Dead power supply in a Tek 2245A

[WattsThat]

I have a nice 2245A that was my daily driver years back to about 2003. One morning it just didn’t turn on and the local Tek house said they were no longer supported or repaired. I replaced it with a TDS220 but I’ve never found it as usable as the 2245A. I could never bring myself to toss it and it’s been in storage ever since. Finally decided it was time in my life to start downsizing and do something with all the stuff that’s been accumulating so it’s on the bench. Downloaded the service manual and I’m just getting started.

The power supply is completely dead, no clicks, no noises but I can tell it’s rectifying line voltage and see initial current as it charges the primary side cap. Absolutely no voltages on the bottom board connector so I pulled the supply board out. It’s very clean, nothing burned, all semiconductors look okay on diode check and no shorted caps.

So it looks like it’s time to start some power on troubleshooting which leads me to the reason for my post - do I need to load anything to test this board out of the scope? I can find nothing in the service manual so it’s time to ask those who have been there, done that. Yes, it will be connected via a 500 va isolation transformer. But I don’t want to just plug it in and do more damage. Can anyone help with the correct configuration for proper board level testing?

[tautech]

Most of what you need to know is in here:
https://www.eevblog.com/forum/testgear/tektronix-2465b-oscilloscope-teardown/

Jump into this thread and ask the guys there and point them back to this thread if that best suits your needs.

[cnqhdszq]

(1) You may check the caps first ,with LCR meter or ESR meter.
(2)check all the resisters of primary side  .
(3) check the transformer  then.

good luck!


http://bbs.38hot.net/forum.php?mod=viewthread&tid=108473
You may need this ,if you can read/speak Chinese.

[David Hess]

It is possible that the circuit responsible for startup has failed.  100 microfarad capacitor C2204 charges from the rectified input voltage through 180 kilohm R2203 in parallel with R2204.  Sometimes this capacitor dries up so it cannot provide enough charge for the power supply to successfully start.  But if that is the case, then the other aluminum electrolytic capacitors are reaching the end of their life as well.

The voltage across C2204 should reach about 14 volts before the switching regulator tries to start.

[aqibi2000]

When the voltage across C2204 it below 12.5V it will no longer start

[WattsThat]

The 44 volt supply successfully starts, remains on for a second or two and then drops to 40 volts when the main power inverter fails to start.

All the semiconductors and caps check fine in-circuit, I’m going to pull Q2209/2210 out and recheck. They certainly are not shorted but I guess could be open and there is a diode in the base circuit that could be fooling me.

Any thoughts/hints/experiences welcome.

[David Hess]

That could be a shorted tantalum capacitor on the secondary side causing the off-line buck regulator to go into current limit.

[WattsThat]

That could be a shorted tantalum capacitor on the secondary side causing the off-line buck regulator to go into current limit.

The main board supply point resistances all check correctly and the power supply board behaves the same in or out of the scope. So, while looks like a current limit issue, I cannot find the cause. What I know at this point:

The voltage across R2201 (the 0.1 ohm current sense) does peak at ~230 mv before dropping to zero when the PWM output goes to minimum pulse width into Q2201. Based on the circuit description, this does behave like there is high current in the 44 volt supply. The MC34060 appears to functions properly going into current limit by reducing PWM duty cycle but it does not enter chirp mode in an attempt to restart.

I can find no reason for the current limit, no shorted devices, nothing out of spec. It’s a stumper at the moment.

[David Hess]

That is high enough to trip the current limit but I wonder why it is not attempting to restart.

[jdragoset]

Perhaps VR 2201 is firing the crowbar at to low a voltage.

[WattsThat]

No, the crowbar is not firing as the 44 volts holds up. Here is what I’ve done with the power supply removed from the scope and on the bench.

Removed wire jumper W2201 to disable the power inverter. Loaded the the 44 volt supply with a 100 ohm 10w resistor (yeah a little small but not leaving it on for long).

Everything in the 44 volt supply is good, it starts and maintains a regulated 41 volts. Yes, a little low, haven’t figured that out but the PWM modulates and all waveforms are correct per the service manual. So, it’s something in the inverter that is loading it down. Nothing obvious, continuing to dig. Even though they all pass diode test, I’ll start lifting the secondary rectifier diodes and see if I can get the inverter to start without sending the pre-regulator into over-current shutdown.

Edit: is it possible that the low primary supply of 41 volts is low enough to prevent the main inverter from starting? Is the design that sensitive/critical that the lower voltage isn’t enough to get the transformer to saturate and cause switching?

[jdragoset]

Try un-soldering the EHT lead from the inverter transformer at the EHT multiplier terminal.
I've used a 75 watt incandescent bulb for a dummy load and it works well.

[WattsThat]

Sorry, it’s my first dance with a Tek scope, not up to your speed.

I understand disconnecting the inverter transformer feed into the HV multiplier module to test but where do I connect an incandescent lamp as a dummy load?

Btw, the inverter bipolar power transistors Q2209/10 never switch fully on, there is a hf oscillation that occurs on startup and the then the amplitude decays over a second or two and both collectors remain high with no switching activity. The gate of the regulator mosfet, Q2214 is at 20V so the channel is fully formed and it’s able to sink current, but there isn’t any current to sink with Q2209 & 10 both off. The start capacitor, C2248 checks fine with the ESR below an ohm and voltage rises to 20V at power up, dropping to zero as expected.

[jdragoset]

The dummy load was only used without connection to the inverter, just to test the 44 volt power supply.
Testing the 44 volt supply powering the scope with the EHT to the multiplier will test if the multiplier is shorted, as the rest of the low voltage power supplies will all be correct while powering all of the scope circuits.

[WattsThat]

Thank you for the info, I’ll use the light bulb instead of the resistor.

Out of desperation, I lifted one end or removed all the secondary rectifier diodes on the inverter transformer. All diodes check fine as they did in circuit.

With the diodes removed, the behavior did change a bit. The unit remains in the incorrect oscillation state with Q2209/10 having a ~ 1 MHz sine wave so they are not saturating as they should. The transistors (2209/10) get hot very quickly so I’ve not left it on for more than 5 seconds or so.

With the diodes in circuit, the incorrect waveform appears only for a brief time, once the PWM output duty cycle starts to decrease, the oscillation stops.

I’m still wondering if the reason for the failure to start is the lower than normal pre-regulator voltage of approximately 40 volts rather than the correct 44 volts. Looking at the circuit, I can’t figure out what actually sets the output voltage level and the circuit description doesn’t provide any clues.

Can anyone provide any insight on what sets the absolute pre-regulator output voltage level? I’m just not seeing it.

[jdragoset]

Q 2208 and divider resistors

[jdragoset]

If you've lifted CR's 2206,2207,2208,2209,2210,2211,2212,2213,2214,2215,2216,2218 and the voltage doesn't come up, the fault must be in T 2204, U 2230 or 130 volt AC supply (P 2204, pin 13)

[WattsThat]

Fixed it 

Unfortunately I had to buy a parts unit 

If I was a betting man (and I’m not), I would have lost money on betting on the source of the problem.

It was T2204, the main inverter transformer. No shorts, no opens but be dammed if will oscillate. As soon as it was replaced with a good part, the pre-regulator voltage came into spec at 44 volts where it had been holding at 40 with the PWM output of the regulator at minimum pulse width. It’s alive and for bonus points, I finally got to use my EEVBlog HPV70 for real rather than play.

If anyone needs any 22xx parts, PM me, I don’t feel like fixing another one!

[philwong5176]

Please email me about your spare 2245A, thanks.

phil1010@verizon.net