Portsdown boxing/build thread

Discussion about this major DATV Project. See also https://wiki.batc.org.uk/The_Portsdown_Transmitter
G8GKQ
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Re: Portsdown boxing/build thread

Post by G8GKQ » Tue Dec 11, 2018 12:59 pm

Hi Gareth

I suspect that your google search found 7 dBm into 600 ohms. Even then, it's not quite right.

Rearranging W = E squared/R gives me E = root(W * R). 7 dBm = 0.005 W, so E=root(.005 * 50) = 0.5 V rms.

However, that doesn't help you as I do not know about DC checking the pass-through of the RF switch. I always use RF to check them, and use my very cheap DVM on the lowest ohms range (200) to check for short-circuits between pins and non-connected pins.

Is it the 4-way or the 8-way that is causing the issue?

Dave, G8GKQ

radiogareth
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Re: Portsdown boxing/build thread

Post by radiogareth » Tue Dec 11, 2018 3:53 pm

Thanks Dave, maths (clearly) isn't my forte.....
Checked my DMM, it gives out 3.1 volts on diode test and 0.66v on Ohms. I DC checked against the diagram, all OK. Every lead was checked through too so I was confident about the build integrity.
Using my RSP-1 as a spectrum analyser (very handy software called RSPSAS), via a lot of attenuation, I followed through the DDS signal. It got as far as the input of the RF amp on the filter board and then stopped. So I unsoldered the chip. It left the RH leg behind so I suspect it was either damaged during soldering (by pressing down presumably if it wasn't already flat when I soldered it) or during the desoldering process.
Anyway, new part soldered in carefully it all works and the exiting signal is a decent size and passes through the various circuits to appropriate sockets on the back panel.
All is now well and the signal levels and MER are looking nice and strong. Onward with amplifiers now, dare I secure the lid??? :-)
Gareth

radiogareth
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Re: Portsdown boxing/build thread

Post by radiogareth » Wed Dec 12, 2018 7:18 am

WOndering how the fault was created I suspect that using pre-soldered pads not of the same level and then pressing down on each chip leg with the soldering iron may well have dislodged the leg. Unlike the SM logic chip for instance, where the legs are supported within the plastic case, on the amp chip they seem to be on the bottom. Time to invest is either solder paste and/or a SM hot-air rework tool.
All part of the learning curve, time to browse Farnell/eBay for something suitable. Suggested brands welcome. Ideally something with a long service life like my Weller TCPs although tips are beginning to get scarce these days.
Gareth

g0mjw
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Re: Portsdown boxing/build thread

Post by g0mjw » Wed Dec 12, 2018 8:06 am

You need to be careful not to overheat these parts as well Standard Silver/Tin/Lead solder with 330C iron. Don't use lead free. Don't pre-tin the pads with those devices, it's not necessary and they are large enough to hold in place easily with tweezers.

Mike

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Re: Portsdown boxing/build thread

Post by G4GUO » Wed Dec 12, 2018 9:23 am

The Rolls-Royce of soldering irons is probably the JBC range :)

https://kaisertech.co.uk/jbc-cd-2be-sol ... l-purposes
radiogareth wrote:
Wed Dec 12, 2018 7:18 am
All part of the learning curve, time to browse Farnell/eBay for something suitable. Suggested brands welcome. Ideally something with a long service life like my Weller TCPs although tips are beginning to get scarce these days.

radiogareth
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Joined: Wed Jan 06, 2016 9:46 am

Re: Portsdown boxing/build thread

Post by radiogareth » Wed Dec 12, 2018 9:39 am

Nice, very nice, but its still 'just' a soldering iron. Would a hot air rework unit with a range of tips(shrouds I presume) not be better?
I have a needle point weller tip but TBH I find my usual 2.3 chisel bit just as usable under some magnification. Even on the 4 way filter board, that worked best for me. I think solder paste might be the next thing to try, assuming that surface tension is sufficient on the unsoldered end to stop 'tombstoning' when the first end is soldered?

g0mjw
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Re: Portsdown boxing/build thread

Post by g0mjw » Wed Dec 12, 2018 9:40 am

There have been some reviews on EEVBlog recently. Obviously you want a metcal if you can find one but there are some fairly reasonable soldering stations around. I am using a fairly cheap solder/hot air station from Farnell. It works but it would be better with a directly heated tip. I recently bought one of those Banggood TS100 irons with a small tip for very small SMD work. They are OK but don't have the thermal capacity of a larger iron.

Apparently the JBC is the one to go for but you will need deep pockets.

Oh yes, and 0.3mm solder is a must for SMD.

Mike

G4GUO
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Re: Portsdown boxing/build thread

Post by G4GUO » Wed Dec 12, 2018 9:53 am

I use one of the cheap Chinese 858D for reflow. If I were buying another one I would go for something slightly more
powerful like an 861DW. Another thing that I am thinking of getting is a ceramic panel to pre-heat boards
as once something is attached to a ground plane it is a pig to remove / re-solder.

I built my own reflow oven using a Arduino to control it but as yet not used it in anger.
The thing I have found most useful was in fact a stereo microscope which I bought used on eBay.

- Charles

g0mjw
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Re: Portsdown boxing/build thread

Post by g0mjw » Wed Dec 12, 2018 9:55 am

I have a hotplate and built a thermal controller to control it. I found in practice that a neural network based approach was much better - that is, me with a thermomenter and control over the thermostat.

G4GUO
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Re: Portsdown boxing/build thread

Post by G4GUO » Wed Dec 12, 2018 11:35 am

Some Youtube Electronics blogger I was watching used small 12v ceramic heating pads to warm up localised areas of a board.
I couldn't find the exact ones he was referring to on eBay and now I have lost the link to his video. Seemed a good idea. I believe they are used in
heating tongs and run off 12 volts, you can buy them with different temperature cut offs between 80c and 220c

- C

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