Roberts R300 Test Link for idle current

[Krolroger]

Hello All,

Can anybody tell me where to find the interruptable test link to set the quiescent current on this set?
The figure I have is 12mA with no signal, volume at minimum, on MW.
As a general rule, do you guys replace any trimmers in the audio stage, or leave them be?

Many thanks,

[GeoffK]

The link is from the centre tap of the output transformer primary but 12mA total quiescent current is correct. If it works then leave it, there is only one adjustable pot RV2 that sets the base bias for the output transistors.

[Krolroger]

Thanks, Geoff.

I was wondering where the link is is a physical sense.

On the solder side of the pcb, presumably; is it a solder trace that can be interrupted?

Regards

[ronbryan]

It is usually a black flexible wire link, on the solder side, near the output transformer and output stage.

Ron

['LIVEWIRE?']

I sometimes replace skeleton presets with Piher or similar enclosed ones.

[Krolroger]

Got it. Thanks.

[Krolroger]

Evening all,

Having changed three AF117s, I finally got around to checking the idle current on my Roberts R300 (using a DMM rather than an AVO 8) and was slightly surprised that I can only get a maximum value of 2.8mA rather than the prescribed 12mA. The volume out of the speaker is what I would expect.

Am I right in thinking that the problem may be due to faulty/leaky components in the output stage (R22, R23 and C20) as this stage is decoupled from the rest of the circuitry?

Unless the use of a DMM has something to do with it...

Many thanks for your help.

[Krolroger]

Well, I may have answered my own question by downloading the ERT service chart which differs from the information I have.

It states that the idle current should be 4mA at room temperature.

Cheers all.

[ronbryan]

The 5.6 ohm emitter resistor R22 has a tendency to go high, which can cause difficulty in achieving the correct quiescent current of 4mA.

Farnell sell 2 watt Panasonic 5.6 ohm resistors, or you can fit a 10 ohm on the top of the pcb and a 12 ohm in parallel on the underside to achieve the correct value.

Ron

[Krolroger]

Thanks Ron.

Do I truly need a 2 watt resistor?

[Al (astral highway)]

Sometimes the metal film types are only available at this power rating. You can buy them from EBay at 1/4 watt rated, though.

[ronbryan]

The Farnell 232-4661 metal film Panasonic resistors I suggested were of good quality, in stock and around the correct physical size for the pcb. The 2W rating came as a bonus. They are grey, with printed labelling though, not stripey.

Ron

[Krolroger]

Spot on, guys. R22 weighed in at 9.00 ohm which I guess accounts for the drift in the quiet current.

Are there any other known trouble spots in these sets that are worth changing on spec? (I've done the three AF117s).

Simon

[ronbryan]

The other 5.6 ohm resistor R13, which defines the negative feedback is also likely to be high, but it is best measured with one leg out of circuit.

Any wax capacitors or brown DCC capacitors will probably be leaky and the Plessey electrolytics dry out and have low capacitance or high ESR. It really depends how well it performs now, and how much time and money you want to spend on it.

Ron