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Old 29th Apr 2009, 8:44 am   #61
cmjones01
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Lightbulb Re: Roberts RT7 - problem

This is my first ever post here, so be gentle with me, and I'm not familiar with this radio, but looking at the circuit diagram above isn't it possible that a break in the track to R30 could be sending the output stage unstable, and all that heat is actually a result of it oscillating violently?

That's my hunch, anyway. I'd be interested to know what the learned company here thinks.

Chris
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Old 29th Apr 2009, 10:52 am   #62
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Default Re: Roberts RT7 - problem

This repair seems to be going nowhere. We have a radio which doesn't work, pulls down the battery voltage and gets hot. All this suggests that the radio is drawing too much current.

I would suggest:-

Investigating the break in the track. If it is indeed a break then bridge it with a piece of flexible wire.

Reconnect the speaker.

Measure the actual current drawn by the set. Meter set to mA range and connected in series with the battery. One lead to battery positive and the other to the red lead of the set which should be disconnected from the battery.

Measure the voltages on all the leads of TR4, 5, 6 and 7. Red lead of meter to chassis. Black lead of meter to the point being tested.

Post the results here for analysis.
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Old 29th Apr 2009, 1:03 pm   #63
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Post Re: Roberts RT7 - problem

I did notice a brown mark on the transformer, which could be an overheating winding causing a shorted turn.

And the solder splutter on the break in the tone correction circuit needs a closer look.

This is a classic radio and should be preserved whatever the outcome.

I have sent a PM suggesting if it cannot be repaired this way that a modern audio amplifier could be fitted instead. If enthusiastic even make one, in a different thread of course.

Geof
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Old 29th Apr 2009, 2:29 pm   #64
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Default Re: Roberts RT7 - problem

Thanks all, I'm on it this afternoon, and will take care. The smoke burn on the T2 bracket looked ominous at first sight. The break is bridged, and I'm making notes as I go. Not done yet.

Derek
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Old 29th Apr 2009, 3:49 pm   #65
geofy
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Post Re: Roberts RT7 - problem

Quote:
Originally Posted by cmjones01 View Post
looking at the circuit diagram above isn't it possible that a break in the track to R30 could be sending the output stage unstable, and all that heat is actually a result of it oscillating violently?

Chris
Welcome Chris, This is a possibility as the transformer was designed to have this as a damping circuit in the collectors which would have the larger audio signal in this part of the transformer.

Geof
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Old 29th Apr 2009, 4:26 pm   #66
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Default Re: Roberts RT7 - problem

First, I wanted to see if there was any difference with the section of track repaired.
TR6 & TR7 were re-installed (the 'legs' are quite long, and I was quick with the iron). Battery at 9.95v. Connect and switched on. Able to receive two channels, voices from the speaker, but fading and accompanied by a whistle.
Connected the multitester on mA between the positive and red wire, and read 1 . in the 20m range - stable for the duration of the test.
Removing the multitester point and re-connecting to the positive on the battery (i.e. switching power off then on) resulted in the voices back, but fading again to nothing. Removing and repeating the process duplicated the result. Switch off, disconnect, battery now 9.8v after approx three minutes connected. Transformers felt cool, TR6 & TR7 warm.

Left it for an hour or so.

Now to check voltages of TR4,5,6,&7.
Tight for space, but managed to get clean contacts on the bottoms of the legs, all components installed.
Tester red lead to chassis, black to component.
Power on.
TR4 Emitter 1.08v; coll. 2.18v; base .77v
TR5 Emitter 1.04v; coll. 5.45v; base 1.10v
TR6 Emitter 1.9v; coll. 4.87v; base 1.76v
TR7 Emitter 3.35v; coll. 2.13v; base 2.10v
the figures for TR6&7 were steadily reducing by a tenth of a v per second.
Now, the output transformer T2 is getting warm on top (the smoke burnt, melted one), also the TR6 & 7 are getting hotter, so too (though I had not checked this on previous occasions) the capacitor C32 adjacent to TR6 & 7 is also getting warm. The others capacitors remained cool.
Switched off and disconnected.
Battery now 7.81v.

I am wondering if, apart from a short somewhere, one or both the replaced OC81's; TR6 & 7 are in fact being destroyed by this current flow. Their values are no longer similar when checked with the meter before I replaced them.

Don't worry, I won't throw it in the skip. I'm ironically getting attached to it.

Haven't lifted any capacitors yet. I suspect the output transformer has an internal short - but that is guesswork.

Derek
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Old 29th Apr 2009, 5:46 pm   #67
geofy
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Exclamation Re: Roberts RT7 - problem

Quote:
Originally Posted by stonehopper View Post
I am wondering if, apart from a short somewhere, one or both the replaced OC81's; TR6 & 7 are in fact being destroyed by this current flow. Their values are no longer similar when checked with the meter before I replaced them.

