troubleshoot Phillips PE1509 power supply

[tonydix]

I have had one of these for years of faithfull service. It has now failed.

The unit powers up and supplies a variable voltage at the output terminals until any load is applied and then shuts down. The circuit (available on line) has a feature that limits the output current to 300 ma.

This feature now seems to limiting the output current to a very small current say 5ma which renders the unit useless.

I believe the current limiting feature uses a Mullard TAA263 IC. I am unable to understand the circuit surrounding this IC. Any ideas about further troubleshooting would be appreciated.

Thanks and best wishes

Tony

[pebe]

Can you give a link to the online circuit?

[tonydix]

http://elektrotanya.com/philips_pe-1509 ... nload.html

Thanks for trying

Tony

[pebe]

Hi Tony,
I’ve downloaded the circuit and found a datasheet for the TAA243. It was made by the old Mullard company and the datasheet is dated April 1969, so not much chance of replacing it if it is faulty.

It is an opamp but with an unusual output. It has the normal push-pull output terminating at pin6, but with an extra NPN emitter follower from that pin terminating at pin7. The differential inputs at pins2 & 3 cause a positive swing at pin7 when the output voltage is lower than was asked for. That turns on TS27 and TS29 (the series ‘pass’ transistor) when more voltage is required at the output.

The current control works like this. The output current flows through R54. It is 2.7ohms and when the current through it reaches 300mA the voltage on its RH side reduces by 800mV. That lowers the emitter of TS31 and causes it to conduct. That turns on TS28 which pulls down pin6 of the TAA243. That pulls down the base of the emitter follower in the IC, and pin7 voltage drops to turn off the output power.

Now for some checks. If you can adjust the output voltage, even with the low current limitation, then I think you can safely assume that the TAA243 and TS27 and TS29 are all working correctly.

The problem may be just that the pot, R2, has got dirty. If so, a tweak one way then the other may clean it. If not, disconnecting pin6 of the IC will disconnect the limiter (it might be easier to temporarily remove TS28 instead). If that cures the problem then we can go on to check the associated components.

[tonydix]

Many thanks for the explanation. I tested the pot R2 in circuit and resistance readings were fine. I also tested voltages at the wiper when powered on and all seemed fine 0 to 32 volts thru the range.

I then removed TS28 as you suggested. The problem still remains. A voltmeter across the output terminals shows a variable voltage 0 to 32 as you move R2 but connect any load and the volts drop to zero and thus no power is delivered.

Its a puzzle but I very much appreciate your efforts. Thank you.

Tony

[pebe]

It should be fairly simple to diagnose the fault. Do you want to continue?

[tonydix]

sure if you can be bothered I will learn something ! otherwise I will use the dc supply and stick in lm317 or equivalent to get it working again.
Thanks
Tony

[pebe]

Here are the steps I would take to find the fault.

Switch on. Set voltage to around 15V. Set current to max. Connect a load that will bring on the fault (ie, reduce current to 5mA).

Bearing in mind its age, probably the most likely fault is that the main electrolytics have dried out. Measure volts across C30 – should be at least 30V. If not, most likely C30 and C31 have dried out. Measure voltage across C26 – should be at least 24V. If not C26 has dried out.

If OK so far, measure voltage between pins 4 & 8 of TAA243. Should be close to 18.6V. If 12.4V then check for voltage across 6.2V zeners GR28, GR29 and GR38 in case one has gone s/c. If a randomly low voltage, suspect TS26 that gives a constant current supply, or a component around it.

If OK so far, remove TS28. Connect meter neg lead to +ve power output (Bu3 on circuit) so all measurements are relative to that rail. Measure volts on LH side of R54 - should be less than 20mV. Measure volts at TS29 base – should be about 0.65V higher. If lower, measure pin7 of TAA243 – should be about 1.5V. If higher, suspect TS27 has an o/c base or R31 is faulty.

