I hope it's not bad form to revive this old thread to add the repair log for my own Marconi 2024.
After remodeling my lab, I finally came around to take a closer look at the Marconi 2024 I bought late last year (thanks CatalinaWOW). It has four issues:
- 1 blank pixel line on the display
- The fan makes a racket after running for a while
- Error 508 Amplitude Mod unlevelled on most frequencies below 1.2 GHz when warmed up
- Inconveniently, all outputs are on the rear-panel (Option 5)
Luckily, the display is a very minor nuisance. I won't bother to fix that.
For the fan, I ordered a replacement (5V, 60x60x15mm) which should arrive soon.
The 508 error appears on and off across several bands, which rules out the band pass filters. I suspect that diode D511 is the culprit. It tests ok with a similar voltage drop as D510. But when I cool it down the error disappears, and when I heat it up, the error appears at more frequencies. I ordered a replacement (Broadcom HSMS-8101) and will report back.
Judging from big solder blobs, it looks like someone already replaced D510 and the adjacent resistor and capacitor. And while poking around, I panicked a bit when I came across this:
But looking at the pictures from Dave's teardown, this seems to be a factory original bodge. I think it bypasses what the service manual calls "printed 2.5 GHz L.P.F." (Sheet 7).
To convert the Marconi back to front-panel outputs, I ordered custom cables from
coaxrf.com. There might be cheaper options, but they are reasonably priced, decent quality and fast (if you're located in the US).
For the RF output I used a 8" RG178 cable with angled SMA male and angled N blukhead connectors. The length worked out well. 7" might work but could be tight. Better long than sorry.
I left the rigid coax to the back in place. Anyone has a rigid coax for the front panel that they want to trade? Also, any thoughts on the impact of this change on calibration?
For the LF Output and Ext Mod Input, I ordered 13" and 25" RG178 cables with straight BNC bulkheads on one end. Both are a bit on the long side, but see above. There are pin headers on the PCB to mount these inputs, so the bare ends of the coax cables were stripped and soldered to 3-pin male 0.1" headers.
A bit of heat shrink and everything looks nice and tidy. The BNCs for the rear outputs are soldered to the board and seem to be routed directly to these headers, so I don't think there are any further changes needed on that end.
Stay tuned for an update on the 508 error.