Haven't lifted any capacitors yet. I suspect the output transformer has an internal short - but that is guesswork.

Derek
I think you are right on both counts.

It looks as if the transformer does have a short circuit turn and the OC81 emitter resistance doesn’t look quite right as the emitter voltage has risen meaning they are not conducting properly so may have been damaged. C32 getting warm could be a separate fault, this can be replaced. The squeal is probably just because the voltage is dropping and messing up the radio section, it is encouraging that this stage is working otherwise.

A more conventional transformer could be fitted, even a pocket radio type. I have made a sketch from another similar radio, the base bias resistors might have to altered and the feedback resistors from the speaker could be retained instead of how it is shown in my sketch. It does mean a fair bit of modification.

I would not turn on the radio anymore until this transformer has been sorted.

I wonder if the well known Eagle LT700 Output transformer would work here ?

The driver stage looks OK

Geof
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Old 29th Apr 2009, 6:55 pm   #68
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Default Re: Roberts RT7 - problem

If C32 is getting hot it is obviously faulty. It is quite likely that this component is pulling the battery voltage down, effectively short circuiting it. Remove it and use the ohms range of your meter to check it. A good cap will initially show a low reading which should rise to near infinity as the capacitor charges. A bad cap will show a continuous low reading. Try the meter both ways as the actual polarity of the leads when switched to ohms may vary.

This is not a foolprooof test, as the test voltage will be low and may not "break down" the fault. You could try connecting the capacitor across the battery with you meter set to mA in series. The current should be very low.

Judging by your test results there are other faults, but this is an obvious one to start with.
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Old 29th Apr 2009, 7:19 pm   #69
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Default Re: Roberts RT7 - problem

The base voltages of TR6 and TR7 are way too high. Check R26 and R27 with your meter on ohms range. You'll need to disconnect one end of each resistor before testing it.
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Old 29th Apr 2009, 8:01 pm   #70
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Default Re: Roberts RT7 - problem

If that transformer is faulty, place an advert for a replacement before you start doing modifications.

I'm very worried about that transformer....

Cheers,

Steve P.
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Old 29th Apr 2009, 8:04 pm   #71
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Default Re: Roberts RT7 - problem

Something doesn't add up here. The service sheet says that TR6 and TR7 collectors should be at 9V, which is the same as the supply voltage. This means that the resistance of the tapped secondary winding of T2 must, by design, be low. Yet the readings taken indicate that about 4 and 7 volts respectively are being dropped in the two halves of the winding. This indicates massive collector current, yet according to post 66 the entire set is only drawing 1 mA. Are you sure this is the correct reading? Some meters have a seperate test lead socket for use when taking current readings.

Similarly the emitter voltages of TR6 and 7 indicate massive current. It's almost as if these transistors were short-circuit from collector to emitter. Are you sure that the new transistors are wired in correctly?
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Old 29th Apr 2009, 8:15 pm   #72
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Default Re: Roberts RT7 - problem

Agreed, but it still 'partly' works.

The collectors both get their power through the O/P transformer, the bases via R26, R27 and T1. (Trader sheet.) Emitters go to earth.

Any solder bridges that shouldn't be there?

Cheers,

Steve P.
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Old 29th Apr 2009, 8:48 pm   #73
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Default Re: Roberts RT7 - problem

As you already measured the voltage at the R26/R27 point as 0.2-0.3V then this should be the voltage you see at the base of both TR6 and TR7, the base current should be so small you should see virtually no voltage drop through the trasnformer windings. If it is not then you either have transistors the wrong way round or there is a problem with the driver transformer or the transistors are broken.


The voltage you measured at the potential divider giving the base volatge was correct.

As the overheating did not occur with the meter in series with supply this may have had enough resistance to stop the thermal runaway you are getting depending upon how good your ammeter is.

Try measuring the resistance of the transformer coils it would then be possible to work out biasing arrangements at it seems the transformer may be faulty causing DC biasing to be incorrect. There are no capacitors that should cause this problem in the output stage.

Jeff
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Old 29th Apr 2009, 8:52 pm   #74
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Default Re: Roberts RT7 - problem

Steve.

Quote:
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
The resistance figures are meaningless because no units are quoted and the resistors were undoubtedly shunted by other components. The 0.28 at the juction of R26 and R27 doesn't look too bad though, assuming that the units are volts.

