Marshall MB4410 output only onheadphone jack

[The Dude]

I was talking about the supply rails for the amplifier/output section- not the +&-15V. Marked V+ and V- on the schematic.

[nosaj]

I was talking about the supply rails for the amplifier/output section- not the +&-15V. Marked V+ and V- on the schematic.

I've got 51.49 and -51.64 I see no reference to what I should have. Should I check that with and without the speaker connected?

Thanks,
nosaj

[Enzo]

What matters is that they are both present, both roughly the same voltage, and both clean. it matters not whether it is 49v or 52v.


Considering the 80v filter caps, I;d say 50v sounds reasonable. On most any amp, you can figure from its rated output what the peak voltage out would be, then add maybe 5v to that for roughly what the rail V will be. Very rare that it would be wrong, more likely it will be missing, or loaded WAY down, or reading low due to loss of filtration, But a smooth DC at say 30v on one side and 50v on the other is unlikely. Of if you get 50v not to worry that they intended 45v.

[nosaj]

What matters is that they are both present, both roughly the same voltage, and both clean. it matters not whether it is 49v or 52v.


Considering the 80v filter caps, I;d say 50v sounds reasonable. On most any amp, you can figure from its rated output what the peak voltage out would be, then add maybe 5v to that for roughly what the rail V will be. Very rare that it would be wrong, more likely it will be missing, or loaded WAY down, or reading low due to loss of filtration, But a smooth DC at say 30v on one side and 50v on the other is unlikely. Of if you get 50v not to worry that they intended 45v.

Ok I put a 8ohm dummy load on and check V- V+ drops to 49vdc. What I'm not getting is why signal passes to output jack until a load is put on. Does this go back to IC1? I'm thinking when I go back to the shed I'll put the load on and check IC1a?

Or am I going the wrong way?

Thanks, nosaj

[The Dude]

My theory is that, although you have signal at the output, it is lower level than it should be. Connecting a load simply lowers it to a point that you can no longer measure it. Can you measure the signal level at the output (unloaded) and tell us what it is? Is it enough to even drive a speaker?

[nosaj]

My theory is that, although you have signal at the output, it is lower level than it should be. Connecting a load simply lowers it to a point that you can no longer measure it. Can you measure the signal level at the output (unloaded) and tell us what it is? Is it enough to even drive a speaker?

10v no load drops to 0 with load

[J M Fahey]

OK, then you have *voltage* at speaker out but no *current* capability, thatīs why you canīt drive a speaker.

That means the "voltage generator driving the load" which is the Power Amp has high internal impedance so voltage drops tremendously when loaded.

You might have:

* healthy amplifier but something open in the output path: broken track/solder/connector , so a weak signal still jumps the break, enough to drive a meter (1 to 10M input impedance) but fails miserably driving 8 ohms.
By the way, I suspect those 10V do not drop down to absolute *zero* but maybe a few mV, check that.

* open emitter resistors or open output transistors (so load is driven by the drivers only) or open drivers (so load is driven by the Vas stage, the voltge gain stage which drives the drivers)
Any of these woud show healthy voltge but die driving a load.

Disconnect load, drive amp to some voltage such as 10V RMS, connect load, watch voltage die at the speaker connector (what you found so far) but remeasure VAC at bthe actual speaker out rail , such as the node: D2-D1 .

Measure continuity across L1 and between node D2-D1 ; if L1 opens (or track/solder fails) R39 (10 ohms) mkay burn open and become, say, 10k or 100k or any residual value its ashes may btake.

That would certainly turn amplifier into a high impedance generator.

* there is the distinct possibility that load or cable or connector or probes are shorting the amplifier speaker output

[nosaj]

OK, then you have *voltage* at speaker out but no *current* capability, thatīs why you canīt drive a speaker.

That means the "voltage generator driving the load" which is the Power Amp has high internal impedance so voltage drops tremendously when loaded.

You might have:

* healthy amplifier but something open in the output path: broken track/solder/connector , so a weak signal still jumps the break, enough to drive a meter (1 to 10M input impedance) but fails miserably driving 8 ohms.
By the way, I suspect those 10V do not drop down to absolute *zero* but maybe a few mV, check that.

* open emitter resistors or open output transistors (so load is driven by the drivers only) or open drivers (so load is driven by the Vas stage, the voltge gain stage which drives the drivers)
Any of these woud show healthy voltge but die driving a load.

Disconnect load, drive amp to some voltage such as 10V RMS, connect load, watch voltage die at the speaker connector (what you found so far) but remeasure VAC at bthe actual speaker out rail , such as the node: D2-D1 .

Measure continuity across L1 and between node D2-D1 ; if L1 opens (or track/solder fails) R39 (10 ohms) mkay burn open and become, say, 10k or 100k or any residual value its ashes may btake.

That would certainly turn amplifier into a high impedance generator.

* there is the distinct possibility that load or cable or connector or probes are shorting the amplifier speaker output

R39 burnt measured 6meg L1 continuity on top but not on bottom after I disassemble everything I could see L1 was broke right at the circuit board. Replaced r39 streched L1 out a bit sanded off some off the enamel soldered it up put everything back together hemostats sure can be handy. Tested amp drives a 15in woofer, have to wait till tomorrow to shake the house as everyones in bed. How often do coils break?

Thanks so much guys

nosaj

[J M Fahey]

Oh, they DO break somewhat often.

Just run a bead of silicone sealant fixing it to the PCB
Hot glue is not good enough because itīs rigid and sometimes cracks.

[nosaj]

Oh, they DO break somewhat often.

Just run a bead of silicone sealant fixing it to the PCB
Hot glue is not good enough because itīs rigid and sometimes cracks.

Would RTV be ok? i've alway been wary of putting stuff on worrying about conductivity.

Juan thank you so much for the detail in why things break in the way that they do I like it so much better that here change this partand be done with it.

Thanks,
nosaj

[The Dude]

RTV & Silicone Sealant- pretty much the same thing. Go for it.

[nickb]

It's best to use a neutral cure electrical grade (acetone) based RTV on PCBS. Otherwise you risk corroding any metal parts that it may contact.