UREI BL-40 Modulimiter

Discussion in 'Amplitude Modulation' started by K4KYV, Jul 13, 2021.

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  1. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    A company called Kenetek in Lexington, KY is currently producing what it calls drop-in replacements for the T4-C. They also offer to re-build original T4-Cs.

    I have an unused spare T4-C; I tested it and with it in-circuit, using a sinewave tone, I get RMS limiting long before any peak limiting reading. With the old one in place, both types of limiting occur about the same time. I suspect my old one is wearing down and becoming sluggish with RMS limiting. Several years ago I modified my T/R relay system to disable audio feeding the limiter when not in TX mode, since shack noise and audio from the receiver were constantly driving the BL-40 into RMS limiting, thus causing unnecessary wear on the T4-C.

    Last night I checked the audio output section of the BL-40 and both output transistors, the U05 and U55 are practically dead-shorted from emitter to collector, measuring between 1.5 and 3 ohms with the ohmmeter in either polarity. Tested them in-circuit, then removed them and tested the naked transistors, same result. Q7,8 and 9 seem OK, checking the base-emitter and base-collector junctions with a Fluke digital multi-meter in diode test mode.

    Before dropping in new output transistors, I need to run some more tests, since something had to have caused both transistors to fail simultaneously and I wouldn't want to send a new pair into instant meltdown the first time I turn it on.

    I prefer to get this thing running to replacing the audio processor, because it so well maintains optimum audio level with my voice with little overshoot and no audible artefacts. Listening on the receiver, without the BL-40, my audio appears tinny and anæmic by comparison.
     
  2. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Since this is a complementary-symmetry circuit with tight DC feedback and some ac feedback, I would recommend checking components in this order:

    Q7, Q8, Q9, and the bias diodes CR8,9.

    The output is fed back via dc coupling to R53 and Q7 which sets the dc operating point for the rest of the circuit.

    Pheel

     
  3. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    A leaky C40 and C58 could also upset the bias as well.

    Pheel
     
  4. W8KHK

    W8KHK Ham Member QRZ Page

    I believe Don mentioned he has already replaced C41, C57, and C58. I don't believe leakage in C57 or C58 would trigger a meltdown, as they are just local filter capacitors isolated from the power bus by R96 and R97. But a leaky C36 could significantly offset the bias of the entire amplifier if there is a DC offset at the gain pot R46.

    I would suggest measuring the voltage at the top of R46, with the selector switch in bypass, then in limit mode. If you see a DC offset at the top of the pot, it could be passed to the input stage, if C36 is leaky.

    Resistor values are not critical, and the feedback should easily compensate, even if some resistors are out of spec by 20%. The global DC feedback should easily stabilize the circuit, but severely out of spec resistors should definitely be replaced, as they may be intermittent in operation.

    Before replacing the output transistors, and after the remaining transistors and resistors are tested, it would be beneficial to test the amplifier without the two output transistors in place. In order to provide a proper bias return for the emitter of Q7, I would ground the feedback bus at the junction of R60, R61, and C4, then pass a signal through the amplifier and test for proper output at the base of (non-present) Q10, then at the base of (non-present) Q11. If all looks well, install the output transistors, and do not forget to remove the temporary grounding jumper.

    The signal, examined with an oscilloscope at these points, should have a very small DC component, and should be a very close approximation of the normal output of the amplifier. Q10 and Q11 are emitter followers, so when they are present, their output signal should be very similar to the signal on their bases, but with a much lower impedance and therefore greater current sourcing capability.

    Smaller wattage resistors at R96 and R97 could function as fuses, protecting the transistors and the power supply if an intermittent problem occurs soon after the replacement of said transistors. Monitor the voltage across the ten ohm resistors, to determine what wattage is required for reliable performance. With the class B amplifier, (assuming the forward bias is just sufficient to reduce crossover distortion), the current demand will of course increase with an increase in signal level, when there is a load on the output transformer.
     
    Last edited: Jul 16, 2021
  5. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    With the right side of R53 grounded, the voltage at the base of Q7 should only be about 0.65V or slightly more, due to the small base current flowing through R50.

    Any additional dc voltage at the base of Q7 would upset the bias(es) through the whole circuit, but no additional dc voltages should be present at Q7's base. What circuitry (not shown) is to the left of C36 that would have a dc bias on it?

    Under normal operation, audio voltages from R49 would attempt to contribute additional bias at the base of Q7, but Q7's operating point would be adjusted by the output of the Q10, Q11 stage via R53 to Q7's emitter.


    It appears that R52, R54, with C37, C38 and R53, form an audio filtering circuit so that only dc will affect Q7's emitter bias.

    Pheel
     
  6. W8KHK

    W8KHK Ham Member QRZ Page

    Phil, I agree with the entirety of your post. First, to address your question, I believe C36 is a blocking (coupling) capacitor to prevent any DC component present on the input pot R46 from altering the base bias on Q7. If there was no potential for a DC bias on the input pot, C36 would be unnecessary.

