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Re: Tektronix 2225 power supply startup ticking / buzzing...

 

I don't really know about the Tektronix power supply, but I had a similar problem a few years ago in my IFR-1500 Communications Service Monitor.? In the IFR power supply, and I believe this is true for most switchers, the ticking or pulsing is an oscillator in the primary power supply.? This oscillator puts out a series of pulses that go to the secondary (part of the) supply.? The pulses into the secondary charges up a capacitor, that once charged to a certain level allows the secondary's oscillator to supply enough voltage to get the oscillator driving the switching FET's on the primary of the transformer up and running.? In my case it was a defective capacitor that had enough leakage that it couldn't sustain enough charge to keep the oscillator running.? You could see this with a scope - the capacitor would try and ramp up but sage down very quickly.? Once in a while it would ramp up enough to get things going for a while, but then it might drop down and the power supply would reset, starting the whole process over.

So, whenever I hear something like this I think, "bad capacitor/s."? Of course it could be something else, but in the case of my IFR-1500, I replaced all the electrolytics on the power supply, about 10 in total, and he's been running fine ever since.

Another time I was helping a friend fix a service monitor.? This was about 20 or so years ago.? It ran fine on 12 volts D.C., but not on 120 V.A.C.? We called the manufacturer's service department and talked to a fellow there who knew the model particularly well.? He said to me, "There are two brown capacitors on the power supply board.? Do you see them?"? I said "yes, I see the two brown capacitors."? He then said, "Replace them, they were blue when the left the factory."? That was my first experience with this problem in a switching power supply.

Be very careful with these things, as I'm sure you are aware, in many switchers the primary is like an AC/DC radio, the negative or cold side of the AC circuit is connected to the ground or negative side of the primary supply.? The primary is isolated from the secondary side by way of the transformer. Connecting test equipment across the primary side can be very hazardous and some times, spectacular.
:>

Just my $0.02 worth.

Burt, K6OQK

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At 03:42 PM 11/18/2017, Chris Smith wrote

Hi all,

I seem to be collecting Tektronix scopes suddenly. After the acquisition of a 475 then a 465B, I obtained a circa 1989 Tektronix 2225 from eBay the other day described as fully working. This was purchased for it's 5mV+10X vertical mag feature which is great for looking at power supply noise. Unfortunately, and perhaps ironically it was not quite as described and has what looks like a power supply fault. This I sort of expect really considering the age.

When I fire it up, the power supply is not particularly happy for between 5 seconds and a minute. This results in ticking/buzzing and trace jumping slightly. Intensity doesn't appear to be affected by this. I've stuck bits of cardboard in around it to isolate where the noise is coming from and it's coming from the inverter transformer. I don't think this is HT as the buzz pitch is variable and doesn't sound like the popping of arcing. It sounds like the power supply is stalling and dropping temporarily into audible frequencies.? Once it has warmed up a bit it is 100% stable for a while and then starts dropping out. The trigger LED comes on when it drops out and the power supply becomes audible.

I measured supply ripple which was slightly above recommended on the -8.6v rail, so I've decided to replace all the electrolytic capacitors in the power supply as a precaution. When I lifted one there was signs of leakage on the bottom. These won't be arriving until Tuesday unfortunately. I swapped out two 2x 470uF 50V parts on the 38v and inverter rails with stock parts I had (panasonic low ESR) and it is slightly happier but perhaps not happy enough. Once I've replaced all the caps I will adjust the power supply again and check the waveforms against the reference ones in the SM and proceed that way.

There's a video of the startup behaviour here: and a video of the behaviour once it has warmed up here: - if you turn the volume up, you can hear the power supply noises.

So, I suppose the question is: are there any other things I need to look for at the same time? Any known issues or problems with these scopes in these areas?

Please don't tell me this is HT related or i'll cry as the inverter transformer and multiplier appear to be unobtainable.

Any help appreciated!

Best regards,

Chris Smith

Burt I. Weiner Associates
Broadcast Technical Services
Glendale, California U.S.A.
biwa@...
K6OQK

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