Roberts R200 1st edition motorboating

[jonnybear]

Has the volume control been replaced? check the 47 ohm R21 it should be connected from secondary of output transformer to the earthy end of the volume control decoupled to chassis by r19 5.6 ohm. also make sure the other end of the secondary winding on the output transformer is taken to chassis
otherwise you have the a makings of a oscillator.
John

[Andy 1964]

Originally Posted by G6ONEDave

If all the elec caps are the correct way round and the resistors are within tolerance, perhaps the characteristics of TR4 have changed slightly. What we really need to know is whether the set always had this issue from new or if it started later in it's life.

Given that the set is a modified version between the first and second generations of circuit. And that the problem goes when fitting a 220K as R18, maybe try say a 47K and note the result. Or even try a 220K pot in series with R18's 47 ohm and vary it for best compromise. It might be a way round the trouble, also in the second generation there is a 3K9 resistor to earth from the other side of the speaker connection, perhaps try that to see if it has any effect.
Dave

Found dodgy resistors, R6,R12,R13,R3,R7 all gone high. Min volume is now much quieter. I think the problem with R18 was always there, so didnt get used, which is why it looks like new.

[ms660]

R18 is not the problem, you measured it at 47 Ohms which is what it should be.

Lawrence.

[jonnybear]

I think this radio has either been got at or is a cross between 1602 and 1449 trader circuits, it seems to have R14 and R15 in circuit (1449) with R21 and R19 (1602)if the volume control earthy end goes to + rail treat it as the 1449 early circuit with R18 being 220k, if the earthy end of volume control is via a 5.6 ohm resistor R19 then it is the 1602 later version with R21 being 47 ohms.
John

[ms660]

It's a production change that took place in the early version as set out in the service info, it gives the serial number starting from 27340 mentioning the 47 and 5.6 Ohm resistors and what else was altered etc etc.

Lawrence.

[Andy 1964]

All done, R18 at 220K.
Original handle, ends and locating bolts re-plated.
Set of new brights, one was missing.
Badge was broken, used one from a RD60.
2 coats of shoe cream. I did find that a 50/50 mix with pva glue primed any little scuffs, and covererd a little wear on the round corners.
I will empty the Plessey caps and lightly glue them over the new ones, as it is looking a little empty in there.

[ms660]

Originally Posted by Andy 1964

All done, R18 at 220K.

That's a frig then.

Lawrence.