Demystifying a (mysterious) Philips OQ0702P Double Balanced Mixer

In the world of electronics, mystery chips are an unavoidable pain-in-the-backside. In a recent project of mine I came across a rather curious case.

It involves attempting to change the carrier frequency of a vintage DQPSK modulator, effectively changing the broadcast standard. This board is fitted with 4 of these performing 3 different tasks. Considering it is littered with adjustments, and that I do not have the service manual – I need a very good understanding of how it works before I can attempt this. This is rather difficult when an unidentifiable chip is doing so much on it.

I am absolutely certain by its usage that it is a double balanced harmonic mixer. I was unable to find any datasheet online, heck, I even went and looked back at the databook archives around the time it was made (1992). Nada. No mention whatsoever let alone a datasheet.

For Philips, world renowned for thorough documentation of their chips and availability thereof, this is almost improbable, especially this being a general purpose type with a plentitude of possible applications. Perhaps this chip goes by a different name? Once again I hit a dead end. None of the mixer chips in Philips’ catalogues had a pin/package configuration like this.

Desperate for answers, I picked up my hammer, and headed to the nearest rocky outcrop. After much digging and bashing, I found something:

Oohhhh. What is this? a double balanced mixer, in a DIP-16 package, from Philips, probably dating from the late Cretaceous period

The last catalogue the TCA240 appears in is dated 1976. This is a really old design.

Is this our mystery chip? Well. One way to find out is to build a test circuit for the TCA240, but using an OQ0702P instead.

TCA240 application circuit: Double-sideband suppressed-carrier transmitter.

The above circuit looks like a nice simple place to start…

But it turns out I didn’t need to bother. A week later an older (5 years prior) version of this piece of equipment lands on my doorstep:

And there they are. In exactly the same circuit.

So what gives?

If this thing is a TCA240, why not just say so? It didn’t suddenly become necessary to re-brand the 555 timer because of its age. The newest TCA240 I could find is dated 1988, however OQ0702’s (that I could find) are all newer.

My guess? Philips officially discontinued TCA240 in 1988, but secretly kept it in production under a different part number for their own purposes, and perhaps for the benefit of a select few large customers. This would be a very convenient way of dodging awkward questions that may arise when a newer sample is inevitably found.

A little bit about the chip

Above is the equivalent schematic of the TCA240. It’s so primitive that it’s almost offensive. If you’re looking to learn about active mixers, this chip is a very good place to start. It has Broadcast credentials, and absolutely no curly details.

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