Does this circuit even make sense? - Page 1
Forum section: electronics / beginners

author: hermitengineer
author_rank: Regular Contributor
timestamp: October 26, 2019, 05:54:59 am
content: Hello all,I am attempting to repair an old game console, just for the challenge of it. The first major challenge, though, was a complete lack of schematics. So I had to reverse engineer the whole thing.Anyway, there is a particular circuit in here that does not seem to make sense. I've pulled the parts and inspected both sides of the board just to verify that I got it correct, and it appears so. But then... is the circuit even useful?From what I learned in trade school, the BJT only allows current flow in one direction, right? So C249 can charge when it turns on, but I see no path for it to discharge. Is there something I'm missing?The only possibilities I can think of are:I'm blind and can't see what should be an obvious traceSome troglodyte replaced the transistor with a wrong partWhat I do know is that the part number of the current BJT is clearly printed on it.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 26, 2019, 06:18:24 am
content: one possibility is to adjustable capacitance between 4.7nF (Q207 turned off) and 4.7047uF (Q207 turned on)

author: oPossum
author_rank: Super Contributor
timestamp: October 26, 2019, 06:27:28 am
content: Collector to emitter current flow is typically better in one direction than the other, but it does go both directions. Some transistors are optimized for bidirectional operation. An audio muting transistor is one example of that.

author: hermitengineer
author_rank: Regular Contributor
timestamp: October 26, 2019, 07:14:52 am
content: Quote from: Mechatrommer on October 26, 2019, 06:18:24 amI agree about the intent, but without a usable drain path, I can't see how the 4.7uF matters in this circuit. I wonder if it's supposed to be a JFET maybe?Quote from: oPossum on October 26, 2019, 06:27:28 amFor BJT's, I've always heard that reverse leakage will be in the nano-amp range though. This circuit is part of the video output generation, and its input is the horizontal sync signal. So nano-amps through a 4.7uF capacitor probably won't drain it by any noticeable amount.

author: oPossum
author_rank: Super Contributor
timestamp: October 26, 2019, 07:17:59 am
content: Reverse leakage would be for BE and BC, not CE.

author: atmfjstc
author_rank: Regular Contributor
timestamp: October 26, 2019, 04:32:27 pm
content: I know it sounds silly, but since you're reverse engineering this off a physical board, maybe double-check that B, C, E have been correctly identified?Also a bit more context for the components upstream/downstream of the circuit might help.

author: AVGresponding
author_rank: Super Contributor
timestamp: October 26, 2019, 04:53:43 pm
content: There is a leakage between emitter and collector, it's just very small.In any case I suspect the circuit shown would make more sense if we could see more context, ie the rest of the immediate circuitry.

author: iMo
author_rank: Super Contributor
timestamp: October 26, 2019, 05:01:14 pm
content: The 4.7nF/4.7uF caps could be part of a timing RC circuit, when the 3904 is on the RC will be xx usecs, when off xx/1000 usecs.

author: james_s
author_rank: Super Contributor
timestamp: October 26, 2019, 05:04:04 pm
content: A little context would be very helpful here, what is the circuit connected to?

author: Zero999
author_rank: Super Contributor
timestamp: October 26, 2019, 09:36:01 pm
content: Quote from: atmfjstc on October 26, 2019, 04:32:27 pmI agree. There are different variations of pinout for BJTs.Another thing to check is there are no internal layers inside the PCB which go elsewhere.

author: hermitengineer
author_rank: Regular Contributor
timestamp: October 27, 2019, 06:20:11 am
content: The pinout question depends: Can two parts with the same part identifier (2N3904 in this case) have different pinouts? I would hope for the sanity of a repair guy that they would always be pin compatible.Anyway, you wanted the whole schematic, so here it is. The output format is SECAM. I've tried to understand and identify each stage, but that doesn't guarantee that the descriptions are accurate.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 27, 2019, 07:14:57 am
content: why the fuck the 74LS624 designer made it so hard to understand what is RNG? Range Input? or Random Number Generator? what a random generator doing in a VCO?

author: hermitengineer
author_rank: Regular Contributor
timestamp: October 27, 2019, 07:52:54 am
content: Quote from: Mechatrommer on October 27, 2019, 07:14:57 amWell from what I've seen on my multi-system TV, it almost seems randomWell, I just got a used test pattern generator today that can output NTSC, PAL, and SECAM. The TV responds properly to PAL/D and NTSC/M, but fails to detect SECAM/L over RF. It can detect it and properly switch through A/V though. Maybe that's a limitation of the TV. I wish I could find the manual for it.Anyway, it appears that the output frequency is determined on the graph page of FC vs RNG. But yeah, it would be nice to know what "RNG" stands for.

author: ataradov
author_rank: Super Contributor
timestamp: October 27, 2019, 07:57:52 am
content: Quote from: hermitengineer on October 27, 2019, 07:52:54 amIt says right in the datasheet that it is a "range input".

