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stonehopper 18th Apr 2009 11:18 pm

Roberts RT7 - problem
Hello, first post here, and a complete novice to radios and their workings. Having fallen for an RT7 - based on 'I like the look of it', I find that after a good clean up of case, chassis, and screen - it don't work! It didn't work before the clean up, though having found a 9v battery to connect to, the speaker makes a small click sound when power is applied or removed, but diddley squat other than that. There are what appear to be two transformers on the chassis, one of which (the right hand one as seen from the back) gets warm to the touch after a few minutes of being 'on'. That's about all I can tell you, other than the controls operate smoothly, and all wires seem attached securely.

Clearly there must be one or more components in need of replacement or repair, but would it be possible to suggest a most likely candidate for failure, or is that a bit like asking 'my car won't start - why?'

Any help in advising a course of remedial action welcomed - but my knowledge of radio terminology is close to zero. Where should I start testing, and what with?


Steve_P 19th Apr 2009 9:17 am

Re: Roberts RT7 - problem
First of all, a bit of good news : The RT7 does not have the AF11x problem.

The circuit is available up top there. You're getting power otherwise you wouldn't hear a plonk when you switch on.

So - Clean the wavechange switches, check all wiring. Can you put a signal in at the Volume Control? A CD player will do - earth to earth, other wire in at the centre pin of R23. (Volume Control).

If it seems OK, then you really need some test gear - a Multimeter at least. If there is no sound, I would go for TR6 and TR7.


Steve P.

Darren-UK 19th Apr 2009 9:44 am

Re: Roberts RT7 - problem
This is probably a wild goose chase, but the RT7 was designed to use the now-obsolete PP10 battery. Derek's example will, presumably, have been modified to use a PP9 or whatever. If the modification included replacing battery leads as well as the connector, it may be worth checking the soldered joints both ends thereof (as Steve suggests above) and also for any heat damage at the chassis end of the leads.

Long shot, but check polarity is correct too - it's not unknown for people to modify radios to accept a different battery, only to incorrectly polarise it.

Very obviously, check the battery isn't duff too. The Forums have had repair threads in the past where the 'fault' has eventually turned out to be a knackered battery. Don't rely on an off load reading, test it on load too.

petervk2mlg 19th Apr 2009 10:09 am

Re: Roberts RT7 - problem
What about "the transformer getting warm to the touch" though?
Doesn't this indicate the possibility of too much currently flowing?

Darren-UK 19th Apr 2009 10:26 am

Re: Roberts RT7 - problem
I'd read post #1 and then forgotten about the transformer issue! If one is getting warm it sounds like a biasing problem at the audio/output stage, faulty transformer, faulty/shorted output transistor (of which there are two of the latter, of course).

stonehopper 19th Apr 2009 11:28 am

Re: Roberts RT7 - problem
Thanks for the replies.
I will have to take your word for the good news Steve, as I wouldn't know what an AF11x problem is!
I have downloaded the service sheet (pity about the crease across the page - it's mentioned in despatches) and located the various components.

With the little PP9 9v battery connected (the 'connectors' are missing, just two bare wires - an indication of past salvage probably) the speaker 'pops' as mentioned. As the transformer T2 warms (T1 does not) so too does the battery. After a couple of minutes being attached and on, the battery voltage reads 8.54v and is as warm to the touch as the T2. As I did not check its voltage before attaching (new battery) it's not possible accurately to say if its shorting or not, but leads me to think it may be - though I'm a novice to radio!

Steve, it's probably common knowledge to most, but how do I get a 'signal' out from a separate appliance - would it be from an output socket such as to headphones? Will I need to find a suitable donor ear phones and strip the wires (which one will be earth?) to make a connection to earth and the centre pin of the R23 - do you mean the centre pin from the front after removing the plastic knob?

When checking transistors, capacitors and resistances, must they be removed from the chassis first? I have a digital multi-meter, and see values on the service sheets, but first will clean up the wave length switches and report.

Darren (or Steve) - how can I check the polarity? Will it have toasted everything if wrong?

Cheers - Derek

Darren-UK 19th Apr 2009 12:09 pm

Re: Roberts RT7 - problem
Incorrect polarity, after prolonged connection (a minute or two) will theoretically cause significant damage, but in reality it'd likely blow out an electrolytic cap before anything else.

Strange, but not unusual, to find bare-ended power leads. This suggests someone has removed the PP10 connector, tried the radio by touching bare wires onto some battery or other, then abandoned it for some reason. Possibly the fault pre-existed your ownership.

