[Lowfer] HD-1420 more tinkering--Sorry, here's with stops....

[Michael Sapp]

Hi All: 

Contunued tinkering with this HD-1420 converter. Not sure if I mentioned on a previous post, 
but it turns out there is no output matching from the MC1496 active mixer in the HD-1420. Data
 sheet says this IC has an output impedance of about 40K....the resistor biasing used may have 
some effect on that (my personal guess, anyone have an informed opinion?). Mixer is run
 single ended, which is a shame to give up the DBM isolation improvement potential. Anyway, for
 a test I decided to try a 10.5:1 BN73 transformer on the mixer output after the output coupling
 capacitor, and I was immediately rewarded with 3~4 S units of improved IF signal level. 
The transformer on the IF output also appears to have improved the RF to IF mixer isolation.
 Previously, there was always some slight hint of 75m signals present with strong 75m stations. 
That changed as I dialed the IF up an a 75M AM station that was full scale S reading on my FT-847
 and HF vertical, yet no IF leakage at all on the same IF frequency through the EWE ant>HD-1420>FT-817---
nice to see a SWAG attempt at impedance matching actual do what the textbooks say and see 
the improved port-to-port isolation as well as improved converter gain.


Next I decided to poke and prod at the LPF between the mixer and preamp. The HD-1420 performance 
seems to roll off gradually above 430 kHz or so, and more sharply at the 600m band. Since I run an
 external BC LPF in front of the converter, I tried pulling the 680pf SM caps in the filter and replaced 
them with 560pfs. That brought up the AM band signals somewhat---some where s9+20, so I tacked 
on a extra 47pf to get into the 610 pf range which kept the strongest AM stations S9 to S9+10. I'm going
 back to the 560pf caps tho, as the filter roll off seems to be attenuating the 600M band at 610pf.

So I popped the lid on the HD-1420 this afternoon to make that change and decided to pull the LPF 
inductors and measure them. They are marked Blue-Grey-Brown---680uh. BUT, my LCR meter shows
 them in the 435~440 uH range. At first I thought my meter might be off---but I happened to have five 120uh
 molded chokes in my parts bin. Each measured close to 120uH, so I tack-soldered them all in series and
 came up with 578uh---4% off of 600uH---so my meter is not suspect---but those three inductors in the
 HD-1420 sure are......

Time for me to model that filter and figure out what's going on. Moral of the story is to measure 
everything possible. I'm heading back to the bench to put the converter back together for another evening
 of WSPR on 600m. I ran my FT-847 on 600m last night with many more decodes than what the HD-1420
 allowed on prior nights.....although the HD1420 is working fairly well from 50 to 400 kHz or so....and decent
 on 137kHz on QRSS60 Sunday evening. Thanks for listening.....

73 Mike wa3tts