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Old 29th Apr 2009, 9:45 pm   #81
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Default Re: Roberts RT7 - problem

Quote:
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 voltage was correct.
I agree with what you say Jeff. With TR6 and 7 out of circuit, but with T1 secondary in circuit the base bias voltage is OK. This points to R26 and R27 being of the correct value or failing that in the correct ratio.

With TR6 and 7 in circuit the base voltage rises, so additional current must be being pulled through R26 increasing the voltage drop across it. This current can only be flowing into the base of TR6 and/or TR7. This is assuming of course that it is actually the bases which are connected to T1 secondary.

Are far as I can see though even if TR6 and 7 collector and emitter were connected directly across the battery it still wouldn't result in the base curent increasing enough to increase the base voltage.
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Old 29th Apr 2009, 9:48 pm   #82
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Default Re: Roberts RT7 - problem

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(snip)
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.
'Roger' that, I'll carry on tomorrow with more checks.

Cheers - Derek
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Old 29th Apr 2009, 10:51 pm   #83
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Post Re: Roberts RT7 - problem

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(snip) Are you sure that the new transistors are wired in correctly?
I 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
I am not fully convinced the transformer is burnt, it doesn't happen that often in a battery set. The transistor polarity needs to be rechecked, it does give the impression the emitter collector is back to front.

The base bias resistors are unlikely to change value very much but they do sometimes break at the leads, if the lower one is open this would give a strange base voltage reading.

But the OC81's are probably damaged by now if they have been getting hot.

G
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Old 30th Apr 2009, 12:27 am   #84
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Default Re: Roberts RT7 - problem

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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.
I'm not sure what the convention is for the colour of sleeving used on transistor leads. However in this set the collectors are connected to the negative side of the supply. This makes me think the collectors should be sleeved in black and the emitters in red. Perhaps one of our more knowledgable members could advise.
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Old 30th Apr 2009, 11:32 am   #85
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Post Re: Roberts RT7 - problem

I have just looked at my Roberts R200 which has its original OC81's and the RED SPOT (collector) has a WHITE lead, bare Base lead, and RED Emitter lead.

The Emitters on all the transistors (AF117 and OC's) are all red as well.

Now I must get back to trying to unblock the shower drain, o joy
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Old 30th Apr 2009, 12:18 pm   #86
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Default Re: Roberts RT7 - problem

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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.
The attached images confirm this. If the red lead is the collector, then the new OC81Ms won't fit the hole pattern.

It seems odd to me though that the collectors are sleeved in red wire and the emitters in black.
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Old 30th Apr 2009, 5:26 pm   #87
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Default Re: Roberts RT7 - problem

Hmmm. I've been reading about testing transistors and see that the advice is to use the diode test facility on a multitester. I have been using the resistance range in 20k ohms - was I wrong?
Because, using the diode setting and testing the now removed rubber jacketed OC31's, they seem to meet the requirements of a sound transistor in all six directions; BE; BC; EC.

Right, that's one thing.

I have removed the C32 capacitor and tested with the multimeter using 29k ohm range. (This is the one getting quite warm, is adjacent to TR6 & 7, and visible in Graham's post 86.)
pos to red cap, neg to black cap - no reading.
But, if I change over;
pos to black, neg to red cap - I get a minus figure that goes through zero and up to around 15 then no reading, and no subsequent reading when repeating the test in that same direction.
If I then go back and reverse my action again, and put pos to red cap, neg to black cap - I get minus figure, going up through zero to around 17 then no reading. Subsequent testing gains no reading.
Am I polarising the capacitor by doing this?
What does it tell me about the capacitor?
And should I not do that?

I have also lifted one end of both the resistors using a croc clip at first, but then thin nosed pliers to pull its tail up through the board as heat was applied.
Tested between the track beneath and the lifted end:
R26 reads 2.53 ohms (20k test range both)
R27 reads .08 ohms

When speaking of 'bias resistors' which ones are they?

The layout of wires on the transistors I fancy might be a red herring, as the original OC81 has its wires in a line - no pattern to match the holes in the base. The newer ones have a triangular pattern, but I fancy they being manufactured forty odd years after the set was made might well be a simple coincidence that they were in a triangular pattern also - albeit the other way. But not being knowledgeable on radios and their possible historic patterns - possibly wrong . . . anyway -

One other point - Heat sinks. TR6 & 7 were mounted in copper clips riveted to a frame support. Surely this is for a reason?

