HI Jim, Yea, you are right, there are a heap of analogue modem parts including M29 and M30 (TL064), and the 4584 Schmidt driver (M35) used for RS232 that are powered by -5v. Any of them drawing excess current will cause the -5v rail to sag.
While the +5 and -5 are off the same transformer, the +5 is designed for a much greater current draw. Regulation on the -5 is simply by zener. A back of the envelope analysis of the Zener supply suggests that with the 180 ohm R97, and a -7.6v supply, it is only rated at 15mA or so, which is not much, so any excessive load will drop the -5v. I would be looking at the 4585 chips driver chip, or the opamps used for the modem, or those caps etc. You could remove the -5v supply to M29 and M38, by carefully cutting the trace to pin 11 - that would remove the -5v and see if that helped. Same for M35, pin 7 is the -5v supply. This will be a complex repair - but when you find it it will be extremely satisfying. Kindest regards, Doug Jackson em: d...@doughq.com ph: 0414 986878 Check out my awesome clocks at www.dougswordclocks.com Follow my amateur radio adventures at vk1zdj.net On Sat, 17 Oct 2020 at 13:26, Jim Anderson <jim.ander...@kpu.ca> wrote: > > -----Original Message----- > > > > The -5V supply is off the same transformer as the +5 v supply. > > I know, that's what really baffles me - how the +5 could be fine and -5 be > so low! > > > The fault domain could be: > > > > 1) D15 Shorted or Open - Check with multimeter > > I did lift a few of the diodes off - D15 shows 0.54 volts forward drop and > open in reverse. I'm not sure if that's a good value for that diode or > not, but at least it isn't shorted or open. > > > 2) R97 Open - Check with Multimeter > > Checked, and it's 180 ohms. > > > 3) Zener D14 Shorted, or incorrect voltage - Check supply voltage Across > > C85 > > I lifted and checked D14 and got 0.73 volts forward drop and open in > reverse (again, not sure if this is an appropriate value or not). I didn't > check the voltage across C85 before lifting D13, D14 and D15 - would it be > any use testing the voltage between the D15 input (terminal 9 on the > transformer) and ground? Or should I put the diodes back in circuit before > powering it on? > > > 4) C86 shorted or leaky > > I did just finish replacing all the caps and it's pretty much the same as > it was with the old caps. I just double-checked C86 with an ohmmeter and > it seems as expected (starts in the megohms and declines into the hundreds > of kilohms over the space of several seconds). > > > 5) Excessive load on -5V supply - Just R97 and see what the available > > supply is between D15-R97 and ground. > > I'm not sure what you mean - is 'just' a typo? Did you mean to lift R97 > and then check the voltage (across C85, basically)? I did this (after > putting D13 and D15 back first) and I get -7.6 volts which is about what > the service manual says it should be (page 4-32) without R97 and D14 to > stabilize it at -5 volts. (I still have D14 lifted and when I touch R97 > back into place the voltage drops immediately to -1.7 volts, so D14 doesn't > seem to be contributing anything to the fault.) > > I've been thinking a bit about this idea of excessive load - I know the > LCD needs VEE but I don't have the LCD module connected to this board, so > what else uses it? I know RS-232 needs negative voltage, and I see it used > several places there for six pulldown resistors and feeding into pin 7 of > M35. After more peering at the schematic (sort of a "where's waldo" for > VEE arrows) I spotted it on M29 and M30 pin 11 (listed as bipolar op amps), > at T1 through R113 (presumably as a pulldown for the buzzer circuit - R113 > tests OK at 3.2 kilohms), and of course the LCD connector pin 3. I didn't > spot anything else - could that be all? > > Not sure where to go from here - assuming I've identified all the places > VEE goes on the motherboard, I guess I'd have to pull M35, M29, and M30 > (one at a time, of course) to see if one of them is the culprit. I don't > have a good desoldering vacuum (just a springloaded handheld one) and I'm > still waiting for the set of desoldering needles I ordered from China to > arrive. I would think that with the desoldering needles I'd be able to > disconnect just one pin from an IC without pulling the whole thing... but > that's just my theory, I haven't ever tried them. > > > Check out my awesome clocks at > > I love these! Time to start saving my pennies again... :) > > > > > > > > jim > >
