Tek 465 HV transformer removal from PCB - Page 1
Forum section: electronics / repair

author: Addicted2AnalogTek
author_rank: Frequent Contributor
timestamp: March 09, 2018, 07:27:30 am
content: Hey everyone!I've got a 465B with a bad HV transformer and a 465 (formerly fully-functional) donor scope, but I can't for the life of me find any specific information on how to properly remove the transformer from the PCB. I've been searching for the last couple hours and haven't come up with a single thread, website, or video that shows how to remove it without breaking either the ferrite or damaging the PCB. It's making me feel like I'm thinking too much into it...I'm thinking that the procedure is going to be something along the lines of heating up the PCB and gently sliding something thin between the PCB and the ferrite, but I'm just not sure if that's really the proper procedure.Does anyone have some experience in performing this procedure or have links to some good info? Help would be greatly appreciated!!!!-Brian

author: Addicted2AnalogTek
author_rank: Frequent Contributor
timestamp: March 09, 2018, 11:30:23 pm
content: I was definitely over thinking the procedure. The transformers come free easily after heating up the area with a heat gun.Just a little background on the project:I received the scope yesterday. It was sold as "powers up with no display". All of the low voltage rails measured good, but the HV was only outputting around -700V. The oscillator functions, is at approx 52Khz, and was measuring about 2V peak to peak. I followed the flow chart procedure for "troubleshooting the -2450V supply" that is in the back of the 465B service manual, and came to the conclusion that the HV transformer was bad. I've now changed the transformer and powered up the scope with the HV multiplier disconnected. The -2450V test point measured around -1000V, oscillator running at 3.2V peak to peak. After connecting the HV multiplier, the -2450V rail dropped back down to approx -750V. Now it's time to swap in a replacement multiplier.

author: Paul Moir
author_rank: Frequent Contributor
timestamp: March 10, 2018, 12:30:50 am
content: If you haven't found it already, here's a homemade multiplier:https://www.eevblog.com/forum/repair/tektronix-465/msg672083/#msg672083

author: Addicted2AnalogTek
author_rank: Frequent Contributor
timestamp: March 10, 2018, 12:50:22 am
content: Thanks Paul!!Unfortunately, I'm stuck scratching my head now... I had -1kV with the "new" transformer immediately after installation with the multiplier disconnected (anode and multiplier ground lead disconnected), so I re-connected the multiplier to check and see if it was good since they're hard to come by. I was still getting over -900V with it connected, so I chose to go through the troubleshooting procedure once again, assuming that the multiplier may be fine. Now I'm not getting more than -750V with the anode lead and ground disconnected. I'm starting to question whether the transformer was ever the issue, or if there's multiple failure points in the circuit. In addition to the 465B (and 465) troubleshooting charts, I've lifted a lead on every diode and checked them, wholesale swapped all of the transistors (then swapped the originals back in since nothing changed), replaced the HV diode, and of course replaced the HV transformer. At this moment, I'm unsure of what is loading down the circuit.If I am to believe the -2450V troubleshooting flow charts, then the HV transformer is bad. I'm just not quite sure where to go from here.

author: Addicted2AnalogTek
author_rank: Frequent Contributor
timestamp: March 10, 2018, 12:53:47 am
content: Also, here's some info for rewinding your own HV transformers:https://sites.google.com/site/linuxdigitallab/home/restoration-an-tektronix-464It also has the information needed for rewinding the 120-0800-01, which is used in the 465, 465B, and 468 scopes.

author: Addicted2AnalogTek
author_rank: Frequent Contributor
timestamp: March 10, 2018, 02:03:59 am
content: Well this is rather odd... I completely missed it before because I went through the scope and checked the capacitors in circuit, but the 5uf 150V decoupling capacitor for the oscillator circuit was apparently oscillating when the circuit was in operation, therefore shorting the circuit to ground. It tests good out of circuit, measuring 7uf (it's a +100%-10% tolerance cap) and measures good for ESR when using a function generator and scope.That's my theory, anyway....SOOO... the whole transformer swap was likely completely unnecessary! Maybe I'll swap it back..