[time-nuts] 8 kHz problem in Wavetek/Rockland 5135A synthesizer

ed breya eb at telight.com
Sun Sep 16 16:25:20 EDT 2018

My (formerly) trusty 5135A crapped out a while back, and I've been 
trying to fix it recently. I have tried in vain to find some particular 
details on the remaining problem circuit. I haven't found any kind of 
manual for it, but found one for the 5120A, an apparently similar, older 

The initial problem was a blown main filter cap in the +5V supply, 
readily fixed to restore operation. However, there is now a persistent 8 
kHz step in the output - a 10.000... MHz setting puts out 10.00800... 
and so forth. There is also some interaction with the 10 kHz decade 
settings. I suspect that the +5V PS failure has damaged something in the 
large LSTTL DDS circuitry.

I have narrowed it down to the "DB1" board, which handles the 1 and 10 
kHz digits, and the output end of the DDS system. The 5120A seems to 
have exactly the same guts as the 5135A here, but there is no detail of 
the circuitry. It's stated as a "send it to the factory for repair" type 
deal, with no schematics or parts info at all, unlike the other sections 
and modules, which are well documented. I suspect that even if I find 
the proper 5135A manual, it will be the same story - no info on the DB1 
or its associated DB2.

I have shotgunned a few of the parts most closely associated with these 
two digits, and have been trying to trace out the actual circuits. Still 
no change in operation. The DB1 does the top two digits of BCD to binary 
conversions, and somehow the counting, sine ROM, and DAC, while the DB2 
does all the lower digits, all using mostly trees of 74LS283 adders - 
lots and lots of them.

So anyway, does anyone know of more info on the 5135A, or of how the 
guts may have been implemented in detail? Is there a repository of info 
on various synthesizers here somewhere? I thought I had seen in the past 
some listings, comparisons, and maybe documents about them here, but 
nothing popped up in my recent searching.


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