Green resistor in PCMA0002F cybernet rigs question. - UK Vintage Radio Repair and Restoration Discussion Forum

Spec-chum · 6th Jul 2022, 7:34 am

I have a Harvard 402MPA vintage cb radio that is fitted with the Cybernet PCMA0002F chassis, similar fitted to the Amstrad 901's, Harrier CBX etc. The radio does work fine but I've noticed the large green 3.3 ohm 3 watt resistor used to limit the tx power has been getting quite hot as it has a large burn mark on its body. I guess the rig has seen a lot of long overs or a used on an antenna with high swr, maybe a mixture of both.

Just as a future failure prevention and causing further circuits to fail, I would like to replace the resistor. I have some here that are the correct value and wattage, but are metal film type. Would they be alright to use or would I need a wireround type?

SiriusHardware · 6th Jul 2022, 8:41 pm

Does the soldering around the output transistor look original? If the O/P transistor ever went short circuit in the past that resistor would have had to bear the brunt until the supply was removed.

I know I have a scrap Mustang (same chassis) in the loft somewhere, I will have a look when I'm next up there to see if it still has its green resistor in good condition.

SiriusHardware · 6th Jul 2022, 10:21 pm

Just been up there to look for something else and found the chassis in question but... sorry to say many parts have already been harvested from it including that resistor and also the output transistor, sorry.

Sparky67 · 7th Jul 2022, 10:03 pm

The Amstrad 901 circuit shows a 3W 3.6 Ohm resistor at R13. If this is the resistor in question it is only carrying DC so any resistor rated at 3W when similarly mounted would be fine. Or maybe two 7.2 Ohm 2W resistors in parallel to give a little headroom?

6th Jul 2022, 7:34 am	#1
Spec-chum Tetrode Join Date: Mar 2019 Location: Grantham, Lincolnshire, UK. Posts: 79	Green resistor in PCMA0002F cybernet rigs question. I have a Harvard 402MPA vintage cb radio that is fitted with the Cybernet PCMA0002F chassis, similar fitted to the Amstrad 901's, Harrier CBX etc. The radio does work fine but I've noticed the large green 3.3 ohm 3 watt resistor used to limit the tx power has been getting quite hot as it has a large burn mark on its body. I guess the rig has seen a lot of long overs or a used on an antenna with high swr, maybe a mixture of both. Just as a future failure prevention and causing further circuits to fail, I would like to replace the resistor. I have some here that are the correct value and wattage, but are metal film type. Would they be alright to use or would I need a wireround type?

6th Jul 2022, 8:41 pm	#2
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,560	Re: Green resistor in PCMA0002F cybernet rigs question. Does the soldering around the output transistor look original? If the O/P transistor ever went short circuit in the past that resistor would have had to bear the brunt until the supply was removed. I know I have a scrap Mustang (same chassis) in the loft somewhere, I will have a look when I'm next up there to see if it still has its green resistor in good condition.

6th Jul 2022, 10:21 pm	#3
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,560	Re: Green resistor in PCMA0002F cybernet rigs question. Just been up there to look for something else and found the chassis in question but... sorry to say many parts have already been harvested from it including that resistor and also the output transistor, sorry.

7th Jul 2022, 10:03 pm	#4
Sparky67 Heptode Join Date: Feb 2008 Location: Great Barr, Sandwell, West Midlands, UK. Posts: 589	Re: Green resistor in PCMA0002F cybernet rigs question. The Amstrad 901 circuit shows a 3W 3.6 Ohm resistor at R13. If this is the resistor in question it is only carrying DC so any resistor rated at 3W when similarly mounted would be fine. Or maybe two 7.2 Ohm 2W resistors in parallel to give a little headroom?