Need to fix old valve radio

...and lastly, the radio has not been used since the last time I tested it about 10 years ago. Then it worked fine, no I tested it again and suddenly there is this problem. It is entirely possible that it was knocked (I hope not dropped) while in storage, but there is nothing visibly broken, and all the valves do glow. BTW, I tried removing the valves one by one and seeing if one valve did not "make a difference", but this approach did not work, any valve removed results in no sound at all.

The power supply. Check the high voltage. Could be the rectifier valve.

Hi...

It's been a long long long time, but I don't believe any of those quoted are a rectifier. Perhaps selenium, or silicon.

Take care.

Ken

Could be selenium ... difficult to tell from what has been posted.

The 6X5 is the rectifier. After 10 years of shelf time, the most likely thing that you should look at is the electrolytic capacitors in the power supply. They are probably dry by now and need to be replaced. Since tube equipment runs hot, you should replace them with 105C temp range units.

Cheers!!

--
Dave M
MasonDG44 at comcast dot net  (Just substitute the appropriate characters in the 
address)

Some days you're the dog, some days the hydrant.

6X5 is a full wave rectifier.

I'd check the main electro (multi part) and the resistors across it.

Tubes/valves rarely go bad just sitting.

I'd start by checking electrolytic capacitors.

--- sam | Sci.Electronics.Repair FAQ:

Repair | Main Table of Contents:

+Lasers | Sam's Laser FAQ: | Mirror Sites:

Important: Anything sent to the email address in the message header above is ignored unless my full name AND either lasers or electronics is included in the subject line. Or, you can contact me via the Feedback Form in the FAQs.

Got a model number? It sounds like a transformer powered all-band

1940's superheterodyne set. Later sets used 12, 35 and 50 Volt versions of those tubes in series diretly across the AC power mains. Then they were replaced by miniature versions of the same type of tubes. The schematics for these sets are all almost identical, the 6say is converter, 6sk7 is IF amp, 6sq7 is detector and 1st audio amp, and 6v6 is audio output, 6x5 is rectifier. Volume control is at input to 1st audio amp. Do you have a scope so you can look at the signal from that point to the speaker? If so, I can walk you thru the testing needed.

H. R. (Bob) Hofmann

6X5 is the rectifier...

Could be bad, but is there any hum? If yes, check the filters, too.

Bob, thank you. I don't have a scope. I think you are right about the "series" thing. They are protecting the valves these way, if someone takes one out everything stops working.

Here is more information:

The model number is very faint, but it ends with E629, maybe IE629

There is also a 4 digits serial number, 2xxx Probably not too many of those about today :-) Made in America Rated 110 to 125 Volts 50-60 cycles 40W Yet (strangely) was always operated from 220V.

I need to change the power cable, it is probably the original and is crumbling in a very dangerous way, so I might get a glimps to the internal components. It looks like a big job simply to take the unit out from the wooden box without damaging the tuning mechanics. I wonder if it can simply slide out or if it needs to be lifted out (bad).

Oh, and another interesting thing, the pickup coil doesn't use ferite core like nowadays, there is a huge drum and on it the coil is wrapped.

Be aware that many radios of this time were 110 volts and used the mains cord as a dropper to obtain that from 240.

--
*Very funny Scotty, now beam down my clothes.

    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.

I have just finished replacing the power cable. The unit came out as a whole, which was easy. I then was able to see all the components inside. It is very messy as in those days they were no printed circuit boards, but the general impression I got was that damaged was caused to the internal part/s due to storage in the attick, and last summer here in England the summer was the hottest on record, so you can imagine... Everything looks shining, a sort of wet-look, this must mean bad news.

On the positive side, I did manage to fix one of the two front panel illumination bulbs - it seemed to be just a contact issue, and after whasing the bulb in some water, it was working again. Now remains only the audio problem.

I have taken some pictures, I will post them soon.

If someone wants this radio, make me an offer. I can ship worldwide, and I accept paypal.

Thanks.

Dave is 1000% corret. The line cord that is crumbling is probably in that condition because it has gotten so hot so many times over the years. The cord has to dissipate at least 20 watts or maybe even twice that amount to drop the voltage down to 120 v when used on a

230V circuit..

The first thing to do is to measure the mains voltage - 230V approximately, and then measure the voltage at the set end of that crumbling line cord when the set is turned on and see if it measures about 115 Volts. If so, then the line cord must not be replaced with a regular line cord or everything in the radio will be fried beyond repair.

The only way to salvage the radio will be to insert a fixed resistor mounted somewhere inside the radio that will drop the voltage the same as the original line cord is now doing. If it is possible, I would suggest measuring the resistance of the two conductors of the line cord, add the resistance of the two conductors together, and use a resistor of that ohms value as the replacement for the old line cord resistance. The wattage of the resistor will be 120V x 120V divided by the resistance, or 14,400 divided by the resistance of the line cord. It will be a sizeable resistor!

Once we get the power input to the radio fixed, we can then start going through the audio stages to see what is wrong. A scope would be nice, but I think we can measure voltages at a number of places and get things fixed.

H. R.(Bob) Hofmann

How's the speaker? Could be blown.

Distortion could be caused by bias failure. Leaky capacitors (probably called condensors when the radio was built) are a big problem with tube (valve) musical instrument amplifiers.

When you plugged the radio in after replacing the pilot light for the front panel, were the lamps the same brightness as before the line cord was replaced. If so, then the line cord must not have been used to drop the line voltage from 230V to 115V. The set must be using a power transformer designed for an input of 230V and not the 115V shown on the back panel that you mentioned. I will see what info I can find. Do you have a high-empedance voltmeter available to check the votages on the tubes? Is the bottom of the radio accesible? Do you know how to count tube pins from the keyway?

Bob Hofmann

following

the

Hello Bob, (and others)

I am not sure about the brightness, since I received you message after I had done the work, I didn't pay much attention to this. I do have a digital DMM. Here are some pictures that I took while repairing the power cord:

snipped-for-privacy@btinternet.com/

following

putting the

I went to "nostalgiaair.com to see if the model number worked. I couldn't find anything close, perhaps you can go to the site and see if anything rings a bell.

Ok, with a digital multimeter, we should be able to get going. Let's assume that the radio did not get fried due to a doubling of the input voltage. We will check the B+ voltage for some sanity, and then go from there. There should be a large chassis-mounted electrolytic capacitor rated 30 uf or larger, at 200V or higher. The can should be grounded to the chassis, and there should be two non-grounded lugs connected to various components.

Check the DC voltage on each of the two lugs. I would expect each of the voltages to be at least 150V DC, maybe as high as 350V, one should be a few volts higher than the other. Is the speaker a permanenet magnet speaker, or is the place where the magnet normally is located actually a coil of wire with two wires leading to it (in addition to the 2 wires that go to the speaker cone)?

If you don't see at least 150V on the electrolytic, check pin 8 of the

6x5. That is the output of the rectifier circuit. It should be connected to one of the electrolytic terminals.

Is the sound clear at low volumes? How does it distort as you turn the volume up, does it just get fuzzy, or does the volume go up and get fuxzzy at the same time.

Bob Hofmann

following

putting the

Just looked at the photos, great. the large metal can is probably the electrolytic I wrote about as there is no chassis-mounted can visible in the above-chassis view. The big loop is the loop antenna, nowadays they use a ferrite stick, back then most coils were flat wound in a spiral form rather than a coil such as your radio has.

Bob H