The plastic shaft was broken so I made a new one
What shall I say, not a good combination. I repaired a bunch of Bose 100V audiosystem panels for this customer and this came with those. Simple but it gave problems. Not so strange if you look under the relais and connector.
This scope is a bit confused. Is it a Fluke or a Philips, or both. Philips on the front, Fluke on the top of the cabinet.
Not much wrong with it. It had starting problems, the PSU wined and there was a broken knob- shaft-potmeter. I made a new shaft for the potmeter, serviced the powersupply and checked the rest.
It is made very service friendly. The cabinet is made to hold PCB’s in a service-position
Because I get more and more secutest for repair, I experimented with several fixtures. Secutests are a pain to work on because there is no room and there are dangerous voltages inside.
There is a PCB with big relais:
This is one of the many versions. The PCB hangs upside down in the cabinet. The view is blocked by most models, by a transformer and many wires. Many problems come from this PCB. I remove the relais, check and service or replace them and clean the pcb. Often there is a lot of dirt under the relais. Do not ask how it gets there, I do not want to know the owners do with these instruments. Most have a hard life but can get quite old with some TLC. G&M does not provide service information, so I reverse engineered it and made my own schematics.
Also a lot of relais that connect test signals to the banana jacks and analog signal handeling.
The rotary switch contacts, LCD display and digital signal handeling.
There are several versions of this board also. The one above is temporary without the 5 relais. I removed them to repair some inner PCB damage. But how to measure in this beast under power without electrocuting your self, it is very dangerous for a service tech inside there.
This setup was no succes, because the short cables and high voltages I do not like it if all pcb’s are loose on the table (and half over each other because the cables are to short.
so I mounted them on a perfoboard. Better as nothing. Then a customer gave me a box with junked secutest parts. I made a fixture from an old cabinet and some extension cables. This was a big improvement. It is flexible and safe. If you want to work on your own secutest be very careful, nothing is labeled. Cable colors are random. No silkscreen. The pcb’s are multi-layer and you need a very good and powerful (de)soldering gear (and desolder skills).
I use a small “thing” to simulate the DUT (device under test). It holds a resistive load, on/off switch, indicator light and some resistors (f.i. a 110M for Riso, a 7 ohm for Rsl, etc) to check all functions.
The problem is there are many versions. Not only with different hardware but also regarding the menu’s and functions.
These burned things began their lives as resistors. The problem is the pcb is fried too.
First step, make pictures and a sketch and then I mill away the burned section.
Then I make an insert and glue that in.
I connect the new pads to the rest and cover it with pcb coating
And this is how it looks like finished
Tĥis is the pcb from a G&M Secutest. It was not working like it should. The problem turned out to be some vaporized traces inside the 4 layer pcb, a LM358 with ventholes, a dead 555 from the boost converter, a dead TL082, a burned inductor, a blown fuse and 4 bad relais.
The the pcb above the LCD pcb turned out bad too. A short between the negative rail and ground. This was harder to find because it was not visible. I found it using my fluxgate based i-prober currentprobe. Repairing was easy, I milled a way a part of the top gnd layer, then the burned out part to remove the short and then filled all up. The new trace is now for safety reasons above the pcb.
Here some more info about the PSU PCB repair. First step was “opening” the pcb so I could inspect the inner-layer and remove the burned residu (that is conductive and can cause leakage current. Not something you want in this type of instrument.)
Here you see a part of the problem. This was not the worst one but I could not made that visible on a picture because there was a groundplane that blocked the view. You see the black “fog” around the trace. This is inside the pcb photographed with a lamp behind the pcb. The traces go to the pins of a relay. 4 traces were burned/vaporized on several places, bizar !
I milled and scraped “channels” so I could remove the burned stuff and layout new traces. I used 0,2mm wire for this. Look at the size of those sot23 transistors to see the scale of things. All this is done under a microscope. I use excavators and mini scrapers that I made myself. It is not difficult but rather time consuming and you need a steady hand and some mechanical experience. I have done a lot of mechanical things for many years, small scale like building scale models (cars, bikes, ships, trains and a small steam engine) but also restored (and complete rebuild) real bikes and cars.