Transistors must be checked following removal, but the OC series don't give much trouble other than at the output stage through a fault or abuse. Capacitors and resistors can be checked in situ providing nothing else is connected in parallel with them.

T2 is the output transformer ie that which drives the loudspeaker. If that and the battery are warm, excessive current draw is indicated. I'd check this with a meter and suspect either or of the output transistors, which I think are OC81's on this model.

You're on the right track regarding Steve's suggestion of injecting a signal. Use the headphone socket of whatever. Imagine a 3.5mm jack with bare-ended wires leading therefrom. Just connect two of the wires to the RT7 as Steve's described. Remember that the jack output from a CD player or whatever will likely be stereo.

Brian R Pateman 19th Apr 2009 12:20 pm

Re: Roberts RT7 - problem
Hello Derek,

In your first post you say that you are a complete beginner as far as radio is concerned.

First of all - don't worry, we all were once and there is a wealth of experience here to help you. You will make mistakes, we all did and I know that I still do, even after over 40 years of fiddling around with electronics.

If you haven't already, you might like to take a look at Paul Stenning's main site here;

There's lots of useful information for the beginner to be found there.


geofy 19th Apr 2009 12:58 pm

Re: Roberts RT7 - problem
1 Attachment(s)

Originally Posted by stonehopper (Post 240145)
what an AF11x problem is!

Darren (or Steve) - how can I check the polarity? Will it have toasted everything if wrong?

Cheers - Derek

Your radio have all the signs of either short circuit output transistors because of the overheating output transformer or the battery connected the wrong way round, and as you have had it connected for long enough to get things warm, if it is the latter then it may have damaged transistors in any case. But the previous owner would have done the same if a battery had been connected wrong.

To find out which way the battery should be connected look at the diagram and either find a large value electrolytic capacitor with the positive side shown and trace this back to the positive battery lead. Or trace the Emitters of the PNP transistors back to the printed circuit via the Emitter resistors of each transistor and this is where the positive side of the battery should go, usually via the on/off switch. The negative side of the battery to the negative side of the electrolytic.

Mullard produced a germanium transistor range with the AF designate such as AF114 AF115 AF116 AF117 that obviated the need for external negative feedback capacitors and resistors to neutralise the self capacitance of earlier transistors like the Mullard OC44 OC45 and the transistors where very good when new. But they had two flaws that led to failures, first the screening lead could break internally and short out the base connection, and more seriously the metal used in the case construction causes fine metal ‘whiskers’ to grow and eventually come into contact with the semiconductor and short out the electrodes to the case.


Steve_P 19th Apr 2009 2:10 pm

Re: Roberts RT7 - problem
How to check battery polarity without a meter.

This set is positive ground by the way.

The battery should have two pins, positive and negative. The positive wire from the battery goes to the speaker. Try and trace this through and see which one - positive or negative, does. The ground may be a large area of the board. It joins the IFTs (small transformer like things) and the metal cases of the two transformers.

If the output transistors are short circuit, you need to buy and fit replacements. If you need help, get back to us and see if anyone on here could do this for you. Or, do it yourself.


Steve P.

stonehopper 19th Apr 2009 6:55 pm

Re: Roberts RT7 - problem
Thanks all, and Brian for the welcome - and yes, I've visited Paul Stenning's site, and got my service sheet from there, though Geofy's talking a foreign lingo! (Appreciated though - I'll learn in time). I can only guess at the symbols on the diagram for now Geof, but I will get there.

I have taken the red lead for the battery to be positive and connected with crocodile clips in absence of the correct PP10 or 9 connectors.

This red lead connects to a tab from the 'OFF' switch.
The black lead connects to several items, C32, T1 & T2 also.
There is a wire leading from one of the T2 connections to the speaker - coloured red.
The black speaker wire connects via a track to several of the little 'torpedos' with coloured bands.
The big C32 in the corner has its red top connected to the perimeter band of solder (I'm sure there's a correct name for the soldered tracks under the board?) which in turn is connected to the metal frame, so positive earth. Fair enough.

The body of T2 shows distinct signs of melted plastic at the top on one side, a dob of wax has collected on the twisted tab holding it to the board (melted most like + gravity) and even a small smoke burn on the metal arch above it, so must consider it 'no longer with us'.

It does look like there may be other issues with items other than this T2 as has been suggested, but until I replace that - short of removing everything and testing for values - I must ask if all these bits are actually available? Where from - and how expensive? I probably paid too much in £7.50, but I'm a sucker for an ugly face.

I've taken a couple of pictures, but I'm sure you all know what toasted components look like.

