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13th Dec 2006, 5:06 pm | #1 |
Octode
Join Date: Jan 2003
Location: Guildford, Surrey, UK.
Posts: 1,958
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R707 Push Button Switch Board Assembly
I'm working on an R707 that came from Howard with no VHF and no MW and LW. It works ok on SW.
The MW and LW problem was found to be an o/c pcb connection between switch M2e and switch S1d, instead of the usual IF module type of fault. Although this is in the aerial circuit, it seemed to stop the AM oscillator in the module from running. The BF195 mixer/oscillator in the VHF tuner module was o/c and this stage was made to work by temporarily substituting a 2N918 for it. This enabled slightly noisy FM stations to be tuned it. A further switch board problem in the whip aerial switching circuit was then found - this time o/c connections between point 43 and S2b and also the S2a to the tuner aerial input. All of the o/c switch board connections are on the component (underside) rather on the (visible) solder side, so I can't easily see what is going on without removing the assembly and possibly removing the switches - a major task which might create more problems. I am unable to determine whether the component side problem is caused by dry joints, cracked print, bad plated holes or something else. I would be interested to hear from anyone who has experienced this problem or something like it. Ron |
19th Dec 2006, 11:25 am | #2 |
Octode
Join Date: Jan 2003
Location: Guildford, Surrey, UK.
Posts: 1,958
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Re: R707 Push Button Switch Board Assembly
The problems with the R707 switch panel were found to be caused by bad (o/c) plated holes that linked the component side tracks to the solder side tracks. These problems existed on four separate circuits. All of the faulty holes were occupied by band switch solder pins, but I could not persuade solder to flow through the holes to wet the joint on the component side.
Rather than link out the errant connections, I decided to see if a less visible repair was possible and removed the switch pcb from the chassis (24 connections and 2 twist tabs). The whole panel was saturated with an oily residue, presumably the result of a previous heavy handed attempt to clean the switches, so I cleaned it off with IPA and kitchen paper. After removing aerial capacitors C2 and C3, I was just able to get my smallest iron, a Weller MCP with a cranked tip, under the switches to add solder to the switch pins and pads on the component side of the board. Fortunately the switch bodies seem to be made of thermosetting plastic and after a few tries I got the joints to 'wet' and confirmed that the connections were good with a meter check. I changed the LP1164 IF module transistors for AF125s while the module was accessible, although the original AF115/AF116s were ok. Ron |
19th Dec 2006, 11:54 am | #3 |
Heptode
Join Date: Jul 2006
Location: North Herts. UK.
Posts: 549
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Re: R707 Push Button Switch Board Assembly
Very interesting Ron. Do you think this radio could ever have worked properly before you sorted this problem?
It's got me thinking about an R600 I have with intermittent good/not very good FM reception, which could well be an aerial switching problem. I haven't delved into it yet, and I don't know if the R600 switch board is made up the same way as the R707, but it's clearly something to bear in mind. Thanks for posting this, and well done. Tom |
19th Dec 2006, 2:29 pm | #4 |
Heptode
Join Date: May 2004
Location: Ellington, Northumberland, UK.
Posts: 815
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Re: R707 Push Button Switch Board Assembly
Well done indeed, what a horribly fiddly thing they are to work on, but well worth the effort (or the cost of getting it done by someone with sufficiently mobile fingers)!
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John. |