Forum Home
Press F1
Thread ID: 43231	2004-03-07 22:55:00	Off Topic. Valve radio parts	Colpol (444)	Press F1

Post ID	Timestamp	Content	User
220942	2004-03-07 22:55:00	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	Colpol (444)
220943	2004-03-07 23:24:00	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-{)	Billy T (70)
220944	2004-03-08 00:13:00	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	Colpol (444)
220945	2004-03-08 00:46:00	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 .	Graham L (2)
220946	2004-03-08 01:01:00	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-{)	Billy T (70)
220947	2004-03-08 04:31:00	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.	Mzee (158)
220948	2004-03-08 05:01:00	Humdinger: noun (informal) "A remarkable or outstanding person or thing of its kind." origin - unknown from Compact Oxford English Dictionary	Laura (43)
220949	2004-03-08 07:54:00	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 ( . portabletubes . co . uk/index . htm" target="_blank">www . portabletubes . co . uk) 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-{)	Billy T (70)
220950	2004-03-08 11:57:00	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	Colpol (444)
220951	2004-03-08 19:26:00	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 :)	Terry Porritt (14)
1 2 3