[Premium-Rx] Fw: Racal 177x PSU

[Michael O'Beirne]

>
> Gents
>
> Before we condemn the RA177x series (or other similarly aged receivers for 
> that matter) I do think we should bear in mind that they were professional 
> products produced for the professional market where the average in-service 
> life was unlikely to exceed 15 years.  That is why after their 
> professional service life they were usually sold off for a song.  Absent 
> the enthusiast market, most of this stuff was long ago written off by the 
> accountants and destined for the dump.
>
> That they continue to give excellent service 30 years on is a testimony to 
> their design and build, though I do feel that the PSU is almost an 
> afterthought and "could do better" is an appropriate end-of-term report.
>
> I suspect John's proposal of a blowing the big heatsink externally will 
> reduce the problems significantally.
>
> I am lucky because my RA1772 looks almost brand new internally and 
> externally, but I remember seeing some RA1779s from M&B, Leeds, many years 
> ago with very distressed internal wiring.  I suspect they had been running 
> in a hot rack for months at a time.
>
> I suggest that some of Nigel's comments are a little harsh.  There was in 
> fact quite a time span between the last of the valved gear (typically the 
> RA17) and the RA1772.  There were the rather poor performing RA217 series 
> in between, though I suspect that their problems were more the trouble of 
> early transistors rather than of inherent design defects.
>
> 73s
> Michael
>
> ----- Original Message ----- 
> From: <GandalfG8 at aol.com>
> To: <premium-rx at mailman.qth.net>
> Sent: Sunday, August 08, 2010 6:26 PM
> Subject: Re: [Premium-Rx] Racal 177x PSU
>
>
>> In a message dated 08/08/2010 17:52:25 GMT Daylight Time,
>> john at crew-green.com writes:
>>
>> Replacing D2 may be worth doing, especially if one can find a  Schottky
>> bridge, but there is another PSU modification that's even more 
>> important.
>> All the RA177x family are excellent from an RF point of view but  the
>> youngest is now around thirty years old and many have led hard lives  in
>> professional service. Unfortunately some aspects of their mechanical  and
>> electrical design were not conducive to longevity. For example, the  main
>> wiring harness between the mains transformer, the rear-mounted PSU 
>> board,
>> the reservoir capacitors and the rear heatsink carrying the PSU  pass
>> transistors was executed in PVC-insulated cable which in service  became
>> very
>> hot. Over the years this caused the plasticizer to leach out  and 
>> vigorously
>> attack any copper it could find. The result is extensive  radial cracking 
>> of
>> the insulation and corrosion of the wiring and  associated terminals. The
>> cure is to rewire the entire harness in modern  PTFE cabling, which is a
>> chore but not inordinately difficult and allows  easy replacement of the
>> reservoir capacitors at the same  time..................
>> -----------------
>> John
>