Interpreting uPD765 FDC result bytes

[TFM]

Very nice machine indeed.    My Genie IIIs is not bad either.

[Bryce]

Replace the diode first. Even if the transistors are damaged, they won't damage the new diode. As far as binding the new transistors (should you need to replace them). You just need them to be pushed tightly together, so any method will do. I usually glue the faces (flat sides) together and then put a cable tie around them.

Bryce.

[JonB]

Very nice machine indeed.    My Genie IIIs is not bad either.

One of these?


http://www.old-computers.com/museum/computer.asp?c=130

[TFM]

No, it's from TCS. Runs CP/M 2.2 and Plus, NewDOS 80, G-DOS and such stuff. Has two Z80 CPU's both with 8 MHz, first has 256 KB RAM, second hat 64 KB RAM. Memories can be mixed. Textmode and GFX mode (hires, 64 KB V-RAM), CRTC 6845 like CPC, but differently connected.
Two 5.25 drives, also two external two 8" drives with 1.8 MB format.
Has two SIO and two PIO.
Was used as computer for a hospital before.
RTC and slots for expansion cards.
Didn't ever get the HD for it
Green monitor, but plug to external monitor too. Only sound is a bit weak.
It's FDC is not as easy to use as the FDC765, but can do lot's of things.

A bit like this, but better ;-)
http://www.homecomputermuseum.de/comp/188_de.htm

[JonB]

8Mhz? OMG... 


TCS Genie IIIs, derived from our old pal the Video Genie. But what a machine. Must be real fast..!

[TFM]

Probably the 3rd quickest. (Dunno if it would be doable to add more Z80 cards with own 64 KB RAM though).


2nd quickest is the RT280 with 12.5 MHz Z280 CPU from Tilman Reh


1st quickest is a self made computer from Dr.Zed with 25 MHz (was it Z80 or Z280, I don't remember).

[XyphoesRe]

It was Z280

[kevin.c]

About VIDEO GENIE III: I like this - "external 5 MB hard-disk"! My youngster doesn't belive me. For him poor HDs start with 250GB!

[JonB]

That's a coincidence... I'm about to make a 25mhz z80 thing using a FPGA.


...thanks to Grant Searle, that is : Grant's MULTICOMP pick and mix computer


..and..


Grant's home-designed CP/M machine


Total cost, about £30.

[TFM]

About VIDEO GENIE III: I like this - "external 5 MB hard-disk"! My youngster doesn't belive me. For him poor HDs start with 250GB!

The IIIs had 10 MB (wohooo!), But my Dobbertin at the CPC has 20 MB, still only 1/3 full. What to do with so much space? 

[JonB]

OK, I have changed the Zener diode 7505 on the PLL schematic and was able to set the 500Mhz test point correctly. I checked the points at top right (250kHz / 500kHz) and they are both correct.  Pin 11 of 7433 (connects to the output - pin 6 - of the VCO) is at 2Mhz.




I am now getting a different result:-

Code Select Expand

C0 00 00 00 10 00 FF

..which seems to be

C0 : abnormal termination because disk ready signal changed state (D6 / D7 = 1)
00 : ST1 - nothing
00 : ST2 - nothing
00 : ?
10 : ?
00 : ?
FF : ?


I am tracing the logic circuits back with a probe. (in the following the component number is given with the pin in brackets immediately after).



The sync pulse is there 7415(2) B1 and 7415(10) B2.
Q1 Output of 7415(13) is pulsing.
Q2 output 7415(5) is the RD signal, pulsing.
7415(11) - CLR2 is high.
7415(3) - CLR1 is high.
!Q1 7415 (also TP1 where you are supposed to get a 1uSec pulse) is pulsing but I can't measure it without a scope.




Q1 7416(5) is pulsing.
Q2 7416(9) is LOW.
J2 7146(11) is pulsing.
K2 7416(12) is pulsing.
7416(13) is HIGH.
7416(2) is HIGH.
7416(1) is pulsing, at 500kHz as expected.


So the only thing I can see at the moment that might be wrong is the Q2 output 7416(9) which is stuck low, but feeds the non inverting input of the 741 op amp. But the op amp is working now, and we can see the output voltage changing in the correct range and setting the VCO to give 2Mhz output at its pin 6 (and thus, to 7433 and out to 7433(4) where the 500kHz test point is showing the correct frequency (as previously stated).



The only other adjustment is the 1uSec test point which cannot be tested without a scope.

[Bryce]

Congratulations. Looks like the hardware is getting closer to doing what it should.

Bryce.

