Had three of these through my hands recently for repair/adjustment, including my own one and would invite comments from members who may have experience with these units.
Only one of them had actual faults (s/C 2N3055's and shattered Crowbar high VCe voltage regen scr drive transistors.
The others just needed setting up as per the manual.
However one unit was totally unstable, bursting into oscillation shorty after switch on (Voltage meter goes hard over on #1 and cant turn Vout knob down past half scale with V control)
Looking at the others , both working into proper 5Amp resistive loads, I noticed that at least one is just on the verge of instability
though needs a CRO on the output to see this, just a small, narrow burst of Rf which does not seem to affect the user controls or calibration.
But it seems that BWD had BIG problems in taming instability in all these units??
For example there are quality low esr "styroseal" caps soldered underneath the main board which appear to be in different places on each 246A looked at, and not shown in the schematic
Even weirder is a long blue Teflon covered wire which snakes out from under the 2N3055 emitter resistor board and returns again via being snagged under one of those extra polystyrene caps several inches down the main board!
All three of the units have this which seems to have been a desperate attempt at taming rogue oscillations in the factory.
Ive fitted A few Ferrite beads which seems to suppress most tendencies to oscillate
Would anyone care to comment on the peculiarities of this model?
When working they are fine, low ripple and noise and, given their age, give good service., but are HEAVY !!!
Ive had two slaved in parallel and that worked OK.
Only trouble ive had with mine was in powering a highly reactive switched inductive load (Kettering type) which made adjustments impossible as the ripple was finding its way into the reg. feedback and rendering it useless, cured by adding an external power diode in series with the 246A output. I don't think the 245's reverse diode across the output (14Amp rated) was fast enough to fully absorb the negative swing caused by the coil turning off.
The crowbar circuit was probably meant to be set to just above the output selected, but is not a reliable circuit. BWD mentions it only to be set to trip at 36v via the rear pot. It does work this way but can be set off by load transients fed back to the output terminals. There is an OFF switch on the pot to defeat the crowbar and I suspect most users had this set to OFF
Can anyone shed more light on the BWD 24x series?
John
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