Repairing an HP 5334B counter and adding the C channel option

While browsing eBay, I came across an HP5334B counter. I already have one, so I don't need another, especially that this one was without opt 003, which is the 1.3GHz C channel. However, this was advertised for cheap, with the comment that it was working until a bang with smoke. This pretty much point to some Rifa capacitor self destruction, thus a low hanging fruit for repair and resell.  Fortunately nobody else wanted the one on eBay, so I got it for the starting price. Unfortunately devices without C channel don't sell for much but I had an idea.

While repairing my other 5334B I acquired a complete front panel while looking for the plastic filter in front of the display. This panel was complete with the proper hole and legends for channel C. The fused BNC connector was damaged, but that could be replaced (more on this later).

So I had a 5334B without channel C, and a font panel with Channel C. The perfect combination to create a working instrument with opt 003 for sale.

First checks indicated a blown main fuse. The reason: Rifa caps. The X cap was shorted.  

I replaced the capacitor and the fuse. Plugged it in and it stared up with the standby led on. Nice. Switched it on it worked for maybe five seconds, then a mighty bang, sparks and blown fuse. What the heck???

Nothing surprising, a tantalum capacitor has exploded, it was literary blown into several pieces. So I replaced that capacitor and the fuse again and since then everything has been working well. 

The next step is retrofitting channel C. It has already been done before, there is an excellent write-up on this. I loosely followed this description.

So what is needed? The 5334B is quite interesting, it seems like channel C became an option at the last moment during the design of this counter. Most of the electronics are already there in the base instrument. The only parts that are missing is the Fujitsu MB506 prescaler IC. It even has a socket, so just needs to be plugged in. The only thing to look out here is to NOT buy a fake IC from eBay.
Besides that two diode bridges need to be soldered in for the peak detector and the limiter. The original diodes are HP 5082-2831. They are still available, but costly and simple to replace with SMD Schottky diodes. Those can be soldered in with a steady hand and a thin soldering iron.

The final component for the C channel is the most difficult if one wants a identical to original retrofit. It is certainly possible to drill a hole on the front and just install a BNC connector, but it would not look original at all. It's not only the connector, but for example the Freq C text is also missing from the appropriate button.

This is where I was lucky. As I mentioned I had a spare front panel, which only needed a new fused BNC. After some research and help from the HP mailing list I figured out the part number. It is an M/A COM 3284-3201-10, panel mount BNC to SMC connector. It is an obsolete part, but seems to be available in abundance on eBay. It also needs an SMC pigtail, which is simple. 

 

Here is the photo of the whole setup. The SMC pigtail is plugged into the connector and solered to the main board. The four diodes are soldered into their place and finally the prescaler is inserted into the socket.

That is it, we now have an operational channel C. The only required adjustment is to set the peak detector so without input signal there is no counting. The prescaler is oscillating with no input, so HP put in the peak detector to actually only allow counting when there is a real input signal.

As usual for the conservative specifications, HP specs channel C to 1.3GHz. In reality it is happily working over 2 GHz.
The only open question now is what to do with an other 5334B.