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?

Derek

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.

Cheers,

Steve P.

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.

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?

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).

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

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.

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;

http://www.vintage-radio.com/

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

Regards,

Re: Roberts RT7 - problem
 

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.


Geof

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.

Cheers,

Steve P.

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?

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 http://www.pasttimesradio.co.uk/index.html.

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

Good luck.

Cheers,

Steve P.

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

Re: Roberts RT7 - problem
 

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.

Geof

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.

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.

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?

Cheers,

Steve P.

Re: Roberts RT7 - problem
 

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.

Re: Roberts RT7 - problem
 

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.

Geof

Re: Roberts RT7 - problem
 

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:
http://www.cricklewoodelectronics.co...cat=191&page=3

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.

Derek

Re: Roberts RT7 - problem
 

The new ones are OC81M which are miniature versions of the OC81 and should be suitable. The leads are in a triangular configuration with the collector next to the black line. The base is the middle lead and the emitter being the remaining lead. The OC81 has the three leads in line with the collector next to the red dot, base in the middle and the emitter being furthest away from the red dot. (Also the base collector spacing is usually greater than the base emitter spacing.

Keith

Re: Roberts RT7 - problem
 

Thanks Keith - but did you mean 'Grey' line? I'm sure you must have!
Looks like I'm on the starting grid then.

Derek

PS Another question gents - The metal bodied version is held in copper sleeves riveted to a frame section, I presume for heat transference.
The new ones are coated in some sort of rubbery material. Putting these in the metal sleeves may retain more heat, as the soft skin acts as an insulator. Should they be put back into the original sleeves, or not?

Re: Roberts RT7 - problem
 

Yes it's the grey line. I was comparing it to an OC81DM I have which has a black line.

Keith

Re: Roberts RT7 - problem
 

I can't see the metal clip, but for this radio if it is used at average listening it is unlikely that the transistors will need any heat sinking. Whilst it is desirable most small output stages just have the transistors without a heatsink as not much heat is being produced. It may be possible to remove the plastic cover though I have never tried this, and the markings would be lost of course. A suitable replacement transistor which is more robust would have been the AC128.

Geof

Re: Roberts RT7 - problem
 

Thanks Geofy, I note the positions.
I shall let these wave in the breeze.
The clips are attached to an angled bracket which is removed for access and visibility, and into which the previous OC81's were pushed. With their rubber jackets these would not take kindly to such treatment.

I'll get the iron warmed. Derek.

Re: Roberts RT7 - problem
 

Well, I got them fitted, and the right way round. Put it all back together and in the case.
Only a little PP3 to hand, and I got a pop from the speaker. The transformer no longer runs hot, though I got nothing on Long wave. Flipped over to Medium, and picked up a couple of stations, but soon lost them and got nothing. Checked the battery voltage and it was down to 7.4, so suspect I need to do the sensible thing and get me a proper 9v battery.

Feeling around the transformer after a couple of minutes connected and with the battery disconnected, I felt the OC81M's quite warm to the touch - the metal cap made me pull my finger away smartish. Would this be normal?

Re: Roberts RT7 - problem
 

I think you need to make a start on checking capacitors and then resistors.
The waxies need to go, don't even bother checking them.
If it were my radio, I would be inclined to replace all of those Plessey electrolytics.
LV electrolytics cost pence each from RS.
When I repair a radio, I only want to do it the once!

Re: Roberts RT7 - problem
 

Start by getting a PP9 9 Volt battery from new.

Change those electrolytics and try it. Also give the controls a clean - they seem to collect dirt on these.

Cheers,

Steve P.

Re: Roberts RT7 - problem
 

While I'm waiting for Screwfix to send a PP9 (£1.95 Ever Ready Silver - but carriage is a fiver, buy some screws or something to make it worth while - still be a fiver carriage), In the mean time - I'll ask a little about these electrolytics.

There are those that are tall and red capped with a yellow label stating 'Electrolytic' and the rating value, and are seven in total. Then there are the 'waxes' which number nine and I can get their value from the service sheet.

For you blokes there will be no problem in knowing what to order by number and component name, and where from. If I go onto RS components website or Cricklewoods, I am faced with pages of semiconductors that bear little relationship visually to those in the set, and no numbers that I can refer to for seeking the correct component. I was lucky (?) with the OC81's - they were listed as OC81! You are going to have to help me out a bit here.

The set does pick up one channel - but only one! I heard a minutes coverage of the marathon, though it was from the area marked Hilversum on the panel. But let's see what a proper battery does before we go there, and a clean of switch gear.

Cheers - Derek

Re: Roberts RT7 - problem
 

I'm surprised you have to send to Screwfix for a PP9. There's a branch of Maplin in St.Albans, surely!

