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A Tale of Two 7D14s
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One of my 7D14s was gating properly; however, the other one would not and, in the one that would not gate, I traced this down to the signal at U733A's "S" pin. The negative-going pulse was only going down to around 2 volts whereas in the working one, that pulse went a lot closer to GND - perhaps between 1 volt and 0.5 volts.
I thought this might be a problem with Q781 and replacing it with a new one allowed gating to work again. I had tested Q781 with my inexpensive component tester and it appeared to be "ok" so I put that transistor in the other plugin (the one that had originally been working) and it still works there which seemed odd to me. I tried that transistor back in the one it came out of but it still won't work there. I've checked the components in/around Q781 and while R780, R782, and R783 are around 10% above their designated values, there's nothing else I can find that's "bad". CR783 also checks good with my Fluke multimeter's diode test function. In the originally-working plugin, R780, R782, and R783 are a bit closer to their designated values and are within the 5% specification. Given that the surrounding components are not egregiously out of tolerance, does it make sense that this circuit would be such that the "bad" Q781 would work in one and not the other? Can it be the "bad" Q781 is "weak" and fails to pull the +5V line enough in one and not the other or are the surrounding components more likely to be the cause? Thanks, Barry - N4BUQ |
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Barry,
Change the resistors that are out of tolerance to see if that makes a difference since you say they are almost double the rated tolerance. A "bad" transistor can work in one area and not another. I have seen this with new in box active devices before. The specs of the one transistor that will not work could be out tolerance, e.g. voltage drop at saturation. This number plugin I would like to get one day. Mark |
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Mark,
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I'll try that. It could just be a "perfect storm" of several parts contributing to the failure. Regarding one of these plugins, one of the custom ICs has one bad output (the 2^0 bit) which causes odd frequency values but most everything else works on it. I keep thinking I can retrofit something to take care of either the entire IC or, perhaps, just handle that one bit's output. I swapped that IC between the two and the problem follows the IC so I know that's the problem. I wish I could source one of those ICs but I think that's just about impossible now. Thanks, Barry - N4BUQ ----- Original Message -----
From: "Mark Vincent" <orangeglowaudio@...> Barry, |
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I think that this IC was only produced in one plugin, and for only a particular run of serial numbers.
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I may have a problem with mine, I may not.? I'd be tempted to replace the whole board with an FPGA or a CPLD working at 500 Mhz, but I don't think I have enough incentive to do so.? If I ever found another 7D15, I'd be tempted to fix it, or mine, depending. Best of luck, I think. Harvey On 1/12/2022 7:57 PM, n4buq wrote:
Mark, |
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Harvey,
If you mean 7D15 and not 14, check the 151-0367-00 transistors for leakage between C and E. The leakage will check as a diode. Checking hfe will give way off results like 0 or way high like a darlington or value jumping around on meter so no need waste time testing them this way. I replaced these in my 15 and it did make it work right. I used KSP10BU as replacements. Those 0367 types are used in plugins, 46/7/85 and others. A 7B92A has a number of them in it and expect them to be leaky. That leakage will cause things to not work right. Mark |
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Nope:? 7D14 of which I have one, and 7D15 of which I have several.
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Very different critters, 7D14 is about 500 Mhz, but not so versatile.? 7D15 is about 200 Mhz or so, and does period, events, and the like. On the 7D14, I measured the input up to the tunnel diode, and that's fine.? I get the waveform that I'm supposed to get (I think) after the tunnel diode, which is the sine wave with glitches (I used a sine input).? What I can't see is the output from the large divide by 2, (that's where they get the 500 Mhz rating), and the rest of the counter chain doesn't toggle. So I'm suspecting the large divide by 2.? Changing the bias on that chip does change readings, but they're not related to the input, but that says that the rest of the counter chain is OK. The 7D15's I have some work, and I should likely go fix the others. Thanks. Harvey On 1/12/2022 8:57 PM, Mark Vincent wrote:
Harvey, |
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I have yet to check them out, but the waveforms I see are sufficiently identical to the ones in the manual, so I'm thinking they're ok.
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From what I know of the circuit: Inputs are conditioned, run through transistors, applied to a tunnel diode which produces very fast spikes, looks like per half cycle. Those spikes go to a very specific, custom made ECL chip (no idea what's in it).? The output of that chip is translated from ECL to TTL, which then is fed to the counter chain. There's a bias adjustment on the custom chip (this is for early serial numbers only). Changing the bias adjustment goes from (I guess) 1) ignore everything 2) respond to inputs 3) oscillate and generate an output 4) likely ignore anything again 1, 3 and 4 work. Since 3 generates outputs that are translated to TTL, I'm thinking that the circuitry *there* works. The symptoms of mine are that it is completely dead. Looking at the inputs to the custom chip, they seem to be correct, although Tek doesn't give me the exact waveform, but mentions what it ought to look like in the adjustment section. So I'm considering that the major chip might be dead.? If it's not, then it's not properly adjusted.? No amount of misadjustment makes the counter work. It's dead, Jim. Harvey On 1/12/2022 9:02 PM, Mark Vincent wrote:
Barry, |
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On Wed, Jan 12, 2022 at 06:07 PM, Harvey White wrote:
...Perhaps input frequency is too high for your scope to show the sharp edges but otherwise sine wave is not expected at U329 inputs, it should look like a square wave. Do you see the correct waveform at prior points, e.g. waveform 13? Ozan |
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Now I'm not sure.? I'll have to go look and find out.? Was using a TDS540A, which should go to 500 Mhz.? I can switch to an active probe and see what goes on.
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Back after a bit. Thanks. Harvey On 1/13/2022 12:19 PM, Ozan wrote:
On Wed, Jan 12, 2022 at 06:07 PM, Harvey White wrote:...Perhaps input frequency is too high for your scope to show the sharp edges but otherwise sine wave is not expected at U329 inputs, it should look like a square wave. Do you see the correct waveform at prior points, e.g. waveform 13? |
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I replaced all three resistors in question and the stubborn transistor still will not work in that 7D14s. I also checked the coupling cap and it was extremely close to the correct value using my inexpensive component checker (which may not be giving me a very reliable reading for a cap that small but it's about all I have for that).
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The same transistor in the other 7D14 will work in either one so I've resigned myself to swap the transistors and let it go. Both 7D14s work that way. Thanks, Barry - N4BUQ ----- Original Message -----
From: "Barry" <n4buq@...> Mark, |
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... and then I tried one more thing. I swapped U731 and putting the "bad" transistor in the plugin where it wouldn't work before, it now works. U731 is a 7400N and one of the inverters is the drive signal for Q781. I had checked the signal out of that inverter (TP799) and it was almost identical between the two units but I wonder if something is not letting that output to have sufficient current to drive Q781. I may need to check the power supply for that chip as well as what drives the input to that inverter.
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Thanks, Barry - N4BUQ ----- Original Message -----
From: "Barry" <n4buq@...> I replaced all three resistors in question and the stubborn transistor still |
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