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Colpol
08-03-2004, 11:55 AM
I recently aquired an old valve radio. It works but the speaker is shot and the volume control doesn't work. Does anyone know of anyone in Auckland, preferably South, Who has parts or knows where to get them.
An old broken one might give me the parts I need.
Cheers
Colin

Billy T
08-03-2004, 12:24 PM
Depends how old it is Colin.

If it has an energised field speaker then you can use an ordinary speaker provided you keep the field coil intact for power filtering. If the field coil is stuffed you need to substitute a suitable choke or high wattage wire-wound resistor to restore proper filtering action and to lower the HT to the right level. Speakers of all sizes are readily available from DSE or Jaycar plus most HiFi/TV/Video repair shops would have some in stock. Surplustronics have a range of older parts too.

Volume controls are no problem as suitable pots are readily available. Depending on age, you would probably get away with any 500K-1M (log) pot. If it is just noisy, spray the inside of the pot with CRC, that can work wonders.

Some older radios used linear pots instead of log and had them in strange locations within the circuit. Look for the letter C (log) or A (linear) on the back of the pot, or set it halfway and measure from the wiper to each end. Linear will be near enough to 50/50, log will be way off.

If you post the make, approximate age, and a list of valve types we might be able to give more focused advice. Don't gut old radios for spares unless already badly damaged or ratted beyond redemption, somebody might want to restore it one day.

Cheers

Billy 8-{)

Colpol
08-03-2004, 01:13 PM
Thank you for your reply Billy.

I know the radio is an RCA. It has 7 valves. It is fitted in a locally made cabinet that I want to restore. I do not know the year. The speaker is very heavy as it has a transformer attached to it and 4 wires running to it. The volume control is a rotary arraingement but is sealed and I dont want to open it yet. The volume control only works at its extreme i.e. full volume.
I suspect the radio it might have been a cathedral? type as the tuning system is a rotary arraingement that turns in a little window or port which is visible from the front. It has tree knobs. The tune knob, The volume control and an on/off tone knob.
Valve types ---mmmm ---Here goes
1 RCA 60 EXO ( 47 on glass)
2 GE 5-09 188-5( 35 and 51 on glass)
3 KEN-RAD ( 24A on glass)
4 TUNG-SOL 322124 ( 24A)
5 KEN-RAD 739 188-5 ( 56 )
6 KEN -RAD ( 35 and 51 )
7 TUNG-SOL322206 (80 on glass)
Hope this lot helps
Cheers
Colin

Graham L
08-03-2004, 01:46 PM
That's definitely a veteran set, Col. Those valves date back to the 30s.

With 4 wires, that speaker has a field coil. Quite often the field coil has failed ... it's lots of turns of very fine wire (up to 5 kilohm). Unless you can find a working replacement, a power resistor is the answer.

It would be a good idea to cut off the power cord, to avoid temptation. :D It's likely to be rubber insulated, and the rubber will be in a state of terminal rot. The power cord must be replaced anyway. A Megger test of the power transformer is a good idea too. They can become leaky, and the high voltages (385 volts ac each side of the HT winding was standard) can cause smoke.

It's amazing how long many of those valves lasted, so it might respond to a little TLC.

The electrolytic capacitors (especially if they are in vertical cans ;-)) are probably dead. The others should be tested for leakage. Valves turn on hard when they get +300V from the previous plate, when they are supposed to be biased at -5, and getting ac signal from the previous stage thought a coupling capacitor.

See if your local library gets Silicon Chip magazine. If so, go through the back files and read the column about restoration of valve radios.

Billy T
08-03-2004, 02:01 PM
OK, the transformer will be the speaker transformer used to impedance-match the output valve to the voice coil.

If there are 4 wires on the speaker plug, two will be the transformer and the other two might be either the field coil (does the magnet on the rear of the speaker look like it is wire-wound?), or the voice coil feed. Are there wires going into the rear of the speaker as well as to the voice coil at the base of the cone? There could also be a hum-bucking coil on the speaker voice coil former, which is very likely of it is an electromagnetic speaker rather than a permanent magnet type.