[JonB]

Thanks Bryce, and a good call on the Zener diode!


..but I'm not out of the woods yet.


I should say that in order to get the TP2 to 500kHz it was necessary to set the potentiometer 4712 and I can see that it is slightly off the factory setting (black gunk now misaligned). So perhaps the 1uSec adjustment has drifted (pot @ 4713). Shame I have no scope... but I bet there will be something else wrong even after adjusting that. It really is that poorly.


Anyway, while you were replying I updated my previous post with various measurements from the circuit... What might I check next?

[Bryce]

Buy yourself one of those cheap shit frequency counter kits: DIY Kits 1Hz-50MHz Crystal Oscillator Frequency Counter Meter Digital LED | eBay
It's a fun little project and is good enough to adjust the frequency accurately. I'd clean the pot first, because all the gunk it gathered over the years is making the resistance very variable.

Or if you're lazy there's a fully built version with LCD: High Accuracy RF 1 to 500 MHz Frequency Counter Tester measurement For ham Radio

Bryce.

[JonB]

I have a frequency counter already - one of these:


Uni-Trend UT60E RMS Autoranging Digital Multimeter with PC Interface | Maplin


It is showing a steady 500kHz at TP2. Do you think I should measure the frequencies at the various test points mentioned in my last post? Those were just to check a signal was propagating through the logic ICs (used this approach to diagnose a faulty PET).

[Bryce]

Then you should have no problem calibrating everything. Yes, rule No. 1 - measure and verify every voltage and known value that you can.

Bryce.

[JonB]

I can't calibrate the 1uSec pulse at TP1 without a scope, unless there is a way to use the meter's frequency counter or hold function. Hmm. Have to look into that. The calibration says you are looking for a 1usec pulse width (between the rising and falling edges of the pulse). I don't suppose I can use the multimeter for this...


As to the other measurements, the only known values are at the two test points and the others as marked on the circuit diagram. So I can do frequency measurements at the points I already measured for logic pulses, but I'm not sure what the expected results should be.


Any idea? I suppose for the dividers I can calculate.

[Bryce]

1µs = 1Mhz. If it's a constant signal you should be able to measure this with the Uni-T. If they are single pulses, then you'll need a scope or at least a counter. Pity that Uni-T didn't include a counter, it would only need some extra firmware.

The normal first measurements when fixing something is:

- Check the input voltage and current - Confirm both are what would be expected.
- Check supply voltage on all ICs (That they are within the specified operation voltage stated in the Datasheet).
- Check any reference points (voltage, frequency, waveform) given in the service documentation.

Bryce.

[JonB]

Well, I have ordered one of these so I should be able to resume the search in due course.


http://www.rigol-uk.co.uk/Rigol-DS1054Z-Digital-Oscilloscope-p/ds1054z.htm#.VS198HCkqrU

Watch this space...

[Bryce]

Well, I have ordered one of these so I should be able to resume the search in due course.


http://www.rigol-uk.co.uk/Rigol-DS1054Z-Digital-Oscilloscope-p/ds1054z.htm#.VS198HCkqrU

Watch this space...

A fine scope. Don't forget to "enhance" it when your 30 days are up

Bryce.

[JonB]

Opinion is divided on the 100MHz bit, but full on memory use and all the rest are definitely a must have. Was going to see how I get on with default features. Might have waited until the warranty was up but its 3 years!

[Bryce]

The "liberation method" for the DS doesn't require opening the device and can be easily un-done, so Warranty isn't an issue.

Bryce.

[JonB]

The service manual has the following timing chart for the FDD (see attached image). I found The RDD signal on the PLL circuit diagram, but I don't know where the other signals are located. Are they aliases for known signals (FDC pins I imagine)?

[Bryce]

The data going to the disk can be measured on pin 30 of the 765. Data coming from the disk can be measured on pin 23. You shouldn't need more than that. U2 could be referring to an IC, but the schematics you posted haven't used "U" numbering on the ICs.

Bryce.

[kevin.c]

U2 refering to 74LS112, it is phase detector with it's outputs pin 5 and 9.  I think that "VFO CLK/2" is erratic, and should be "VFO TP2"  (500kHz).
There is no CLK/2 = 1MHz signal in this phase detector.
Your diagram shows phase regulation process. To see that on real circuit, you need 2 channel scope synchronised with TP2 clock. Phase regulation is dynamic process, and always changing. In this case it depends on disc revolution (rotation?) stability. So actually this TP2 clock will not be exact 500,000kHz, but will be drifting around 500kHz. It is still chaseing Read Data frequency and phase.