Roberts R200 1st edition motorboating

[Andy 1964]

This is my fourth R200, one in each colour. SN 34017, in blue and looks almost brand new. But its motorboating on anything less than full volume, an Ipod connected to the red wire on the volume knob and grounded to the chassis comes through loud and clear.

Thoroughly cleaned wave change switch, and fitted new rubber grommets to tuning mount, gave an improvement, but no cure.

Checked continuity of aerial wires back to pcb, OK.
Replaced electrolytic caps and volume C19, some where bad, no change.
Measured transistor voltages, according to service sheet 1449, not exactly on the numbers but very close. (no dodgy AFxxx in this set)
Looked for dry joints on pcb.
Cleaned board with isopropanol.

So now I am stuck.

[ronbryan]

If your set has brown moulded DCC (Dubilier) capacitors, they are almost always leaky and could upset the biasing of the mixer/osc and IF stages, making motor-boating more likely.

I presume that you have changed all electrolytic capacitors from the statement in your original post.

Ron

[Andy 1964]

Yes, these ones.

[jonnybear]

May be a good idea to take a picture of the printed panel so others can get an opinion.looks like there are still a few wax/paper caps that have not been replaced.
John

[Andy 1964]

I will test the remaining brown caps. Is there likely to be a resistor to examine?

[jonnybear]

I would replace all those brown bakelite decoupling paper caps one at a time, they would be my suspect components, resistors are generally ok and would not normally cause instability unless they are really out of spec.
John

[Andy 1964]

Just about to do that

[Andy 1964]

Its non of them, they were leaky, so replaced anyway.

[Andy 1964]

If I tune into a station, turning down the volume if like detuning! Good reception at full volume, poor reception at half volume!

[Al (astral highway)]

Hi Andy,

Sounds like you're half-way there!

I know it sounds unlikely, but have you checked your battery voltage under load?
It can produce this effect in some transistor sets if it's below the design voltage.


Not familiar with this set specifically but did you also check the cap/s in the AGC line

[Andy 1964]

Battery under load with no staion is a steady 10ma. The only caps I havent changed are the very thin ones. The aerial cap C3 is spot on.

[Andy 1964]

If I turn down the volume it goes up to 140ma

[Andy 1964]

My other R200 2nd edition does not change with volume

[Al (astral highway)]

Originally Posted by Andy 1964

Battery under load with no staion is a steady 10ma.

Andy, that's the quiescent current...

You need to check the voltage, which is across the battery terminals in situ in the set, not the current through the set.

It should be the full design voltage of the battery, not below, when the set is switched on and at listening volume.

[Andy 1964]

Voltage 9.25v full volume, 9v min volume

[ms660]

Originally Posted by Andy 1964

If I turn down the volume it goes up to 140ma

A slight increase then.

Lawrence.

[Andy 1964]

Could it be diode OA70? I have lifted a leg to test. Ir=5.6uA Uf=758mV C=0pF No capacitance, data sheet list 1pF

[ronbryan]

It is likely that the increase in battery current is caused by the output stage oscillating.

If you look at the circuit, there three feedback loops shown - the audio negative feedback, the base bias for the last IF transistor derived from TR4 emitter and the automatic gain control feedback to TR2 base via R5.

The amount of audio negative feedback appears to depend on the position of the volume control, as does your motorboating problem. What happens if you disconnect the 220k resistor R18 (lift one end), to remove the AF stage feedback - does the motor-boating stop or continue unabated?

The forward voltage you measured for the point contact OA70 detector diode seems high. I would have expected a few hundred mV at low currents (e.g. 1mA). What test current does your meter use?

Ron

[G6ONEDave]

I take it that you have replaced C20 a 2uf electrolytic, as well as C21 a 100uf.
Dave

[Andy 1964]

Lifted R18, now its very loud and no motorboating. That must mean something but I dont know what. The tester is Bside ESR02 PRO and used 5.19v