Problem with Datron 4600

[perdrix]

I'm trying to get my Datron 4600 working, but I'm not getting very far.

The external symptom is that with 0V applied to the input terminals, and an ammeter across the output terminals, when I press "Output On" the Overload (Over Compliance) LED flashes and the unit turns the output off immediately.

In this condition, when I measure the voltage across the bus-bars where they connect to the sense PCB, I see +6.2V to +6.3V which is definitely going to trigger the overcompliance circuit as the comparison limit is 2.5V.

With output off, the input voltage isn't connected to the input amplifier on the Sense PCB, and pin 6 of the OP27 on the Output PCB measures 0V.

In circuit diagram terms the heat sink which is connected to the collectors of all the output transistors of the output amplifier is sat around 6.3V.   I *think* that normally this should be at 0V under the conditions above (turn it on, measure voltage across bus-bars).

I've scanned a few pages of the A3 schematics on my A4 scanner and stitched them back together in a PDF which I've attached.  It has the schematics for the key analogue parts of the Sense PCB and of the Output PCB.

I've measured the voltage drops across the 0R47 emitter resistors, and all measure around 0.465V so the quiescent current draw for both the top and bottom halves of the output amp is 9 * (0.465/0.47)A = 8.9A which is correct.

Additional info: Voltage across R109 was 1.6V which gives a current of about 2.35mA which seems a tad high given the current limiting diode just above it which is rated for 2.4mA (+/- 20%).

Any thoughts on this would be very helpful.

PS I can't take live measurements at present as I have the Output PCB in bits to test all the transistors

PPS Both the +/- 7.5V rails from the SMPS are good.

Thanks
Dave

[perdrix]

The fault, dear Brutus, is not in our power-amp, but in our MoSFETs.

To be more specific it looks like something is amiss with Q402.  When I press "Output On" the gate voltage sticks at -15V, whereas the drive voltage at the other side of a 10K resistor goes up to +15V.   Strangely an Ohms test doesn't show a short between Source and Gate   

If Q402 doesn't turn on then both Q401 and Q403 remain short and therefore no signal gets to the Sense amp input.

I don't think this is all that's wrong, but it's a start.   I'm pretty sure there are other faults in the Sense Amp (schematic 4).

Of course I don't have a single BS170 or even a reasonable cross for it

Ho-hum, so ist das leben
Dave

Cheers
Dave

[lukier]

I've reported the same fault in the main thread:
https://www.eevblog.com/forum/metrology/wanted-schematics-datronwevetek-4600/msg1263153/#msg1263153

I thought you were aware of the other thread, so I didn't reply as I've assumed your problem is different than Q402.

[perdrix]

I too think there is more to my problem that this MOSFET.  I only discovered this was part of the problem today.  I suspect I will have quite a bit more debugging to do.

Dave

[perdrix]

Well now that was interesting, and a lesson to be learnt: Don't trust the text marked on the PCB - it may be wrong.

Replacing the MOSFET restored input signal to the op-amps in the Sense Amp. With TL304 at position B (to disable the Over-Compliance shutdown), I got 26A and some out of the output terminals.    Ooh-err! That's not right!

Looking at the description of the Sense Amp. TL401 and TL402 are provided to allow testing of the sense amplifier circuit, and the jumpers should be in position A for normal operation according the circuit diagram.  They were according the legends on the PCB next to TL401 and TL402.

However looking at where the traces went it soon became clear that the lettering for TL401 had been switched at some point.  The PCB layout drawing in the schematics shows TL401 position A on the right of the jumper and B to the left.  Whereas my PCB was marked with A on the left and B on the right.

Here's a picture of the relevant section of the board with the jumper on TL401 in the correct position for normal operation.



Dave