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.

Posted in Circuit snippets

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