author: AVGresponding
author_rank: Super Contributor
timestamp: October 27, 2019, 08:17:21 am
content: As ataradov says, it's on the datasheet...So it looks like the transistor is being used to switch the 4u7 cap in and out of circuit to alter the frequency range of the VCO.Given that the transistor gate connects back to the VSECAM connection and SYN (synchronisation?) pin on the (CPU?) IC it looks like a region selection for PAL/SECAM maybe.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 27, 2019, 08:28:22 am
content: range what? freq range or amplitude range? do i have to read all those text to understand what is what, and then there's FC, another freq select what? the table only shows their input voltage range.. its my miserable day...

author: drussell
author_rank: Super Contributor
timestamp: October 27, 2019, 10:49:20 am
content: Quote from: Mechatrommer on October 27, 2019, 08:28:22 amHuh? You're complaining that you can't understand the datasheet without reading it?The first paragraph (at least of the 1980s TI datasheet) talks about the basic features of the chip, like havingFrequencyControl and FrequencyRange voltage controlled inputs. These basic features and attributes are then referred to multiple times throughout the datasheet.Do you expect to just intuit the functionality of the chip through osmosis by waving your hand over the datasheet or something?

author: hermitengineer
author_rank: Regular Contributor
timestamp: October 27, 2019, 11:19:07 am
content: Quote from: Mechatrommer on October 27, 2019, 08:28:22 amFor that much, you'd start by referring to figure 3. It gives you a frequency graph with 3 plots for FC=5-RNG volts, so that you can choose your extremes and choose a capacitor to base it on. Looking at it from the analyst side, use the external capacitor value to find the frequency ranges. This is 56pF+, depending on the setting of the capacitor, so it's going to be towards the left side of the graph, in the MHz range (as expected).Figure 1 gives you a 50pF reading with FC on the horizontal axis and plots for RNG=0V to RNG=5V, so you can see what the expected freqs are at that setting. Figure 2 is for 15pF. Figures 4, 5, and 6 don't apply to this particular chip.SECAM blue starts at 4.25MHz and drops to 3.9MHz at bluest. SECAM red starts at 4.406MHz and moves up to 4.756MHz at reddest. So naturally the VCO needs to be in that range.The second capacitor is because there's supposed to be a bandstop filter centered at 4.286MHz, right between the two colors but closer to blue.So anyway, the 74ls624 should be somewhere in the 3.9-4.75MHz orbit.Quote from: ThickPhilM on October 27, 2019, 08:17:21 amBut does a 2N3904 provide an adequate discharge route for this? If not, then the first charge of the capacitor is also its last. That's why I wonder if it's the wrong part.Quote from: ThickPhilM on October 27, 2019, 08:17:21 amIt's SECAM only, but SECAM is quite complicated in its color sync patterns. I can't say I fully understand it myself yet, but from what I do understand:There's a time in the vertical blanking period where 9 color sync pulses are sent out. They alternate as red and blue, with the last one identifying what the first line represents. This is one way that SECAM syncs up the colors. A second way is to provide a smaller pulse on each horizontal sync line. I suspect that the synchronous counter IC is part of setting up the former.The pattern of a color sync pulse is to begin at the innermost frequency and sweep out to the outermost in 15us (red) or 20us (blue). During the blanking interval, the sweep stays at the outermost until that line is over. Because of the bandstop filter, the amplitude begins low and increases with the frequency sweep, creating what people have termed "green bottles" on the scope trace.SECAM also uses a four-field interlace instead of the usual two, but I am still struggling to understand that part. But it probably explains why this circuit is so complex.Anyway, further reading on what I've been able to find here:https://radios-tv.co.uk/Pembers/World-TV-Standards/Colour-Standards.html#SECAMAlso, ITU-R BT.470-6

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 27, 2019, 12:21:21 pm
content: Quote from: drussell on October 27, 2019, 10:49:20 ammaybe i'm used to microchip/atmel way of explaining things... table describing each pins is what is unavail in that VCO DS so i browsed the text looking for RNG, nowhere, how can i relate "freq range" as RNG? it might be FC might it not? anyway it put the guess to the reader.Quote from: hermitengineer on October 27, 2019, 11:19:07 amits not clear either. ok looking at figure 1 and 2 explained it in somewhat acceptable manner. so i guess your "doesnt make sense" circuit is to control how slow or how fast the freq ramping up or down. or maybe its somesort of diy PLL or feedback control.

author: atmfjstc
author_rank: Regular Contributor
timestamp: October 27, 2019, 01:53:04 pm
content: Quote from: hermitengineer on October 27, 2019, 11:19:07 amLooking at the broader circuit now, it is clear that there are many other components who can discharge that capacitor while it is in circuit, e.g. the Z209 buffers. So the transistor doesn't need to discharge the cap, it just takes it into and out of the circuit. While it's not in circuit, it will likely retain whatever voltage it had before the transistor cut off, and slowly discharge it through leakage.If you have an oscilloscope, it would be nice to probe SYNC and the cap to see exactly what is happening.

