Consumer VHF-UHF receiver measures 100vac from chassis to ground - Page 1
Forum section: electronics / general

author: videobruce
author_rank: Frequent Contributor
timestamp: March 17, 2021, 04:33:45 pm
content: I have two similar scanning receivers (Radio Shack Pro4004 & Pro2006) of similar design that I just discovered both have a 'hot' chassis. Measuring 100vac (no load) to earth ground (grounded roof antenna). It's anon-grounded line cord.They both have a 'old school' power supply (metal enclosed xformer), not switching that is mounted to the metal chassis. There is a 1.8 meg resistor from the neutral to the chassis.There is no other component between the AC line cord and the xformer.They were designed in the late 80's by GRE and were very popular.If I de-solder the resistor, the leakage voltage drops to 9vac. So the question is, is it ok to leave it disconnected? I assume the 1.8 meg resistor is to drain any static charges off the chassis .The odd thing is I have had one of these for probably 30+ years and never noticed this which I did now by re-connecting the antenna cable and holding a external metal enclosed FM broadcast filter and the antenna cable connector.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: March 17, 2021, 05:18:32 pm
content: Is there a schematic for this thing anywhere?

author: tom66
author_rank: Super Contributor
timestamp: March 17, 2021, 05:18:45 pm
content: Looks like you have swapped L/N in your socket or cable somewhere. (Or is that the reversible American cord that has no polarity?)1.8Meg to chassis is probably not an issue. Leakage current is <1mA. But may be worth testing that.

author: videobruce
author_rank: Frequent Contributor
timestamp: March 17, 2021, 05:37:34 pm
content: It looks like that is/was the problem.

author: BrokenYugo
author_rank: Super Contributor
timestamp: March 18, 2021, 04:44:15 am
content: That's typical, no need to disconnect anything. The 66 microamps you'll get if the outlet is wired wrong aren't going to hurt anything or one. A couple orders of magnitude off from tripping a GFCI. Modern switching supplies and metal cased 2 wire equipment will show similar leakage if you go measuring them.Hot chassis, strictly speaking, is when one side of the mains is directly connected to the chassis, mostly found in series string table radios from the 30s.

author: videobruce
author_rank: Frequent Contributor
timestamp: March 18, 2021, 09:44:40 am
content: I realized it wasen't really a 'hot' chassis, I was more concerned about possibly damaging other devices running off other antennas from the same ground.

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 18, 2021, 12:40:39 pm
content: Add a 10M resistor in series with the original. And replace the plug with a polarized one if it's not already.

author: amyk
author_rank: Super Contributor
timestamp: March 18, 2021, 01:28:01 pm
content: Quote from: BrokenYugo on March 18, 2021, 04:44:15 amI believe 250uA is the limit according to the safety standard, so 66uA is not unusual nor a problem.

author: videobruce
author_rank: Frequent Contributor
timestamp: March 18, 2021, 01:49:59 pm
content: The plugs were polarized, it was my error, that I didn't catch, same error on both line cords, just different quick connect connectors wires crossed.Ironically, I don't know if it is just coincidental, but a hand held radio I had connected that hasn't been used for a couple of months, the xmit is messed up (non-tech term), it indicates the swr is high with a series of beeps when xmitting, but swr is not the problem. I'm figuring the CPU xmit function is toast. It's a older Moto P1225.