The volume control sounds historically right in the middle of the period during which the modern volume control arrangements were introduced. Prior to automatic gain control of the RF/IF sections, volume level depended on signal strength, so control was often applied via RF/IF gain instead of at the input to the audio stages. The former does funny things in the absence of a broadcast signal so your control may not be faulty at all, though it does sound suspicious. Measuring it with a meter might be an idea.

Pots are usually sealed but with some of the older types you could prise the back-cap off and spray the inside with contact cleaner. Drilling a small hole and spraying cleaner inside was another trick.

A word of warning! If you take the innards out of the cabinet and run it on the bench, check that there is no voltage between the speaker metal and the radio chassis. Those EM speaker coils used to break down to the speaker metalwork, but nothing bad happened because the speaker was not touching the chassis. I remember trying to adjust the volume on one such radio while holding the speaker magnet with the other hand and the whole shebang went flying.:O

After that I always checked first.

Cheers

Billy 8-{)

Mzee
08-03-2004, 05:31 PM
It sounds as if you have a "humbucking-coil" "humdinger".
The idea of this was to create hum frequency in the opposite direction to the hum in the speaker coil. This eliminated the audible hum. This was an alternative to smoothing & choking.

I have a 1930's radio which had a 'humdinger". I fitted a later 4ohm speaker and smoothed the HT with a 40watt lamp bulb with a large capacitor across the output. Works well & the bulb brightness fluctuates with the base :)
The beauty of the lamp bulb is that you can adjust the voltage by changing it, & heat is not a problem.

Laura
08-03-2004, 06:01 PM
Humdinger:

noun (informal)

"A remarkable or outstanding person or thing of its kind."

origin - unknown

from Compact Oxford English Dictionary

Billy T
08-03-2004, 08:54 PM
Not quite Mzee, close but no cigar.

I have never heard the term "humdinger" used in that context, so maybe it is a local variant. The principal reason for humbucking was the low values of filter capacitance available from the wet-electrolyte "condensers" of the day. This, coupled with the uneconomic size of filter choke necessary to compensate, meant that most sets had quite audible 100Hz hum.

Because the speaker field coil is used as the filter choke for the power supply and carries the the 100 Hz AC ripple from the full wave rectifier output, this ripple signal on the speaker's magnetic field causes significant hum levels in the sound.

To counter this 100Hz hum, a "hum-bucking" coil is wound on the speaker pole piece right next to the field coil. From there it is connected in series with the speaker voice coil so as to oppose the AC ripple in the field coil and cancel the 100 Hz hum component. (If the leads to the hum-bucking coil are accidentally reversed, the hum level will be much higher, as many an apprentice found in my day.)

Hum bucking was usually only applied to the big speakers in larger cabinets where low frequency output (hum levels) were accentuated by the speaker cone size and relatively efficient cabinet baffle. Smaller speaker/cabinet combinations were too inefficient to put out enough low frequency to worry about.

This Site (http://www.portabletubes.co.uk/index.htm) has a wealth of useful information for restoring old radios Colin. Don't be put off by the site name though or its difficult style. Use the text-based site plan at bottom-left to navigate, it is much easier.

Cheers

Billy 8-{)

Colpol
09-03-2004, 12:57 AM
Thank you so much guys. Plenty to think about but getting a bit techo.
Anyway here is a bit more info.

"It would be a good idea to cut off the power cord, to avoid temptation. It's likely to be rubber insulated, and the rubber will be in a state of terminal rot. The power cord must be replaced anyway. A Megger test of the power transformer is a good idea too. They can become leaky, and the high voltages (385 volts ac each side of the HT winding was standard) can cause smoke."

The cord fitted at present is strange. It appears to be plastic. The neutral ?wire is in the insulation. The other two wires are in their own insulated area with no indication of Phase or Earth but looking at the way it is wired to the present plug tells me which is which.
I am reasonably sure that at some time in the past it has been modified as the mounting of the transformer shows that a different one was originally fitted. The mounting holes are there for a larger one than that presently fitted.

