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Old 24th Apr 2009, 6:18 pm   #21
KeithsTV
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Default 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
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Old 24th Apr 2009, 7:40 pm   #22
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Default Re: Roberts RT7 - problem

Quote:
Originally Posted by KeithsTV View Post
(snip)
The leads are in a triangular configuration with the collector next to the black line.
(snip)
Keith
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?

Last edited by stonehopper; 24th Apr 2009 at 7:45 pm.
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Old 24th Apr 2009, 8:23 pm   #23
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Default Re: Roberts RT7 - problem

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

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Old 25th Apr 2009, 8:32 am   #24
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Post 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.

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Old 25th Apr 2009, 10:32 am   #25
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Default 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.
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Old 25th Apr 2009, 7:38 pm   #26
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Default 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?
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Old 25th Apr 2009, 8:37 pm   #27
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Default 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!
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Old 25th Apr 2009, 9:04 pm   #28
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Default 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,

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Old 26th Apr 2009, 2:12 pm   #29
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Default 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
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Old 26th Apr 2009, 2:30 pm   #30
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Default 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,

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Old 26th Apr 2009, 3:44 pm   #31
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Post 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
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Old 26th Apr 2009, 3:53 pm   #32
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Default Re: Roberts RT7 - problem

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

Cheers,

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Old 26th Apr 2009, 3:59 pm   #33
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Default 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.
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Old 26th Apr 2009, 7:18 pm   #34
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Default 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 doesnt show its the same value anyway.
How about an equivalent for the .04uf value would something higher suit?

Wont know voltages across the OC81s really until a proper battery is on, but I will check.

Surprised to learn the little PP3s would last longer than the big lump. How about doubling up on the mAmp/hrs by putting two PP3s 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, Im 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
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Old 26th Apr 2009, 7:33 pm   #35
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Default 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,

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Old 26th Apr 2009, 8:33 pm   #36
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Post 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
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Old 27th Apr 2009, 2:48 pm   #37
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Default 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.

Last edited by stonehopper; 27th Apr 2009 at 2:58 pm.
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Old 27th Apr 2009, 3:11 pm   #38
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Default 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.
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Old 27th Apr 2009, 5:26 pm   #39
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Default 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
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Old 27th Apr 2009, 5:50 pm   #40
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Default 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.
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