Chinon Floppy drive.... - Page 1
Forum section: electronics / projects

author: jrmymllr
author_rank: Regular Contributor
timestamp: November 12, 2022, 05:09:38 pm
content: Here's a strange request.I have a Chinon FR-506 that I stopped using 25 years ago and stole some parts from about that long ago. Now I want to get it working again, because why not....Luckily the only parts I took are a few SMD capacitors, and I've had good luck determining the correct value for most of them by asking someone on a vintage computer forum to perform in-circuit measurements of their drive and comparing a schematic of a FZ-506. I can't completely trust in-circuit measurements though.Unfortunately there is no schematic for the FR-506, and no datasheet for the Chinon-branded controller (T8745F). However I believe Chinon may have sort of combined the two chips on the FZ-506 into their custom part on my drive. The area I am missing a value for is in the red boxed area. I've reverse-engineered this area on my drive and it's essentially identical to this boxed area, except some values are different and the IC8 analog switches are internal to the Chinon IC.The values on my FR-506 are the same as in the boxed area except:L5 and L6 are 220uHC9 I don't know, won't measure in-circuitC20 I removed and measured as 1000pFC12 or C13 removed and measured as 800pFC12 or C13, whichever doesn't have the 800pF, is unknown. If you look carefully, C12 and C13 can be individually switched in parallel to C20. I got someone to measure theirs in circuit, but they got 15nF which seems really high. I get it though because the 800pF didn't measure at all until I removed it.Anyone have any ideas? I'm not great at RLC circuits, but I would guess what's in the FZ-506 schematic is trying to do the same thing as my FR-506.

author: harerod
author_rank: Frequent Contributor
timestamp: November 12, 2022, 08:01:51 pm
content: Hi, I'm happy that you want to atone for your past vandalism.The switched capacitors change the cut-off frequency of the second lowpass in that bandpass. Replacing your missing components could be done in several ways:1) remove all components in the affected section and replace by the known set in the FZ-506 schematic2) calculate the transfer function from the FZ-506 schematic and add components to you ruins to match the remaining parts (Hints attached, after the tranformation the rest is pretty straightforward.)3) if you don't want do the math, build the FZ-506 schematic in LTspice. Then replace with the known components from your drive and fiddle until you have a matching transfer function. *meh*Checking your math with LTspice would also be a cool thing.At any rate, this is just a filter circuit. If you get the components a bit wrong, the drive should still work. You'll lose performance margin, though.You said that you're not familiar with LC-circuits. Never mind in this context.Z(R) = R / Z(C)=1/(2*pi*f) / Z(L)=2*pi*fAnother thing: One could even ignore the initial highpass capacitors and just look at the 2nd order lowpass and rebuild that.There is not much chance to doing any permanent damage. Just play with one test disk. Format, write, read. Then read another disk that contains pre-written data.Have fun.

author: jrmymllr
author_rank: Regular Contributor
timestamp: November 13, 2022, 02:09:52 am
content: Quote from: harerod on November 12, 2022, 08:01:51 pmThis is great advice. I would guess the filter network would be the same for any 5.25 PC drive, so matching the FZ-506 filter parameters is a valid course of action even for a different chip?And many thanks for the drawings. It's been awhile since I did this in school, and have never used it. But it looks fairly straightforward.

