02-09-2020, 08:28 AM
(This post was last modified: 02-09-2020, 08:30 AM by ppppenguin.)
Turned on the PM14/1A this morning for the first time in ages. Guess what..... It didn't work. I could hear line whistle and it autoswitched to 405 when given a 405 line input. No EHT rustle. Remember this has separate EHT and line scan so you can hear line whistle even without EHT.
Drag it out of the rack on to the bench and probe around. About 2kV EHT. Normal-ish waveform on the EHT output transistor collector but something is clearly not happy. There's a 1R resistor where it's convenient to see how much current the EHT output is taking. I don't know what it should be but 2A is much too high.
Disconnect EHT smoother and EHT comes up with much discharge around the disconected point. One of the photos is of the EHT transformer with the cap just visible bottom right. This was taken in 2013 when I had much fun fixing the EHT generator. The output transistor had blown and I had fitted a BU208A. This was fine on 625 but overheated on 405. Took a while to find that one. The BU208A has a much lower current gain than the original BU102 and was coming out of saturation at the end of the forward stroke. Somebody gave me a BU102 and that fixed it. I also rebuilt the EHT rectifier assembly which was very prone to corona. There should probably be an anti-corona metal cup on the EHT cap terminal but I didn't have one and it would have been awkward to fit. I think the original mechanical design of the EHT stuff was a bit iffy and I got it to work OK without corona. EHT is about 16kV.
The EHT smoother cap (1000p, 20kV) was near enough short circuit. Look in spares drawers and by a miracle there's a similar looking cap. See both in other photo. Try to fit replacement and find it's got a 2BA screw hole instead of 4BA. Not hard to fix but I'll have to take out the EHT board which is a hassle. Also ream out the tag without damaging anything. Or replace it.
In theory the monitor ought to be able to work without an EHT smoother, just relying on the tube's graphite. If I can get it to stop corona'ing like made without the cap I'll give it a go.
Drag it out of the rack on to the bench and probe around. About 2kV EHT. Normal-ish waveform on the EHT output transistor collector but something is clearly not happy. There's a 1R resistor where it's convenient to see how much current the EHT output is taking. I don't know what it should be but 2A is much too high.
Disconnect EHT smoother and EHT comes up with much discharge around the disconected point. One of the photos is of the EHT transformer with the cap just visible bottom right. This was taken in 2013 when I had much fun fixing the EHT generator. The output transistor had blown and I had fitted a BU208A. This was fine on 625 but overheated on 405. Took a while to find that one. The BU208A has a much lower current gain than the original BU102 and was coming out of saturation at the end of the forward stroke. Somebody gave me a BU102 and that fixed it. I also rebuilt the EHT rectifier assembly which was very prone to corona. There should probably be an anti-corona metal cup on the EHT cap terminal but I didn't have one and it would have been awkward to fit. I think the original mechanical design of the EHT stuff was a bit iffy and I got it to work OK without corona. EHT is about 16kV.
The EHT smoother cap (1000p, 20kV) was near enough short circuit. Look in spares drawers and by a miracle there's a similar looking cap. See both in other photo. Try to fit replacement and find it's got a 2BA screw hole instead of 4BA. Not hard to fix but I'll have to take out the EHT board which is a hassle. Also ream out the tag without damaging anything. Or replace it.
In theory the monitor ought to be able to work without an EHT smoother, just relying on the tube's graphite. If I can get it to stop corona'ing like made without the cap I'll give it a go.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
