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Success Stories If you have successfully repaired or restored a piece of equipment, why not write up what you did and post details here. Particularly if it was interesting, unusual or challenging. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 17th Feb 2018, 1:36 pm   #1
vinrads
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Default Roberts R600 repair

This radio as been on the round to it pile for ages , the fault was no sig audio ok , so I did the usual thing removed the IF module and replaced the three AFII7's

Re assembled switched on was greeted by aah just the same , Bu--er so removed the module again , wired it in with six inch leads so as to work on it , checked all the previous work all ok .
Now for some fault finding all ok till I checked the volts on the first IF amp , the collector read low less than two volts , duff new transistor me think's eer no the base was high dragging down the collector , now where is the high base volts coming from having isolated several components I came across the culprit , A 47k decoupling capacitor leaky. I think in all the years in the trade ,some fifty two. I feel old all of a sudden I have never had a component except the transistors fail. , anyway back to the Roberts faulty cap replaced all working fine . That's one for the book as we used to say. Mick.
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Old 17th Feb 2018, 2:12 pm   #2
ukcol
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Default Re: Roberts R600 repair

I would have approached that repair in exactly the same way as you did. Faced with a duff IF module containing AF117s and the inconvenience of lashing up the module for fault finding, I would have concluded that changing the transistors and reassembling would have - in the vast majority of cases - cleared the fault.

I don't remember ever having had that particular type of Mullard capacitor fail in a transistor set but its the odd random and unusual fault that makes things interesting.

Below is the circuit of the Mullard LP 1164/1 module reverse engineered by Rob Bryan. The capacitor in question is C5.
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Old 17th Feb 2018, 2:50 pm   #3
vinrads
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Default Re: Roberts R600 repair

Wow thank's for that Colin! I was wondering if there were a cct for this we should all say a big thank's to Rob for taking time to reverse engineer the circuit , it's nice to see it in the diagram and why it caused the fault , kind regards Mick.
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Old 17th Feb 2018, 5:06 pm   #4
ronbryan
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Default Re: Roberts R600 repair

I fondly remember Hancock's Half Hour, where in one episode the local press mistakenly referred to the main character as Tiny Hitchkick. Just for the record, my name is Ron Bryan.

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Old 17th Feb 2018, 5:28 pm   #5
ukcol
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Default Re: Roberts R600 repair

Apologies Ron.

Mind you it could have been worse, I could have called you Rob Brydon.
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Old 17th Feb 2018, 11:22 pm   #6
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Default Re: Roberts R600 repair

Well done finding that Mick. I went through the pain of refurbishing a module in one of these and fitting it only to find that it didn’t work. What a pain it was having to undo all that hard work. I do remember having to change one of the Mullard tropical fish capacitors once but only as it was way out of spec. I now have a donor R600 and R707 with clips on wires attached to where the module used to be so as I can test any that I’m repairing. Ron’s schematic and layout have been extremely useful so thanks once again Ron.

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