Also BTW the engineer I bought the scope from also gave me what looks like a home made isolation box. I'm guessing it gets the Scope out of the way from reading itself or anything plugged in or nearby.
He told me to use it so I am. The generator however is plugged into a wall outlet.
He told me to use it so I am. The generator however is plugged into a wall outlet.
The top one is what I needed to see.
With no signal and no speaker load, what is the DC voltage measured directly across the emitter resistors of each channel? Give the voltage to 3 decimal points (0.000v).
Do you see any noise on the center legs of the output transistors? Check the positive and negative rails. Set the scope to 2ms, 5v/div and AC coupling. Do this with the same speaker load and signal that you previously used in the video.
With no signal and no speaker load, what is the DC voltage measured directly across the emitter resistors of each channel? Give the voltage to 3 decimal points (0.000v).
Do you see any noise on the center legs of the output transistors? Check the positive and negative rails. Set the scope to 2ms, 5v/div and AC coupling. Do this with the same speaker load and signal that you previously used in the video.
If the Emitter resistors are the 2.7k ones near Q16 / Q17 on one side and Q68/Q69 on the other then the measurements are as follows:
Right channe: 32.93v
Left channel: 32.81v
I'm sorry but my DMV can only display 4 digits at a time. Its a Fluke 10. If I try to force it it to three decimal places it beaps at me with overload.
Checking output transistors in a minute...
Right channe: 32.93v
Left channel: 32.81v
I'm sorry but my DMV can only display 4 digits at a time. Its a Fluke 10. If I try to force it it to three decimal places it beaps at me with overload.
Checking output transistors in a minute...
The emitter resistors are the large resistors directly connected to the emitter leg of the output transistors.
0.000v across the emitter reisistor clusters. The PCB has 8 of these clusters, 4 resisters in each cluster. These clusters are the ones connected to the TIP35C transistors.
I cant figure out what to ground the Scope probe to for measuring these output transistors.
I cant figure out what to ground the Scope probe to for measuring these output transistors.
To look at the signal on the center legs of the output transistors, connect the probe ground to a non-bridging speaker terminal.
After you look at the voltage on the center leg, try adjusting the bias to increase the voltage across the emitter resistors to ~0.002v. Does the noise change?
After the bias is increased, the outputs will tend to get hot so keep them clamped tightly to the sink.
After you look at the voltage on the center leg, try adjusting the bias to increase the voltage across the emitter resistors to ~0.002v. Does the noise change?
After the bias is increased, the outputs will tend to get hot so keep them clamped tightly to the sink.
Non-bridged terminal - thats what I had a hunch on so i went with it. I dont see anything much with scope set to 2ms 5v/div, but if I set it down 5uS 50mv/Div I see this across almost all transistors except the power fets. I also see this signal with audio source and load connected, but I have to 'zoom' way in with the scope to see it on any sin wave. This noise is not coming from the generator. Maybe I'll swing my PC275 on the bench and see if this noise exists on it's good Left channel.
I'm not sure what you mean by Bias controls. Is that like the amp's gain or the little set screws near the rising boards?
I'm not sure what you mean by Bias controls. Is that like the amp's gain or the little set screws near the rising boards?
The bias pots are the 100 or 220 ohm trimmer pots near the driver boards. Turn them slowly while monitoring the voltage across the emitter resistors.
Got it. I dont want to say its fixed, but it looks slightly better after moving those pots a little. I did see the voltage across the emitter resisters rise a little but not much. I saw the signal get a little better on the scope but the stuff in my previous video was still there. Maybe 25-50% better but nothing more.
I put the adjustments back to a measured 0.000v without any source or speaker connected so the problem is still here.
You said these adjustments were for bias which I presume is power related, but then again I know these little boards have something to do with signal processing.
I put the adjustments back to a measured 0.000v without any source or speaker connected so the problem is still here.
You said these adjustments were for bias which I presume is power related, but then again I know these little boards have something to do with signal processing.
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Only slightly better whenbias closer to 0.010v. Not perfect. Set back to just lower than 0.001 now.
Are you thinking there might be something fishy with the driver boards next to the bias pots? The other day I measured 100ohms from different points on them. I thought those boards cause trouble for each channel independantly of the other. Unless they've both degraded in the same manor; there might be something wrong with a commong supply with these boards. Any thoughts?
