Chuck Harris via groups.io <cfharris@...> wrote:

On more than a few occasions I have found that a scope
probe will excite a high speed, high impedance op amp
circuit into oscillating.

But sticking the probe didn't change anything in the audio. Wouldn't it do
it, if this was the cause?

Anyway, an update, if someone's still interested.

After frying SMPS boards I decided to make linear regulators (which I
generally wanted to do anyway) and I'm back to square one. QMX buzzes just
like it did, but op-amp doesn't self-oscillate anymore.

5V line is clean too (with the SMPS it wasn't, there was some strange
distorted waveform there, I can provide a photo if someone's interested;
but the waveform is not there anymore and the symptom didn't go away, so
it's probably irrelevant).

Some observations:

1. With IC403 removed the buzzing stops (it only hisses, normal radio
static noise).

2. With one of BPF capacitors (C401, C402, C404) grounded on the IC402
side, buzzing stops (only hiss / static noise is present), of course the
capacitor that does this depends on the selected band. Probably because
grounding it removes the biasing done by R401 + R402.

3. Grounding these capacitors on the other side (RX_IN) does nothing, the
buzzing continues.

I ordered two new LM4562 amps, I want to blindly replace them and test,
but it's just that -- blind waving of the soldering gun with no real idea
what can be going on.

I have nine new 3253's (I had ten, but one replaced IC403 that developed a
short after frying these SMPSes), but I don't want to replace IC402 just
yet -- something tells me this chip is OK, I just see no reasons to
suspect it, it's just a switch. Or are there reasons?

Further down the line we have Q508 and all these PIN diodes and filters.
Frankly speaking, I don't see anything here that could cause this buzzing.
Should I try removing Q508, even if grounding its source made no
difference? This is so tiny and fragile that I don't want to remove
components without any suspicion.

So, ideas are still welcome...

Thanks.

Hi,

I just replaced SMPS boards in my QMX with a linear regulator board, and
removed the power switch (I always found it annoying). Some tips, if
someone wants to do the same:

1. 3V3 regulator (I used the 78M33 which was normally used to "bootstrap"
the CPU before SMPSes took over) will get bloody hot. QMX draws around 100
mA on this line, which isn't much, but at 12 V it translates to 870 mW of
power dissipated in the regulator. Make sure you provide as much ground
plane for heatsinking as possible. I did, and my regulator is still almost
90°C. I'm not sure how it will behave long term.

2. CPU needs two fake signals: ADC_5V (2.5 V) and ADC_3V3 (1.65 V). The
reason is that it still tries to drive nonexistent SMPSes, expecting
feedback on these two lines. It won't boot without them. Just provide
resistive dividers, like in the original design (resistor values are not
critical, I used 18k).

3. Lines PWM_5V, PWM_3V3 and LIN_REG_EN can be ignored (unconnected).

4. 100 uF capacitors (instead of 470 uF ones used in original boards) seem
to be sufficient. Maybe even smaller ones would be OK.

When it comes to the power switch removal:

1. V_IN can be hooked directly to +12V (but you lose reverse polarity
protection this way).

2. PWR_HOLD signal can be ignored.

3. PWR_ON signal can be ignored, but the original SMPS board has a 100k
pullup (to V_IN) on this line. Make sure your board contains this pullup
as well (although I used a much lower value, 4k7). If you keep this line
floating, then the (very sensitive) gate of Q202 will get triggered by
literally anything.

4. QMX will power on automatically. When you want to turn it off by long
pressing the knob, it will show "Shutdown!" and hang. At this stage, the
MCU releases PWR_HOLD line, but without the power switch this does
nothing. Only power cycle seems to be able to recover from this state
(understandable).

5. CPU seems to save current state (mode, frequency) to EEPROM only upon
shutdown, so if you just remove the power without shutting QMX down with
the knob press, current state won't be retained. This can be a problem or
an advantage.

Hope it helps.

Thank you Stan?

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