    In BYPASS mode, input to R46 is taken from the output of IC1, which in turn is fed from the secondary of transformer T1, which should have no DC offset. So if this circuit is operating as designed, it should NOT pass any DC offset, only the AC component of the input signal should appear on R46 in bypass mode. However, in LIMIT mode, R46 is fed by IC 4, the peak program amplifier. The signal input to this op-amp is capacitively coupled, so assuming C20 has no leakage, the audio input to this device should not contribute any DC offset bias. However, the peak balance pot, R33, does contribute a DC offset to the feedback input of IC4, which could be propagated to the gain control, R46. In any case, it is prudent to capacitively couple the input of a direct-coupled amplifier to avoid any potential DC offset from the program input. It is for these reasons I suggested measuring the DC offset at the input to R46, and if there is an offset, suspecting leakage through C36.

    R52, R54, with C37, C38 are simply audio filter components, but R53 provides both audio (AC) and bias (DC) feedback.

    My reason for suggesting the grounding of the feedback loop is twofold. First, with no output transistors in place, there is no DC ground return for the first input transistor, Q7. If the amplifier was working will all components in place, the output DC offset, if any, would provide the DC return path. With balanced plus and minus 18 volt supply rails, the output should be very near 0 volts DC. If it was offset, the negative feedback through R33 would tend to reduce the output offset. The second reason is that, in my experience, when troubleshooting these directly coupled transistor amplifiers, it is beneficial to first remove the feedback to determine what is working properly and what is not. If the output swings high or low, it is much easier to identify the cause without feedback. Grounding the source of the feedback loop sets the input to emitter of Q7 to the potential that would be present if feedback was working and the output stage was balanced, with no DC offset present. But the critical issue here is that without grounding the feedback loop, there is no DC current path for Q7 emitter, so any test with it floating would be invalid and inconclusive.

    Another valid option for testing without the output transistors present would be to connect the feedback input to the collector of Q9, which would be virtually the same as the normal output, sans the emitter followers. But I still believe that testing first with the feedback disabled would reveal the circuit fault, without it being masked by the effect of the DC feedback loop. There are many approaches to troubleshoot this type of circuit, but the method I described has been expeditious in my experience. I am of course always open to learning new methods and techniques.
     
  7. W8KHK

    W8KHK Ham Member QRZ Page

    An Afterthought....

    If there is a DC offset on the input, and all else is working as designed, then there would be a DC potential across the output coupling capacitor, C41. Even this fault would not instigate a melt-down of the output transistors. The output capacitor would simply charge to the offset voltage. If the output were directly coupled to the output transformer, then the DC offset could potentially cause a high level of dissipation in either of the output transistors, causing thermal runaway. But that is likely not the case here.

    Considering Don has found that both emitter-follower output transistors have failed in a shorted condition, I would start looking for a situation that caused both transistors to dissipate an excessive amount of power. The voltage developed across the two bias diodes controls the forward bias to the output transistors, therefore I would be looking at them first, and possibly replacing them to avoid any possibility of a recurring melt down.
     
    AC0OB and KJ4YEV like this.
  8. KJ4YEV

    KJ4YEV Premium Subscriber QRZ Page

    It depends on the polarity of the DC offset. The output capacitor, C41 is a polarized electrolytic. If the offset voltage is positive, that is reversed polarity voltage across the electrolytic. I'm surprised they didn't use a nonpolar cap there.
     
  9. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Totally agree with your analysis and I earlier suggested testing and or replacing those two bias (CR8,9) diodes.

    I suspect (just hypothesizing here) a domino effect such that IF C36 became leaky and allowed a dc voltage to offset/upset Q7's bias, then those two bias (CR8,9) diodes may have been stressed to failure further causing Q10, 11 to fail. To pass enough current to cause them (CR8,9) to fail, then Q8 and or Q9 would also be suspect, since they are relatively low current devices:

    https://www.onsemi.com/pdf/datasheet/2n5087-d.pdf

    Pheel
     
  10. W8KHK

    W8KHK Ham Member QRZ Page

    All these things are possible. And it is also possible that either Q10 or Q11 failed first, resulting in the other output transistor failing, with no fault of the rest of the circuit.

    After Alfred's comment, I looked again at capacitor C41. Usually, when the output is balanced, and referenced to ground or the common power supply output, no coupling capacitor is employed. It is strange that a polarized capacitor is used in this location. And the input side of the capacitor is negative. Looking further at R52 and C37, it appears R52 and R53 form a feedback attenuator for AC (audio) with capacitor C37 allowing the DC feedback to go unattenuated.

    The polarity of C41 and C35 indicate that the output of the amplifier may have a slightly negative DC offset. This would provide a slight forward bias to Q7 through R50. So testing with the feedback loop connected to the collector of Q9 may be preferable to grounding the input to the feedback loop. Perhaps Don will measure and record the DC voltages when he gets it operational.
     

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