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Hafler P-500 Restoration
Hafler P-500 Restoration
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Old 17th June 2017, 03:30 AM   #21
mjstriker5 is offline mjstriker5  United States
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Quote:
Originally Posted by phase View Post
Watts Per Channel...
Rms was around 325@8ohms or something way more than the 255 listed in the literature.
Ok, you made me go find it again;
Hafler DH-500 Power Amplifier Review price specs - Hi-Fi Classic
Way cool. Thanks for the information
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Old 17th June 2017, 03:52 AM   #22
kct is offline kct  Canada
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Hafler P-500 Restoration
I wonder in such diamond configuration all parts want to be somewhat matched, Q12 to Q13 and Q11 to Q8 so as the diodes D12 to D13 and D11 to D14. Have you been able to find reasonable matches ?
Also, I am not sure if 5% resistor tolerance is ok, I would rather see 1% used preferably with good temp. coefficient.

Can you set P2 such that Q9 conducts the most, e.g. minimum bias.

Can you trim the amp output to zero with P1 ? If you can, this should equalize the voltages across the circuit to symmetry, then with P2 you should be able to increase the voltage spread on basis Q12 to basis Q13, also even with symmetry.

What are the resistance values for R21, R30 ? Are these 1/4 W ?

With respect to ground, what voltage do you get on anode of D16 and cathode of D15 ?
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Old 17th June 2017, 03:59 AM   #23
mjstriker5 is offline mjstriker5  United States
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So right now I seem to have equal voltages feeding Q10/Q11/Q13 as based on comparing to the positive section. Test points D and C are equal (albeit +/-) meaning everything feeding into Q10/Q7 are the same. The change occurs at Q11 collector as it is around -0.75V and should be -1.2. The positive side is +1.14V. When I bypass the bias section I get around +0.4V meaning I have lost -0.4V somewhere. Since the change is at the collector of Q11 I have been testing and changing everything around this point.

D10/D14/D13 are all new. C14 is new and I confirmed proper orientation. Q13 is new and appears to be functioning as it is dropping voltage appropriately (it is just being fed too low a voltage from Q11). The only other components in connected to this point are C13/C17/C11/C19/C20. I have a few 0.1 polyester caps I can use but some of the smaller value ones I will have to dig around or order.

The only other thing I can think of is the output section which is just the three mosfets and three resistors. Is it possible I am back feeding something though Q13 to cause the voltage issues?

If that is the case. Can I remove Q12/Q13, eliminating the output section and safely test the circuit? Now that I think about it I could just remove the wire at J9 and see what that does.
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Old 17th June 2017, 04:10 AM   #24
mjstriker5 is offline mjstriker5  United States
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Quote:
Originally Posted by kct View Post
I wonder in such diamond configuration all parts want to be somewhat matched, Q12 to Q13 and Q11 to Q8 so as the diodes D12 to D13 and D11 to D14. Have you been able to find reasonable matches ?
Also, I am not sure if 5% resistor tolerance is ok, I would rather see 1% used preferably with good temp. coefficient.

Can you set P2 such that Q9 conducts the most, e.g. minimum bias.

Can you trim the amp output to zero with P1 ? If you can, this should equalize the voltages across the circuit to symmetry, then with P2 you should be able to increase the voltage spread on basis Q12 to basis Q13, also even with symmetry.

What are the resistance values for R21, R30 ? Are these 1/4 W ?

With respect to ground, what voltage do you get on anode of D16 and cathode of D15 ?
I did not match Q8/Q11/Q12/Q13 as I was under the impression that the more upstream transistors were more critical of matching and given their price I wasn't about to buy a bunch to match.

Currently I have the bias section bypassed. Prior, when I adjusted the bias too high the relay would kick in even though the bias reading across the resistor was barely over 150mA. Also when I adjusted the bias and read the voltage off Q8 and Q11 I could see the voltage change over Q8 (positive side) but wouldn't change the voltage over Q11. This would just stay at the -0.7ish volts.

Offset can be adjusted to zero and is fairly steady around that value.

R21/R30 are both 82 ohm and this coincides with the schematic. They are 1/4 watt per the schematic.

For D16/D15 I can't remember what the exact value was but I am definitely sure they were equal. These were replaced as I had exact replacements available.
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Old 17th June 2017, 04:11 AM   #25
kct is offline kct  Canada
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Hafler P-500 Restoration
Yes, you could temporarily disconnect the OPS on point 4 and 9. You could then also temporarily install two equal dummy resistors connecting 4 to 6 and 9 to 6 so the driver transistors would mimic a weak OPS. If things then balance out as expected, the problem then likely is found in the lower half of your OPS. Perhaps on of the power Fets is dead or half dead.
You could do quick measure if the gates float or not.
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Old 17th June 2017, 04:29 AM   #26
mjstriker5 is offline mjstriker5  United States
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I did pull the output gets and measure them with a DMM. I used this method:

https://m.youtube.com/watch?v=gloikp9t2dA

Is this what you mean by checking if the gates float? If so the all passed. I'll remove the output stage from the equation tomorrow to try and further narrown this down.

Is it possible one of those caps I mentioned is the culprit. I don't have a way of testing them but I don't want to change all these parts just because and most likely for no reason.
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Old 17th June 2017, 04:42 AM   #27
kct is offline kct  Canada
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Yes, simple test of the Fet suffices for now.
I am not sure if a cap could throw off the balance just slightly like that, usually caps fail badly shorted or wide open and remain so.

It is possible that Q8 vs. Q11 and/or Q12 vs. Q13 have to much difference in current gain as they are of opposite types. Generally these want to be reasonable well matched in current gain.

If you have extra spares, and if you have a DMM with hfE test, see what difference you get when testing the opposite types.
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Old 17th June 2017, 07:25 PM   #28
mjstriker5 is offline mjstriker5  United States
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Found another set of transistors for Q8/11/12/13

HFE test:

2n3440- 98
2n3440- 97
2n5415- 118
2n5415- 111

I have o total of three sets with the other two already installed. I can pull the others and match these more closely if this isn't case enough.Seems pretty darn god to me.
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Old 17th June 2017, 07:32 PM   #29
kct is offline kct  Canada
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Seems very reasonable.
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Old 17th June 2017, 08:00 PM   #30
mjstriker5 is offline mjstriker5  United States
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Well I that eliminates these transistors. This amp is becoming quite the head scratcher. The next step is to eliminate the output stage. Pretty stumped after that.
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