"The electrolytic capacitors (especially if they are in vertical cans ) are probably dead. The others should be tested for leakage. "

Reference the vertical cans. The set has two. Turning the unit over reveals that they are not conected and a number of modern? capaciters have been wired in.

If there are 4 wires on the speaker plug, two will be the transformer and the other two might be either the field coil (does the magnet on the rear of the speaker look like it is wire-wound?), or the voice coil feed. Are there wires going into the rear of the speaker as well as to the voice coil at the base of the cone? There could also be a hum-bucking coil on the speaker voice coil former, which is very likely of it is an electromagnetic speaker rather than a permanent magnet type.

The magnet on the rear looks like it is wire wound. Two wires go from the Transformer to the base of the cone. the other two wires go directly ( via a mounting plate) into the main coil and from there down to the cone. The area between the point where the cone ends,and where it enters the coil appears to be wound with a very fine wire.

I must add that the speaker still works and the output can be understood but the quality is poor

"Pots are usually sealed but with some of the older types you could prise the back-cap off and spray the inside with contact cleaner. Drilling a small hole and spraying cleaner inside was another trick."

I have tried to take the back off but it doesnt want to budge so will try drilling it.

I have had it running on the bench and it runs fine. Before I removed it from the cabinet I noticed a bit of " insulation" ??? at the point where the chassis was in contact with the speaker. I will be carefull to keep the two apart just in case.
Once again thank you for the expert assistance.
Cheers
Colin

Terry Porritt
09-03-2004, 08:26 AM
A humdinger was just a pot across the AC heater supply with the wiper connected to earth. It was adjusted for minimum hum.

If the electrolytics have failed, then the purists carefully open them up to remove the innards, and fit new smaller ones inside. High voltage electrolytics are not as easy to get these days.

I have an early Bakelite 1932 vintage cathedral Echo TRF, that's been in the family since new, nice old set.

Another 2 cents worth :)

Misty
09-03-2004, 08:40 AM
I have a "Clipper" sitting in my garage. Bought it many (many) years ago on a whim. Was going then, but one of the valves conked out. Have not wanted to get rid of it because how many "old" things do individuals have these days ! Know nothing whatever about its intricacies.
Misty
:)

Billy T
09-03-2004, 09:46 AM
> Thank you so much guys. Plenty to think about but
> getting a bit techo.
> Anyway here is a bit more info.
>
> "It would be a good idea to cut off the power cord,
> to avoid temptation. It's likely to be rubber
> insulated, and the rubber will be in a state of
> terminal rot. The power cord must be replaced anyway.
> A Megger test of the power transformer is a good idea
> too. They can become leaky, and the high voltages
> (385 volts ac each side of the HT winding was
> standard) can cause smoke."
>
> The cord fitted at present is strange. It appears to
> be plastic. The neutral ?wire is in the insulation.
> The other two wires are in their own insulated area
> with no indication of Phase or Earth but looking at
> the way it is wired to the present plug tells me
> which is which.
> I am reasonably sure that at some time in the past it
> has been modified as the mounting of the transformer
> shows that a different one was originally fitted. The
> mounting holes are there for a larger one than that
> presently fitted.

It is common for the power cord and transformer to have been replaced. If he radio goes, the transformer is OK at present though it may die during the first 20 or so hours of use. don't leave the radio unattended during that time.

> "The electrolytic capacitors (especially if they are
> in vertical cans ) are probably dead. The others
> should be tested for leakage. "
>
> Reference the vertical cans. The set has two. Turning
> the unit over reveals that they are not conected and
> a number of modern? capaciters have been wired in.

Common practice, should be OK but the replacements could be over 40 years old already. Post the brand names, that helps date them.

