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Old 12th August 2007, 01:32 PM   #1
GeeVee is offline GeeVee  Australia
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Default Need help with a power supply

Hi all.

I am in need of some assistance.

I am rebuilding a power supply from a preamp, that died several years ago.
So far I have been able to get most of the correct voltages restored, but a have two problems that I cannot locate.

The +15V and + 35V rails are OK.

The -35 volts is reading -45.35V (??)

Now for the strange one. The -15V rail takes about 8 seconds to ramp up to this voltage. I have not come across this problem before.

With respect to the -35V reading -45V I thought that the 10v reference zener was shot, beceause it was exactly 10V out. This has been replaced, and had no effect.

(Supply voltage is 30-0-30 vac)

Any advice will be greatly appreciated.

Regards,
George.
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Old 12th August 2007, 02:30 PM   #2
jaycee is offline jaycee  United Kingdom
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Check the capacitor C44 and the other transistors Q42, Q44 and Q46 (i think).

Does the -35 rail take 8 seconds to come up as well ? If not, you might want to check that C46 and C48 arent damaged, along with the 7915
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Old 12th August 2007, 02:33 PM   #3
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It looks like Q44 is not turning on so the pass transistors are fully on. Q44 is supposed to shunt base current (supplied from Q42s collector) away from the pass transistors. Or Q42 could be shorted. The base voltage of Q44 should be about -.7 volts.
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Old 13th August 2007, 08:02 AM   #4
GeeVee is offline GeeVee  Australia
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Quote:
Originally posted by jaycee
Check the capacitor C44 and the other transistors Q42, Q44 and Q46 (i think).

Does the -35 rail take 8 seconds to come up as well ? If not, you might want to check that C46 and C48 arent damaged, along with the 7915

Hi Jaycee.
Thanks for your assistance.
The -35v or -45v as its currently stands appears instantly, and does not suffer the same delay as the -15v.

The 7915 regulator appears to be OK, and when I changed to a new one last night, I was getting the same results.
I will check the caps and Q42, 44 and 46.

Regards,
George.
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Old 13th August 2007, 08:03 AM   #5
GeeVee is offline GeeVee  Australia
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Quote:
Originally posted by sawreyrw
It looks like Q44 is not turning on so the pass transistors are fully on. Q44 is supposed to shunt base current (supplied from Q42s collector) away from the pass transistors. Or Q42 could be shorted. The base voltage of Q44 should be about -.7 volts.

Hi sawreyrw
Thanking you for your assistance.
I will check the voltage on Q44.

Regards,
George.
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Old 13th August 2007, 10:21 AM   #6
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Default runaway voltage..

Hi,

I did encounter a simalar problem a few years ago.

It turned out that the power supply (with no or only resistive load) was oscillating.

Introduction of 100uF capacitor's at the output of PSU cured the problem.

In normal service it never occurred because the amp it was built for had the 100uF caps sitting on the amp PCB....

Jos
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Old 13th August 2007, 02:20 PM   #7
GeeVee is offline GeeVee  Australia
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Default The problem is getting worse

Hi again.

I have undertaken the testing as advised, but still no joy.

Firstly, I must admit that I had no load connected to the 15v rails during the first test.

Things are realy starting to become strange.

First test.
I connected the LED across the 15V rails, to provide a visual indication that power was on as shown on the original schematic, and boy did i get a surprise.
The ramp up of the -15v rail has stopped, but it is now only -0.82v.
I suspect that this is a current problem, as the LED is pulling this rail down.
The -35v rail is still at -45v.

Second test.
Replaced Q46 (BC640), and now I get -11.80v on the -15v rail, and -12.54v on the -35v. Interesting that there is about 0.7v difference.
After approximately 1 min, both of these rails jump down by approximately 0.7v.

Changed Q42, Q44 and Q48 no difference

I have checked the base voltage on Q44 (BC557), and it measures approximately 45v with respect to the -35v rail (output side), and the 0V rail.

I am sure that it is obvious to all that I am not very experienced, but usually I can muddle my way through.

This one has me stumped.

Once again, thanks in advance to all for the assistance.

Regards,
George.
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Old 13th August 2007, 02:32 PM   #8
SY is offline SY  United States
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Let's try to take it one step at a time. Disconnect the 15V regulators and we'll get the 35V rails working first.

Put a dummy load across each of the 35V rails to ground. The dummy is a resistor sized to draw the same current as the preamp (watch the power rating). For example, if the preamp draws 100mA under normal operation, each dummy load would be R = V/I = 35V/100mA = 0.35k. 330R is close enough. P = VI = 100ma x 35V = 3500mW = 3.5W, so use 5W resistors and understand that they're going to get a bit hot.

Once the 15V stuff is disconnected and the dummy loads are connected, power up. Measure output voltages. If they're not right, check that the Zener is turned on. Check each b-e voltage on the transistors and see if any of them are not turned on or are shorted.

With a voltage map of critical voltages in hand, we can then proceed to the next steps.

Also, I would recommend buying, begging, borrowing, or stealing an oscilloscope. As someone mentioned earlier, an oscillating supply will show all kinds of weird DC voltages, and without a scope, you're working blind.
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Old 13th August 2007, 02:39 PM   #9
GeeVee is offline GeeVee  Australia
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Originally posted by SY
Let's try to take it one step at a time. Disconnect the 15V regulators and we'll get the 35V rails working first.
Thank you so much for your asistance.
I will get things ready tomorrow night. It is now midnight here and a bit late for me.

Yes you are correct, it is time i purchased an oscilloscope. I have my eye on one at the moment, just trying to settle on a price with the seller.

Kind Regards,
George.
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Old 14th August 2007, 12:34 PM   #10
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Quote:
Originally posted by SY
Let's try to take it one step at a time. Disconnect the 15V regulators and we'll get the 35V rails working first.

Hi Sy.

Ok I am under way.
I have disconnected the 15 v regulators, installed a dummy load, and the -35V rail is reading approximately -11.7V (see photo).

I have done the other checks and these are the results:

1. The 10V zener is on and OK (measure 10.04V)

2. Q42 (BC547) b-e 0.178V
3. Q44 (BC557) b-e 13.02V
4. Q46 (BC640) b-e 0.575V
5. Q48 (TIP32) b-e 0.642V

It would appear to me that there is a problem at Q42 and Q44.

I am looking forward to your response.

Thank you.

Regards,
George.
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