Wier 423D - UK Vintage Radio Repair and Restoration Discussion Forum

Refugee · 29th Oct 2012, 12:38 am

Did it really have to be stripped down this far

Well i suppose it did really. It is one for the awkward fault gallery.
First it had no output apart from a little bit of biasing from the sensing wires.
I did a check of the transistors with a diode tester and found that the voltage sensing transistor was open circuit collector. It was BCY70.
I changed it for an NOS one with gold leads it was also a BCY70.
The problem was not corrected with this

I took many measurements and compared it with the good 413D that shares the same case.
I got down to a wiring fault and that is why it ended up being a major strip down.

It was loosing the drive to the power transistors with the voltage pot being shunted down. This may have happened for the first time when the fault that caused the transistor failure occurred.
I found that the floating unregulated supply that holds the negative side of the sensing was open circuit from the rest of the unit.
Where was it then.
The fault was in the sense circuit all right and I got closer and closer to the front panel.
It was the binding posts on the front. They were tight and had a good solid feel to them but all the same they were open circuit between the little PCB behind them and the nuts that held it all together.
Once I had restored order there I calibrated the meters and put the cover on again.
It was a pain to find though.

Herald1360 · 29th Oct 2012, 1:23 pm

Well found.

Sounds like a classic example of why it's a bad idea to have any insulating material (the PCB here) in a compression contact stack up.

Refugee · 29th Oct 2012, 2:05 pm

I will monitor the situation and if it gets dodgy again I will fit a wire and an extra tag as the PCB is single sided.

29th Oct 2012, 12:38 am	#1
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,553	Wier 423D Did it really have to be stripped down this far Well i suppose it did really. It is one for the awkward fault gallery. First it had no output apart from a little bit of biasing from the sensing wires. I did a check of the transistors with a diode tester and found that the voltage sensing transistor was open circuit collector. It was BCY70. I changed it for an NOS one with gold leads it was also a BCY70. The problem was not corrected with this I took many measurements and compared it with the good 413D that shares the same case. I got down to a wiring fault and that is why it ended up being a major strip down. It was loosing the drive to the power transistors with the voltage pot being shunted down. This may have happened for the first time when the fault that caused the transistor failure occurred. I found that the floating unregulated supply that holds the negative side of the sensing was open circuit from the rest of the unit. Where was it then. The fault was in the sense circuit all right and I got closer and closer to the front panel. It was the binding posts on the front. They were tight and had a good solid feel to them but all the same they were open circuit between the little PCB behind them and the nuts that held it all together. Once I had restored order there I calibrated the meters and put the cover on again. It was a pain to find though. Attached Thumbnails

29th Oct 2012, 1:23 pm	#2
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Wier 423D Well found. Sounds like a classic example of why it's a bad idea to have any insulating material (the PCB here) in a compression contact stack up. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

29th Oct 2012, 2:05 pm	#3
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,553	Re: Wier 423D I will monitor the situation and if it gets dodgy again I will fit a wire and an extra tag as the PCB is single sided.