Marshall MA50H current drain OK or excessive?

[Stratfordade]

Investigating a MA50H head that came to me with no output, but on powering up found there were no channel switch LEDs or internal illumination (it has bulbs to add to glow of valves!) so immediately suspected +15V supply. Found internal T630mA fuse protecting +15V psu blown and it’s been behaving fine since replacing that fuse. As well as channel switching and reverb the +15V PSU also feeds heaters of 2 ECC83 preamps with 12.6V.

i was curious about why the fuse had originally blown but not since replacement. I measured current through the fuse and by coincidence it’s 0.63A. The preamp valve heaters each draw about 0.19A so in spec. With V1 and V2 removed the draw is 0.35A.

Should a slow blow fuse be drawing current at it’s rated value, or should I be checking the transistors and LM393 op amp for excessive current draw, or is it normal for a circuit protected by 0.63A slow blow to draw 0.63A in steady state without problems?

[Helmholtz]

It wouldn't be wise if rated fuse current equals operating current.
Especially as heaters draw increased current at turn-on.

[Stratfordade]

Thanks Helmholtz. It's an anti-surge fuse I expect because of the two heaters being powered by the same supply. The T630mA installed as per schematic hasn't failed again after many power ups so seems able to manage the short surge when heaters cold, but I've no idea whether the steady 0.63A draw is normal. I have found a post on another forum where an MA50H was repeatedly blowing the same fuse, and it turned out the steady current was 1.5A till an op-amp in switching circuit was changed, though the post didn't say what the current draw was after replacing the op amp.

[Helmholtz]

Fuses come in different qualities and may be rated to different standards.
A UL rated slo-blo fuse may blow at rated current after >4h operation, while a quality IEC rated T fuse should hold rated current forever.
So not sure if a 0.63A current is normal.

[Stratfordade]

Thanks that 4H figure is very useful. I’ve ordered a new LM393 to see if that drops the current. 0.35A sounds a lot for a transistor/op-amp switching circuit even with a few PCB relays so will update after the new LM393 installed.

[g1]

Your numbers from the first post add up to just over 700mA, so not sure what the discrepancy is.
Is it AC, DC? Some combination (DC with high ripple)? If so, your meter may not be catching all of it.

[Stratfordade]

Your numbers from the first post add up to just over 700mA, so not sure what the discrepancy is.
Is it AC, DC? Some combination (DC with high ripple)? If so, your meter may not be catching all of it.

Sorry I’ve checked my notes and I made a mistake in the post. It’s AC I’m measuring and the AC current drawn by the two heaters is 370mA combined, and the drain with V1 and V2 removed 260mA so 630mA total (excluding the two illumination bulbs which are not connected while chassis is out, but will add to total). This is the same as the slow blow fuse rating of 630mA. It feels odd not having much safety margin in the choice of a 630mA fuse, and a recipe for the fuse blowing.

[Helmholtz]

It would help to see the complete schematic or at least the part with the heater supply and fuse.

[Stratfordade]

It would help to see the complete schematic or at least the part with the heater supply and fuse.

Yes of course, sorry.

Lower left area of first schematic shows the power supply circuit fused by FS2. Next schematic shows the switching/mute circuitry that this power supply drives (aside from the reverb drive/recovery that is also solid state). Everything else in the amp is valved and uses different supply.

[g1]

I'm attaching the entire schem. as everything solid state runs off that line (except FX loop).

circuitry that this power supply drives (aside from the reverb drive/recovery that is also solid state).

The 14V line for the reverb section also comes off the 15V line.

[Helmholtz]

I guess you measured FS2 current by connecting the AC ammeter in place of the fuse?

[Stratfordade]

Yes ammeter in place of removed fuse