Military Microphone Amplifier - Amazing Old Tech  - Page 1
Forum section: electronics / projects

author: willeye
author_rank: Contributor
timestamp: June 18, 2020, 07:55:16 pm
content: Got hold of 2 of these amazing little boxes. Both Labelled 608/1/33746 Microphone Amplifier. Not sure exactly what they were used in but they look so nice. I have just built with the help of the beginners forum a condenser mic pre-amp and then i spotted these and thought the looked interesting.They seem full of very spiffy components and the op amps look top spec.Starting to reverse engineer them but I thought you guys might like a look too. Anyone know anything about this sort of kit? I cant find anything on them online, just need to work out if i can use them for anything interesting.

author: james_s
author_rank: Super Contributor
timestamp: June 18, 2020, 08:10:18 pm
content: I don't see anything to suggest military. Those look like they were hand made, some sort of very low volume custom production for some specific application. It's hard to say what they were built for but the construction reminds me of some doppler ultrasound devices for checking the blood pressure of pets that I've repaired, same sort of PCB that looks like it was built in some guy's garage. The LM301 is a rather mundane op-amp, not something I'd call "top spec", if I recall correctly it's inferior to the lowly LM741.

author: willeye
author_rank: Contributor
timestamp: June 18, 2020, 08:15:17 pm
content: They came from ebay and the listing stated "2 identical microphone amplifiers taken out of military test gear. Both are in full working condition. operate from 24v DCnot sure of gain or pin out. "Thats why i think they are military.

author: james_s
author_rank: Super Contributor
timestamp: June 18, 2020, 08:24:49 pm
content: What is "military test gear" though? They could be custom test gear hand built by a technician in the military to test something else. They certainly are not the sort of thing you'd find in actual military hardware, ie installed in tanks or airplanes or whatever. It's possible they were part of a bench test rig for checking headsets or other comm gear.At any rate they probably work just fine as microphone pre-amps, but I wouldn't expect any kind of spectacular performance, they're made with very early op-amps.

author: phil from seattle
author_rank: Super Contributor
timestamp: June 18, 2020, 08:30:18 pm
content: socketed dips are definitely not military, at least anything that would be in the field. Given audio frequencies, they really don't need high spec opamps, though. Also, the military would have used LM101 or maybe LM201 which have a wider temperature spec. The 301 is only rated down to 0C.By the way, the NatSemi data sheet for that is a great example of how datasheets should be. Makes me nostalgic.

author: willeye
author_rank: Contributor
timestamp: June 18, 2020, 08:36:38 pm
content: It looks very nicely made but i dont recognise some of the components, like the black cubes near pin 1 of the opamps. Whats the best way to test them safely?

author: magic
author_rank: Super Contributor
timestamp: June 18, 2020, 09:04:25 pm
content: I have seen similar looking boards in serially produced commie era products. No soldermask, appear to be half hand-drawn and half hand-drawn with a ruler and that's because the masks probably were prepared exactly that way.IIRC LM101 has similar performance to µA741 except no internal compensation. Both came in the late '60s.The discrete NPNs may be used for a low noise input stage.editYep, the top ones have their emitters tied together and to the collector of another one and their collectors go to the top opamp and 10kΩ resistors.

author: StillTrying
author_rank: Super Contributor
timestamp: June 18, 2020, 09:47:08 pm
content: Those tracks will have been laid out with sticky tape.www.eeweb.com/profile/alex-tamari/articles/eras-of-pcb-design-the-land-before-cadIt seems to have 3 plated through holes, I don't know how they would have been done.They must havehavehad a lot of spare military standard pink wire to use up.

author: willeye
author_rank: Contributor
timestamp: June 19, 2020, 12:19:03 pm
content: So when would you guys guess it was made?I like the pink wire made me think of Big Clive.I still am not sure what UK and IT would stand for, I see OV is ground but...

author: GLouie
author_rank: Regular Contributor
timestamp: June 19, 2020, 03:06:00 pm
content: Odd item. Looks like a date code on the transistors of 45th week of 1980 (8045). 85 degree caps. Inspection stamp on the PC board. Nice engraved lettering. They made enough of them to make all that worthwhile.Curious that it has no ordinary connectors, or even a mark on the 24v to indicate which one is +. The labels are odd, too - UK and IT? Maybe designed to be permanently mounted and soldered in.It's quite possible that it would be terrible for a modern studio microphone compared to a contemporary design. The levels, gain, impedances coud all be for a very specific device. Let's see the reverse engineer!