Edit - PS: Fixing the shower would be a doddle!

Here are the old OC81's. The insulation has welded together at the top most ends:
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Old 30th Apr 2009, 5:37 pm   #88
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Default Re: Roberts RT7 - problem

R27 should read 51 Ohms. Do you have a lower scale, say up to 200 Ohms. Measure it here. The other one, R26, should read 2.2k, or 2200 Ohms.

If you're in any doubt, swap them anyway. Without the transistors in, the base contacts (on T1) should be 0.2v.

Cheers,

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Old 30th Apr 2009, 5:53 pm   #89
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Default Re: Roberts RT7 - problem

R26 and R27 values sound plausible if you interpret as 2.53K and 0.08K ie 2530 ohms and 80 ohms. That would give around 0.275V base bias for the output transistors at zero base current. I would expect each OC81 to need about 200uA for (say) 10mA collector current which would drop the bias voltage by about another volt which would reduce it to <0. Which couldn't happen, so what it means is the standing current in the output devices should be somewhat less than 10mA which would be fine.

The description of the capacitor test on the DVM sounds fine- at the low voltage from the meter it charges up and the resistance appears to get higher and higher; when you reverse the meter it reads the charged up capacitor voltage as negative resistance until it has recharged it positively again. The capacitor is not leaking when charged backwards since the applied voltage is low. Someone suggested testing the capacitor by hooking it across the battery in series with the DVM on current which might expose some weakness that the simple resistance check doesn't, I think that would be useful to try.

Have you tried disconnecting the output transformer from everything else and looking for any leakage resistance between its separate windings- that could be a path for excessive current though it wouldn't make the output devices overheat.

A similar problem in the driver transformer could cook the output devices, however.

Keep trying, there's only a dozen components and some of them are already known to be OK. The answer will eventually seem obvious, but only after the event

Chris
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Old 30th Apr 2009, 6:06 pm   #90
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Default Re: Roberts RT7 - problem

Thanks Chris, I'll try the capacitor on a battery with the meter in between. If it shows a problem - what will the readings be?

Derek

PS I'll work on removing the T2 next.
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Old 30th Apr 2009, 6:08 pm   #91
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Default Re: Roberts RT7 - problem

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R26 reads 2.53 ohms (20k test range both)
R27 reads .08 ohms
R26 and R27 are indeed the bias resistors. They form a potential divider across the battery. There should be about 8.8 Volts across R26 and 0.2 Volts across R27. By "across" I mean one meter lead connected to each end of the resistor. Thus with TR6 and TR7 in circuit you should see about 0.2 Volts on their bases. NOT 2 Volts.

What sort of meter are you using stonehopper? Is it an analogue one with a needle or a digital one with a digital display? I ask this question because I'm having some trouble interpreting your readings.

R26 should be 2.2K or 2.2 Kilo-ohms or 2200 ohms. You say it reads 2.53 ohms. If that's true it's about 1000 times off its true value. Could it in fact be measuring 2.53K or 2530 ohms?

R27 should be 51 ohms. You say it reads 0.08 ohms. Once again this indicates it's way off value. If however it's 0.08 Kilo-ohms that's the same as 80 ohms, which once again is way off value.

Regarding C32. Could you clarify what you mean by no reading. Do you mean Zero/0 ohms (short circuit) or infinite resistance (open circuit). You can replicate a short circuit by touching the meter leads together and an open circuit by holding them apart.
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Old 30th Apr 2009, 6:25 pm   #92
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Default Re: Roberts RT7 - problem

Regarding the leads of the ORIGINAL transistors. According to "Towers" which is the bible on the subject OC81s came in a X02 package with an LO3 lead out.

Your picture in post 87 shows an X02 package all right. However with an LO3 leadout the collector should have a bigger gap between it and the base than there is between the emitter and the base. It's difficult to tell from your picture, but it appears that the bigger gap is adjacent to the black lead which would make that the collector. Can you confirm which is the bigger gap please.
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Old 30th Apr 2009, 6:48 pm   #93
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Default Re: Roberts RT7 - problem

Hello Graham, it's a digital. Pretty average, and I'm transferring exactly what comes up on the screen for the given ranges. That's not to say I am using it correctly though! Nor can I vouch for the last time it had a new battery - a while back.