>
> The magnet on the rear looks like it is wire wound.
> Two wires go from the Transformer to the base of the
> cone. the other two wires go directly ( via a
> mounting plate) into the main coil and from there
> down to the cone. The area between the point where
> the cone ends,and where it enters the coil appears to
> be wound with a very fine wire.

That is an EM speaker alright, so it cannot be easily replaced. One issue is the loss of humbucking, though that can be retained if you keep the field coil.

> I must add that the speaker still works and the
> output can be understood but the quality is poor

The poor quality may not be the speaker at all. Unless the cone is badly ripped and/or rattling, or the voice coil is audibly rubbing on the magnet pole-piece it is probably OK. Those radios did not produce very high quality sound at all, despite some people's nostalgic memories of "lovely tone." You can test without replacing by connecting another speaker in parallel on leads long enough to get you away from the original, or if you have the equipment, unsolder one lead frn the speaker to silence it and check the sound on the replacement.

There are many reasons (other than speakers) for poor sound on old valve radios, the most common being gassy or low emission valves, leaky coupling capacitors, RF/IF oveload for older sets or AGC faults for the more modern. Don't leap to conclusions.

> "Pots are usually sealed but with some of the older
> types you could prise the back-cap off and spray the
> inside with contact cleaner. Drilling a small hole
> and spraying cleaner inside was another trick."
>
> I have tried to take the back off but it doesnt want
> to budge so will try drilling it.

"Sleeve" the drill with tubing so that it can only just break through the back plate. You need only 2-3 mm of drill showing, otherwise you could butcher the internals. Don't trust to manual control, it is all too easy to go too far.

If you don't already have one, buy a cheap analogue meter for testing purposes. I have recently purchased one from Jaycar that is excellent for this work.

Catalogue number is QM-1020, price $39.95. It uses 9 volts at the meter probes for Hi ohms reading which makes it very good for checking capacitors for leakage and it has 5 well spaced ohms ranges for accurate readings on high value resistors & pots etc. Don't bother with a digital meter, they are great for voltage and current but not good on ohms unless measuring resistors. I have about five digital meters but I still need and use an analogue meter regularly. The latest purchase was to replace a 25 year old Hioki AS100D that finally died of old age.

Cheers

Billy 8-{)

Billy T
09-03-2004, 09:59 AM
> A humdinger was just a pot across the AC heater
> supply with the wiper connected to earth. It was
> adjusted for minimum hum.

Eh? Say what Terry?

I learn something new every day! I never heard that called a humdinger before so perhaps it was an industry sector/regional thing.

I spent my apprenticeship a small factory/service shop where we made heaps of custom equipment for commercial clients (andthe boss's golf course buddies). We used to call that a hum-cancelling pot and we only installed them in hi fi gear where the filaments were fed from an isolated 6.3v winding. The pot was used to provide a "tunable" virtual-earth to cancel out heater-cathode leakage and induced voltages. Filament wiring was wound up to a tight twist with a hand drill to minimise radiated fields!

It was varied and very enjoyable work.

Cheers

Billy 8-{)

Terry Porritt
09-03-2004, 11:33 AM
That's right Billy. With many older sets, at least the UK ones, the humdinger was a wirewound pre-set straight pot of about 100 ohms.
Humdingers were also used fairly recently too on some valve amplifiers for hum reduction, instead of centre tapping the 6.3v secondary to earth.

Another common fault with old sets is the poorer quality enamel used on the transformer windings, leading to open circuits. Very easy for 48/50swg wire to oxidise right through, especially in intervalve transformers.

I forget now how many thousands of turns of 48swg I calculated to fill the bobbins on the old EKCO, good job I had the use of a coil winder :), else I'd still be winding by hand 15 years later.

(EKCO not echo, was short for E.K. Cole, but I forget what the E K stood for, they were one of the first radio manufacturers to use the new wonder plastic material "Bakelite" for the cases.

Terry Porritt
09-03-2004, 11:39 AM
Ah-Ha, just found my notes on the intervalve tranny. 14,000 turns 46swg on the primary, and the secondary was 2 separate windings each of 14,000 turns 46swg.