Anyway - Electrolytic Capacitors from Cricklewood Electronics:

2 uF : Try 2U2R150
10 uF caps - Well 15 uF will do : try 15R160
100 uF caps - try 120 uF ones like 120R25.

Others:

0.01 uF - CFH10N
0.1uF - CFH100N
0.25uF - CFH220N

Cheers,

Steve P.

Re: Roberts RT7 - problem
 

If the transistors are still getting warm then the original fault could still be there, I would, without having a circuit diagram to verify, suspect the red and yellow electrolytic capacitor nearest the two transistors could be leaky, if this is the emitter bypass of those two. Or the driver transistor also an OC81 could be faulty.

I personally would not at this stage start changing all the red/yellow caps on mass, they are not that unreliable. And this set doesn't have waxies in the same sense as older valve sets and the smaller value caps will not suffer leakage in the same way.

For experimental purposes a PP3 battery is satisfactory, and an alkaline PP3 will outlive a zinc carbon PP9, which of course can be fitted for authenticity if this is one of the criteria, at the risk of battery leakage if forgotten about for to long whereas an alkaline will be less likely to leak. I use a rechargeable 200mA/hour PP3 which gives several weeks normal listening.

Just my opinion of course

Geof

Re: Roberts RT7 - problem
 

What are the voltages on the two transistors? Base, Collector and Emitter.

Cheers,

Steve P.

Re: Roberts RT7 - problem
 

Personally, if it were my radio, I would change them en-masse.
However, as the OP is a novice, I would suggest changing one at a time, and re-try the radio after each cap has been changed.

Re: Roberts RT7 - problem
 

No Maplins in St Albans, nearest Luton. Only 12 miles but I needed fixings and Maplins want £6 against £1.95 at Screwfix, and I had assumed the bigger lump was beneficial in terms of power. Thanks for some numbers Steve – did you mean 2U2R160 – 150 doesn’t show – it’s the same value anyway.
How about an equivalent for the .04uf value – would something higher suit?

Won’t know voltages across the OC81’s really until a proper battery is on, but I will check.

Surprised to learn the little PP3’s would last longer than the big lump. How about doubling up on the mAmp/hrs by putting two PP3’s in parallel? – Plenty of room!
Could a transformer (or whatever - like the in car chargers for mobiles) be used to step down from 12v dc external supply? If so what and how?

Rather than change one at a time, I’m with Grommet on renewing for certainty, but is there any likelihood of the resistors causing trouble? Lots of them little devils in there.

Better send before I think of another batch to bother everyone with!

Cheers - Derek

Re: Roberts RT7 - problem
 

No. They don't have 100uF but they do have 150uF. That'll do.

0.04uF - Try 0.047uF. Part no: CFH47N

Try it with a new battery - a new PP3 will do for this - and measure the voltages like before.

Cheers,

Steve P.

Re: Roberts RT7 - problem
 

An Alkaline PP3 battery will last a long time, not much point in paralleling as the discharge from each may be a little different and become unbalanced.

The other possible cause is the driver transistor being damaged, though as you have had some sound this seems to be working. A damaged output transformer is one possible cause of the output transistors failing, as the new ones are still overheating from what you have described, this would be harder to replace unless old spares can be found.

I personally would only change the larger value electrolytics at the moment, it is unlikely that the small value non electrolytics would cause this type of fault unless one has gone short circuit in, say, an input to the driver stage and this doesn't happen very often. But if you are ordering it might be more economical to get everything in one go, depends how much you want to spend on it.

Geof

Re: Roberts RT7 - problem
 

Update: Screwfix were on form - arrive 07.15 this-morning.

Battery before switching on - 10.1v. Connected up and switched on, got a channel on Medium wave (just one mind) but carrier wave only.
Checked voltage between the new OC81's; collector to emitter 9.6; collector to base 9.6; emitter to base 9.6. The other was the same. I could smell something getting warm, and it wasn't the transformer which stayed cool to the touch. Twiddled the dial a bit more, but failing to get even a carrier now, re-checked the voltage at the OC81's. This was now down to 3.4v all round on both - and getting very warm. Switched off, disconnected, and checked battery volts - 9.1 and recovering. Stabilised at 9.6 after two minutes.

Something is shorting out. I'm happy to keep at it, though the thought of trying to find a replacement transformer is disconcerting if none are available! I am guessing I would need to know the correct output voltages at the various terminals to discover if it is duff or not, though as it's no longer getting warm it could mean it's either dead, or OK?

Or is the next step change the electrolytics?

Might I have cooked my new (and somewhat pricey) OC81's?

Does any of this help you distant Doctors?

Note: My description of 'carrier wave' might be wrong. As I turned the dial onto and past the station point, a high pitched signal was heard which reduced in note to inaudible, then increased back up through high pitch to inaudible.

