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

    My BL-40 audio processor just crapped out. Looks like the problem is in the audio output amplifier section. The output transistors draw excessive current, pull down the regulated + and - 18v to about 16 volts, causing the power transformer to overheat and stink (because a previous owner had substituted the 1/4 amp fuse with a 3 amp one), and the low-ohm resistors in series with the collector and emitter of the output transistors to overheat. I found a replacement transformer, but the old one seems to work OK; I grabbed the replacement just in case.

    A couple of electrolytic capacitors were swollen and oozing, so I replaced all electrolytics in the power supply, the ones in the +/- 18 v lines to the output amplifier, and the main coupling capacitor to the output transformer, as well the overheated resistors in the output stage, but that didn't solve the problem. Temporarily disconnected the +/- 18 v lines between the power supply and the output amplifier, and the voltage jumped back up to normal, and both the RMS and peak limiter seem to work normally. I suspect one or more of the transistors in the final audio section bit the dust.

    So it should be a simple fix, a 3-stage transistorised audio amplifier, using three small-signal transistors followed by a complementary par of PNP/NPN power transistors? Not so fast; evidently many of the transistors used in this 1974-76 era unit are no longer available. I can find the NPN output transistor, but the matching PNP one is apparently unobtanium. Same with the small-signal ones used in the driver stage; two are readily available, but not the third one.

    I haven't yet tried to test the individual transistors to maybe pinpoint the bad one(s), nor the electrolytic capacitors I didn't replace (a PITA because that requires removing them from the circuit board and re-soldering back in good ones (without lifting a circuit board trace), but the problem remains finding replacements for bad semiconductors no longer available. Haven't found much help in on-line substitution guides so far.

    That's one problem with solid state equipment. There must have been a jillion semiconductor types manufactured over the decades, and types are being discontinued all the time, often a couple of years or less after they first came out. A manual covering semiconductors in the manner of the RCA Tube Manual would be like a New York City phone book (back in the days when those were published). On-line searches bring up all kinds of irrelevant crap and occasionally something that might look useful. And don't even bring up the subject of SMT.

    Anyway, I am hoping to get this thing back into operation before summer static season is over. The output transistors are types U05 and U55. The U55 is the one apparently made of unobtanium.

    The small-signal transistors are 2N5087 (still available and cheap) and 5117 (no longer available). A 2N5088 was suggested on one of the guitar amp/audiophool sites, but the characteristics between the 5087 and 88 don't seem to match up.

    This is one of the reasons I have stuck mostly with hollow-state. It's easy to find substitutions for tubes and parts for equipment made 50-85 years ago, or often, fabricate one's own. This thing is "only" 47 years old and many parts are unavailable. I am much less knowledgeable about how to re-design transistorised circuitry to make it work with currently available semiconductors, than I am modifying tube stuff to keeping it on the air.

    Oh, yes. The rest of the unit is comprised primarily of little round metal-case plug-in op-amps. Like the audio transistors, some of the op-amp types are still available, and some are not, or at least not in the round metal case wire pin format. Hopefully, none of those will fail any time soon. At least I do have a spare T4C optic attenuator, a proprietary item but I have seen replacements on line (not cheap).

    No way will an expensive modern-day transceiver last as many years as a 75A-4. Probably some $5 proprietary IC or other component will soon be discontinued and turn it to a costly door-stop.
  2. WZ5Q

    WZ5Q Ham Member QRZ Page

    If you want to chance it, NTE offers the NTE210 (NPN) & the NTE211 (PNP) as replacements for the NSDU05 and NSDU55. At a glance they look like direct drop in replacements. They might work great electrically then again they might not, it's hard to say what transistors NTE ultimately repackaged for them. It's a roll of the dice, but I generally have fair luck.

    As you said, more research and comparisons would have to be done on the specifications to find exact suitable alternative replacements, but it's doable.
  3. K5UJ

    K5UJ Ham Member QRZ Page

    I just went through that last week with a pc board for a coffee maker (yeah, coffee makers are now run with chips). If the caps are axial lead, just clip the leads where they enter the cap to make stubs, then wrap the leads on the new ones into quigs that can be slid over the stubs and soldered. In my case the caps were radial so I had to unsolder them off the board which was a PITA because the board was conformally coated, and double sided with both SMT and through hole parts.

    Yeah, I worry more about lightning taking out my old Orban, CRL and Hnat-Hindes audio gear than I do a plastic radio. Tubes forever.
    WA3VJB likes this.
  4. W8KHK

    W8KHK Ham Member QRZ Page

    Don, I sent an email to you.
  5. WA3VJB

    WA3VJB Ham Member QRZ Page

    Sorry to hear this, Don, yes, with the increased sophistication that accompanied "Solid State" gear, there is a huge risk of obsolesence from components that are limited production, proprietary, and/or superceded in function by incompatible successors.

    What's ironic to me is how those who touted the solid state era said it would be wonderful compared with replacing fragile vacuum tubes.
  6. K5UJ

    K5UJ Ham Member QRZ Page

  7. KJ4YEV

    KJ4YEV Premium Subscriber QRZ Page

    Don, I have a small stock of U55 and U05. I'll be glad to send you a couple free.

    I used to be a UREI/Universal Audio service center.

    My Email is good on QRZ.

    BTW, in this application, it's not critical. Almost any PNP/NPN complementary pair that will withstand the voltage (+/- 18V, as I recall) will work.

    I have successfully used metal can types 2N3440 and 2N5451 for this application. You have to cross a couple leads, as the pinout is different./

    But original types are recommended if you can get them.
    N1BCG likes this.
  8. KJ4YEV

    KJ4YEV Premium Subscriber QRZ Page

    BTW, the 2N5088 will NOT replace the 2N5087. The 5087 is PNP, the 5088 is NPN. I would use MPS 6534 to replace that, which I can supply also.
  9. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    Here is a schematic of the audio section. I replaced C41, 57 and 58. One of the latter was swelled and oozing; replaced all three for good measure. Did not fix the problem; in fact the swelled and oozing capacitor checked OK on the capacitor checker.

    Attached Files:

  10. N1BCG

    N1BCG Ham Member QRZ Page

    Awesome unit. I was given a BL-40 when I was in college and told that because the RMS limiter electroluminescent panel was worn out and no longer available from UREI it "can't be fixed".

    Just the thing to tell a college student who doesn't understand "can't".

    I took apart the metal can that held the panels and photocells to check it out and found the panel with the manufacturer's name on it. Contacting them by phone, a sales rep asked how many I needed (in thousands). When I replied "just one" he said that was impossible but then asked what it was for. I told him the story to which he said "Oh, it's for a college project? Okay, I'll drop a few in an envelope. Good luck with the experiment!"

    Now that's old skool customer service!
    Last edited: Jul 15, 2021
    AG5CK likes this.

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