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.

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.

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.

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.

Ron

Re: Roberts R600 repair
 

Apologies Ron.

Mind you it could have been worse, I could have called you Rob Brydon. ;D

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.

Regards

Graham