More food for thoughts; just before bed I checked up on the LM837N opAmps in this PC2150 - theres 5 or 6 of them on the PCB. On the scope, the opAmp perpendicular and closest to the left channel emitter resistors appears to have the most noise on output pins 8 and 16. Could that be the culprit? I think I need a better way for testing these opAmps before I say anything in conclusion.
More food for thoughts; just before bed I checked up on the LM837N opAmps in this PC2150 - theres 5 or 6 of them on the PCB. On the scope, the opAmp perpendicular and closest to the left channel emitter resistors appears to have the most noise on output pins 8 and 16. Could that be the culprit? I think I need a better way for testing these opAmps before I say anything in conclusion.
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I don't think there's anything wrong with the driver boards.
Does the noise on the output transistors change when you set the crossover to low pass?
Does the noise on the output transistors change when you set the crossover to low pass?
Good morning Perry;
The noise does not change with changing the crossover between Low, High, or Off.
I was thinking about building a quick active 6db LP crossover to put inline with the speaker and terminal just to see if the noise would go away. I think the noise is much higher of a frequency than what I've been supplying. Again this picture 'zoomed' way in on the output transistors:
The noise does not change with changing the crossover between Low, High, or Off.
I was thinking about building a quick active 6db LP crossover to put inline with the speaker and terminal just to see if the noise would go away. I think the noise is much higher of a frequency than what I've been supplying. Again this picture 'zoomed' way in on the output transistors:
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Those switching transients are common in car amps. If you set the vertical amp on the scope low enough, you can find them on virtually every component in the amp (including the heatsink).
There's a screw in the corner of the board near the LED. It connects the heatsink to the amp ground. Is it in place an tight?
EDIT
If you set the scope to 2ms and 1v/div, is the voltage on pin 9 of the 3525 a straight line or does it have ripple?
There's a screw in the corner of the board near the LED. It connects the heatsink to the amp ground. Is it in place an tight?
EDIT
If you set the scope to 2ms and 1v/div, is the voltage on pin 9 of the 3525 a straight line or does it have ripple?
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Screw near LED is securely fastened and has good continuity at the PCB.
You must be talking about pins 3 & 4 of the unconfirmed noisy LM837N opAmp? If yes, I'll check it when I get home from work and report.
Also I goofed and said that opAmp was noisy at pins 8 and 16 which is incorrect. I meant to say pins 7 and 8 which are the ouputs closest to the emitter resistors.
You must be talking about pins 3 & 4 of the unconfirmed noisy LM837N opAmp? If yes, I'll check it when I get home from work and report.
Also I goofed and said that opAmp was noisy at pins 8 and 16 which is incorrect. I meant to say pins 7 and 8 which are the ouputs closest to the emitter resistors.
This opAmp. Red circle shows what I believe to be slightly dirty outputs. Still ened to confirm 100% on this.
(Not my amp. Pic taken from ampguts)
(Not my amp. Pic taken from ampguts)
No ripple when the speakers and RCAs are disconnected. I can make it ripple about ~1 notch on the scope's Y access and the DVM measures ~.5vAC if I drive about a 2v 60hz frequency through the RCAs with speakers connected. Amp gain is all the way off. qbass off. -12DB button set to 'on'.
Edit: While measuring ping 9, if I change modulation frequency from the generator the scope also changles wavelength or ripple. Its invertantly a very low voltage signal based on the frequency supplied via RCA only while speakers are also connected. The 3525 does not like seeing the miniAmp touching pin 9, shield at general ground. We're still working though
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Have you checked the basic things you did on the 275 (10 ohm resistors behind RCA jacks, 280 ohms between shield and amp grounds with no RCAs plugged in...)
I don't think the problem is in the op-amps because the low pass filter doesn't remove it. I think it's power supply related. With the scope set to 2ms and 5v/div, do you see excessive noise on pins 4 and 11 of the 837s. Scope probe ground on the non-bridging speaker terminal.
Did you try biasing both channels higher at the same time to see if the noise was eliminated?
I don't think the problem is in the op-amps because the low pass filter doesn't remove it. I think it's power supply related. With the scope set to 2ms and 5v/div, do you see excessive noise on pins 4 and 11 of the 837s. Scope probe ground on the non-bridging speaker terminal.
Did you try biasing both channels higher at the same time to see if the noise was eliminated?
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