This is a follow-up to my previous post about repairing a Tektronix CG551AP/CG5001 Calibration Generator.

The CG551 had been sitting for many months and I thought it was time to give it a quick check. Shortly after turning on the TM5000 mainframe that I had plugged the CG551 into, I was assaulted by the tell-tale burning tantalum capacitor smell. Of course I turned off the mainframe with considerable alacrity and pulled out the CG551 in order to assess the damage.

I should explain that this particular CG551/AP is really a CG5001: Although the serial number (B010129) suggests that it was an earlier model, several of the critical boards had been replaced with newer revisions including the A9 (CPU/GPIB), A8 (High Edge), and A7 (Output). On this later revision of the A9 board, for example, the potentially leaky Ni-Cd battery was replaced with a lithium primary cell. And the GPIB interface is built onto the A9 board rather than on a daughter-board like earlier models. The point is that this CG551 is OK to be plugged into a TM5000 mainframe.

The shorted tantalum capacitor turned out to be A6C1622 (15uF, 20 V) which was connected across the +15v rail.

At this point I decided to replace all the dipped tantalum capacitors. As there were several of these inside the A3 module, I replaced the single aluminum electrolytic capacitor there as well.

The construction of the A3 module is interesting enough to warrant a few extra images. For example, the outside covers have silicone heat transfer pads which contact some the chips on the various sub-boards in the module.

The A3 module is compartmentalized in cast aluminum:

A3A7 Board Removed from the module. This board has one dipped tantalum capacitor:

A3A7 (100MHz VCO) ground plane and connection pins:

The boards are held in place by captive (nominally) hex head bolts. The bolts are (nominally) held captive to the boards by E-clips. This allows removal of the boards from the connector pins in steps, by turning the bolts. However some of these clips were prone to fall off and potentially cause short circuits:

I replaced all the the old E-clips with new ones on the boards which I removed from the A3 module.

There was one aluminum electrolytic capacitor in the A3 module: A3A6C1010

More dipped tantalum capacitors on the A6 card:

There were two more aluminum electrolytic capacitors on the “front” side of the A2 board. These are A2C1241 and A2C1612, 10uF 20 V capacitors. Every other board in the CG551 must be removed in order to replace those, so they were the best place to start.

Very curiously, I found that one of the factory installed dipped tantalum capacitors had been installed “backwards” (reversed polarity) originally: A7C1713. Luckily, it was in a current limited circuit, and simply became a 400 ohm resistor rather than a short circuit. It did not heat up enough to get discolored. I do not know how many hours this unit ran with this reversed capacitor.

Correctly installed C1713 on the A7 board:

Incorrectly installed C1713 on the A7 board:

The components replaced in this round:

The dipped tantalum capacitor with the resistor was the one that was installed reversed from the factory. I was testing it with a 100 ohm resistor in series to simulate the actual DC circuit. The two large resistors A7R1726 and A7R1728 were 82 ohm 5% resistors which had drifted up and out of specification.

Most of the higher quality, military grade, or hermetically sealed tantalum capacitors were left as found. They all tested within expectations. I found that the original dipped tantalum capacitors had (generally) higher ESR than that of the modern replacements. I also fitted several new aluminum polymer capacitors as replacements where appropriate. I will be interested to see how these perform over the years.

I have put together a spreadsheet including all the capacitors I have replaced in this CG551 along with my replacement choices and part numbers.

 

Here is a link to the full spreadsheet: CG551 Capacitor Replacement

My next task will be to calibrate this unit which comprises many adjustments, judging by the number of adjustment points in the manual.