author: harerod
author_rank: Frequent Contributor
timestamp: November 13, 2022, 01:26:26 pm
content: Thanks. However, only your results will show, if the advice was helpful...Regarding constraints for portability of filter designs:- frequency response / transfer function (primary design goal)- input impedance of source (the filter puts a load on its source (in this case the preamp output). The source must be capable of driving that load. See CX20185 datasheet.)- output impedance of circuit as seen from the differentiator input (See CX20185 datasheet, circuits.)If you look at the remaining components on your board, you could make some guesstimates.About measuring component parameters: I wouldn't accept any measurements while the component is mounted on that PCB. If you don't have a sensitive enough tool for direct measurement, other options would be:- oscilloscope and signal generator/switch- R's and C's with known parametersYou build simple circuits and measure the step response. L's may be easier to measure via resonance frequency, using a sine-generator and a known capacitance.If you haven't done LTI network analysis in a while, there may be some items worth pointing out:- one can calculate with Z's, just like R's. If you are satisfied with setting up the transfer function with Z's, you just fill in the complex impedances given by the simplified model of that component. I am hedging here, to let the smart alecs know that I know that I only give a rather simple, albeit sufficient LTI model.- voltage sources are a short-circuit that happens to produce a voltage. This is the basis for the transform suggestions that I posted. It allows to move the components around as suggested. It also works for determining the filter output impedance - just set the source at zero Voltage, all that remains is a short circuit.Thanks for your post, I had a great time yesterday, digging around the internet for info on that old stuff. Since this is my work, I rarely do electronics for fun, these days.I haven't dealt with floppies in a long time and used the opportunity to read up on some old questions. There are some interesting differences between SD/HD, e.g. frequencies and field strengths.A "simple question" like yours could keep one occupied for while...

author: jrmymllr
author_rank: Regular Contributor
timestamp: November 13, 2022, 09:24:29 pm
content: Happy to give you something different to work onSome of this began coming back to me as I looked at it more. Although I remember doing homework with phasors, I got impatient trying to relearn it. So then I reasoned that if I assumed a frequency, I could calculate the impedance for each component and then it becomes an easy resistor network. I figured out that 250kHz is likely about what frequency is present on this circuit based on scope plots I found online of someone aligning their drive (easier than getting my scope out and hooking everything up to another drive). 250kbit/s is also the 5.25 drive data rate so I think that makes sense.But, the impedance of the circuit on my drive is much lower than the one in the schematic. So then I realized maybe I should be looking at how much the circuit attenuates at that frequency since it's going into, what should be, a hi-Z input on the IC, and overall impedance wouldn't matter much at all.Tired of all the manual calculations and feeling lazy, I simulated this in LTspce with a 1V p-p 250kHz sinewave input. On the FZ-506, the capacitor at the output is 390pF with an option to add 100 or 220 more pF. So the attenuation for each of the 3 possibilities (assume it doesn't use both 100 and 220 together):390pF: 0.814490pF: 0.781610pF: 0.719On my FR-506 it has a 1000pF cap at the output with an option to add in 800pF and some unknown capacitance:1000pF: 0.6521800pF: 0.4351000pF + unknown: ?So the attenuation values on my drive already attenuate more, and I see no relationship between the the 3 steps on the FZ-506 and the two I know of on mine. And on the FZ-506, the fixed cap is 390pF, then there's one that is about half (220) and another that is about half of that (100). On mine, the fixed is 1000pF and one of the switched is 800, not close to half or a quarter of the fixed 1000pF. So nothing makes sense.Short of someone with this drive actually pulling that part off and measuring it, because no capacitance meter I have will measure it in circuit, my only real option is taking a guess. I never have much hope for in-circuit measurements anyway given all the other passives connected to the one I'm measuring. I'm thinking of going with something about half of the 800, maybe 390 or 470. Maybe 300 since that value is already used on the board. It might not matter much and may work anyway. In any case, I'll order multiple values around this since I need some others anyway.Your advice was definitely helpful. It didn't guide me to the exact answer, but took me as far as possible and showed that I perhaps can't get the exact answer from what is known.EDIT: Something came to mind later. I simulated this to see what kind of frequency response both have. The FZ-506 has a bandpass, with a peak pass near 250kHz, so that's interesting (and expected). Mine, FR-506, has a low-pass, with a noticeable drop somewhere around that same frequency. I think that's just weird, but there's certainly more to this than I know.

