faulty PSU in Herald Pentara PABX - UK Vintage Radio Repair and Restoration Discussion Forum

Dave Moll · 16th Jan 2015, 5:21 pm

First, my apologies to any THG members here who may have already seen this request, but I thought it worth running this past those on here with experience of dabbling around inside electronic equipment. Also, I have taken my investigations a bit further since I raised this in the THG.

Second, a bit of background for those not familiar with this equipment: The Pentara has a combined power supply consisting of two units - first, one deriving an AC ringing current and a 50V DC supply from the mains; second, a DC-DC converter to derive from the 50V DC supply a number of lower-voltage DC outputs:
* +12V
* -12V
* +5V
* +24V
* +1.5V

There are test points for each of these outputs, and although I get the correct reading at the 5V and 1.5V test points, the +12V, -12V and +24V all give a reading of 0V.

My first suspicions were directed at three large electrolytic capacitors - the usual suspects. I find, however that one had a voltage of 5V across it and is thus presumably working. The other two each gave a reading of 0V, so warranted further investigation. I desoldered each of these from the circuit board and found that each charged up nicely with a resistance meter across them.

Any thoughts on further lines of investigation would be appreciated.

16th Jan 2015, 5:21 pm	#1
Dave Moll Dekatron Join Date: Feb 2005 Location: West Cumbria (CA13), UK Posts: 6,130	faulty PSU in Herald Pentara PABX First, my apologies to any THG members here who may have already seen this request, but I thought it worth running this past those on here with experience of dabbling around inside electronic equipment. Also, I have taken my investigations a bit further since I raised this in the THG. Second, a bit of background for those not familiar with this equipment: The Pentara has a combined power supply consisting of two units - first, one deriving an AC ringing current and a 50V DC supply from the mains; second, a DC-DC converter to derive from the 50V DC supply a number of lower-voltage DC outputs: * +12V * -12V * +5V * +24V * +1.5V There are test points for each of these outputs, and although I get the correct reading at the 5V and 1.5V test points, the +12V, -12V and +24V all give a reading of 0V. My first suspicions were directed at three large electrolytic capacitors - the usual suspects. I find, however that one had a voltage of 5V across it and is thus presumably working. The other two each gave a reading of 0V, so warranted further investigation. I desoldered each of these from the circuit board and found that each charged up nicely with a resistance meter across them. Any thoughts on further lines of investigation would be appreciated. __________________ Mending is better than Ending (cf Brave New World by Aldous Huxley)