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Final Setup and Assembly Questions

beta24

Re: Final Setup and Assembly Questions

Postby amb » November 28th, 2017, 3:26 am

Oops, sorry, that's what I get for relying on my (bad) memory, haha. Make sure you have the correct zener diodes installed in D11 and D12. The voltage across each one of them should be approximately 12V.
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Re: Final Setup and Assembly Questions

Postby muskyhuntr » November 28th, 2017, 9:17 am

I'm getting 14.06 on D12 and 12.21 on d11. D12 seems stable but D11 has a very slow rise (about .01 per 5 seconds.) 22W resistors getting very hot with probes attached. I shut down the measurements after a couple of minutes because of the heat from the 22W resistors.

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Re: Final Setup and Assembly Questions

Postby amb » November 28th, 2017, 2:13 pm

14.xV across D12 is a bit high (that's beyond the tolerance of the BZX55C12 zener), but it shouldn't be a cause of Q45-Q48 getting too hot. Those transistors shouldn't run hot at all, actually, only a few mAs should be flowing through them.

The 22Ω current limit resistors should get pretty hot when your amp is biased up to the specified level. You'll have two output stages each flowing 200mAs each, so the total of 400mA through 22Ω will be 3.5W of power dissipation on the resistor. There should be 8.8V drop on these resistors.
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Re: Final Setup and Assembly Questions

Postby muskyhuntr » November 29th, 2017, 4:21 am

I'm getting between 6.3 and 6.5 (goes back and forth) on one of the 22 ohm resistors. I did not measure both legs. I'm going to try to verify associated resistors in circuit. Any other ideas? Other than D12, any other parts I should have on hand for possible replacement? I have a few hours today, but will have to take a break until after holidays.

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Re: Final Setup and Assembly Questions

Postby amb » November 29th, 2017, 4:39 am

I think you should do a BJT sanity check on all the BC550C and BC560C transistors (not applicable to FETs). Also carefully inspect the board for any mis-installed or wrong-value parts, or cold solder joints. Measure the resistors to be sure, noting that some resistances will read lower than expected because of in-circuit interactions with other resistances.
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Re: Final Setup and Assembly Questions

Postby muskyhuntr » December 10th, 2018, 5:43 am

I'm back, a year latter. Problems associated with the above posts were found to be a solder bridge and a poorly soldered leg on one of the small transistors.
I was able to do a proper set up of the amp. Sound with test tones on a small speaker were good. Unfortunately, what appeared to be a bad insulating pad on one of the power transistors, that part of the board burned up irreparably. Disgusted, I took a long break.

The replacement board is now finished and set up according to instructions. Again, I have run test tones through a test speaker with good results.
I have also run the board for several hours at a time also using a small speaker with a test tone with no problems.

A question;

While reading the amp for connection to a scope, I had no speaker connected but while running it with a 1K Hz test tone, the 5w resistor in the Zobel circuit burned up. I replaced Rz but the new one gets hot in a few seconds. The amp does not get hot when this happens. This does not happen with the speaker attached. What's happening here? Some measurements:
1. Input from signal generator had .001 vdc across input
2. Input signal was 99mv
3. 105k from neg terminal to ground
4. 124k from pos terminal to ground
5. 200k from pos to neg terminal
6. 20k across Cz ( I have no means to measure capacitance)
7. 10.3r across Rz

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Last edited by muskyhuntr on December 10th, 2018, 1:14 pm, edited 2 times in total.
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Final Setup and Assembly Questions

Postby amb » December 10th, 2018, 8:31 am

You need to be careful about connecting any ground-based test equipment to the β24 outputs. Both the + and - speaker terminals are actively driven. When you connect a regular scope, you’ll effectively be shorting the - terminal to ground. A similar thing happens when you’re measuring a 3-channel active-ground headphone amp.

Speakers are passive, floating devices, so they don’t pose an issue.

To properly use a scope you’ll need a differential input adapter (Tektronix makes them).

Alternatively, you could measure half of the differential amp at a time, by connecting the scope’s ground clip to ground (power supply ground star point) rather than to the - speaker terminal. You’ll see only half the voltage swing in this case.

P.S. I hope the resistance across R1 isn’t really only 10 ohms!
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Re: Final Setup and Assembly Questions

Postby muskyhuntr » December 10th, 2018, 1:13 pm

Thanks for the reply.
My mistake R1 is actually Rz. I have edited my original post to reflect this.
That seems to be the problem! Interestingly, Rz over heats with just the ground probe/clip from the scope attached to the neg. post and nothing from the scope attached to the pos. post. It seems the amp is finding that path to ground with only the ground probe/clip connected. Also, I found it also happens with and without a 1Khz sine wave is applied.

Thanks again,
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