So the intervalve transformer had a 2:1 step-up ratio :D

Billy T
09-03-2004, 12:43 PM
Just musing here Terry, but wouldn't that be a 1:2 step up ratio?

Coil winding was my least loved job. Winding RF & IF coil sets out of Litz wire for marine RTs was an awful job and if you counted the turns wrong the boss would pull the offending coil out and throw it at you! On one memorable Monday a set built from scratch over the weekend by a team of two (suffering from a Friday night hangover) for an urgent sale was found to have coils that were somewhat outside the required specs. The Boss swore & cursed for a while, then there was an almighty crash as it hit the floor, then he dragged it out to the workshop by the Mic cord and told us that jobs were on the line if it wasn't fixed by the next morning.

Nostalgia.

Cheers

Billy 8-{) :8}
Coil drying oven was a ZC1 valve case with a 75W lightbulb
in the bottom and a coil rack across the middle. Meths lamp
to burn off the fabric cover and the varnish (no easy-soldering
polyeurethane varnishes then) and sealing wax to secure the
windings. After termination and drying they were dipped in hot
wax to keep moisture out.

Terry Porritt
09-03-2004, 01:55 PM
Of course, a 1:2 ratio.

I tried to buy some litz wire some 12 years ago or so to wind a high Q frame aerial for medium wave, but couldnt find a supplier. There was some mention of heavy duty litz wire for , I think, switch mode power supplies, or was it inverters? Anyway at exhorbitant cost.

Graham L
09-03-2004, 03:19 PM
Isn't that 1:1:1 ? A single ended driver to push-pull output? ;-)

I let Beacon Radio wind mine.:D

Terry Porritt
09-03-2004, 03:35 PM
Whilst in rambling mode, I always get confused wth transformer ratios, it's all in the words and symbols, eg a one to two step up ratio, or symbolically 2:1 as 1:2 really means 1/2. So maybe I was right when I said a 2:1 ratio??

The secondary winding was in a divided bobbin, and the original was continuously wound, I had to make new bobbins as the original was distorted, and made a tapping so that I could either have 1:1 ratio or a 2:1 ratio, or is that 1:2 ?:|

Anyway 14,000 primary and 28,000 turns total secondary.

The valves were MS4B screen grid HF valve, 41MHL detector and af amp, and PM24M output pentode.
These numbers dont mean much to the southern hemisphere colonists who used ungodly American types :D

Graham L
09-03-2004, 03:57 PM
Col: That mains cord seems to be a bit naughtily wired. The earth wire should be the one in the middle. Phase and neutral are usually insulated as well as embedded in the sheath.

The vertical can electrolytics have obviously been terminated with extreme prejudice. If the modern tubulars haven't exploded they may have reformed and be OK. :D

All the connections on the speaker plug are at high voltages. The wire going to the field could could have well over 400V DC. The input of the speaker transfomer will be fed from 300-350V. You can get a "good" belt from that. :_|

It might be simplest to replace the volume control. It will probably be marked 500k, or 250k. The "modern" 470k or 270k values would be perfectly OK. If DSE or Jaycar haven't got a suitable physical size (shaft, mounting ferrule) a television serviceman would almost certainly have some in the junk box. :D

Colpol
11-03-2004, 12:25 AM
Thanks Graham. I was getting a bit lost there. To tecko for lil ol me. Anyway you are right. Now that I look closer the plug is wired the way you say.
Thanks for the warning about the vcoltages to the speaker. I will certainly watch that. A earlier Reply suggested that other things may cause the speaker to play up so I will take your advice and replace the volume control.
I have checked the back of the control as you sugested. There is text there but it is almost impossable to read what it says and none that I can desypher.
The unit has three terminals. Mesuring between the centre terminal ( Common)and each of the others gives readings of 0 at one extreme and 150 at the other for both sides( Does that make sense to you??)