author: harerod
author_rank: Frequent Contributor
timestamp: November 14, 2022, 08:46:24 am
content: Ah, I love to work with people who are actually willing to do their homework. I can't spare you from doing all the physical research, because I haven't had access to a 5.25" drive this side of the year 2000.I assume that the filters have to do something with the two different rotation speeds 300rpm/360rpm. In that case they could function as noise filters.Those filters are only second order. I see that switching of the RLC-part mostly as tweaking for optimum reading performance.I attached another schematic of an older model drive, which also uses the CX20185, but doesn't switch the filter.Just to make sure that I understood you correctly:The drive that you are trying to resuscitate uses a higher intergrated chip, not the CX20185? In that case we may have a hard time guessing if the missing capacitor is the larger or the smaller adjustment. Same as you, I'd assume the latter.Never mind the attenuation being different in your drive. We should assume that the preamp is adjusted in a way to fit the whole signal chain and circuit sensitivity.Another thing: I don't see a connection between the read and the write circuitry. This means that your drive should write just fine. Only the read performance may be a tad off. That might bite you, when you try to save old data.After all that preparation: Just MacGyver a 390pf in and try to write/read a test disk.Regarding your edit: have I sent you in the wrong direction, by writing "bandpass"? In the FZ-506 C14/15 act as a really low frequency highpass, one should say "as a DC filter". What follows is a second order lowpass.The large capacity C14/C15 (47nF) are mostly to get rid of offsets, AC coupling makes the design simpler.I assumed that in your drive the circuit topology was the same?

author: jrmymllr
author_rank: Regular Contributor
timestamp: November 14, 2022, 05:25:40 pm
content: Correct, mine uses a Chinon branded "T8745F" and an octal line driver for the stepper. That's the only ICs on the main board.Luckily I have two more 5.25 drives, both TEAC, and no old data on those floppies, so the only reason I'm doing this is nostalgia because I'm a retro-nerd engineer. Of course if it eventually fully functions that's a bonus.Right, the 0.047uF look like DC blocking caps. I've reverse engineered that section of my drive from the PCB traces and the topology is absolutely identical. The only "difference" is there are no IC8 analog switches, but I can clearly tell that it's being done inside the T8745F. Those DC blocking caps (C14, C15) and R7/R8, and R9 are the same values so no issue there. On my drive, L5 and L6 are 220uH, C9 is 300pF, and C20 is 1000pF on mine. On mine, one of C12/C13 is 800pF, the other is the mystery value. I measured all caps out of circuit, and the inductors are through-hole and color banded.What I was referring to with the bandpass/lowpass is the LTspice simulation results. If I sweep through a frequency span centered around 250kHz and use values from the FZ-506 schematic I get what looks like a bandpass, where the attenuation increases above and below ~250kHz. If I use my values, even with C12 and C13 not included, I get a low pass where the attenuation stays fairly constant, then attenuation increases roughly around 250kHz. So at least I'm not seeing crazy non-sensical simulation results, but why did they move away from the bandpass to a low-pass instead? I guess nobody will ever know. But I'm thinking my drive is newer with the higher integration IC and 1992 date codes, so there must have been a reason to go that way.Once I get capacitors I'll try this, but I may wait until I need something else from Digikey since I'm too cheap to pay shipping and not in a hurry (and Digikey is only a few hours away ha ha). The really important ones are on the motor control board. I am missing two there that are for speed control, and they are package sizes/values I don't have.

author: harerod
author_rank: Frequent Contributor
timestamp: November 14, 2022, 06:47:24 pm
content: I wouldn't trust an engineer who isn't a nerd. It is just that the work for my clients allows me to play with so much cool stuff, that I rather go bird-watching in my spare time.Mmmh, I am not sure that I follow your simulation results. Share the .asc, maybe?

author: jrmymllr
author_rank: Regular Contributor
timestamp: November 14, 2022, 08:01:00 pm
content: Sure, attached is the .asc for each. FR-506 is my drive without either of those additional capacitors added in. FZ-506 is the one in the schematic, also without either add'l capacitor.And I realized I had the reference point on the bottom right side, which may not have been correct, so I moved it to the bottom left side on the AC source. So now both response curves look about the same except the FR-506 has more attenuation overall. But...maybe it was correct on the bottom right to begin with given this is a differential configuration. I'll have to play with it more. Take a look if you'd like and tell me what you think.

