RGD record player P10

[Cave Miner]

Hello
HELP Needed
I have an RGD record player model P10. It has a dead amplifier, the two output transistors being short circuit. Before replacing this PNP/NPN pair I want to do some circuit checks but am compleatly unable to find a schematic for the 4 transistor amplifier contained in the box. If anyone can help locate a circuit diagram or offer any help on this machine I would be most grateful.

Thanks
Dave

[Mark Hennessy]

You're right to not just swap them without looking for why they failed. There's a chance that the biasing mechanism is allowing the transistors to conduct too much - perhaps a dirty contact on the quiescent current adjustment preset? Hard to say without a schematic, but you might end up having to reverse-engineer the schematic from the unit. Given some good photos, we might be able to help...

[Mike Watterson]

Is it same as an ITT?

ITT KP1020  BC131 BC107 AC131 AC186

The RGD P10 is ITT branded on vents / cover
http://www.radiomuseum.org/r/ittgb_kp1020.html

OR

More beefy ITT KP1080   BC131 BC131 BC236 BFS60K AD161 AD162
http://www.radiomuseum.org/r/ittgb_kp1080.html

(They look similar to RGD P10 and both have schematics)

Anyway can't be hard to trace. I sometimes brightly light underneath, or scan/photo both sides and do semi-transparent merge.  It ought to be simple.

As Mark says the most common issue is pot or diode or transistor bias on input to last stage.

[Mike Watterson]

PS.
This is ITT version of the P20, which might be stereo version of P10?
http://www.radiomuseum.org/r/ittgb_kp043.html  (schematic available)

KP043
So each amp would be BC109 BC187 AC128 AC176

A lot of them seem to be BC154 or BC109, BC107 or BC187, and output AC128 AC176 with near identical circuits, or else the louder AD161 AD162 outputs.

[Cave Miner]

Thank you Mark and Mike,
It is the same as the ITT Now downloaded schematic and will check as described. Will let you know next week as am away at moment
Once again many thanks

Dave

[pwdrive]

KP1020 & (P10 ?) courtesy of the KB Museum:

http://kbmuseum.org.uk/itt_service_manua...20_ITT.pdf

EDIT: Whoops...Post crossed.

Lawrence.

[Mark Hennessy]

Well that looks pretty straightforward (looking at the one linked in post #4 - let me know if that's the wrong one).

Nothing beyond a single resistor to bias the output stage, which is rather crude, but perhaps more reliable than the alternatives. Make sure that resistor (Ra13 or Ra28) is OK.

After that, remember that TO1 devices can suffer from tin whiskers. It's not as commonplace as the TO7 package (AF117, etc), but it happens and could cause damage.

[Cave Miner]

Mark,
Thanks for that I will check it out when i return on Sunday, I have two new output transistors but will carry out as many passive checks before I power up., particularly R13 bias resistor. This simple repair has used up much more effort and time than I antisipated. Can't wait now to hear some sound coming from it.
Dave

[Crackle]

Hi Dave
This is the same as the ITT-KB KP1000 and KP1020. Have you checked the output capacitor C7 (on the KP1020 service manual) , could this cause the transistors to fail if it went short circuit.

Mike

[Cave Miner]

Hi Mike and everyone who helped
Checked all resistors and transistors and found , as you suggested Mike , that the output capacitor C7 was open Circuit. It was probably my erroneous testing of the output transistors that led me up the garden path (lesson learnt) but replaced anyway. Replaced C7 and the amplifier burst into life. Now working fine. In future I will be less suspect of transistors and check passive components and PCB track first. I have also ordered a transistor checker from eBay rather than relying on the diode check on my multimeter. Anyway thank you all again for your wisdom.

Dave