HP 1955 Inverter Problems

[budm]

Re: HP 1955 Inverter Problems

Not sure exactly either, it has been a long thread with more then one original poster, but I think he should check or replace those two tuning caps as you suggested.

[1lostsock]

Re: HP 1955 Inverter Problems

I have largely left the original working inverter alone. On the borken channel the .22 capacitor has been changed for a new part.
Q812 was also replaced but I will try and find a moment to squeeze my ancient oscilloscope on the the gate (I might have to solder a test lead to run with the CFLs attached).
C801 & 802 show no physical signs of damage so I have left them alone. I only have basic caps (i.e. not much better) around me to replace them with.

Since the circuit seems to run well with the FU9024N bypassed I will concentrate on the switching circuitry.

Thanks for your time as always Gentlemen.

[1lostsock]

Re: HP 1955 Inverter Problems

OK. It has been a while and I did find the venerable oscilloscope and wired it in for a peek.

The hot Q812 is because the switching waveform is not a square wave.
The frequency seems to be about 80 kHz and the duty cycle is about 5-10% on (= voltage high) from pin 7 of TL1451.

When pin 7 goes to ground (internal transistor conducting) the waveforms on the scope at pin 7 and the output of Q810/Q811 both cut off nicely.

However when Pin 7 stops conducting the voltage waveform does not abruptly step up, there is an RC decay curve towards the new voltage level.
At the low duty cycle it begins to look like a saw tooth wave.

The poor switching causes linear FET operation and way too much heat.

The local resistors have the right values and all the local semi conductors have been replaced - maybe TL1451 needs replacing but I am clutching at straws there.

I would like to think that the test leads and scope didn't introduce that much capacitance.

At this point it may be easier to get another inverter board from EBay...

[budm]

Re: HP 1955 Inverter Problems

That FU9024N Q812 is the P-CH so if you have 5-10% High that means the MOSFET is on 90%, P-CH is on in this circuits when Gate voltage switch toward ground. the harder the Q111 turn on (more off time from pn7) the harder Q812 will turn on. make sure to set the switch on the probe to X10.

[1lostsock]

Re: HP 1955 Inverter Problems

Thanks for your quick rpely.

The probes were at 1X, switching to 10x produced similar results.

I have attached a picture of what I am seeing - the top trace is Y1 and is taken from the FET gate, Y2 is Pin 7 of TL1451.

Digital cameras - giving every 'scope a digital storage option!

[budm]

Re: HP 1955 Inverter Problems

What you should see at pin 7 is that the positive pulse width should increase as you dim down the backlights, you should be able to see that on the other section that is working OK. Look at pin 10 on how it behaves when you adjust the backlights brightness control.

[moda]

Re: HP 1955 Inverter Problems

If the inverter is switching on for few second and then goes off on TL1451 then replace
RT resistor 10k to 100k, this may help.( pin no 2 )