UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Horrid intermittent on Transistor Black Box 1004

Sideband · 20th Dec 2011, 10:52 pm

In your first post, you mention changing VT7 which was faulty but not VT6 which is the other 'half' of the output pair. I would always advise changing both even if only one measures faulty (it was always a requirement at Philips). Admittedly sometimes you can get away with it but generally it's better to change the pair. Excess current through one or the other could cause problems and I have had intermittent faults caused by output transistors being faulty. The problem with all of these sorts of output stages is that because they are DC coupled, a problem in one area can affect the voltages all over the circuit. An AC127 for VT4 and an AC128 for VT5 will be fine as drivers. I think that's what we used as subs anyway. Make sure that one of your speakers is not going O/C (they're in parallel to give 7.5 ohms). Will give reduced output (though not distorted).

Rich.

20th Dec 2011, 10:52 pm	#13
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,580	Re: Horrid intermittent on Transistor Black Box 1004 In your first post, you mention changing VT7 which was faulty but not VT6 which is the other 'half' of the output pair. I would always advise changing both even if only one measures faulty (it was always a requirement at Philips). Admittedly sometimes you can get away with it but generally it's better to change the pair. Excess current through one or the other could cause problems and I have had intermittent faults caused by output transistors being faulty. The problem with all of these sorts of output stages is that because they are DC coupled, a problem in one area can affect the voltages all over the circuit. An AC127 for VT4 and an AC128 for VT5 will be fine as drivers. I think that's what we used as subs anyway. Make sure that one of your speakers is not going O/C (they're in parallel to give 7.5 ohms). Will give reduced output (though not distorted). Rich. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman..... Last edited by Sideband; 20th Dec 2011 at 10:58 pm.