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 Vibrator Dropping Resistors
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 Return to top of page · Post #: 1 · Written at 9:25:08 PM on 5 August 2019.
Alarum's Gravatar
 Location: Toowoomba, QLD
 Member since 6 July 2019
 Member #: 2359
 Postcount: 12

Good evening all,

I'm trying to make sense of a mess that is a number of old wire wound resistors leading into the vibrator.

The vibrator is an Oak synchronous V6724 with 24V input hence would I be right in assuming the radio is a 32V DC farm radio?

Many of the resistors are missing the resistance values and in some the wire winds are visible. They seem to be around the 200-300 Ohm range and 3W rating.

The main question is: what sort of resistor would be suitable to drop from 32V to 24V and what resistance/wattage rating should it have?

Cheers

Oak Vibrator
Oak Vibrator


 
 Return to top of page · Post #: 2 · Written at 10:36:09 PM on 5 August 2019.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 5262

If you haven't already come across it, there is a useful page on those vibrators here:

http://members.iinet.net.au/~cool386/msp/msp.html

As to the question of resistance and wattage for a voltage dropper, first you need to know the current flowing, then apply Ohms law:

R= E/I (Voltage drop divided by current)

E.g. 32 minus 24 = 8 volts to be dropped. Say the current is 2 amps, then the value of R = 8/2 = 4 ohms.

Power = I*I*4 (I squared times R)

E.g. 2 x 2 x 4 = 16 watts.

As you can see, the current squared factor has a big impact on the power rating required.


 
 Return to top of page · Post #: 3 · Written at 9:51:58 AM on 6 August 2019.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3707

It might be an ideal if possible to identify what it came out of. Unfortunately all sorts of strange things happen after stuff gets out into the world.

It may have actually been, or is a 24V radio? 24V is still used in aircraft and trucks. It may have had the resistors added to use it on a 32V Lighting Plant? These points cleared up, may make life a little easier. 32V lighting plants were very common, in rural areas.

What is often overlooked when dropping DC volts is the diode. Irrespective of the current, the voltage drop across the diode will remain constant. Also do not overlook the panel mounted resistors, which can be mounted on a steel chassis wall, causing it to become a heatsink: RS is one that has them.


 
 Return to top of page · Post #: 4 · Written at 12:35:50 PM on 6 August 2019.
Alarum's Gravatar
 Location: Toowoomba, QLD
 Member since 6 July 2019
 Member #: 2359
 Postcount: 12

The radio has a "Music Masters Radio Co" dial and the valve lineup seems to indicate that it is an A5 model made for DC power. I'm not 100% sure on the model as two valves are missing the valve numbers, though the other valves and capacitors seem to line up well with the schematic.

At the moment, there are 5 resistors in parallel running into the vibrator, which I assume is dropping the voltage from 32V to 24V. However most of the resistance values have been rubbed off and are unreadable.


 
 Return to top of page · Post #: 5 · Written at 10:18:11 PM on 6 August 2019.
Brad's avatar
 Administrator
 Location: Greenwich, NSW
 Member since 15 November 2005
 Member #: 1
 Postcount: 5505

Photos uploaded.


‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
A valve a day keeps the transistor away...

 
 Return to top of page · Post #: 6 · Written at 12:27:06 AM on 7 August 2019.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 5262

most of the resistance values have been rubbed off and are unreadable.

Put an ohmmeter across each of them.


 
 Return to top of page · Post #: 7 · Written at 12:44:09 AM on 7 August 2019.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3707

The A5's appear to be mains sets V5 is a vibrator set and they're all 6V. V6724 is a 24V Synchronous vibrator, Max 1.7A 100 cps

This is looking like king size hack. The only set I see in AORSM's 32V is V525M 7 it did not use octal valves. Those resistors look like a dogs dinner and I have every doubt as to them being in any way, original. I have had to deal with a mains STC with a resistor mess like that. He was trying to fix the bias after his mod: I fixed it, I put it back to original. Lots of radios work well when you do that.

What are the numbers on the valves you have? I suspect something like 1C7 and there are five valves with their filaments in series. If so this may have been a 6V one?


 
 Return to top of page · Post #: 8 · Written at 7:49:00 PM on 7 August 2019.
Robbbert's avatar
 Location: Sydney, NSW
 Member since 18 September 2015
 Member #: 1801
 Postcount: 1074

Ugh, what a mess. I can see 2 resistors marked 100 ohms. The dead giveaway of a horrible hack is all those resistors soldered to a metal ring next to one of the valve sockets. It's obvious someone was trying to match incompatible voltages. Did they succeed?

I'd expect all those resistors to be close to their original values, despite the grotty appearance, so just disconnect and measure each one.

After that, you'll have to sit down with a calculator and do a bit a of maths to work out what is and what should be.


 
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