I bought this unit in early May. From the beginning I noticed that when it was first plugged in, I would need to push the POWER button 3x or so before the power supply would start. Then, if the unit remained plugged into the AC wall outlet, it would respond normally to the POWER button.
I decided to ignore the problem which was a *big* mistake since I had a limited right to return on this eBay sale.
Well, the problem got worse over time and I decided to open up the unit in early June for a look. I attached a few photos here for your viewing enjoyment:
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I followed the troubleshooting tree in the TDS3000 service manual, which indicated the main board was at fault. I figured I had nothing to lose at that point, and decided to dig further.
Since there is no theory of ops in the service manual, I wanted to share with you what I figured out:
The basic concept of the power supply is that a 120VAC to +15VDC switching power supply is the main source of power, running at around 90 kHz. This remains powered up at all times while the unit is plugged into AC power.
A battery board follows the AC power supply and allows an optional 15V battery to be plugged into the unit. A Schottky dual rectifier with common cathode allows power input from either the optional battery or the AC power supply. The battery board has the POWER switch which turns power on/off to the main board.
The battery board contains an LTC1435 switching regulator which converts +15V to +5V to run the logic circuits on the main board. The board also produces +15V/-15V for the optional powered probes.
Finally, one terminal of that battery board (pin 12) produces a weird voltage: ~+5.5V without load, and ~+3V when plugged into the main board. I do not know what the purpose of that voltage is (more to follow below.)
I found the AC power supply board was working great. The battery board, when disconnected from the main board, also produced all of the spec voltages per the TDS3000 repair manual.
Further, when I used lab power supplies to provide +5V and +15V to the main board (ignoring pin 12) the scope worked. I determined the current flow to the scope, and used resistors to prove that the battery board would produce the required current output.
So the question was, why when the battery board was plugged into the main board, did the unit refuse to power on? I replaced the through-hole electrolytics on the battery board but that did not help.
After many hours, I figured out that pin 12 was the issue. The battery board will stop producing output if the resistive load is lower than ~1M ohms. However, the main board resistance on pin 12 was ~300 ohms.
When I used lab power supplies to power up the main board without providing power to pin 12, I noticed a warning msg about the unit switching from 50 ohm input to 1M ohm input due to excessive input voltage. Of course there was no input to the vertical channels during my testing. Does that help anyone figure out what the purpose of pin 12 is?
The main board stopped working totally after all of this testing. So I was really dead in the water.
I contacted the seller of my unit, explained my predicament, and the seller was willing to have a tech look at the problem. The tech confirmed my findings and of course they could not do anything about it.
Meanwhile I contacted Tektronix and found that neither the main board or battery board were available for sale. However they would attempt to repair/calibrate the unit for $1,900 plus shipping.
So, I asked the seller to forward the unit to Tektronix. Initially I was told that the unit was not repairable as both the main & battery boards needed to be replaced and parts were not available. I asked for more info about what was wrong with the main board.
After some drama, the latest is that the main board had failed (no surprise) and that they were able to replace it. The battery board was OK. I was also told that the purchase price of the replacement part (had it been available) was $4,500 so obviously it would make no sense to buy parts.
In hindsight it was a bad idea for me to buy this unit. I share my experience with the group, encouraging you to think long and hard before spending four-digits to buy one of these scopes, since repair documentation and parts are not available and the repair cost at Tek is high.
This is like an Italian sports car: when it works it is wonderful, but if it has a failure, forget it.
