25-07-2011, 11:03 AM
Rob just beat me to it with the mention of a bench power supply, but allow me to emphasise how important this is 
If possible, try to get one that does fully adjustable current limiting. While any bench power supply will have some sort of built in current limiting (so that it doesn't get damaged by an external overload), the next consideration is the device under test. When I power up a new radio for the first time, here's the general procedure:
Basically, observing the current is a bit like using a stethoscope - you can learn a lot about the health of a set from doing it. Yes, you can use batteries and an Avo to monitor the current, but it's so much easier with a "proper" power supply. And being able to set a the current limit makes everything so much safer.
I made up some leads with 4mm plugs on one end (into the PSU), and PP9 battery snaps on the other. Of course, I have a load of similar ones terminated in miniature crocodile clips as well. Highly recommended :D
A lot of cheaper power supplies don't have the variable current limit and don't really lend themselves to having it retrofitted. I'd recommend looking for something like a Farnell "L Series", or the Thurlby-Thandar PL320 range - the latter comes with digital metering (I like, but not everyone agrees) and allow you to set the current limit *before* switching on the load. These are industry standards, but fairly affordable now. And importantly, service data is readily available...
If you can find one, a dual PSU is really convenient for those sets that have 2 batteries that aren't just wired in series. Sets like the Hacker Herald RP10, or the Mini-Herald RP17, to name a couple that I'm very familiar with... Plus, you can put them in series or parallel for extra volts or amps. The extra voltage is helpful when "zapping" away the tin whiskers in AF11x transistors :P
I actually couldn't tell you how many power supplies I have! Over the years I have made many, and still have most of them. I did a design for work which satisfies the above criteria, and could post details of it if you wish. It uses a dual op-amp, half a dozen transistors and a digital meter module. If you have a suitable case and heatsink, it could be built for less than £50...
Hope this helps,
Mark
* A good friend exported a radio to the US. A while back I furnished him with a nice L-Series PSU, but he's not very technical, and not yet in the habit of using the power supply. So he failed to notice that when the power switch was set to the off position, the set was still powered! His customer complained that the PP9 batteries were only lasting 4 days! Luckily, this person had a friend that was technical, so the problem was sorted out fairly quickly without having to send it back here, but it was still rather embarrassing :s
If possible, try to get one that does fully adjustable current limiting. While any bench power supply will have some sort of built in current limiting (so that it doesn't get damaged by an external overload), the next consideration is the device under test. When I power up a new radio for the first time, here's the general procedure:
- Set the voltage to the required level, set the current limit to around 50mA - perhaps a little less.
- Check that the radio consumes 0mA when switched off*
- Switch on, look to see how much current is consumed with the volume at "min" - hopefully less than 30mA, but check the manual if you have one.
- If there is a fault such as a shorted output transistor, the 50mA current limit will stop the rot spreading to the other transistor.
- If the fault is a heavily leaking capacitor, you should see the current slowly fall away over the course of a minute or two as the capacitor reforms itself - while it should be found and replaced, it tell you what you are dealing with...
- If the set is dead, but the amplifier responds to the audio checks Yorkie describes, check the current. If it's about 10mA too high, it will be a short on the power supply to the IF stages. Perhaps caused by an AF117, equally possibly caused by a shorted electrolytic...
Basically, observing the current is a bit like using a stethoscope - you can learn a lot about the health of a set from doing it. Yes, you can use batteries and an Avo to monitor the current, but it's so much easier with a "proper" power supply. And being able to set a the current limit makes everything so much safer.
I made up some leads with 4mm plugs on one end (into the PSU), and PP9 battery snaps on the other. Of course, I have a load of similar ones terminated in miniature crocodile clips as well. Highly recommended :D
A lot of cheaper power supplies don't have the variable current limit and don't really lend themselves to having it retrofitted. I'd recommend looking for something like a Farnell "L Series", or the Thurlby-Thandar PL320 range - the latter comes with digital metering (I like, but not everyone agrees) and allow you to set the current limit *before* switching on the load. These are industry standards, but fairly affordable now. And importantly, service data is readily available...
If you can find one, a dual PSU is really convenient for those sets that have 2 batteries that aren't just wired in series. Sets like the Hacker Herald RP10, or the Mini-Herald RP17, to name a couple that I'm very familiar with... Plus, you can put them in series or parallel for extra volts or amps. The extra voltage is helpful when "zapping" away the tin whiskers in AF11x transistors :P
I actually couldn't tell you how many power supplies I have! Over the years I have made many, and still have most of them. I did a design for work which satisfies the above criteria, and could post details of it if you wish. It uses a dual op-amp, half a dozen transistors and a digital meter module. If you have a suitable case and heatsink, it could be built for less than £50...
Hope this helps,
Mark
* A good friend exported a radio to the US. A while back I furnished him with a nice L-Series PSU, but he's not very technical, and not yet in the habit of using the power supply. So he failed to notice that when the power switch was set to the off position, the set was still powered! His customer complained that the PP9 batteries were only lasting 4 days! Luckily, this person had a friend that was technical, so the problem was sorted out fairly quickly without having to send it back here, but it was still rather embarrassing :s
