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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 17th Jun 2015, 12:15 pm   #1
learnerTech
Retired Dormant Member
 
Join Date: Jun 2015
Location: Melbourne, Australia.
Posts: 1
Default Marconi CR300 issues

Hi all, I apologise as this will be long winded.

Hopefully seeking guidance/direction with my Marconi CR300, The story behind this unit is, it was in storage when floods hit my region somewhere between 1998 & 2002, and has been sitting in storage unattended until about 6 months ago when I took possession and saved it from a fate worst than death.

As you can imagine I had a job on my hands with this one, with the covers off, I can clearly see the flood water marks, what struck me is virtually every solder joint I could see was dry (I can see these joints giving me problems down track) I noticed a few wires had come away from their solder points due to very dry joints, these were resoldered to their respective points,Without even touching my multimeter I could clearly 3 x 470 ohm resistors with burn marks in the centre of their body's, I changed these three straight away. A fair number of other resistors were 20% or more high so I replaced those, there were still a few resistors which were fine these were left alone.

The heater wiring at some stage in its life had been changed to 6.3v so I rewired this to ship voltage 24V, I had to build a power supply, its giving around 240vdc B+ & 26vac for the heaters so I’m happy with that.

I don't have a variac and know there is a risk in firing the receiver up without one, after more checks I decided to connect the psu and flip the on switch, I was happy the dials lamps lit up, didn't hear anything unusual or smell anything burning, but the bands were dead, the only noise I heard was click thru the speaker when I changed phone/off/cw, that was promising I thought.

Power off, I restuffed with 630v red Mylars the 0.1uF 3 in 1 capacitor can closest to V1, power up the receiver again and bingo the AM broadcast &the lower half of band 4 came to life At this point I changed all of the 0.1uF 3 triple capacitor cans, when I powered up the receiver, this time, all the bands came to life, I can even get sideband on 80 & 40 mtrs, but there are issues now that I'm hoping some of the experts might be able to give some guidance/direction with..below

“HF gain only comes in at the 2/3 point when rotating, LF gain is near perfect

"Even with the HF & LF gain at minimum, there is a rumbling, raspy static like noise, low level which is always present & not the usual RF static like noise"

"At about 14Mhz on band 7 when tuning at that point there is really strong, really nasty squelch like noise that tunes in, is worst in CW than PHONE mode"

"The overall receiver gain appears to be lacking, in PHONE mode there is heaps of gain but in CW mode it seems to be down quite a bit"

I'm by no means a tech, no formal training just self taught, it would be great if anyone can give me some advice/guidance in trying to fix these issues.
Many thanks in advance for taking the time.
Cheers Ron
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