Quote:
TR6 Emitter 1.9v; coll. 4.87v; base 1.76v
TR7 Emitter 3.35v; coll. 2.13v; base 2.10v
With TR6 and TR7 fitted the base voltages have risen to the 2 volt region. What can be causing that? It implies a base current of about 1.5mA for each transistor. Even a dead short in a transformer winding shouldn't cause that. 2200*0.003 is approximately 9-2=7 volts.
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Old 29th Apr 2009, 8:59 pm   #75
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Default Re: Roberts RT7 - problem

Suppose it's partly going to deck through the transformer?

Cheers,

Steve P.
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Old 29th Apr 2009, 9:01 pm   #76
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Default Re: Roberts RT7 - problem

I'm not sure I would know if there were solder bridges that shouldn't be there Steve, but I cannot see anything under the board that looks out of place or 'extra', though if it had been done well, maybe I would not notice.

As regards the meter readings on current, yes - there is more than one connecting socket on the meter for the red lead, and it is possible I have used the wrong one. I switched round to the 20m range in the 'A' quadrant, and got 1 . with the red lead in the V/ohms/mA socket.

Now I've just been back to the bench and carried out another quick check with power on, and got 0.03 with the red lead in the 10ADC socket, and switched round to the single 10A range.

Told you I didn't know what I was doing! Got to be led by the hand everywhere . . .


OK, looks like I'm on a transformer hunt. Meanwhile, I'll lift the C32 capacitor and see what readings I get. Should I be successful in getting a replacement T2 I can plan to move on from there.

Lots of interest in this thread and I must thank all involved very much for the advice given.

Duff forecast for tomorrow, so might get started on pulling the T2 from the board, and perhaps a visit to Maplin. Don't think they'll have what I want, but never been before.

Thanks all round gents, I'm still listening.

Derek

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Old 29th Apr 2009, 9:03 pm   #77
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Default Re: Roberts RT7 - problem

Quote:
Suppose it's partly going to deck through the transformer?

Steve. If that were the case the voltage would be high even with TR6 and TR7 out of circuit. Yet apparently with these transistors out the voltage is OK.
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Old 29th Apr 2009, 9:12 pm   #78
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Default Re: Roberts RT7 - problem

Quote:
Originally Posted by Steve_P View Post
Suppose it's partly going to deck through the transformer?

Cheers,

Steve P.
Ah! - Now I did do a simple continuity test on some of the dubious bits of bared wire protruding from the T2. I say dubious, as they were devoid of any form of insulation, and not covered with the red varnish (?) that T1 and the other side of T2 has. Between those three points and the chassis there was complete continuity.

If you go back to post No.20 and check out the thumbnail furthest right, you will see over the top of the smoke burnt bracket, the tips of bare metal from where I made that continuity test to chassis.
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Old 29th Apr 2009, 9:25 pm   #79
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Default Re: Roberts RT7 - problem

Quote:
Ah! - Now I did do a simple continuity test on some of the dubious bits of bared wire protruding from the T2. I say dubious, as they were devoid of any form of insulation, and not covered with the red varnish (?) that T1 and the other side of T2 has. Between those three points and the chassis there was complete continuity.
If you mean T2 primary which is on the left of the diagram then that is what I would expect, as the centre tap is connected to the chassis. True there should be some resistance in the two halves of the winding rather than "complete continuity or zero ohms, but I would expect it to be quite low. For resistance measurements to be meaningful TR6 and 7 emitters would need to be disconnected from the primary winding.

Quote:
As regards the meter readings on current, yes - there is more than one connecting socket on the meter for the red lead, and it is possible I have used the wrong one. I switched round to the 20m range in the 'A' quadrant, and got 1 . with the red lead in the V/ohms/mA socket.
In that case 1 . probably indicates an overscale reading.

Quote:
Now I've just been back to the bench and carried out another quick check with power on, and got 0.03 with the red lead in the 10ADC socket, and switched round to the single 10A range.
0.03 Amps is 30 milliamps which is way too high.

I think you're being too hasty in suspecting the transformer. Transformers can overheat and still remain perfectly servicable. I'd concentrate on R26, R27 and C32.
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Old 29th Apr 2009, 9:45 pm   #80
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Default Re: Roberts RT7 - problem

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Originally Posted by Station X View Post
(snip) Are you sure that the new transistors are wired in correctly?
I double checked before fitting, and when re-fitting. The old OC81 had a spot of coloured paint by the collector wire which is insulated with red pvc in the shots in post 20. The new OC81's had a rubber jacket with a grey line by the collector. Assured this was correct, that is how they were fitted, same location as the red insulated wire, though the layout of wires as the exude from the base of the units is an a triangle that is opposite to the triangle of holes in the board. The original OC81's wires were located in line, so the pattern appears irrelevant.

Derek
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