Re: Roberts RT7 - problem
 

You might have cooked them. Sorry if you have. Take them out and test them, Base to Collector, Base to Emitter, Collector to Emitter. The Base should read one way but not the other. There should be no reading Collector to Emitter. If this is not the case, put them in the bin.

There's two resistors, one 51 Ohms and one (on the battery line) considerably more. You should see them on the sheet. Check them, especially the 51 Ohm one. (You have a meter I take it).

Cheers,

Steve P.

Re: Roberts RT7 - problem
 

Yes, I have a meter. Do I test in the 20k range?
Also - can they be tested in situ - or removed?
When you say 'battery line', is that the line nearest the off switch - positive (earth side) terminal?

I see the 50ohm, but the service sheet has been photocopied with a crease, which inconveniently obliterates the values of R29 & R 30! R 29 looks like it might have a three figure number in ohms - could be 100, but R26 IS in that same line and 2.2k ohms - is that it?

I'll go and take the OC81's off and test. Get back soon.

Derek

Re: Roberts RT7 - problem
 

Take them out and test them. Good luck.

The two resistors you want to test are R27 - 51 Ohms and R26 - 2.2k. Then check the volts with the transistors not in the set.

Cheers,

Steve P.

Re: Roberts RT7 - problem
 

OC81's in the 20k ohm range on my meter;
base - collector: 2.97
collector - base: no reading
base - emitter: 3.16
emitter - base: no reading
collector - emitter: 1.64
emitter - collector 18.25

base - collector: 3.25
collector - base:no reading
base - emitter: 3.67
emitter - base: no reading
collector - emitter: 1.84
emitter - collector: 19.19

Looks like the bin.

Re: Roberts RT7 - problem
 

They are probably OK. So its something else round there. Check them two resistors.

Cheers,

Steve P.

Re: Roberts RT7 - problem
 

The two resistors R26 & R27:
Checked using the 20k setting, no power, and in situ with the meter points held at each end;
R26 1.05; R27 .08

R26 With battery connected and on, voltage at the 'Gold' end: 9.72 'Red' end: 0.28
R27 'Gold end': 0.28 'Green' end: 0.00

Derek

Re: Roberts RT7 - problem
 

Hmm, they are OK then. What are the voltages around TR5?

Cheers,

Steve P.

Re: Roberts RT7 - problem
 

While you are checking voltages elsewhere the two OC81's should have their collectors or emitters lifted out of circuit if these are still drawing excessive current, of course this can cause an additional problem of having to unsolder them. But they can't be left in drawing excess current and overheating, something is biasing them on far to hard. The base driver stage of the output stage should be checked, this is the other (driver) transformer and its components on the board.

What should have been mentioned earlier is to monitor the battery current rather than wait until things get hot, by putting the milli Ampere meter in series with the battery lead. The current should only be around 10 milli-amps or so, if the needle shots off the scale then this is why the things are getting hot and draining the battery.

(With a good radio at normal volume this current with increase and fluctuate in time with the signal)

Geof

Re: Roberts RT7 - problem
 

TR5 power on; Red lead 9.5v; Black 1.83v; Base(?) 1.95v.

Should I not be testing these components for resistance off the board?

Derek

PS Thanks Geof. The OC81's are currently off the board. Battery levels for the minute or so checking for voltage has remained stable - same before and after checking (so far). But with OC81's attached, there was a drain - it drew the PP9 down from 10.1v to 9.1, which has since recovered to 9.93v.

Re: Roberts RT7 - problem
 

TR5 looks like it is OK, this can be checked for forward and reverse, collector to base, base to collector and so on, in situ with no power applied of course as you did with the OC's.

resistors can give misleading readings in circuit as all the other resistors are also in the circuit with the one being measured, an end has to be lifted to measure a resistor, so only check those which give cause for concern as most will still be ok and should not be unsoldered for no real reason.

The fault is somewhere in the driver transformer secondary (output OC81 base) stage as far as I can tell without having the circuit to look at.

Geof

Re: Roberts RT7 - problem
 

I'm not sure of the best way to do this, but I'll try and get a diagram across.

PS Must shut up shop for tonight - Cheers for now.

PPS Geof, I'm aware of copyright infringements posting the circuit, so will send in a separate PM.

Derek

Re: Roberts RT7 - problem
 

If you're thinking that Geofy, then R28, R29, R30, C33, The speaker or the Transformer.

Could be a bit nasty.

Cheers,

Steve P.

Re: Roberts RT7 - problem
 

All this taking out and putting back could generate more problems. These old single-sided SRBP boards were't very robust, and this one has had plenty of time for the glue to deteriorate.
Alan