 3 wire flex ( you can still get 3 wire flex / trurip ) if thats your original theme ...or Cotton braided TPS flex ... that looks the bom .
Earth the chassis for your own sake .
Also make sure that the rectifier tube and output tube are in the corrct places .
Use a 100w lamp in series with the set when you power it up for the first time ..or use a variac if you can get one . I use 110v on 230v sets for at least 15mins to check for possible problems , while studying current draw . Good luck -

Alright, I finally had a short 5 minutes to look at the circuit and it looks like there is a resistor that I'm not too sure about. It's a big carbon film resistor that looks in bad shape compared to the others, I mean pretty bad. I can hardly distinguish the colours, and the white is starting to chip off by the look of it. It connects on the terminal block next to the filter capacitors and the power circuit, and looks like it eventually connects to the first valve socket. Any ideas? There's a good image of it here http://graphicgraffiti.com.au/storage/radio23.jpg
It's the big carbon film one near the pot.

Thanks,
-Tim

Not having the set under one's nose is not helpful.

That resistor could well be R12 or 11?

If it is the circuit I have appears to indicate 270 Ohm (Red Violet Brown)
R11 should be smaller and 33 Ohm which may be pure brown with the resistance marked on it, or orange orange black.

R12 comes off of the centre tap of the HT secondary (Outsides go to pins 1 & 7 of the rectifier (6V4). The negative end of C17 (33 mfd) should connect to that.as well as the 470K grid leak from the 6AQ5

The positive end of C17 should connect to pin 3 of the 6V4.

Should R11 or 12 be seriously burned, you will need to check all of the valves for heater cathode shorts. That can be done with an ohm meter.

If you need the pinouts, post request.

Marc

Hi Marc,

I've finally had time to have a good look at the circuit and I've found a few things that I might question. That seriously burnt resistor that I've mentioned before is neither Red Violet Brown or Orange Orange Black. It's like Black Black (Maybe another black, very hard to distinguish) Silver. I've also found what may be the short, but i'm not entirely sure. Two uninsulated wires coming out of the speaker transformer, one is sitting on the chassis, the other just misses the chassis. Here's a photo (http://graphicgraffiti.com.au/storage/image1.jpg). I'm going to replace every resistor now that I've got time, because most, if not all look pretty bad. I got the multimeter out and checked continuity on the 6AQ5 socket, which resulted in good results (no continuity). Some of the solder joints have gone like a liquid brown, maybe flux, not sure, also the tuning mechanism only goes half way, I can't tune to the top end of the AM dial (I haven't turned it on, this is just physical, it stops moving halfway). Hopefully that helps. I'm ordering the resistors when I can so I can rule the resistors out of the equation.

Thanks,
Tim.

Something else probably worth noting that I don't think I've mentioned before is that when I turned the radio on (a fair way back), the dial lamp was bright at first then dimmed and then went bright again and kept doing that. There are two dial lamp sockets, only one was plugged in (the other was missing) so yeah, just thought it would be worth noting.

Thanks,
Tim.

That image fails to load? That resistor could be 100 Ohm, circuit should say, if you are able to identify its position. If it's back bias, cooked indicates problems with too much current.

The flickering bulb could be just loose, replace the second one & see if they both do it. If they do then its possibly a cathode flashing over. Check all of the cathode resistors where fitted for bad earth or going open.

No circuit in front of me.

Marc



Marc

Image should work now. I can have another look at the circuit after.

Tim.

Alright, I turned it on to check the globes with both in. One globe is burnt out, the other didn't dim at all this time. There was a hum present (still) and a slight crackle and the smell was still there. The tubes lit up (as I've mentioned before). Do you reckon replacing all of the resistors would fix all of this, or do you think it's a lost cause (meaning do you think the transformers have gone bad).

Tim.

Those wires coming out of the speaker transformer are copper and going to the speaker, then thats the secondary winding and not likely to be the hassle.

That is an ISO speaker transformer, you could disconnect the primary wires & see if they have shorted to its case? One should come off of pin 3 (cathode pin) of the rectifier the other pin 5 (plate) of the 6AQ5.

The only way to find out what is going on with the transformer is to unload it.

Rip out all of the valves & see if it crackles & growls then? The fact that the valves light up merely tells me their heaters are working and little else.

If it growls & crackles then the short is either within the transformer, on the heaters (unlikely) or on the AC side of the rectifier.

There should be about 230VAC between the centre tap and pin 1 of the 6V4 socket (no valves installed) and the same between pin 7 and the CT. If there is no voltage between either of these pins and ground, then one of the back bias resistors R11 & 12 are open. Their resistances should be checked.

If it does not crackle & growl, then all of the valves are going to need short checking and the wiring gone over with a "fine tooth comb" to find the error / short.

My work bench is almost clear of the Midwest, with its two speakers, Variac & Stepdown transformer (Its 110V) That's another option. Even that I have 4 radio's in the line to replace it.

Midwest chassis is the full width of the table (18 valves abt. 35+ Watts out 4x 6F6).

Marc