If I am using the 20k ohm range and it shows 2.53 in the display, I would interpret that as 2.53 x 1000 = 2,530 ohms. Correct me if I'm wrong. Is 2.53k as it seems to be, too far away from 2.2k as per spec?

R27 does seem to be way out at 80 ohms.

Sorry "no reading" - I should of course be saying 'open circuit' - the numbers didn't change from 1, the resistance of air.

Derek
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Old 30th Apr 2009, 6:59 pm   #94
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Default Re: Roberts RT7 - problem

On that basis it appears that R26 is 2530 ohms and R27 is 80 ohms. If you retest R27 on the 200 ohm range this should confirm this. Both resistors are off value. R26 will probably be OK, but R27 should be replaced with the nearest available value which is 47 ohms.

C32 appears to be OK. If you test it battery positive to capacitor positive, capacitor negative to meter positive, meter negative to battery negative, meter on 20mA range you should see a high cuurent flow initially which will drop to near zero if the capacitor is OK.
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Old 30th Apr 2009, 7:01 pm   #95
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Default Re: Roberts RT7 - problem

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Regarding the leads of the ORIGINAL transistors. According to "Towers" which is the bible on the subject OC81s came in a X02 package with an LO3 lead out.

Your picture in post 87 shows an X02 package all right. However with an LO3 leadout the collector should have a bigger gap between it and the base than there is between the emitter and the base. It's difficult to tell from your picture, but it appears that the bigger gap is adjacent to the black lead which would make that the collector. Can you confirm which is the bigger gap please.
Yes indeed - small though they are, there is a wider gap between the red insulated lead and base than black, red being the lead against which there is a dob of white paint on the metal cylinder. I've bent the leads away for a better view: (sorry, focus not quite the ticket)
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Old 30th Apr 2009, 7:11 pm   #96
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(snip) If you test it battery positive to capacitor positive, capacitor negative to meter positive, meter negative to battery negative, meter on 20mA range you should see a high cuurent flow initially which will drop to near zero if the capacitor is OK.
Wilco. Bit like repairing by braille with gloves on?

Derek
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Old 30th Apr 2009, 7:11 pm   #97
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If you retest R27 on the 200 ohm range this should confirm this. Both resistors are off value.
R27 or at least the driver lower bias resistor is in fact the volume control, this could be faulty given strange bias.

The driver stage is coupled via this by a 2uF electrolyic from the first audio stage OC71, if this was leaky it could put to much dc on the base of the driver stage.

G
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Old 30th Apr 2009, 7:20 pm   #98
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Default Re: Roberts RT7 - problem

Are we singing from the same hymn sheet? R27 isn't the volume control. R23 is the volume control.

As far as I can see the base bias voltage on TR6 and TR7 is governed solely by R26/27 and any base current drawn by TR6/7.
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Old 30th Apr 2009, 7:53 pm   #99
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OK.
Battery voltage 9.54v.

Tested in the 20mA range (3 o'clock on my meter) as per.

No readings at all. All Zero.

But if I dial up the 20v range, I get battery voltage to begin with which reduces steadily down. After a minute of that, and reaching 5.5v, I stopped. Rechecked for readings in the 20mA range - nothing.

Battery voltage still 9.54v

Derek
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Old 30th Apr 2009, 7:57 pm   #100
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Default Re: Roberts RT7 - problem

Me again....

Given that R26 has gone a bit high (2.53K vs 2.2K), the best replacement for C27 would be 56 ohms. If you replaced R26 with 2.2K an easy approximation to 51 ohms is two 100 ohm resistors in parallel.

So far as the capacitor check with battery and meter is concerned anything more than a few microamps is bad news, so any current reading at all on your meter would be suspect.

FWIW I use the same meter for my "so cheap I just chuck it in the toolbox" meter. (I think the last offer on them was two for a fiver or something similar). It's actually lasted several years so far and seems OK.

Doesn't it have a transistor check function? The small round black socket on the left, used with the switch set to hFE. I wonder if it will cope with a germanium device's leaky nature compared to silicon? I feel a quick fiddle in the shed for curiosity's sake coming on Dunno if I've got any OC81s not in sets but I'll try something similar.

Just spotted reply above... when the current is at zero on the 20mA range, try switching down to 200uA (two clicks anticlockwise) if it's still zero or a few uA, nothing wrong with the capacitor leakage wise.

Chris
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