Ah, excellent. Thanks! Josh steven stengel wrote:

Tony Duell wrote: Yep, of course :). Fuse is good, power supply is getting AC... Tested it, seems to be OK. I get about 320V. There's a pair of 100K resistors on the other side of the smoothing capacitors and they check out OK. After looking over the schematics pointed out to me (http://oldcomputers.net/Attache_Schematics.pdf -- see pg. 8) I metered out the voltages at the op-amp and I'm getting 8V (which, if I'm reading the schematics correctly, is not unexpected). I'm not sure where else on the board to check (I'm still learning this stuff, but I'm getting better, honest!). I get nothing at all at the actual power supply outputs. Thanks for the help thus far! Josh

ard at p850ug1.demon.co.uk wrote: Yep, 8V from U1. (well, 7.92V actually, I assume that's within acceptable range?) Hmm, I get 6.75V (measuring from Pin 7 (gnd) to 14.) 7.91V on pin 8. (I have a datasheet at http://yahozna.dyndns.org/scratch/tl494.pdf). I scoped it out, and pin 5 is oscillating, a nice sawtooth. 0.06V on pin 3. 0V on pin 9. I won't, not that I have any clue how to do that at this point :). Thanks again for the help! Josh

That's fine. Ouch!. According to the TI datasheet I've just looked at, the reference should be between 4.75V and 5.25V. Either the TL494 is defective or soemthign else is pulling it up (which may have damaged the TL494 anyway). Or I suppose youre meter could be way out of calibration, but since the 8V line checks out I think that's less likely That sounds as thouhg it's not oscillating and that the output transsitor is never turned on. OK, the oscillator is running, but it's not driving the output stage. Well. if it's as low as that the outputs should certainly be oscillating. But shouldn't the protection circuit designed to shut the supply down if the 8V rail drops too low have operated at this point? It compares the 8V rail after 5 diode drops with the reference voltage, if the former is lower, then pin 3 is pulled high, shutting the supply down. So check pin 3 again... I am beginning to suspect that ther TL494 may have failed. It's not a difficult-to-get chip. -tony

Hi Josh Are you still getting the high voltage on the vref pin? I don't recall you mentioning the value of pin 4? The only place I can see that would cause vref to be pulled high is if D15 were shorted. Also, what is the voltage on pin 8? Dwight _________________________________________________________________ Want to do more with Windows Live? Learn ?10 hidden secrets? from Jamie. http://windowslive.com/connect/post/jamiethomson.spaces.live.com-Blog-cns!5…

Hello, I have a PDP8/e, with RX8E and a dual drive RX01 floppy subsystem. I have a bunch of unformatted 8" Single Density, Single-Sided, Soft-Sector floppies that will work with these drives. I also have a working PC04 paper tape reader/punch, and a console connection at 4800 baud (to a PC running either Windows or Linux). All of the hardware listed above has been tested using DEC diagnostics (loaded from paper tape), and all tests pass. What I'm looking for is a boot floppy(RX01) for OS8 V3D or better that will support this hardware configuration. The issue is this: If I can lay hands on a bootable floppy image, how can I make a real floppy from the image? Is there some kind of standalone tool that will take an image file from a PC, and send it over a serial port, and write the image out to a floppy via the RX8E? I'm looking for something kind of like VTSERVER for the PDP11, but written for the PDP8/e, and small enough to either toggle in from front panel (perhaps a few hundred words, I'm pretty patient), or be able to be punched to tape and loaded with the BIN loader? Also, is there anyone that has BIN-loader loadable paper tape images for the various diagnostics for the RK8E disk controller/RK05 disk drives? The system has an RK8E in it (along with the diode boot board for the RK8E/RK05), but there seems to be a problem, as when I try to boot from a known good RK05 pack with OS/8 on it, the machine hangs. I have a bunch of the DEC diagnostics on paper tape, and have run all of the ones that I have, but there's none for the RK8E/RK05. All of the other diagnostics (memory checkerboard, CPU tests, etc.) run with no errors. The system has 24K of memory, with 8K (2x4K boards) of core, and 16K of solid state memory. Also, I'd need documentation for said test programs in order to know how to run them. Thanks in advance for any help that the collective may have. Rick Bensene