Billy T
11-03-2004, 10:07 AM
Hi Colin

Those figures won't mean anything unless you tell us the meter range. It could be 150, 1500, 15000, 150,000 or 1.5 Megohms. None of those options sound like a standard pot, so set the control to the middle, measure between the two ends and see what that says, then measure from centre to either end. Post the readings & meter type (analogue or digital) and resistance scale (x1/x10/x100 etc).


I suspect that it could be technology from the earlier days as per one of my earlier posts where the volume control is not at the input to the audio stage but controls RF/IF gain instead. Can you measure any voltages on the pot when the radio is going?

If you only have zero or full volume available, as I said earlier your speaker may not be faulty at all. The problem could be gross overdriving of a tired audio output valve. Do voices sound like they are gargling in soup, or do you have rattles and scraping noises?

Do you have a makers name or model number for the radio?

Cheers

Billy 8-{)

Colpol
11-03-2004, 02:22 PM
Ok
I got my son whos eyes are better than mine to check out the back of the pot. The numbers on the back are 79-006-1504. using my digital meter on Ohms ( 200k range) with the control in the middle the reading accross the two ends is 155.
55 in one direction and 115 in the other. I suspect that the end mesurements would be the same if all was ok.
There is a small voltage present at either end of the pot 3-8 volts if I am reading the meter right.
I had a rush of the smelly stuff to the brain and improved the position of the aerial. This has improved the quality of the speaker output to allmost normal :-)),(bearing in mind an earlier post about these early speakers not having the nice mellow tones of the later one which is what I was expecting) Turning the pot back from the full position by a very very small amount reduces the volume to a level that does not distort much so I suspect that the speaker getting full volume was more than it could handle
The radio is an RCA. The only numbers apart from the patent numbers is C169778 which I think is the serial number.
I tried to check for voltages on the speaker. I got some fairly large numbers and also some reverse readings??????

Billy T
11-03-2004, 02:47 PM
OK Colin

Sounds like a logarithmic pot of 150-175K. Does it look original?

Voltages should not be present on the pot, which indicates either an RF/IF gain control (pot carrying valve bias voltages) or an open circuit grid leak resistor on the output valve allowing signal rectification to take place if it is a conventional volume control. This causes the audio to provide variable bias on the valve through signal rectification, hence awful distortion. Other possible explanations are too complex to traverse here.

If the distortion slowly clears after a signal strength change then it is probably an agc or audio bias problem. Putting your meter probe on the grid of the output valve (with the other probe grounded) should improve the audio if it is a bias problem but if a digital meter the impedance will be a little high and the effect may be limited.

Getting better audio quality by changing the signal input also suggests either IF overload (if automatic gain contol is not present) or simply the variation in audio level with different signal strengths affecting sound quality per the previous paragraph. Remember, our local radio station field strengths may be significantly higher than the poor old thing was designed for. To test this theory, reduce aerial size or find weaker stations.

Take a look at the diagnostic advice on the website I posted, find the audio output valve and check the signal grid and cathode voltages.
The output valve will be the one wired to the speaker transformer via the plug & socket if the transformer is mounted on the speaker. Grid will be the one that goes to the wiper of the volume control and cathode will go to ground via a resistor of less than 500 ohms.

Gotta go do some work now, but keep us informed.

Cheers

Billy 8-{)

Billy T
11-03-2004, 06:06 PM
Further update:

Valve types

80 - Rectifier
47 - Pentode Audio output
56 - Triode, possibly audio preamp, especially if right next to the 47.
24A - Tetrode RF/IF amplifier. Most likely second and third IF/detector in this radio
35/51 - variable mu (gain controllable) Tetrode. Most likely RF (gain controlled) and first IF (gain controlled)

Seven valves is quite a number so this could have been a reasonably up-market radio.

At a guess, the 80 will be back in one corner, then the line-up might be 35-51, 35-51, 24A, 24A, 56, 47.

Of course I could be miles out here, and there is always a chance that the 24As and the 35-51s have been swapped around.

Where is PF1's vintage radio expert when we need him?

Cheers

Billy 8-{)