Roberts RIC1 very high quiescent current

[Hammonds]

Good afternoon, All

I've just received this nice looking Roberts RIC1.

I set up my power supply to 9 volts and set the max current to 100mA.

Connected up and switched on. There's a bit of crackle as the volume control is adjusted, but no other sound. The current went straight up to 100 mA and the voltage on the PSU went down to 4.5V! I'm assuming the total quiescent current should be about a total of 20mA.

I jiggled RV2 and RV3 slightly to see if that made any difference to the current, but no change, still 100mA.

I connected a milliameter in the red link and adjusted RV2. The minimum quiescent current I could achieve was 12.9 mA.

Measuring the voltage between the junction of TR2 and TR3 emitters and ground is 0.282V instead of 5.2V on the trader sheet, regardless of the setting of RV3.
Is this pointing towards a problem with TR3?

Any ideas, please as to where to go next.

Thanks in advance

Dave

[paulsherwin]

Remove TR2 and TR3 and test them. Your initial diagnosis is probably correct.

[orbanp1]

Hi Dave,

You are correct about suspecting TR3.
But do check all the transistors!

It is Lin-type PA stage (like an op-amp) with negative feedback, so any faulty semiconductor in the amp could cause the DC imbalance.
Part of that amp is in the IC.
Here is a datasheet on the TAD100:
http://www.datasheetarchive.com/dlma...Book-10532.pdf

Interestingly, the service sheet from RM does not show what are the final transistor, but from the pinout it looks like Ge transistors.
If TR3 is faulty, you can replace all three transistors with Si devices.

Regards, Peter

[paulsherwin]

The transistors are a standard AC187/8 pair.

It may be possible to replace them with a silicon pair like a BC327/337, but it may be tricky to get the bias right. Easy enough to experiment.

[Hammonds]

Hi again

Thanks for your responses so far.

I've taken out TR3 (AC188) and checked base/emitter and base/collector voltage and got about 0.23V for each, so short circuit there.

I've now found out how hard it is to remove transistors without lifting the copper foil!!

I'm now reluctant to have a go at removing TR2.

Is there a trick to removing components from these sorts of boards without lifting the copper?

Dave

[paulsherwin]

There are several techniques, depending what stuff you have available. I apply tension to the wire before heating the solder. The wire then pulls out as soon as the solder melts, and the heat can be immediately removed. I still get caught out sometimes though.

If you have no experience of doing this, try practising on a scrap circuit board.

[DangerMan]

Don't forget, the tin whiskers that can grow inside these transistors can also short any of their terminals (E,B or C) to the case as well as (or instead of) to each other, so with the case now isolated the fault can go unseen.

So as well as checking all leads to all others, check from all leads to the case with DVM/Avo on ohms range to see if this has happened... this can be done in situ... but note that a collector lead of one of the transistors actually should be connected to its case via the circuitry (that's if the heatsink is connected to one supply rail, of which I'm not sure).

Pete

[Herald1360]

Also, with Ge transistors you can get a C-E short with normal looking B-E and B-C junctions. Check also that the C-E shows some resistance and that this increases significantly when B is shorted to E.