Presto 86-C Control Panel

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Hi All,

I just picked up a 28N system. It is wonderful and in really beautiful condition for its age...

Between the two 8N tables sits a 86-C control panel, which is a passive switching unit to aid continuous/simultaneous recording. Has anybody ever used one of these???

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It features some lovely Daven attenuators and two pretty large UTC transformers that are, according to the schematic, type HC 19X. Can anybody shed any light on these??? The schematic just shows how it is connected, no info about windings, though it appears to be for amp-level signal splitting. The model 301/type 20 meters are wired across the amplifier terminals and seems to be the last thing the signal sees before feeding the cutterheads. 0dB is referenced as 1.73V...

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It seems that the amplifier output is to be connected directly to terminal strips on the bottom of the unit. They are marked 250ohm on the schematic. This is where things get a little confusing. The amplifier that came with the system is an 87-type, but it is non-standard and there seems to be no schematic available.... I have begun the process of reverse-engineering it. There are 5 individual preamp sections (all right off the book) routed through a stepped attenuator into a 6SJ7 driver, 6J5 phase splitter and two 6L6's in push-pull configuration (common cathode bias). Something of a relief as I can replace all the valves for the price of two 2A3's from a standard 87-type! I'm in the process of re-capping it and replacing a few badly drifted plate resistors, but the amp passes signal. The OPT is marked as 'primary p-p (plate to plate I assume) 6600ohm' and 'secondary 4, 8, 16 and 500ohm'. The 8ohm tap is connected to the speaker jack and two other wires are connected in parallel to the Cutter/Pickup switch. The other OPT secondary taps are terminated on lugs.
In the manual, the brief section dedicated to the control panel states that the connected amplifier should be:

1) At least 85dB gain (ok - fine)

2) Better than 50dB SNR (ok)

3) At least 10W with less than 3% distortion (ok)

4) flat from 30 cps to 15k cps (to be assessed)

5) A low effective output impedance (ok)

Now, it is my understanding that if i were to connect the 4, 8 or 16 ohm amplifer output to a 250ohm load it would cause all sorts of problems due to a reflected primary in the several hundred k-ohm range. So, my question is, do I need an OPT with a 250ohm tap or am I missing something? Was this panel never actually intended to be used with the 87-type amplifer?

More photos etc to follow once I've finished cleaning, recapping etc...

Thanks!

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OK, for those interested, it seems the 28N system was intended to be used with the 200A rack, together making the post-war 'Model A' system. The 200A rack featured two 85B recording amplifiers with an output impedance of 500ohms.

[tragwag]

very cool! did you ever get a hold of that schematic?
here's the 87A schematic I have

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Yes, I did. Thanks Tyler. The amp I have is quite different to a standard 87. The only similarities I can see being the switching. Even the bass/treble contols have been replaced with stepped RC filters. What is really throwing me is the fact that the 500ohm secondary OPT tap is terminated to a lug. Both 1-D heads are 500ohm (DCR is ~120ohm for each). The only OPT secondary that is doing anything is the 8ohm tap going to the speaker jack....
I'll have a better idea once I've finished recapping it and can attempt to follow a signal around, it's just taking a while with all the cutting I've got to do...

I did have a bit of a eureka moment with the control panel this weekend though. Those UTC HC 19 X units are actually multi-tapped LC circuits which, together with the 160A slider, form an RLC circuit for each head, controlling the 'peak' frequency - so not transformers at all! All the splitting takes place on two switches.

Also, that 1.73V=0dB reference.... Seems to be related to the 6mW 'single frequency level' stated in 1-D documentation:

V=sqrt(PR)=sqrt(0.006x500)=Sqrt(3)=1.73

When the 'standard' 0dBu ref voltage is 0.775, based on 1mW and 600ohms:

V=sqrt(0.001x600)=0.775

I'm curious where this 0.006W reference comes from since the 1-D documentation also states that normal recording level is +23dB (1.2W). The meters on the 86-C only measure up to +6dB (above 1.73V).

This is so much more fun than relentlessly cutting plastic...

[buckyhold]

I see you mentioned a manual. I am searching for a manual for the presto 8N lathe. If you have one for your 28N would be willing to share a scan of it with me. Thank you and great work on the restoration.

[CutR]

You should have a chat with Ben @ raretonemastering as he has a 28N too.

[buckyhold]

Thank you I will reach out him and see if he has any beginner advice on it. Also I dug more in the forums and I stumbled across a PDF manual. I am stoked. I’ve been searching for so long.

[apoklis]

Here is a schematic of the 86-c . Im setting one up between two 88 amps .

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