Normally to much work for an old Secutest, but I just love doing this sort of things so now and then as extra practice.
I had to remove some of the top traces.
It looks a bit messy because I covered the new toptraces with epoxy.
And after testing all connections the parts are tested and placed.
A peltier powered dryblock from Isotech that stopped working. There was no current flowing to the TECs (peltier elements). That can be caused by a dead controller, dead PSU or dead TECs. They should have a very low resistance so that is easy to test first. I measured a few megohm. The once used here should measure an AC resistance around 2,2 ohm.
I made pictures from the tear down for those who are curious about the construction of these instruments.
The first step was easy, removing the cabinet. This one has a motor with a rotor that holds a magnet. It is the thing in the Telflon ring on the picture above. It should be glued but had come loose.
After removing the motor and the rest of the cabinet we see the dryblock in its full glory. Check the 4 screws on top of the cabinet, the whole block is only mounted by those tiny things
Cutting away the foam is a dirty job but not difficult. Just be careful not to cut the wires.
Here most of the foam is removed. The pressure-plates are pressed together during mounting and then “locked” by the M3 threaded rods screwed in the brass studs. They are not used to set or change the pressure, only to keep the pressure constant.
The foam was burned where it touched the anodized dryblock core.
After removing the threaded rod the plates come loose and the TECs are visible. The core looks like it is milled with a flycutter, the alu plates too. The TEC’s are lapped and sealed versions. The sealing is nothing more as captan tape and kit.
Breaking away the last foam. The TECs are stacked in pairs of 2. There are 2 strings of 4 TECs. one string is made so there are 3 in series. “One” of the 3 is made of 2 parallel. A construction that is often used in dryblocks. All TECs are the same. Marlow DT-12-6 in the sealed and lapped version.
As said, 3 are toast. Nothing to loose and curious is a good combination so time for some experiments on the dead ones.
A TEC is made from a lot of small blocks semiconductor n and p material. They are all mounted in series (electrically, thermally they are parallel). This is done by soldering them on small metal strips. This is all mounted between two ceramic plates. When a voltage is applied to the free ends of the semiconductors there is a flow of DC current across the junction of the semiconductors causing a temperature difference. The side with the cooling plate absorbs heat which is then moved to the other side of the device where the heat sink is.
So what was wrong with them ? I have seen and measured a few other dryblocks that where here for repair. Datasheets from the TEC state often the AC resistance. If a dryblock does not meet his specs anymore and the controller etc is OK, there is a good chance the resistance from the TEC is increased. I think they first come a bit loose, increasing the contact resistance and finally end open. You can measure the resistance per row and find the one that is wrong. In this case the problems where all on the outer rows nearby a corner. As an experiment I managed to resolder them, that turned out to be not so difficult and the result is a resistance within the datasheet specs. I will only use the repaired ones for some experiments and tests to see how they hold.
This one had a few fried resistors due to a short caused by damaged isolation from a wire.
I first removed the resistors. The picture above is after cleaning with IPA. The pads are loose but not damaged. The black stuff is burned epoxy. You can see the structure from the, now epoxy-less, FR4 glasfiber. You can pinch a needle through it as if it were cardboard.
This is the other side.I milled away the damaged part, glued in a new piece. I then drilled holes and placed some vias. Then milled the new pads and traces and soldered new resistors in place.
Not very beautiful, but this sort of things are hard to photograph and always look worse then the reality. I could have painted it and use some filler but that is a waist of time. It must be good and that it is.
Fluke uses a resistor divider to tell the processor in what position the switch is. It is made from a sort of carbon film. At the picture above you can see it. This meter has also a problem with a partial dead IC so a nice victim to practice my PCB transplanting skills.
In this meter the black stuff is worn beyond cleaning, believe me I tried all the tricks from cleaning to pencil. I made a circle from very thin pcb, copied the layout from the black stuff to that. Then milled a “pocket” in the meters pcb to hold the new pcb. I made the diameter a few mm biger so there was room for resistors. Fluke changed this systemj later to one that used resistors. Glued it in place, soldered some wires to replace traces that had to go. The result was successful. It still does not measure but it now can switch into the functions.