What's an 'emitter' look like, and what does it do?

Steve_P 19th Apr 2009 9:52 pm

Re: Roberts RT7 - problem
Right - the Red is positive, the Black is Negative.

The Emitter is one of the three pins of a transistor. The other two are Base and Collector. There's a diagram on the sheet you have.

The torpedo things are resistors by the way. Quite a good description that!

I'd say, judging by what you've told me that the Output Transistors are shorted. Did happen with Roberts sets. The transistors are probably getting red hot which is what the wax is from.

Now - you need some basic tools and a soldering iron. Also a meter. Get down to Maplin, they'll do all this.

For parts, I don't think Maplin do OC81's. Try Past Times Radio at

Don't put transistors in the wrong way round. And check your wiring before you switch on.

Good luck.


Steve P.

stonehopper 20th Apr 2009 7:49 am

Re: Roberts RT7 - problem
Thanks Steve, the soldering iron's been with me since '65, though I will be after one of those 'suck-it-up' sprung loaded syringes. I'll do some shopping. Might have to put it on the shelf for a few days, but will get back to you all.
Thanks again - Derek

geofy 20th Apr 2009 3:26 pm

Re: Roberts RT7 - problem

Originally Posted by stonehopper (Post 240336)
I'll do some shopping. Might have to put it on the shelf for a few days, but will get back to you all.
Thanks again - Derek

Might seem like a foreign language Derek even though I was pitching in at a fairly basic level. You should try to get a book on basic electronics to help you understand the subject better.

The type you are describing is a solder sucker, not an iron, a useful device to have though care has to be taken not to overheat the printed tracks which can lift off the board under the recoil of the solder sucker.


stonehopper 20th Apr 2009 10:24 pm

Re: Roberts RT7 - problem
Thanks Geof, I had wondered about that, so I've ordered a bit of Solderbraid as well. never used that before, but then never needed to. Pair of heat sink tweezers as well. - And dropped a line to Past-times radio about parts.

stonehopper 24th Apr 2009 1:50 pm

Re: Roberts RT7 - problem
Oooops! Did I do something wrong?

I've browsed the site for component suppliers, and looked at Cricklewood Electronics who list OC81's. They have arrived, but are much smaller than those fitted to the set.

Have I slipped up, or has technology advanced enough to compact the required function into these OC81's that have arrived. They are made by Mullard, black cylinders with a small metal recessed cap, three eights of an inch long, with inch and a half long tails none of which are insulated - or marked as to which tail goes where. The malfunctioning OC81's are a full inch in length, two wires insulated red and black, in an all metal cylinder. Oooer.

Steve_P 24th Apr 2009 2:07 pm

Re: Roberts RT7 - problem
Put them in (the right way round!) and heatsink them. That's why they are clamped down.

Try and see. It's no use as it is, now is it?


Steve P.

Darren-UK 24th Apr 2009 3:06 pm

Re: Roberts RT7 - problem
1 Attachment(s)
I don't think I'm familiar with OC81's that are a full inch in length. Attached is a pic of a Mullard OC81; this is about 15mm long (the can) and is the type we see commonly.

If you're not sure how to connect them, the transistor can usually has a coloured dot adjacent to the Collector. See this page.

geofy 24th Apr 2009 4:49 pm

Re: Roberts RT7 - problem

Originally Posted by stonehopper (Post 241266)
The malfunctioning OC81's are a full inch in length, two wires insulated red and black, in an all metal cylinder. Oooer.

A picture or two would be helpful as too much guesswork is going on. The two wire black and red sound more like a diode such as the AA series which is used in some biasing arrangments.

The bare leads of the OC81 are collector, (with a paint dot near it) then the middle base, and then emitter. You will need to familiarise yourself with these as you have already said you haven't a clue what they mean.


stonehopper 24th Apr 2009 5:49 pm

Re: Roberts RT7 - problem
3 Attachment(s)
Gents, my 'inch' was a lie. Must use calipers. The existing OC81's are 15mm in length x 6mm dia. The new ones are 10mm x 6.5mm dia.

These new ones were listed in Cricklewoods transistor section under base numbers Nxxx - Sxxx ; P3:

The first pic. is the new 'uns, the second shows both transformers with some 'melt' on top of the left hand one (T2 on the sheet), and the third shows the two existing OC81's side by side between the T2 and the C32 capacitor.

Wrong ones?

PS I suspect base is the middle wire of the three, and perhaps(?) collector aligning with the grey line on the new ones. but I'm not doing anything until further guidance or confirmation.


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