author: harerod
author_rank: Frequent Contributor
timestamp: November 15, 2022, 09:36:11 am
content: I moved the 0V reference (a.k.a. ground) to the bottom of C4 for greater convenience and extended the sweep from 5kHz to 500kHz.The filter responses (voltage on top of C4) are indeed a bit different, the FR-506 having a much flatter response than the FZ-506. The latter even shows some mild resonance.Even adding capacitance to the FR-506 C4 doesn't produce resonance around 250kHz.At any rate - look at the overall dampening level. 3dB are 36% and 6dB are 50%. You shouldn't have any trouble reading, even with slightly off capacitances.There are two items that I am uncertain about:- how often can I mention "C4" in a post before making the no-fly lists?- how confident are you about the measuring results of your components? <- shouldn't be much of an issue. It just irks me that one circuit shows a bit of resonance and the other doesn't.

author: jrmymllr
author_rank: Regular Contributor
timestamp: November 15, 2022, 01:04:27 pm
content: Quote from: harerod on November 15, 2022, 09:36:11 amThat's good to hear. And knowing this wasn't Chinon's first floppy drive, I assumed they had things pretty well figured out and likely have good margins.C4:The caps I removed to measure are (per the .asc simulation designators) C3, C4, and the remaining switched cap not in the .asc. I used both my Amprobe AM-140A and LC-200A on the removed parts to make darn sure I had the correct value. For C1 and C2, one was just hanging on the footprint by one end and measured about 0.05uF and so I assumed the other was the same. That lines up with both schematics of other Chinon drives. So I'd say these are the correct values.So now I just need parts. Maybe I have to make up a reason to do a DK order.

author: harerod
author_rank: Frequent Contributor
timestamp: November 15, 2022, 01:35:49 pm
content: Ah, thanks for reporting back on your equipment.I am not sure what made me ask this. Three years of isolation and remote testing my designs by doing what we callSchimpansenfernsteuerung(remote monkey control) didn't make me less suspicious of anything.I shouldn't complain too much, though. Today we have video- and datalinks to most places on the planet. 25 years ago I logged days in transit, just to upgrade some 64kB of firmware on the other side of the world.

author: jrmymllr
author_rank: Regular Contributor
timestamp: November 15, 2022, 02:39:39 pm
content: Quote from: harerod on November 15, 2022, 01:35:49 pmFair question though! I have learned, as I'm sure also have you, not to completely trust what someone tells you else one gets led on a wild goose chase. Reminds me of an occurrence years ago when I was doing tech support over the telephone. The customer tells me their CDROM didn't work because when they click on it in Windows they get an error. After too much time troubleshooting I finally ask "do you have a disk in it?".Valuable lesson there.

author: jrmymllr
author_rank: Regular Contributor
timestamp: November 19, 2022, 10:44:45 pm
content: Replaced all the missing capacitors except I left the unknown one off for now. Works fine

author: harerod
author_rank: Frequent Contributor
timestamp: November 20, 2022, 01:13:12 pm
content: Classic situation. Typical overengineered stuff.That being said, in my line of work I have the luxury of building expensive equipment that has to last a decades in the field. Look at any of my designs and you may take away two-thirds of the components - it will most likely work, under controlled lab conditions. Who needs all that EMI stuff?

author: jrmymllr
author_rank: Regular Contributor
timestamp: November 22, 2022, 08:40:37 pm
content: The drive was apparently not Muntzed. And neither are your designshttps://en.wikipedia.org/wiki/Muntzing

author: harerod
author_rank: Frequent Contributor
timestamp: November 23, 2022, 10:38:11 am
content: Yep, I know about him. However, even the Wiki very carefully mentions the limitations of his approach.Which brings us to the point of "good enough for the job at hand".One must be very careful, not to confuse rookies with anecdotes like this.I'd rather stay true to myself - an old fashioned German engineer, who is rumored to wear a belt and a pair of suspenders...