Roberts R707 had mains in battery terminals!

[Robsradio]

Evening,
I bought this some time ago from a car boot sale and was concerned at the time to see a figure 8 socket connected to the 12v battery contacts, and even more concerned when the seller said it doesn’t work as he had plugged it in… to the mains to try it!
I decided to bite the bullet today and see what damage had been done.
No visible damage, connected to 12v, but nothing from the speaker and too much current being drawn.
The shopping list wasn’t too extensive and surprisingly both Mullard modules survived and worked.
AC187 and AC188 output transistors were fried, along with one of the 1 ohm emitter resistors, one Lockfit PNP device was fried, and one NPN tested ok but produced awful distortion in circuit.
Along with the usual switch and pot cleaner, it now works fully and sounds very nice.
I’m surprised at how lightly it got off considering.
Why on earth somebody fitted a figure 8 onto the 12v contacts we shall never know.
Another set saved and joins the working radio collection.
Cheers
Rob

[Nickthedentist]

That’s amazing, I would have expected much worse. Well done

[deswradio]

I've repaired lots of Roberts radios via a repair cafe. And a significant proportion of them have been damaged by using the wrong power supply. Incorrect polarity of the barrel connector is common - there's lots of confusion about matching voltage, max current, polarity and barrel diameter. I really have heard people say "Surely any power adaptor will do..."!

Des

[Robsradio]

Cheers gents, I've seen a few that have popped due to centre pin +ve in a centre pin -ve radio, but never mains on the battery connections.
Nick, me too, I was expecting a total wipeout of transistors, low value resistors and some electrolytics, I just got lucky.
Cheers
Rob

[G6Tanuki]

When I occasionally fix things like this for friends, I am happy to tack a suicide diode across the power supply rails, something like an 1N5401 so that it will short out any future reverse polarity connected supply.

[Nickthedentist]

...which is a very worthwhile mod, unless the mains-up-the-DC-input scenario happens

[ortek_service]

It also relies on the PSU having a reasonable current limit - With CB's and Car Stereos it was common that people had removed the original in-line fuse in the power-lead.
And when connected the wrong-way round, the reverse-polarity protection diode then failed s/c, taking out a short length of track to it - as these were usually on a spur, so the protection was no-longer present!

I also had a mobile 12V CB where someone had replaced the power lead with twin Brown & Blue mains-cable, as twin Red & Black cable was harder to come-by. And someone-else, not familiar with these, had then apparently connected it to the mains!
I did actually manage to fix it, as liked a challenge - But it did need all the IC's and the odd transistor etc. replacing that I luckily had spares of all!

[Robsradio]

Yes, i was expecting a larger shopping list of dead components!

[Michael Maurice]

It’s very fortunate that the thing didn’t go up in flames!