UREI BL-40 Modulimiter

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

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  1. W3SLK

    W3SLK Ham Member QRZ Page

    K4KYV said:
    I'm glad I'm not the only one that noticed this Don. Many times I left my parts jobber, (I love that name of electronic parts stores!), wondering if I asked for the proper wattage resistors!
  2. W8KHK

    W8KHK Ham Member QRZ Page

    Don, considering the miniscule current required by the output amplifier transistors, I believe you will find that it is possible to size the wattage of the resistors such that they are small enough to fail in an open condition if there is a fault that would risk the power supply components, but, at the same time, exhibit no noticeable heating issues under normal operation. It would require quite a large current to cause significant heating in these 10 ohm resistors.

    An example comes to mind, when I performed a foot-in-mouth error when I was testing the series modulator tubes in the widow-maker breadboard setup. I connected one of my HP function generators to the grid circuit of the modulator driver tube. I forgot there was a DC bias pot connected to the grid, and I neglected to use a coupling capacitor. Fortunately, there was a 600 ohm, 1/4 watt resistor within the enclosure, in series with the output terminal, providing a 600 ohm source impedance. The resistor was vaporized by the bias supply!

    I did not have a manual for that device, so, when searching online, I discovered a reference in an HP journal, discussing new features (at that time) in the article. The last few lines of text indicated the instrument was inexpensive enough to be used in educational institutions, and it intentionally included a 1/4 watt fuse for the same reason - bonehead moves like I committed!
    WA3VJB likes this.
  3. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    Definition of jobber: one that jobs: such as

    (1) : specifically : a wholesaler who operates on a small scale or who sells only to retailers and institutions


    Years ago I set myself up a "consulting service", simply by printing a batch of business cards and letterheads. Numerous times it has enabled me to purchase items or subscribe to publications from outfits, including electronic parts suppliers, that had a policy "not to sell" to individual customers.
  4. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    Looks like one of the op-amps in the BL-40 is bad, thanks to my carelessness. I received the rest of the components needed to finish repairing the audio output stage, and after installing them, looked down and discovered that I had forgot to turn the damned thing off before starting to unsolder components. When I tried it out, the amplifier now works perfectly, but the peak limiter section is dead. I suspect that a piece of solder wick I used to remove the old components managed to flick over and short something to one of the 18v rails or to ground.

    Traced it down to the negative-peak gain reduction IC. Fortunately, that one, an LM-318 high speed op amp is still available, but only in 8-pin DIP format while the ones in the unit are all 8-pin TO-99 (can type). Some of the other ICs and transistors are pure unobtanium, not to be found anywhere. I was dreading the thought of attempting to fabricate some kind of adaptor, when I found this company on line. They make a variety of adaptors such as TO-99 to DIP and vice versa. The one I need costs $2.50 each. I ordered several, just in case I lose another IC, or run into a similar problem in another piece of equipment.

    WZ5Q likes this.
  5. WZ5Q

    WZ5Q Ham Member QRZ Page

    I've been using their products for years. You will not be disappointed in function or quality.
  6. K5UJ

    K5UJ Ham Member QRZ Page

    Thanks for the progress report Don; we're all looking forward to your return to the low bands. Kevin W9FMH asked about you last night and I told him what you were doing.
  7. W3SLK

    W3SLK Ham Member QRZ Page

    I have one of their equalizers. It was sitting in my friend's basement for years and sitting down in mine did it no good either. I need to return to the hamfests in search of the ever elusive 'roundtuit' so I can get started on a lot of these projects.
  8. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    I can get on the air without the processor, but I don't have much incentive to do so when I turn on the receiver and the AM signals I hear are buried in static. I don't usually get out to the shack until well after sundown while most of the activity, at least the good readable signals, seem to be on in late afternoon and early evening. By the time it's fully dark, they've started rolling up the sidewalks.
    K4TML likes this.
  9. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    Here it is a month later, and I still haven't got the BL-40 peak limiter section to work properly. Successfully repaired the audio section, and the RMS limiter function is OK. One of the problems is that most of the semiconductors inside are obsolete and hard to find or unobtanium. The ones in the peak limiter appear to be critical, and having to come up with usable substitutes is not easy, plus the serial crap-outs continue (I can now think of 9 unrelated major things round the house and shack to go south on me since this past February). Last week my audio generator died, and yesterday the 10x probe I use with the scope abruptly quit while I was using it. Fortunately I have usable substitutes. Still hoping to get it going before static season ends; it sucks trying to operate without any audio processing.
    K4TML likes this.
  10. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    Finally got it going, and seems to work normally. Some of the parameters are out of spec, according to the manual. For example, the overall gain is about 8 dB under specs. Some of the others are off a few dB and adjustments won't bring them back. Had to replace R34 (470k) with a 240k resistor to achieve the proper peak balance adjustment, but none of this seems to matter; it has plenty of input and output gain to spare and adjustment for proper threshold of limiting is easily achievable; I doubt it was ever dead-on to specs according to the manual from the time I got it.

    What had been throwing me off was the out-of-spec FET Q4. The gate voltages for threshold of conduction and full saturation were both about 1 volt more positive than they were supposed to be, then the other day in middle of testing, it started acting flaky and then abruptly jumped to normal, or else (unlikely) something was flaky in my test set-up. I swapped Q4 with the same type FET Q16 in the (disabled) phase-flipper just to (hopefully) make sure the gate voltages won't shift back out of spec again, and finally got the peak limiter functioning once more. Tried it on the air last night; after a few tweaks of input and output signal levels it now seems to work just like before all the crap-outs.

    At least now I know the circuit in detail from one end to the other, and in the process learnt a lot about op amps and FETs.
    K4TML, K5UJ and WZ5Q like this.

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