author: hermitengineer
author_rank: Regular Contributor
timestamp: October 27, 2019, 07:31:42 pm
content: Quote from: atmfjstc on October 27, 2019, 01:53:04 pmBut wait... don't all circuits involving the capacitor necessarily include the transistor? And we're back to: Doesn't a 2N3904 (BJT) only allow current flow in ICE direction? At least, the datasheet doesn't even bother to show current flow in the opposite direction.Quote from: atmfjstc on October 27, 2019, 01:53:04 pmOh, now you want real-world measurements?SYNC (magenta) vs Q207/C261 junction (yellow)SYNC vs FC pinSYNC vs FC pin with C261 completely removed from the circuitBasically, I see no difference with C261 present or not.

author: capt bullshot
author_rank: Super Contributor
timestamp: October 27, 2019, 09:58:24 pm
content: Quote from: hermitengineer on October 27, 2019, 07:31:42 pmA bipolar transistor can be operated with C and E swapped. Parameters like hfe get worse then, but works to some extent. So in a circuit like this, the transistor is driven into saturation by plenty base current, the C is a near short to E for small C currents. If C polarity gets negative in respect to E, the transistor operates in "reverse" mode, and for small enough currents, C is still a near short to E.So effectively, the transistor connects the capacitor to the respective node if the base is driven, or opens the connection if the base is not driven. A very simple and cheap analog switch. Works as long as the collector current is small enough, this kind of circuit is sometimes seen as a mute switch for audio in / out.

author: hermitengineer
author_rank: Regular Contributor
timestamp: October 28, 2019, 04:42:54 am
content: Quote from: capt bullshot on October 27, 2019, 09:58:24 pmInteresting. This is one thing my semiconductors teacher never mentioned. Maybe I should do some breadboard experiments to get a feel for it.Questions to experiment on: In a reverse current flow, is it still VBE that determines the current flow? Or is it VBC? What's the order of gain difference for reverse flow?BJTs are usually drawn as equal size N regions (for NPN), but as I recall, the collector has a much larger N region. I guess that's why gain is so much smaller?Okay, I played with a breadboard with the following circuit:Press the top button and the LED lights, but goes out immediately. Press the left button while the top button is pressed, and the LED goes out until the capacitor charges. Release the left button, then release the top button. LED goes out. Then press left button and LED lights up until capacitor discharges.Demonstration of reverse-active mode successful.But is this what they call reverse-active mode? Because it says that reverse-active mode requires negative VBE bias. But the simplified circuit still has forward VBE here?Of course more specific to the logic board in question: Why does removing the capacitor entirely make no difference?Obvious answer: Q207 is dead?

author: hermitengineer
author_rank: Regular Contributor
timestamp: October 29, 2019, 08:51:16 pm
content: The part of the diagram before the 74ls624 is two buffers driven by a couple of flip-flops. Guessing purely by what I know SECAM needs, I referred to this as the color burst strobe. But after modeling it in a state diagram, that's apparently not it.Instead, each flip-flop is clocked by a different source: Z201B by the output of the 74LS624, and Z201A by a 4.45MHz resonant circuit. When this block is enabled at all, both flop-flops will go high on their respective clock cycles, and when both are high they are (almost) immediately reset simultaneously. The Q output of the first mixes with theQoutput of the second to drive the range voltage, so this has the effect of pulsing the output, which the capacitor(s) will help to smooth. Because the pull-up resistor is 220K, the quiescent voltage is only around 1V with the buffers detached, so the combination of buffers can easily pull it towards 0V, 2.5V, or 5V (but 5V is only for a propagation delay of time), and 2.5V seems to be the most common.So, this seems to be a PLL circuit. Or am I off the mark?

author: capt bullshot
author_rank: Super Contributor
timestamp: October 29, 2019, 09:59:33 pm
content: Quote from: hermitengineer on October 29, 2019, 08:51:16 pmAgreed. Looks like the phase detector (that appears kind of similar to the edge based phase detector in a 4046) locks the '624 mid frequency to the crystal frequency using the RNG input, while the SECAM FM is done through the FC input. Q207 / C249 change the loop time constant (I guess it's slow while the visible content of TV line is sent, and fast while or near the sync pulse to allow for whatever burst).According to the datasheet, nboth RNG and FC have influence on the VCO output frequency, so it looks like the voltage at RNG sets the center frequency (to the crystal frequency) and FC controls the FM deviation.

author: hermitengineer
author_rank: Regular Contributor
timestamp: October 30, 2019, 06:53:17 am
content: Confirmed by zooming the scope on the 624 output during sync pulses. So the PLL circuit only activates on the red cycles. Makes sense, as 4.45 is just slightly into red. It activates on the visible lines, at least. Presumably also on the vertical blanking. This is probably for line-based field identification. Since the DC level on the PLL is affected by the tuned frequency of the 624, I bet this has something to do with Q207/C261's purpose too--different FC inputs will bounce the DC around every time it re-centers. Without vertical blanking (as is the case when no cartridge is inserted and it's just executing junk), this level holds steady and is only affected by tuning C281.On vertical blanking, it appears that the primary players mess with the FC side. Z208B activates on blue during VBI. Z200D also activates during VBI, but only when the PLL is inactive, and ultimately cascades into Z207A or Z207C depending on color.Anyway, I gussied up the schematic a bit.