HP 8657A magic smoke...

[Nick]

So, my trusty HP 8657A signal generator made a nasty hissing sound... followed by the tell-tale smell of dead capacitor. We all know that one.

It was the IEC inlet - a Schaffner FN 370 2/21 which is a combined mains filter and voltage switch, so lots of wire. The grot had spread itself over the neighbouring mains transformer too, so that had to come out for a clean... (see attached photo)

It's really poor how many of these fail - I know that they are normally "on" 24/7/365, but I've replaced loads over the years and that's the second HP one this month. I found the right one for 20 quid + VAT at RS (that was for the 6A one, the -6/21 model) - the 2A one was 43 quid!!!! I guess they don't sell too many of those...

As my unit was originally used for ATE using GPIB, the RF out N-type is on the back which is a pain - there is a blanked-off cutout on the front panel for this...

So, I'm using this opportunity to make up some semi-rigid RG 402 with SMA connectors to route it to the front. I was going to use RG 316/U, but the rest of the unit uses RG 402, so that's what I'm going with....

   

   

[ppppenguin]

I had a Schaffner mains inlet/filter fail on my Tek 1755A vector/waveform monitor. PCB mounted, it got hot enough to melt the solder. I found a filtered inlet in the spares box, not PCB mounted, that fitted with a bit of care. Otherwise I'd have just fitted and ordinary IEC inlet without a filter.

[Mike Watterson]

I added a filtered IEC socket to a transformer I was checking last week. Immediate burning smell. Quickly unplugged. I was surprised that the solder had started to melt on the socket terminals and the metal case was almost too hot to touch. I forget the make. I never encountered it before. The transformer was fine, but didn't have the o/p voltage I wanted. I write the tested o/p volts on the transformers before putting them away with a permanent marker.

[Mark Hennessy]

I went through exactly this recently with my HP3314A. The part number I needed was FN370-2-22. RS had close-enough equivalents for around £20, but the right part was more than £40.

I found the right part on Mouser for a sensible price, and added a few other parts to the order to reach the free shipping threshold. Very impressed with the service from Mouser - delivery was very quick indeed.

There was no opportunity to use an unfiltered unit because it combines a voltage selector and fuse(s) into a snap-in package and there's no room on the rear panel to introduce a plate to bodge a standard inlet (plus I'd have to find a voltage selector switch - not straightforward). Besides, my unit was otherwise mint, so although pricey, fitting the right part was the only option really. Just grateful the correct part was still available. The original was a little over 30 years old, so it did OK really...

I have used unfiltered replacements in other gear. Ironically, that was probably a greater sin, as they all used switched-mode power supplies. My Tek audio analyser definitely deserved better, and I will get the right part one day - some recent testing indicated that it's probably putting some hash out via the mains cable. But in most cases where I've put a simple unfiltered socket in place of the original, it was because the originals were not readily available and no equivalent could be found. For example, a Philips frequency counter that used a filter with wires emerging at right angles from the side, and there no way anything else with a filter will fit the space because the chassis practically touched the bare end of the original.

That picture looks rather like a HP8904A to me. Luckily, a friend bought one of those, meaning I don't have to! Not exactly a high point in UI design

[Nick]

Yep. It is an 8904A (posted wrong photo) - I got one NIB with all features for a song many years ago. The IEC on that failed last week - also a Schaffner - you can see the new one in the photo...

The FN 370's voltage selection switch is used on many bits of HP & Tektronix kit. I just wish they were a bit less prone to failing messily and smellily! (is that even a word?)

Your */22 one is identical to my */21 except you have the USA 6.3x32mm fuse and I have the European 5x20mm fuse

[Nick]

... For example, a Philips frequency counter that used a filter with wires emerging at right angles from the side, and there no way anything else with a filter will fit the space because the chassis practically touched the bare end of the original.

That picture looks rather like a HP8904A to me. Luckily, a friend bought one of those, meaning I don't have to! Not exactly a high point in UI design

As mentioned, it was the HP 8904A that I replaced the IEC on last week - however, it uses a Schaffner FN 326-3/06 which is, like the one in your Philips kit, a right-angle exit type... you can just see the replacement at the far right of the photo...

[Nick]

Looks like I will need a special bracket for front-mount the N-type - the mount seems to be the same as on the 8657B - I will have to find the p/n and see if one is around (hah!). I have got some RG402 and the correct SMAs, so that's going to be fun...

The fan was making a lot of noise, then a "clang", then lots of vibration - see the photo. Got a very good replacement from Farnell with the same current & airflow - nice and quiet now...

Took the opportunity to check supplies, reference clock etc. and all within spec - amazing really as I haven't looked at this for about 10 years.

Note that the allowed frequency range is stated as 48Hz to 440Hz - that's a wide range for a transformer to handle...

Unit now working fine except that the CW amplitude seems incorrect. Maybe I'm being thick...

                   

[Reprobate]

... For example, a Philips frequency counter that used a filter with wires emerging at right angles from the side, and there no way anything else with a filter will fit the space because the chassis practically touched the bare end of the original.

That picture looks rather like a HP8904A to me. Luckily, a friend bought one of those, meaning I don't have to! Not exactly a high point in UI design

As mentioned, it was the HP 8904A that I replaced the IEC on last week - however, it uses a Schaffner FN 326-3/06 which is, like the one in your Philips kit, a right-angle exit type... you can just see the replacement at the far right of the photo...

Hello, new to the forum after I found your post, and also thank you for the documenting of what you did, the part numbers in the post led me right to the part I needed for my 8657B, I actually ordered the  FN370-2-22 instead of the  FN370-2-21 by mistake, but my luck would have it my fuse was in fact the 6.3x32mm out of the old unit (not included in new unit) so I just copied the old wiring and moved the cap over to my new unit, and it worked. Lucky that was all it was as when I saw that smoke I thought the worst, so again thanks very much for the documenting including the model number!

Now, I wanted to ask a favour, I saw you also had an option 2 unit like myself, and your outputs are at the back. I don't suppose you have more info on what I would need to do in order to move them to the front (at least the RF)? I have the 8657B but I saw you said it's a similar procedure?

Thanks again!