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Old 28th Mar 2017, 7:03 pm   #21
vinrads
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Default Re: Bush TR82 radio - why is power supply reversed?

Have I missed something or are you running this on a five volt power supply? Mick.
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Old 28th Mar 2017, 7:04 pm   #22
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Default Re: Bush TR82 radio - why is power supply reversed?

A point regarding Earth Polarity. Many Car radios made in the mid to late 60s used PNP Transistors, so the Negative side was the supply rail as has been mentioned, but these radios were also Dual Polarity, so could be set for either Positive or Negative Earth. Service sheets usually gave the readings as being Negative w.r.t. the Positive line.
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Old 28th Mar 2017, 7:42 pm   #23
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Default Re: Bush TR82 radio - why is power supply reversed?

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Originally Posted by indigo.girl View Post
Aha - I see That explains it then. What is confusing is that they have labelled the voltages on the -ve rail as positive volts when it is actually a negative voltage wrt +ve rail.
The emitter of any circuit always goes to the polarity of the emitter for the transistor to function, a P emitter goes to the plus rail, an N emitter goes to the minus rail. Radios like this should remain original without additions or modifications, just my opinions on this. If the detector diode is faulty a temporary bridge with an OA90 diode or similar germanium diode will show if this is where a fault is. It tends to be cheap radios that use two legs of a transistor junction as a diode that suffer this failure more.
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Old 28th Mar 2017, 8:22 pm   #24
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Default Re: Bush TR82 radio - why is power supply reversed?

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Have I missed something or are you running this on a five volt power supply? Mick.
Yes, Mick - for convenience in testing as I do not have a 9V battery to hand. Of course all the voltage readings are 5/9s lower than quoted but nevertheless I am getting a very acceptable volume output.
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Old 28th Mar 2017, 9:33 pm   #25
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Default Re: Bush TR82 radio - why is power supply reversed?

Stick two 5V PSUs in series with a silicon diode?
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Old 28th Mar 2017, 9:48 pm   #26
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Default Re: Bush TR82 radio - why is power supply reversed?

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My thought on these 'in a can' moments is "made by man (probably an underpaid lady worker at the time), fixable by anyone with a bit of nouse. Unless it's potted, these are not.

Digressing slightly, I bought a Perdio transistor set from a charity shop, the lady that took my money said she used to make them. I popped the back open and we had a very nice half hour discussing the innards.
Yep - one sees a little bit [sometimes a lot!] of 'humanity' in a hard-wired hand assembled set ..... much if not all of which disappeared when PCB's and then fluid-bed soldering etc. etc. appeared.

A lot of ladies do seem to have done a lot of this type of assembly work - photos of the Philco 444 ['Peoples Set'] assembly line in Greenford don't feature any men at all. Maybe the ladies were more patient and attuned to these repetitive tasks? Certainly, in most instances, they made a pretty neat and tidy job of things.
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Old 29th Mar 2017, 6:54 am   #27
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Default Re: Bush TR82 radio - why is power supply reversed?

I would get a 9 volt supply before going too far and before you start replacing components.
I have had about 4 of these sets pass through my hands and none have needed any capacitor changes, so far, but the red and black capacitors can be iffy after all this time. So use a decent 9 volt supply to give them a chance of reforming.

Any wax or paper capacitors will probably be OK as well, the low voltage in these transistor sets does not result in a lot of leakage.
I like to insert an amp meter in the 9 volt supply, it will give you a good indication of the health of the circuit and components as you turn the radio on. (as well as later checking voltages on the various points around the transistors).

You can sometimes pickup decent lab type variable low voltage supplies (0 to 30 volts) for not a lot.

By the way it is important to not damage the black point on the transistors as they are light sensitive inside and can generate noise if light leaks in.
Nice radios to work on, and also fairly decent sound as well.

Mike
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Old 29th Mar 2017, 10:32 am   #28
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Default Re: Bush TR82 radio - why is power supply reversed?

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Originally Posted by indigo.girl View Post
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Originally Posted by vinrads View Post
Have I missed something or are you running this on a five volt power supply? Mick.
Yes, Mick - for convenience in testing as I do not have a 9V battery to hand. Of course all the voltage readings are 5/9s lower than quoted but nevertheless I am getting a very acceptable volume output.
If the radio and your signal source are running from the same 5 volt supply and the signal source is grounded directly to the radio chassis then that would short out the 5 volt supply (I think)

Lawrence.
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Old 29th Mar 2017, 1:09 pm   #29
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Default Re: Bush TR82 radio - why is power supply reversed?

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I would get a 9 volt supply before going too far and before you start replacing components.
I have had about 4 of these sets pass through my hands and none have needed any capacitor changes, so far, but the red and black capacitors can be iffy after all this time. So use a decent 9 volt supply to give them a chance of reforming.

Any wax or paper capacitors will probably be OK as well, the low voltage in these transistor sets does not result in a lot of leakage.
I like to insert an amp meter in the 9 volt supply, it will give you a good indication of the health of the circuit and components as you turn the radio on. (as well as later checking voltages on the various points around the transistors).

You can sometimes pickup decent lab type variable low voltage supplies (0 to 30 volts) for not a lot.

