RCA Solid State Pre Info Needed

[guitarrock04]

I just scored a pair of interesting RCA preamplifiers, and haven't been able to find any information on them whatsoever. They're modular, appears to be out of some form of console or desk, and contain a Kenyon input transformer. There's no output tranny. The odd part is that they contain a retractable, hard wired bk-6b microphone on a spring loaded reel. It's a pretty cheesy gimmick, but cool in it's own right. The pair put me out $25

The label says it's a MI-40624B, part no. 8529782-504.

Has anyone ever seen anything like this? Any information is greatly appreciated, maybe even a schematic.

 

[emrr]

Seen them sold on ebay with the schemo several times, never seen any available data.  From looks they appear to be talkback modules for TV product line. 

 

[guitarrock04]

Through the generosity of a nonmember forum browser that happened to know Larrchild, I was informed that this preamp originated from an RCA TR-70 quad tape recorder. This would've been their top of line solid state video recorder in the late '60s/early '70s.

Through the generosity of yet another person, a video editor and conversion facility found online, I was able to receive a schematic from the TR70 manual.

It appears that the preamp is only a small portion of the stuff going on in the module. I'm not clear on exactly what the left most portion of the circuit does. Q3-5 is the actual preamp. Are Q1-2 relevant to using this as a standalone preamp? Anyone foresee any output impedance issues either way?

 

[NewYorkDave]

The functional blocks are labeled. It says "400Hz cue [something] oscillator." It's hard for me to make out the details on that image. Besides that and the mic pre, it looks like the rest of the circuit is switching. Again, it's hard to make out the details.

We used to have RCA Quad machines here. I might still have the books somewhere.

 

[PRR]

> It says "400Hz cue [something] oscillator."

"Mark". I bet it drops a beep on the tape.

I never seen quite that form of oscillator. But it is just an R-C-R-C-R-C phase-shift job. R6 C5. R4 C1. R5 C4. With one amp and equal RC values it needs gain of 29; improved forms can oscillate with less gain. R6:R8 give gain of 30, R2 R3 cut that back, but Q1 buffers... I accept that it beeps.

Oh, the mike amp. It's quickie/cheapie but not junk. We can not know the mike impedance or T1's ratio from the data shown. Q3 Q4 and their NFB give gain of 20 to the top of the volume control. The volume control will NOT fade deep bass to zero because it is "grounded" in C12, but it will reach -57dB at 50Hz which is pretty near nothing. Q5 gives gain near 2.5 into one or two 10K loads. Total gain is 50 or 34dB plus T1 which may be 1:5 or 14dB.

Supply voltage is used carelessly, and can't be over 24V (note 25V cap on B+). It may be as low as +12V... the bias works better down there, and in a hi-buck machine they should not be running 24V on 25V caps. Taking 4V max peak output, input overload is 200mV at Q3 base. Assuming T1 is 1:5, that's 40mV at mike. Assuming dynamic mike, that's 120dB SPL, which seems unlikely even on a close-talk mike.

Do whatever you want. But I submit that it's a great toy just the way it is. Hot-wire +12V onto it, see if transistor voltages seem plausible. Tap C3 with a 10K resistor, connect to point 12, to mix beep and slate-mike. Put doorbell button in power line to oscillator. Maybe mike-amp too.

Pull out the mike, tap beep button, hold mike button, say "Take 47!", then wave at the talent.

On playback, you can learn to hear the beep even when it is FF-ed into a chirp. Or you could get clever and trigger a relay from 400Hz tone to start or stop the tape. A steady 400Hz tone is fairly easy to spot in P-Tools or other waveform display.

Q1 Q2 "could" be bodged into a preamp, but it would be easier to build a 2-Q preamp than to hack this up into something it isn't.

The reel is the killer attraction. Nobody in your town has a slate-mike on a retracting leash. All the rest is boring.

 

[PRR]

me> Hot-wire +12V onto it

Wait!! It's all Positive ground!!. The live-lead is negative!

Ah ha. 2N220 is Germanium, and I bet the others are also.

2N220 breakdown is 10V, which comports with a negative 12V supply.

This also fits with the way the transistors are mounted way away from their pads... Germanium can be melted at solder-temp.

And you say late-1960s.... this would indeed be SOTA for that day.

Well, you got yerself a ruff-n-redy Germanium! preamp. Is that cool or what? What. I don't recall Ge days with nostalgia, but some folks do.

 

[emrr]

The reel is the killer attraction. Nobody in your town has a slate-mike on a retracting leash. All the rest is boring.

This is the crux of the matter.  Yep, all germanium.  Probably the same input as on the BA-71's which is 50/150/600:1200, I recall. 

 

[guitarrock04]

Thank you so very much for your input.

I apologize, but the schematic posted is only partial. Farther right on another part of the scan, it gives the voltage to pin 7 as being -20v. You recommend having it lower to bias properly, but I'm afraid I don't quite follow. 2N220 datasheet states recommended voltage is 22v...

The Kenyon input tranny is 150/375/600:1.2K I believe. I know the primarys are correct but I'll have to double check that secondary.

I'm thinking simple here. I'll most likely re-rack this in a 1/2 rack with two pres and a power supply. Probably front 1/4" input bypassing the tranny as well. The cable on the reels is pretty rotten, so they're probably not worth monkeying with.

I realize that it isn't anything horribly special. But I got two more pres that are built significantly better with significantly better components than are common today. I don't have any germanium pres, but the Volumax is germanium and I really dig that piece for the character it imparts.

 

[PRR]

> the schematic posted is only partial.

I know.

> it gives the voltage to pin 7 as being -20v.

I would believe RCA more than me.

A 12VAC transformer, FWB, 1,000uFd, 220R, 1,000uFd power supply would give about the right voltage clean enough for testing.

Three 9V batts would run several hours, and if you screw-up the connections the batts may not be able to smoke anything.

> 2N220 datasheet states recommended voltage is 22v...

DATA sheets don't usually "recommend"; and the one I found said 10V max. That does not contradict 20V supply: the enormous emitter resistors soak-up most of the supply voltage. Also the "10V" rating is probably low-balled, parts like these were 99% sure to be good for 15V or 35V, but the sorting would turn up a few low-volt parts which could still be sold under the 2N220 number.

The input impedance is generally good. If we agree it may be a 1:3 ratio transformer, the actual input Z is 1K or higher, just like we want for most dynamic studio mikes.

The output is unbalanced and 3K (plus any 10K resistor). This will feed all your unbalanced inputs, and most modern balanced inputs, but not a vintage true-600 input. (The Volumax may be true-600.)

Anybody know? Can you still get the BIG Germanium devices used on Delco radios? One of them, or even a solid 200mW device, plus a 470R resistor, could put OK levels in 600 loads.

Jack has a couple popular 150mW Ge parts, the 2N404 and the 2N1309: http://www.muzique.com/pcb.htm

SurplusGizmos has a box of the '404: http://www.surplusgizmos.com/2N404A-PNP-Germanium-Transistor_p_1352.html

Use 'em after C14 like this:

Trim R2 to get 6V to 9V at Collector. If you can get good thermal contact to cool the transistor, do it. The '404 is handy IF you can still get TO-5 heatsinks. The older Ge-part cases were odd sizes. We used to bend scrap copper heat-clamps to hold these things to chassis; be sure you get good all-round contact, and do not crush the case (white stuff leaks out, no good).

> The cable on the reels is pretty rotten

That's sad. That's the part that tickles my fancy.

 

[guitarrock04]

Thanks once again for your insight, PRR.

Here are a couple photographs for anyone interested.

As received:

And what's left after trimming the fat:

A power supply and enclosure are soon to come.