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Old 13th Mar 2018, 12:23 pm   #1
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Default Roberts R600 - Very High Current Draw!

This post is about the Roberts R600 I received from greg_simons.

When the radio was connected to a PP9 battery and switched on nothing could be heard. The radio was completely dead!

I used my Avo8 to measure how much current the radio was drawing. To my surprise It was drawing 2A of current!

Thinking that something was seriously wrong I put the radio away for a few days.

I had a look at the circuit diagram thinking that a component was shorted somewhere. Measuring voltage readings on one of the transistors revealed that barely any voltage was getting though.

This made me wonder If C24 (across the battery terminals) was shorted. I removed it and a resistance check with my DMM showed it to be shorted.

Once C24 was replaced the radio bust into life and performed well on all wavebands.

I would post some pictures but my mobile will not comply.

Thanks for reading.
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Old 13th Mar 2018, 12:45 pm   #2
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Default Re: Roberts R600 - Very High Current Draw!

Well done.

Just goes to show that a bit of testing and fault diagnosing is much better than Compulsive Component Changing.
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Old 13th Mar 2018, 12:57 pm   #3
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Default Re: Roberts R600 - Very High Current Draw!

I've not met anything quite this drastic, but was reminded of an RP33 Autocrat where an electrolytic in the same circuit position - C11 on the amplifier board - was doing its share of mischief. I was given the set by a friend who had already given it away twice, only to have it returned to him on each occasion by the recipients, who each reported that, while it was a fine radio with good tone etc., they couldn't afford to keep it in batteries. It was indeed in full working order, but checking quiescent current consumption showed it to be, if memory serves, somewhere around 250-300 mA: without affecting operation at all, that electrolytic would have reduced the life of a pair of PP9s to a very few hours.

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Old 13th Mar 2018, 6:06 pm   #4
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Default Re: Roberts R600 - Very High Current Draw!

I've repaired a number of 1980's/1990s Hewlett-Packard calculators where the capacitor across the battery (here used to power the memory device to preserve its contents when you change the batteries) had gone very leaky and would flatten the batteries in an hour or so.
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Old 13th Mar 2018, 6:23 pm   #5
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Default Re: Roberts R600 - Very High Current Draw!

Originally Posted by OldTechFan96 View Post

I used my Avo8 to measure how much current the radio was drawing. To my surprise It was drawing 2A of current!
Ouch! The internal resistance of a standard PP9 is roughly 3R (so the battery behaves as if this is in series with a 9V voltage source - I recall this from somewhere in Horowitz and Hill...).

So a fresh 9V battery can source an astonishing 3A, but only for a minute or so! It must have got pretty hot in seconds!

You'd have temporarily measured a voltage drop of 2A*3R=6V from circuit +ve to ground, so displaying just 3V, assuming this internal resistance.

Good thinking to work out what could produce such short-circuit conditions!
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Old 13th Mar 2018, 8:32 pm   #6
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Default Re: Roberts R600 - Very High Current Draw!

This particular fault illustrates why it is a bad idea to fault find a transistor radio powered from a bench power supply that can source a lot of current. Instead of just replacing a short circuit capacitor, you would have had burnt out print to deal with as well.
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Old 13th Mar 2018, 8:41 pm   #7
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Default Re: Roberts R600 - Very High Current Draw!

As I frequently find myself saying, lamp limiters work just as well at DC

In lieu of a current-limited bench PSU, just put a suitable lamp in series. Automotive bulbs are available in a wide range of powers - something like 5W would be as high as I'd want to go for a transistor radio (1 or 2 watts would be better) - and the holders are readily found (or just solder wires directly to them). They also double up as a handy dummy load for testing power supplies, etc.
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Old 13th Mar 2018, 9:01 pm   #8
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Default Re: Roberts R600 - Very High Current Draw!

I don't wish to hi-jack the thread, but several Roberts sets seem to suffer this fate, and maybe other sets of that era too - it's to be expected after more than 50 years. Depending on which caps go short circuit, will determine whether it just flattens the battery, or harms the output stage.

A little while ago I had a Roberts R505 which has three caps in particular which can go dead short. Two will only flatten the battery as in the R600 which is the subject of this thread, but in the R505, if C40 (680uF) has a high leakage rate, the output pair (AC187/188 germanium transistors) will draw excess current and despite being heatsinked, will get very hot. If left in that state for long, they can be written off.

A classic symptom is that when the set is switched on, it works for maybe two seconds then goes dead.

It was covered in a thread here:

If the electrolytics aren't tested, because the output pair get very hot, it would be easy to jump to conclusions and to assume that they've developed shorts. That could lead to fruitlessly removing them to test, and if we're lucky, they'll test OK, but if they're dead, and replaced with substitutes, they'll suffer the same fate too as the fault that caused their demise hasn't been diagnosed.

In my R505, C40 (680F) on test was identified not as a capacitor, but as a 0.53 Ohm resistor - a dead short from the output transistors via the speaker speech coil to chassis. I tested the other electrolytics and though there were no other shorts, while the set was dismantled and on the bench I replaced the others as it may only have been a matter of time for them to suffer the same fate.

While I was at it, in checking the Robert's Service Data on the tests to be conducted on the audio stage, it stated: 'inject a 1kHz sine wave into S1E and with a 'scope across the speaker, adjust the pre-set pot R36 for symmetry at the onset of clipping'. I checked that, if only out of curiosity, and found that it was clipping quite markedly so I tweaked it.

Firsts pic shows part of the R505 output stage with the three electrolytics that can cause problems, notably C40, which I guess could also damage the 4 Ohm speaker speech coil as that, (rather than the 330 Ohm parallel resistor), would take most of the current until the output transistors became 'three legged fuses'.

Second pic shows C40 under test, identified by the tester as a 0.53 Ohm resistor.

Third pic is the scope trace of a 1kHz sine wave before adjustment of R36.

Fourth pic is the scope trace after adjustment.

Very pleased with the performance of the set after the repairs and tweaks.

I appreciate that not everyone has a 'scope and signal generator, but lots of us do, and maybe we should use them more often - I know I should!

Hope that might help someone.
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