Bush TR82 radio - why is power supply reversed?

[indigo.girl]

Hi, I'm restoring an old TR82C bush radio and am looking at the circcuit diagram. I have noticed that the 9V battery is connected with -ve on the positive rail and +ve on the ground rail. At first I thought it was a misprint but it says the same on the traders and the manufactures data sheet. When I inject audio at the test point I only get speaker output with the polarity as shown in the circuit diagram (although the voltage measurements come out negative rather than positive).

I've never see this before. Is this a common set up and why is it done this way? Here's a screen shot of the diagram from manufacturers datasheet.

[Herald1360]

That polarity is quite right for PNP transistors where collector is negative and emitter is positive.


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.

Have a look at the TR82's close relative, the VTR103. Don't know why Bush did it that way!

[julie_m]

In circuits using PNP transistors, it was usual for the positive to be connected to chassis.

[ms660]

It's because the transistors are PNP types, they need -ve on the collector with respect to the emitter.

With the later NPN types the battery -ve would be connected to the common signal rail (ground/chassis or whatever) because they need +ve on the collector with respect to the emitter.

EDIT: Post crossed.

Lawrence.

[indigo.girl]

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. Thanks for the quick response guys

[Sideband]

As above. Strictly speaking, the supply rails are not reversed, although it may look like it. It's just that the earth rail of sets with PNP transistors is positive whereas the earth rail for NPN transistors is negative. So in your case the polarity is correct for the technology used.

[Sideband]

Originally Posted by Herald1360

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

Understanding this rather depends on when one started playing with radio Pre 60's all valve, positive on top. 60's lots of PNP about, got used to negative as the 'rail'. After that mainly NPN and positive got to be the supply of choice. I thought a negative supply was wrong in the late 60's (I was less than 10) until I read a bit more.

[ms660]

Originally Posted by indigo.girl

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.

It can be confusing at first, to measure the voltages in that receiver connect your meter +ve to chassis and all will be well.

The detector diode in that receiver is known to go faulty if there's a no reception fault.

Lawrence.

[vinrads]

Can you imagine back in the sixties , I was thrown in at the deep end and told this is your job repairing these transistor radios . Nothing lit up what's more they were made upside down Happy days .Mick.

[indigo.girl]

I presume this is just something we see in battery operated circuits. For convenience I have powered my circuit from a 5V USB power supply - I'm presuming the transformer I'm using that's converting the 240V AC to the 5V DC USB output is isolating the output from the mains so this doesn't matter.

When I inject my signal do I still connect my ground to the circuits ground, even though this ground is +ve?

[ms660]

Yes, connect your signal ground to the receivers chassis.

The chassis is the common signal rail, ie: it's common to the base/emitter input signal and the collector/emitter output signal, just like in a valve receiver, ie: grid/cathode signal input and anode/cathode signal output.

Lawrence.

[indigo.girl]

Originally Posted by ms660

The detector diode in that receiver is known to go faulty if there's a no reception fault.

Well predicted Lawrence - I have a very noisy output when I inject at any point prior to the volume control. The collector output of TR3 passes through a diode on its way to the volume control so I suspect this is faulty. Its inside a metal box so not sure if its replaceable which is annoying...

[ms660]

If you suspect the detector diode then you should be able to test it without removing the IF can, disconnect the inner screened lead from transformer pin 5 then measure the diode both ways between transformer pin 5 and chassis.

By the way I don't know what signal source your using but pre detector injection would need an RF signal source of the correct frequency.

Lawrence.

[Nuvistor]

It's replaceable, just a bit awkward to get at.

Frank

[Tractionist]

Mmmmmm ........... that's an early model - so at least it's not infested with AF117's etc. Those earlier glass-enveloped trannies are pretty reliable .... but they are getting on a bit in years!

By the same token - I would as a matter of course check [and probably replace all the electrolytics .... particularly the colourful Plessey units] ..... similarly, the wave change wafer always responds to a good scrub with Servisol [regularly!].

Nice easy sets to work on .... once you've prised the tuning 'knob' [great name for something that doesn't really look like one!!] off ....... They don't perform too badly either ..............

[Refugee]

Those diodes usually go open circuit and many repairs are done by just fitting a new one across the pins under the chassis.

[David G4EBT]

Originally Posted by Nuvistor

It's replaceable, just a bit awkward to get at.

If you just leave the duff diode in its 'metal coffin' and fit the new diode externally across the IFT pins observing the polarity of the diode, it can be done in a matter of minutes. The old diode is open circuit so won't have any adverse effect on the new one.

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.

[m0cemdave]

Back in the day, British-made cars had positive earth too...