Hello everyone, for an old amp's restoration i went to buy a little lot of 2N6488 transistors, ONsemi of course.
These transistors must to replace same components, but originals vintage Motorola, not more availables at selling, in the power supply.
The results is the power supply in self-oscillation, don't works.
I tried all fix but impossible tu run.
So i measure the HFE of some old Motorola for a medium value of 40.
Then, same measure with ONsemi for a medium value of 540 (!!), and ONsemi data sheet says HFE 20-150.
What do you think?
These transistors must to replace same components, but originals vintage Motorola, not more availables at selling, in the power supply.
The results is the power supply in self-oscillation, don't works.
I tried all fix but impossible tu run.
So i measure the HFE of some old Motorola for a medium value of 40.
Then, same measure with ONsemi for a medium value of 540 (!!), and ONsemi data sheet says HFE 20-150.
What do you think?
If the old semiconductors had not failed, do not change them.
If you use faster and higher gain semiconductors, redesign the schematic accordingly.
If you use faster and higher gain semiconductors, redesign the schematic accordingly.
Thanks for contribution.
Unfortunately the old semiconductors had failed, so i must to replaced them.
But my doubt is: why the ONsemi's gain too much more high than data sheet?
Maybe if the gain respect the data sheet value, i haven't any issue.
Unfortunately the old semiconductors had failed, so i must to replaced them.
But my doubt is: why the ONsemi's gain too much more high than data sheet?
Maybe if the gain respect the data sheet value, i haven't any issue.
More gain is not usually an issue as the overall gain is controlled by a feedback loop.
I would think the frequency response will be the issue.
If MHZ range a 22p cap Collector to Base should stop it otherwise study the schematic and reduce the HF gain to suit the take off frequency.
I would think the frequency response will be the issue.
If MHZ range a 22p cap Collector to Base should stop it otherwise study the schematic and reduce the HF gain to suit the take off frequency.
First problem is the component don't respect the data sheet specs.
Otherwise how a person can to buy the specific components?
In my case unfortunately i don't have the scheme, and not is available.
And i'm not a technician, impossible for me to suit it.
I did believe in the substitution simply.
Otherwise how a person can to buy the specific components?
In my case unfortunately i don't have the scheme, and not is available.
And i'm not a technician, impossible for me to suit it.
I did believe in the substitution simply.
Power supply stability trouble....
This is the only clue we have.
Can you send us a photo of this power supply ?
As said, try with a capacitor at the transistor collecter to base.
Not sure the hfe is that wrong, I think it is specified at high Ic while your meter measured it at a low Ic.
This is the only clue we have.
Can you send us a photo of this power supply ?
As said, try with a capacitor at the transistor collecter to base.
Not sure the hfe is that wrong, I think it is specified at high Ic while your meter measured it at a low Ic.
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I thank you mchabin, but maybe i don't explain it well.
Yes, i measured with a multimeter, but 2N6488 Motorola has HFE 40, 2N6488 ONsemi has HFE 540(!).
540 is over thirteen times 40. (13!)
The same transistor.
Or, if you want, almost four times the maximum declared in the ONsemi specs.
I understand that a suitable instruments can be better measure, but i don't believe a so great error with a multimeter.
The fourfold? bah...
Yes, i measured with a multimeter, but 2N6488 Motorola has HFE 40, 2N6488 ONsemi has HFE 540(!).
540 is over thirteen times 40. (13!)
The same transistor.
Or, if you want, almost four times the maximum declared in the ONsemi specs.
I understand that a suitable instruments can be better measure, but i don't believe a so great error with a multimeter.
The fourfold? bah...
The specs and graphs for current gain in the datasheet are for high collector currents: 0.2 - 15A. Very possibly your hand held meter tests at a much lower current.
All the same, there is never a guarantee that new transistors will be problem free in an old design.
What is surprising is these are in regulators... How do you know they are indeed oscillating? How do you know other components in the regulators are not also blown?
Then the fourfold of specs can be normal wth the low current of multimeter?
But i repeat, same transistors (it appears), same instrument for both, and same measurement.
If much lower current, is th same for both, or not?
Maybe the value it isn't anomalous, but the great difference between two same transistor remains.
I know that new transistors can cause problem in an old design, but the too much gain it remain out of specs, i think.
So i cannot to try nothing.
The power supply run with five couple of these transistor, with one couple it works ok, when add second couple the regulation fail and power supply go in overload current.
This is a old car audio amp, and for try to fix it maybe is better to open a thread in the car audio forum's section.
Here i did write for the huge difference between same transistors.
I'm sorry but for me is an inadmissible difference.
But i repeat, same transistors (it appears), same instrument for both, and same measurement.
If much lower current, is th same for both, or not?
Maybe the value it isn't anomalous, but the great difference between two same transistor remains.
I know that new transistors can cause problem in an old design, but the too much gain it remain out of specs, i think.
So i cannot to try nothing.
The power supply run with five couple of these transistor, with one couple it works ok, when add second couple the regulation fail and power supply go in overload current.
This is a old car audio amp, and for try to fix it maybe is better to open a thread in the car audio forum's section.
Here i did write for the huge difference between same transistors.
I'm sorry but for me is an inadmissible difference.
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Yes, modern ON semi TO92 transistors have huge gain. Yes, they exceed the datasheet. I'm reading about 300 gain at 60v on a MPS8099. Allows me to have 250k input resistance on an amp circuit specified at 100k with a generic 2n5551. This $.08 part performs like a specially selected $1 bc547c of years ago.
That gain would have been improbable in 1975, but comes in handy now. If it is too much, add b-e bleedoff resistors or do as suggested : increase feedback (lower feedback resistor) or slow down slew rate with capacitors. Extra resistors are time wasters, but tenth watt resistors are about $1 a hundred, no big deal.
You are supposed to download the datasheet the day you buy from a major distributor. Suffix codes change meaning, For example I bought some On semi TIP107 which usually come in TO247 but the suffix code meant I received TO220 which has much less surface area to waste heat to the sink. Would have been nice if Mouser had changed the picture, but since there was a link to the datasheet the change was totally my fault, they say.
That gain would have been improbable in 1975, but comes in handy now. If it is too much, add b-e bleedoff resistors or do as suggested : increase feedback (lower feedback resistor) or slow down slew rate with capacitors. Extra resistors are time wasters, but tenth watt resistors are about $1 a hundred, no big deal.
You are supposed to download the datasheet the day you buy from a major distributor. Suffix codes change meaning, For example I bought some On semi TIP107 which usually come in TO247 but the suffix code meant I received TO220 which has much less surface area to waste heat to the sink. Would have been nice if Mouser had changed the picture, but since there was a link to the datasheet the change was totally my fault, they say.
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I think a huge gain can be helpful in some cases, but not always.
Howewer i thanks all the thread participans, and i will try to fix the amp with all suggestion.
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