By the way it is important to not damage the black point on the transistors as they are light sensitive inside and can generate noise if light leaks in.
Nice radios to work on, and also fairly decent sound as well.

Mike
By and large I agree with you Mike - but I too have worked on any number of these sets, and ..... have needed to replace a few caps in my time! However, running those electrolytics up on a decent 9v source is very sound advice - similarly so the issue of Ge glass envelope tranny photo sensitivity.

On the subject of those Ge diodes [AKA detectors] ... I have also had need to replace one or more of the FM discriminators in Bush VTR 103's .......

Finally, in every instance - I've had to replace the tuning cap suspension. This can actually be done without removing or de-soldering the component itself - careful severing of the old fibre grommets [with a craft knife or razor blade] and replacing with an appropriately sized neoprene grommet one at a time is my preferred MO. This measure will prevent the tuning scale needle and dial knob ultimately scratching and removing the scale legend etc. etc.
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Old 29th Mar 2017, 6:08 pm   #30
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Default Re: Bush TR82 radio - why is power supply reversed?

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Originally Posted by crackle View Post
I would get a 9 volt supply before going too far and before you start replacing components.
I have had about 4 of these sets pass through my hands and none have needed any capacitor changes, so far, but the red and black capacitors can be iffy after all this time. So use a decent 9 volt supply to give them a chance of reforming.
Thanks Mike - I have already recapped and all seems to be working fine on the 5V. I would prefer to keep the 5V supply because I also want to run a 5V Bluetooth from it so the radio can also play digital audio.

Apart from reducing the max volume by using 5V instead of 9V is there any other disadvantage to changing its original supply like this?
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Old 29th Mar 2017, 6:19 pm   #31
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Default Re: Bush TR82 radio - why is power supply reversed?

P.S. And definitely check the quiescent prior to wrapping it all up - just to ensure that those old Ge's aren't having a private party! (A properly sorted TR82 will run for months on end on a new PP9).
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Old 29th Mar 2017, 6:56 pm   #32
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Default Re: Bush TR82 radio - why is power supply reversed?

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I would prefer to keep the 5V supply because I also want to run a 5V blutooth from it so the radio can also play digital audio.
If you are going to run an PNP circuit (positive-earth) and an NPN circuit (negative-earth) from the same power supply, then you must connect only the "live" connection of the audio signal between the two, and only via a capacitor. If you use a screened audio cable, make sure the outer is only connected at one end (earthing the sending end usually works best, but try the receiving end if you get excessive hum and noise pickup).
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Apart from reducing the max volume by using 5V instead of 9V is there any other disadvantage to changing its original supply like this?
Not really.

5V is below the output of even a pretty well spent 9V battery (usual practice is to aim for operation down to 0.9V per cell), and if the set will work at all in that state, then you should consider it a happy accident.

Running on a reduced voltage won't damage the set; but it could cause underperformance including reduced output volume, audible distortion, whistles, farting noises, loss of sensitivity and inability to tune in certain frequencies. You're basically out of the set designer's comfort zone and into the hands of the Gods.

Some plug-in USB PSUs are hackable, though! You can change a resistor to adjust the output voltage. (At least, you can change it if you have a fine-tipped, thermostatically-controlled soldering iron, a steady hand and the patience of a saint!) Look for the larger of two resistors between a pin on the IC and the negative of the primary-side smoothing capacitor (the smaller one will be for setting the current). Wire in its place a suitable potentiometer, adjusting it to the same value as the original resistor to begin with; then connect a voltmeter to the output, with a 470Ω, 1W resistor as a load, adjust the pot for 9V output and finally replace with nearest preferred value. How long the smoke will stay in is anyone's guess, but it's got to be worth a punt. If all my tools and components weren't in storage pending laying a new floor after flood damage, I'd have a go myself! Be sure to change the output connector of this hacked power pack, for obvious reasons .....
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Old 29th Mar 2017, 7:51 pm   #33
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Default Re: Bush TR82 radio - why is power supply reversed?

You might have success with the radio but I think it would work best with two separate power supplies, one for the radio and one for the Bluetooth unit.

Lawrence.
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Old 29th Mar 2017, 8:36 pm   #34
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Default Re: Bush TR82 radio - why is power supply reversed?

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What can really blow your mind is PNP radios with a negative chassis. It can be done- the signal doesn't care which polarity is ground but the circuit looks kinda inside out and upside down.
Yes I've seen that. I wonder if anyone did NPN sets with a positive chassis just to be awkward
Never seen that, but I have come across valve (hybrid) car radios with a positive chassis! For positive earth cars, PNP output stage, all fine there, till you follow back to the valve stages! Then the anode loads get returned to chassis, the cathodes and screen-grids are bypassed by capacitors to chassis, but the cathode feed and grid-leak resistors have to go to the upper (negative) rail!
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Old 31st Mar 2017, 5:32 pm   #35
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Default Re: Bush TR82 radio - why is power supply reversed?

Re #11, in 1968 when I was a vacation student at the old STC Submarine Cables lab at North Woolwich, I was given a project to update a circuit that used PNP germanium transistors with a -ve supply rail, to use Silicon NPN transistors with a +ve supply rail.
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