Substituting Si for Ge in P/S output stage - advice? - Page 1
Forum section: electronics / projects

author: HarryDoPECC
author_rank: Regular Contributor
timestamp: June 27, 2023, 06:34:45 am
content: Amateur seeking advice from the experts.I have an early 1970s regulator (develops -11.5v for Mostek 4008 RAM chips, so keen not to hurt them....).This regulator ran OK during early restoration of the host machine but then failed and the output stage driver transistor was dead. It's a GT544 - appears in no datasheet searches. Based on similar numbers I suspect it's a Ge but of course cannot test.The only Ge I have are 2N404, these are switchers and so may not be the best choice.So I'm looking for advice about using Si for Q2 instead. Main issues seem to be adjusting bias and recognising/dealing with more gain.Original schematic attached, relevant area is around L2. I have redrawn for understanding, notes below refer to the KiCad schematicMy amateur understanding of the circuit is:- LM723 reference is hung between the externally regulated +5 and the -11.5 output of this regulator, so the -11.5 will be referenced to the +5- a proportion of this reference output is compared with a proportion of the +5 to -11.5 voltage and this result drives the LM723s output QNow I think the output stage works by R5 biasing Q2-Q3 to produce an output nearer -12 and then the LM723 drives current into R5 to turn Q2 off and bring the output level back until regulated voltage is achieved. Zener on the LM723 output is ? to shift the setpoint nearer to 11.5 (7v5 LM723 reference + 4v3 = 11v8?). Don't really understand how the zener is working.If this is on the right track then I think I could:- put a BC558 or similar into Q2- disconnect Q2 from zener and adjust R5 until output is -12 or so, an excess to the target level.- reconnect Q2 and see if it regulates around the correct range- consider if I need to introduce something for R6. Current circuit does not have R6. My first experiments got the BC558 working but the output had prominent supply ripple that was not on the other rails. Maybe excessive gain was bringing this up? Maybe tame this with R6???I have made a few experiments but decided to stop and try to understand better. Grateful for advice from experts. Thanks!

author: Kleinstein
author_rank: Super Contributor
timestamp: June 27, 2023, 07:02:18 am
content: Normally the circuit should also work with Si transistor. There should be no need to adjust R5. If at all R5 provides a kind of very crude current limit, not any voltage stabilization.The voltage regulation is via R1,2,3,4. The circuit is a bit strange, as it looks like the regulation is relative to the +5 V and not relative to ground.The function of the zener diode should be some level shifting to get an output closer to the negative supply of the LM723. The zener is on the amplifiers output and would thus not directly effect the output voltage, just the voltage range (in this case reduce the minimum for the rw -17 V) the regulator can work with.Another function of the zener can be for the start up, so that regulator initially gets at least some -4 V.If there is hum, the first suspects would be electrolytic capacitors. There may be a raw supply (epsecially the raw -17 V) with too much hum.

author: floobydust
author_rank: Super Contributor
timestamp: June 27, 2023, 08:35:55 am
content: Lots of ECN's. Q4 was 2N1540 (PNP Ge) then 2N6246 (PNP Si, not Ge). No data on the GT544 transistor but I think it's nothing special and would substitute a Si part. I believe the pass transistor is normally full on due to R5 2k2 and the 723 shunts/turns it off. You have pin 2, 3 flipped so it's inverted pin 3 goes to the trimpot?Zener D2 is 6.2V in the DIP version of the 723 compare with 4.3V here. I'm not sure what it's about originally.Here I think it's to prevent avalanche of the E-B junctions so the 723 output near 0V is still -4.3V at Q2 base.Something important is the rail sequencing on power up for these old DRAMs. Double check MK4008 datasheets but the MK4027 mentions "MOSTEK recommends sequencing of power supplies such that VBB (-ve rail) is applied first and removed last. VBB should never be more positive than VSS (0V) when power is applied to VDD." But there is no VDD used on the MK4008.MK4006 +4.75V/-11.4V and MK4008 +5.25V/-12.6V operating.I usually add reverse-diodes on old gear to ensure loss of a rail does not let it get pushed to the opposite polarity.The hum and ripple I would go through it with a scope, but this is old gear and the capacitors surely dried out.

author: HarryDoPECC
author_rank: Regular Contributor
timestamp: June 29, 2023, 08:52:57 am
content: Thank you both, I'm still wrestling it.Have confirmed the LM723 voltage reference.Then lifted the zener on the LM723 output and found:- adjusting the trimmer for equal voltage on pin 2 and 3 gives full + output on pin 9 -> error amp is suspect- despite this full+ output from error amp, LM326 pin 6 is sitting near to pin5 voltage -> output stage suspect- output zener after LM723 output is off-spec, starting to conduct at 2v5 and full on by 3v8So maybe when Q2 was destroyed, damage was propagated back into the 723I think I will replace the 732 and the zener, see if this alters the observations above.Thanks again