Recently, I got a defective HP 3325B, it is a very useful generator even for today’s standard. It features some highly linear ramps, has great frequency resolution and a powerful output (10 Volts p-p into 50 Ohm). This unit reportedly had major issues, no output, and failures with startup. So even before switching it on, I removed the panels to check. Nothing obvious at first glance.
After a quick power on, some smell from the output section, and clearly, there are some burned resistors, and one of the power stage transistor is terribly hot, so hot that the solder melts… don’t burn you fingers!
Removed the board altogether (take care not to damage the connection flat cables!), and even the solder had some spray by heat effect, so I cleaned the area well.
To get access to the resistors, and to also do a proper test, all the transistors in the area were removed, and the transistors desoldered. All cleaned up pretty well, the board seems to be of good quality.
The 3-440 transistor aka 1853-0440, cut open. It has a tiny chip, difficult to see the damage with my means, but it is shorted to base.
The resistors, the only issue is a slightly discolored 47 Ohms carbon composition resistor, part EB4701, a 0.5, 10% tolerance resistor. Quite expensive to get, and the part, despite some signs of heat, tested good and within tolerance. So I decided not to replace this transistor, because it has an effect on the high frequency performance of the circuit.
The power amp, it is a marvelous push-pull design. It relies on complimentary NPN-PNP transistors that have high frequency power.
Nowadays, the PNP RF transistors of this sort are rare, probably they even were rare and expensive during their time.
The damaged resistors, fortunately, after a good amount of searching, I found the bags here in by temporary Japanese workshop.
The transistors, these 3-440 are equivalent to the 2N5160, and I happened to have 3 of these back in Germany, new old stock. Purchased them some years back, because they are generally not easy to get.
After these replacements, I run the adjustments and performance checks as per service manual, with no trouble at all. Also the self test passes flawlessly. We can call the generator fixed.
Out of curiosity, I checked with ebay, and there are very reasonable offers of what appear to be Chinese copies of 2N5160 transistors. They have the Motorola label, but to my knowledge, the date code is much past the obsolescence of these parts at Motorola. So I am waiting to receive these parts, and will give them a good test and study, to see if these are good replacements, or just fake.