If pin7 is 1.0V or lower then check voltages on pins 2 & 3 of TAA243. If pin3 is higher than pin2, then remove C27 in case it is s/c. If still the same, then TAA243 is faulty (but can be replaced with another type).

Hope that helps, and I would like to know what the fault was.

[tonydix]

Thanks very much for your suggestions. Volts across c30 27v,C26 17v, both below your suggestion, both tested ok with a blue tester but I dont have a meter to measure capacitance so I am hunting in the parts box !!
We live in the rainforest jungle of panama and the nearest part shop for electronic components is on the other side of the great divide !!
Thanks I will keep you posted
Best wishes

Tony

[tonydix]

It looks like it was the Electrolytics and the BC179.
Replaced c30 and the unit worked with no current control.
replaced bc179 (which had been removed ) and unit stopped working
So I must now search for an equivalent of bc179.

Many thanks for all your help. Much appreciated. I owe you a beer or even a case !!
Tony

[pebe]

So far, so good. You can use any small signal PNP in place of the BC179. Googling for 'small signal pnp transistors' will give a good selection - there's a lot to choose from.

I assume you have checked that you have 18.6V supplied to the TAA243? If not, one of the C6v2 zeners will not be conducting and that could give false symptoms.

[tonydix]

The mystery goes on !!! I found a replacement for BC179 installed it and the unit stops functioning, so presumably that was not the problem. Removed the unit firstly created a load with a 6 volt bulb and everything seemed to work, light got brigter as control was advanced. I worried about blowing the bulb so replaced it with 6.7 ohm wire wound resistor and an ammeter in series. As you wind up the control volts on the meter go to about 5 and do not increase any further and the ammeter shows current increasing to about 5.5 ma but no further.
Wire wound gets nicely warm so it is doing something !!
I was suspicious about C6v2 since it showed a voltage across it of about 4.7. I tested it in circuit and it showea a resistance on a diode test of about 700 ohm in both directions so I removed it. Out of circuit it tested fine o/c in one direction 700 ohms in the other. Any further ideas ?

Thanks again

Tony

[pebe]

You need 18.6V at TS26 collector to make the three C6V2 zeners (GR28, GR29 and GR37) conduct. It is essential that they conduct to ensure that TS28 and TS11 are correctly biased so that they work OK.

TS26 is wired as a constant current source and it needs 4.45V across R27 (5.1V across GR27) to make it work, so in total you need at least 4.45V + 0.5V (c to e) + 18.6V = 23.55V across C26.

You said in your post on Friday that when you measured the supply voltages, the voltage across C26 was only 17V. That's your problem. Change C26 - it has dried out.

[tonydix]

OK so I swapped out C26 it made no difference. I tested the original C26 it read 165 mf so presumably it was ok.
Took some more voltage measurements on GR26 in 14.5ac out 17.2 dc
Did the same on GR33 in 21,0ac out 27 dc.
Do you think the transformer winding s3 could be bad ?
If at any stage you get fed up with this troubleshooting please dont hesitate to say so. You have been great. Thank you
Tony

[pebe]

I did not realize that after changing C30 you still had only 27V across it. You cannot get a 30V stabilized output with only 27V input to the regulator, TS29. So it looks like both outputs are low. That could be caused by several things:
1. The transformer could be on the wrong tapping, ie. tapped at 125V if your supply was only 100V. But unless you somehow changed it when you were faultfinding, then if it was OK before it should be OK now.
2. One of the two secondaries has a high load on it.
3. One of the three windings has shorted turns.
4. Your meter is reading incorrectly.

!. So first measure the AC mains supply and check for the correct tap being used. You know what mains voltage to expect so that should also rule out a meter fault.
2. Then disconnect one side of S2 and S3 from their bridge rectifiers and measure the voltage output of each winding. I would expect the rectified and smoothed output of each to be about 1.25 to 1.3 times the AC voltage going into its rectifier, so you are looking for 25VAC or more across S2, and 21VAC or more across S3.

Can you let me know the results?