A while back, I wrote a posting regarding RX01 OS/8 boot images for my PDP 8/e. A lot of folks responded off-list with very helpful information. As it turns out, with the gracious help from everyone, I was able to find some programs (dumprest) that did what I needed. I was able to get a working OS/8 boot floppy image onto an actual floppy, and able to boot up from it just fine. I was also able to make some paper tapes of the various MAINDEC diagnostics and load them up and run them. I want to thank everyone who wrote to me with suggestions. It's quite gratifying to have this old machine running with floppies again. Back in the 195/1976 timeframe, I had a summer job that involved a PDP 8/e with dual floppies (I think they were RX01's), and a PC-04, and I wrote a number of math library programs for it, as well as maintaining the system. It really brings back memories when I use PAL, and assemble programs, with the floppy drives clattering away. My next question relates to the RK8E disk controller. The problem with the controller is that it causes apparent conflict on the Omnibus. When the RK8E boardset is installed in the bus, the front panel functions don't work properly. Pressing LOAD ADD with 0's in the switch register results in all 1's showing in the address (but not in the extended address registers), and pressing EXAM causes the address to advance, but weird flickering of bits in the MD register for perhaps a 50ms or so, and then settling down. It's almost as if two or three memory reads are occurring. I can't run any diagnostics or anything, since the RK8E's presence on the bus causes the machine to malfunction. When the RK8E isn't in the system, everything works fine. So, I'm suspecting a few potential areas to look at one the RK8E: A) Omnibus interface chips may be bad (bus drivers/receivers) B) Data break logic (falsely asserting single-cycle data break when it's not valid) C) Timing/Control circuitry I've downloaded the printset for the RK8E, and also have the 8/e Maintenance Manual (Volume 3) with the theory of operation for the RK8E. With this information, I should be able to track down the problems. My initial idea is to replace all Ombibus driver IC's...those which drive the Memory Address (MA) bus, the Memory Data bus, as well as those which drive the Data Break and Interrupt signals. I know it's a shotgun approach, but without an Omnibus extender, or a practical way to power up the boardset outside of the 8/e, it seems the best way to go. Would it make sense to simply remove the 8881's, and then put the boardset in, and see if the machine behaves itself (since the bus is actively pulled up by the Ombibus Load board) to get an idea of the drivers might be bad (or logic that enables them is bad)? The problem is with parts. The RK8E uses SP380's and 8253s' for bus receivers, and 8881's for bus drivers. The 8881 are quad 2-input NAND buffer/drivers, with open collector outputs. I've got a good number of vintage TTL 7401 ICs which match pinout, and are open collector devices, but I'm not sure if the 7401 will work as a replacement for the 8881, since I can't find data on the 8881 I don't know if they are electrically compatible. I do have a few spare 8881s, but not enough to replace all of the 8881's on the RK8E. I don't have any replacements for the SP380s nor the 8253's, which seem to be rather unique, which could be a problem. Anyone out there know if there are 7400-series TTL replacements for these parts? Thanks again, Rick Bensene

At 02:40 PM 9/23/2008, Rick Bensene wrote: Could be any of the above; what I'd do for starters is to pull the RK8E and insert the boards one at a time. You might find that only one of the boards breaks the bus; that may cut the number of possible problems by a third. The RK clearly won't work that way, but it's a potential way to troubleshoot. (After that, divide and conquer to narrow down the problem further by masking pins on three of the four edge connectors. If you get to the point where you've identified the dead component and need 8881s, SP380s, etc. I have a few that you could have. -Rick

Hi Josh Can you take some more voltage readings. It would be good to see Pin 4 and Pin 9 of U4. In fact, it would be good to see a repeat of all the pins of U4 again. Can you also take readings of the pins on U5 as well. This will help quite a bit on trying to understand what is causing the failure. Dwight _________________________________________________________________ Want to do more with Windows Live? Learn ?10 hidden secrets? from Jamie. http://windowslive.com/connect/post/jamiethomson.spaces.live.com-Blog-cns!5…

Hi Josh Getting closer. Pin 9 should be almost zero. The two transistors ( Q17, Q18 ) form a latch. Normally this latch is not tripped and Q17 is on while Q18 is off. Two sources of signals can trip the latch. One is an over voltage on the 5 volt line and the other is an over current in the switcher transistor. The over current is detected by the optical coupler ( D1 in the switcher board ). All the levels on U5 look as expected under the current conditions. There are a number of possible reasons that the Q17,Q18 latch has fired. Not all are normal operation of this circuit. 1. D18 may be shorted and any voltage on the output trips it 2. Replacing the bad U4 has caused the voltage adjustment to be too high, causing the circuit to trip. 3. Q18 may be shorted. 4. Q17 may be open. 5. Something else We know that U4 was bad so it may have been the only bad part. This makes #2 a likely possibility. Try adjusting the pot R39 such that the wiper is closest to R38 ( maximum resistance ). Cycle the power switch. Normally I wouldn't suggest adjusting any pots in a non-working circuit but this one should be harmless. Don't worry about remembering the location on this pot as it would have needed some adjustment anyway, with the replacement of U4. Do not fiddle with any of the other adjustments! Dwight _________________________________________________________________ See how Windows Mobile brings your life together?at home, work, or on the go. http://clk.atdmt.com/MRT/go/msnnkwxp1020093182mrt/direct/01/

Hi Tony I'd put these under #5. When finding a failed part, I generally don't suspect other parts unless I can find some connection with the failure I know. Not saying I've not seen multiple failures in old equipment, just that it isn't as likely. Checking the diodes for the output is a good idea in most any case. I guess we should recommend that he connects a dummy load. Connecting a 6V car lamp is usually enough. As you say, the turnon spike with no load might be enough to trip the overvoltage latch. That is why I said that this should be a safe adjustment. Like I said, he shouldn't touch any of the other adjustments. I'd suspect he could handle this one. The difference in U4 chips could be enough to trip the overvoltage detector. I'd rather he did this than defeat this circuit ( although it would be simple to defeat ). Any problem of too much current, as you suggested, would be letting too much smoke out. I was involved with designing a 4 quadrant switcher supply once. Before I located the input to an OpAmp being inverted, I popped a number of transistors. I suspect that I could have done the debugging in a safer way but it was hard to convince my self that there was a design error that I'd made my self. Sometimes one just has to burn ones fingers a little to except reality. Dwight _________________________________________________________________ Want to do more with Windows Live? Learn ?10 hidden secrets? from Jamie. http://windowslive.com/connect/post/jamiethomson.spaces.live.com-Blog-cns!5…

dwight elvey & tony duell wrote: Thanks (again!) for the suggestions from both of you; I'll do some more testing later this week and see what I find. Thanks, Josh

That sounds fine. OK, the input rectifier/smoothing ciruit is fine. Right. I sent my last message before I'd had a chance ot look at the schematic. Those resistors are called 'bleeders', their job is to discharge the smoothing capacitors when the supply is turned off. There is no startup resistor. The startup supply comes from a mains transformer and 8V regulator. Since you're getting 8V on the op-amp chip I think this is working correctly (but check it). See my other message for more ideas. -tony