Hello all,
after quite a few years of absence from this group and from my trusty 7000-series equipment, I am back.
The reason is quite obvious. I have a special measurement job which needs my 7A13.
As expected, after almost 10 years of sitting around my 7A13 has quite a few issues, but this is a story on its own.
As the subject already tells all the relays within the 7A13 have gone bad. Some seemed to work fine, but on closer inspection every relay has its own problems.
Mostly contact resistance of the relays is highly unstable. Sometime you can get the relay down to 100mOhm, sometimes the same contact has up to 10 Ohm.
Of course, I tried all the recommended reviving techniques like repeated switching at high frequency under load, etc. but nothing really helped.
I remember that I replaced 2 of the relays already 10 years ago by a self-made free-floating solution to fit a new relay into the old socket.
This was quite tedious and I don't want to repeat the same thing again. Leave alone the fact that this free-floating solution might not be capable of providing the right HF and Isolation performance.
By searching for a better solution in this group I saw the solution of Tom Ford back in 2019. (
/g/TekScopes/album?id=91736)
Unfortunately, he didn't provide any design data and he seems to be offline since then. Therefore, I went to action and designed my own little adapter PCB. And I will also try to document my work as good as possible, to give anyone else enough information to do this on his own. Related pictures can be found here and maybe on my GibHub.
/g/TekScopes/album?id=299851My first Idea was to replace the internals of the original TEK relays with a modern relay with the original TEK Housing still in place. Therefore, the PCB had to be so small to fit inside the original housing and connect to the relay and the header pins.
In addition to the relay there also should be a small bridge rectifier to get rid of the polarity issue most modern relays have (most modern relays are polarized) and optional a little SMD LED to see if the relay is activated.
As the space within the relay is really tiny, I came up with the idea of using a 2layer Flex PCB which would be bent and hold the relay, a bridge rectifier and an LED + resistor.
This concept (concept 1) looked really good but I had quite a few problems:
1. Crosstalk between channels and isolation between open contacts was not that good because of relatively long tracks in the flat flex running in parallel.
2. Flux residue between the flat flex and the header caused leakage which cause "enormous" leakage currents. Not the required +/-0.2nA more the +/-15nA or even more.
3. Due to the tight space within the relay the bending radius of the flat flex was quite tight and only in the range of 1-1.5mm. This caused on some trials problems with broken tracks.
All in all, the ideas sounded great but the realization was not the great at all.
For my next concept (concept2) I reverted back to the idea of Tom Ford and created my own little adapter PCB for both needed relay types used in the 7A13.
At the moment this is an unproven design as the relays are still on back order and I'm waiting for the distributor to dispatch my order.
For the relay I did a longer market study to find the best fitting relay. In the end I decided to use the AXICOM IM06TS which is really tiny and still provides relatively good switching and HF performance.
The footprint of the relay is a little issue as the row spacing between the pins is only 5.08mm. This makes the pads slightly interfere with the through hole pads of the socket header. To still accommodate the socket header and the relay on the same PCB without interfering I decided that the pins of the relay need to be bent outwards a little (~.7mm per side). This is not a really nice solution but I assume it will work fine. If the socket header would have been available in a SMT version the use of a SMT relay would have made everything a lot easier.
I presume Tom Ford used the BAS3007 as bridge rectifier. Unfortunately, this part was not available at my favorite distributor. Thus, I selected the BAS40DW-04 which is not exactly a bridge rectifier, but it can be connected as one while still being really tiny.
As I already wrote I also added a small surface mount LED together with a resistor to the PCB because I experienced on my unit some issues with the Cam-Switches which could have been easily debugged if there had been a little indicator in place.
All in all, the BOM for a 148-0034-00 relay replacement looks like this, for the 148-0035-00 it is basically the same, you only need to remove some pins from the pin header rows.
1x Relay (IM06TS)
0.1x Pin Header (W+P 952-09-080-2 cut into 10 pieces)
1x SMD Resistor 0603 1.8kOhm
1x SMD LED 0603 yellow
The resulting PCB is measuring 8 x 14.5mm and is still smaller than the original relay (8.5 x 16mm). My favorite Chinese PCB manufacturer charges me the same no matter if the PCB is 8x14.5mm or 100x100mm. Therefore, I decided to make my own custom panel which utilizes the 100x100mm as good as possible. Now I have 5 panels of 66PCBs each lying next to me waiting for assembly. Each PCB holds 36 148-0034-00 compatible PCBs and 30 148-0035-00 compatible PCBs.
As already stated above I was not able to do any tests, but I will definitely do tests as soon as I have some relay. As soon as my concept of the PCB is tested and proven good, I will give you all a follow up on the status and will also publish the design data in case you want to replicate or improve my design.
I will not need all 330 PCBs, maybe 15 to 20. Therefore, I am willing to sell these to anyone interested in getting some relay replacements. As I also ordered a solder paste stencil I will wait how big the interesst in these PCB is before i start populating a whole panel. I think it would be the easiest if I sell these PCBs with the SMT parts already soldered on.
Best regards,
Wolfgang
John Williams
Jean-Paul