Pioneer qx-8000 : capacitor or transistor problem? - Page 4 - UK Vintage Radio Repair and Restoration Discussion Forum

PJL · 20th May 2022, 11:10 pm

Don't like co-incidences, try lifting one end of R6.

jmcilkley · 23rd May 2022, 3:48 pm

OK, progress. I took out Q2 and checked for any shorts between the connection for it and there were none. So I found a BC337 and put it in and checked the voltages on the base of Q1 and it was 13.2v which seems about right. The emitter of Q2 was 33.2v. So I resoldered in the resistors to reconnect the outputs but left the boards powered from B2 still not connected. The results were :
B1 : 54.8 B2 : 33.2 B3 : 22.4 B4 : 22.6 B5 : 11.7 B6 : 33.2
So now too high on B2 and B6!

Measuring around Q1 and Q2 :
Q2 53, 33.2 , 33.8 Q1 13.2, 12.6, 33.8

frsimen · 23rd May 2022, 5:21 pm

The output voltage of the regulator is about right now. The exact voltage will depend on the exact values of the potential divider resistors, the Zener diode and the VBE of Q2. What you have seems reasonable.

Without a load, the voltage on B2 will be too high. Once the external load is connected, it will drop to the correct level.

It looks like you have repaired the fault, so well done on your persistence!

Paula

jmcilkley · 24th May 2022, 8:34 am

More well done to you for persevering with me. I guess I was fooled by the fact that the original Q2 tested fine.
Many thanks. I will now get on with re-capping the other boards.

20th May 2022, 11:10 pm	#61
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Pioneer qx-8000 : capacitor or transistor problem? Don't like co-incidences, try lifting one end of R6.

23rd May 2022, 3:48 pm	#62
jmcilkley Tetrode Join Date: Jan 2020 Location: Ilkley, West Yorkshire, UK. Posts: 92	Re: Pioneer qx-8000 : capacitor or transistor problem? OK, progress. I took out Q2 and checked for any shorts between the connection for it and there were none. So I found a BC337 and put it in and checked the voltages on the base of Q1 and it was 13.2v which seems about right. The emitter of Q2 was 33.2v. So I resoldered in the resistors to reconnect the outputs but left the boards powered from B2 still not connected. The results were : B1 : 54.8 B2 : 33.2 B3 : 22.4 B4 : 22.6 B5 : 11.7 B6 : 33.2 So now too high on B2 and B6! Measuring around Q1 and Q2 : Q2 53, 33.2 , 33.8 Q1 13.2, 12.6, 33.8

23rd May 2022, 5:21 pm	#63
frsimen Heptode Join Date: Jan 2011 Location: Croydon, London, UK. Posts: 772	Re: Pioneer qx-8000 : capacitor or transistor problem? The output voltage of the regulator is about right now. The exact voltage will depend on the exact values of the potential divider resistors, the Zener diode and the VBE of Q2. What you have seems reasonable. Without a load, the voltage on B2 will be too high. Once the external load is connected, it will drop to the correct level. It looks like you have repaired the fault, so well done on your persistence! Paula

24th May 2022, 8:34 am	#64
jmcilkley Tetrode Join Date: Jan 2020 Location: Ilkley, West Yorkshire, UK. Posts: 92	Re: Pioneer qx-8000 : capacitor or transistor problem? More well done to you for persevering with me. I guess I was fooled by the fact that the original Q2 tested fine. Many thanks. I will now get on with re-capping the other boards.