author: Bud
author_rank: Super Contributor
timestamp: June 19, 2020, 03:26:58 pm
content: Wire soldering is neat. Not looking amateurish. But not sure about the point of using sockets for the ICs. Perhaps that was a thing back in the era.

author: magic
author_rank: Super Contributor
timestamp: June 19, 2020, 03:32:01 pm
content: Yeah, post the schematic. The resistors around the input stage look like standard, single-opamp differential stage with the inputs AC coupled through the big elcos.0V might be for "zero volts".

author: nfmax
author_rank: Super Contributor
timestamp: June 19, 2020, 03:39:14 pm
content: Pink wire + laced cable assemblies + style of feedthroughs + '/' in the part numbers all make me think this was made by RACAL, probably as part of an in-house test system, sometime in the early 80's. IT may be Input Transformer, but no idea what UK might stand for

author: Bud
author_rank: Super Contributor
timestamp: June 19, 2020, 03:40:40 pm
content: Quote from: willeye on June 18, 2020, 08:36:38 pmCould be capacitors. Aren't they marked with something?

author: CatalinaWOW
author_rank: Super Contributor
timestamp: June 19, 2020, 03:55:23 pm
content: To me this looks very typical for 1970s era military development work. Prototypes and sometimes test boxes would often be built with non-mil spec components. Layout is standard for Rubylith hand layout. CAD didn't take over until the 1980s. The drilling was definitely not done by some average Joe in his shop. Probably not CNC, but done on a mill or equivelant. Socketing active components was common also as the fab and test practices of people who grew up on high voltage vacuum tubes were not kind to semiconductors. And the jelly beans of today were still the prized and rare parts of recent memory. The box is definitely designed to be tossed, thrown and driven over.Somewhere in a warehouse there is probably amazingly complete documentation of this device. Requirements, design notes, masks for the PWBs, schematics, test procedures and the like. Military contracts tend to be one size fits all so even the prototypes get the full production treatment. But the documentation might as well not exist because there is no way to retrieve it. No one knows where it is and there is no searchable index, even on paper.Based on the serial number there were at least either 8 or 108 of these made. And probably at least one revision, hence the B series serial number. Which might also imply even larger numbers.

author: magic
author_rank: Super Contributor
timestamp: June 19, 2020, 08:05:36 pm
content: Quote from: Bud on June 19, 2020, 03:40:40 pmSure, 104JThe large ones are between the rails of the opamps, the small ones are probably compensation.

author: james_s
author_rank: Super Contributor
timestamp: June 19, 2020, 09:28:23 pm
content: 104 is a 0.1uF, the J is either a temperature range or a voltage rating.

author: magic
author_rank: Super Contributor
timestamp: June 19, 2020, 09:36:09 pm
content: J is20%tolerance and I made it up because it's a supply bypass capDamn, it seems J actually means 5%. I could swear most generic film caps I see are J though. Apparently they are better than I thought

author: duak
author_rank: Super Contributor
timestamp: June 20, 2020, 01:11:00 am
content: In the 70s and before, for custom and small production runs, I found it was quite common to have ICs in sockets, especially for RAMs and multipin packages that were hard to unsolder. It was a trade off between bad chips vs bad sockets. I remember chasing down an octal register that would get blinky when warm. The board cost me over $1000 and I didn't want to damage it. I had a replacement chip so I just cut the pins off and pulled them out one by one. It wasn't until the Japanese showed how to make and deliver known good parts (practically zero defects) in the early 80s that sockets became passe.

author: willeye
author_rank: Contributor
timestamp: June 22, 2020, 05:26:37 pm
content: As i got two i thought i would open the second and see if there are any changes and it looks llike this is a newer version. The original is serial B108 and this one is B101. As this is the first thing i have reversed it is more difficult than i thought

author: StillTrying
author_rank: Super Contributor
timestamp: June 23, 2020, 01:52:33 am
content: Quote from: willeye on June 22, 2020, 05:26:37 pmI hate doing it, but that looks quite simple.I'd start by drawing three separate diagrams of the 3 op amp's 8 pins and the components connected to them, by the time you've done that you should notice some places where the 3 op amp diagrams connect to each other, and then can draw a full diagram from the 3 partial ones.

author: magic
author_rank: Super Contributor
timestamp: June 23, 2020, 05:07:02 am
content: Yeah, it's a trivial circuitThe mic input part is a standard four resistor + opamp differential stage, with a discrete input stage added to the LM301 externally.Haven't looked what the other input and its signal path is doing but it can't be much more complex than that. The third amp probably combines the outputs of the first two in some way.