After fully refurbishing and fixing the MSR-904A receiver, and some months of service, another look at the AM detector. It is working fine, but at times, is seems to have some microphonic resonance and random shifts of level. Nothing that prevents use of the machine, but a small flaw worth correcting.
Locating the faulty assembly – pretty obvious when knocking at the A3B5 assembly. This takes in a sample of the 250 MHz IF signal, and performs the AM demodulation, and also generates the AGC signal.
These are the inner workings, the signal is first attenuated, about 20 dB, then amplified, about 25 dB, using an Avantek GPD-202.
The signal is then futher amplified via a tuned transistor amplifier.
Demodulation is performed by a zero-bias HP Schottky diode, a HSCH-3486. This was state of the art at the time, and still, there aren’t many diodes around that are any better.
Turns out that this diode must have suffered damage earlier, and it was fixed with a drop of epoxy!
The HSCH-3486 are not so common any more (with the SMD package HSMS-2850 being widely used), here a comparison:
The HSMS-2850 and HSCH-3486 use the same technology – the HSMS-2850 is more or less a SMD version of the earlier HSCH-2486 (glass package, similar to DO-35).
Now, should I replace the flaky diode with a HSMS-2850? Well, after some thought, rather keep the parts as they are, for such a rare instrument. With some effort, this is what I found:
Glad I didn’t have to buy at USD 20 each, that’s what some vendors are asking for.
Finally, a quick sensitivity and microphonic test – no issues at all.
Quite interestingly, someone must have fixed this assembly before – note the ECG opamp, 1991 datecode (it is a LM1458 replacement; for the AGC circuit).