Hi All,

My first post on the forum, I'm hoping someone may be able to offer some help & guidance with a Marconi CR300/1 receiver.

I apologise for this being long winded, This particular receiver was in storage when floods swept through my region somewhere between 1998 & 2003, it was completely covered by the flood waters and has sat in storage ever since until I grabbed it late last year. It was complete but minus the PS unit.

I wanted to get the set restored and set about giving it a thorough visual, replaced 3 resistors to start with as these 3 had burn marks in the centre, I had to build a separate PSU for the set and with that built, I fired the set up, I knew it was risky without a variac, but at least when the power was switched on there was no crackling or sizzling noise no smell of smoke, the only sound was some crackle when changing from Phone to CW setting.

I then decided to change all decoupling & coupling capacitors after this was done switched the set on again, this time the broadcast band came alive but also heard a hell of a lot crackling noises, the other bands close to dead.

I decided to change all resistors, When I got approximately a quarter of the resistors changed I switched the set on again CW & SSB came up on 80 & 40 mtrs but sill a lot of crackling noise through the speaker could be heard.

I've now changed all resistors and some of the small value caps in the Aerial, HF & Oscillator assemblies along with new caps in the IF's 1 - 4.

the voltages thus far

B+ 235 vdc

V1 V2 V3 V4 V5 V6 V7 V8

Pin 1 nc nc nc nc nc nc nc

Pin 2 6.8 6.1 6.3 6.2 6.5 6 6.2

Pin 3 215 213 211 192 150 0 0

Pin 4 68 74 103 98 0 0 0

Pin 5 0 15.6 15.6 3.4 0 0 0

Pin 6 nc 103 nc nc nc nc

Pin 7 6.8 6.1 6.3 6.3 6.5 6 6.2

Pin 8 14.8 2.4 15 3.4 2.8 0 0


In CW mode I hear SSB but BFO cannot decrypt/decipher the signal, I can clearly hear SSB but the signal is muffled/unintelligble.

I've also changed some of the low value caps in the Aerial, HF & Oscillator assemblies and have changed the caps in IF's 1 - 4, waiting on caps to arrive to change 5 6 7 IF's

The HF gain control only kicks in at the 3/4 point when turning it.

The AM broadcast band seems ok but bands 4 through 8 don't seem active at this.

Manual is here http://www.vmarsmanuals.co.uk/archive/1678_CR300_Manual.pdf

So I'm hoping someone may be able to give some guidance on where to next with this. I'm not a electronics tech just a hobbyist


Any help would be greatly appreciated,

Cheers Ron

I am not wrapt in your methodology. The American term for this is shotgunning.

As it has been in flood water that there is likely mud & debris inside tuning gangs, IF transformers and trimmers. These would need cleaning. And dirt and corrosion on the tuning gang plates is a common cause and you may have bent a plate when fixing.

To me you have changed parts that are unnecessary & liable to throw coils off frequency, some are not standard values and that was deliberate.

If it had paper caps these & any remaining electrolytics should have been changed and the resistors checked as you went, changing those that were damaged or out of spec.

There may be water in the speaker transformer arcing but we have not got enough detail including circuit.

The fact it did not crackle until the last time you touched it, would suggest that, this is the point that you introduced a fault and it is that which needs checking, perhaps you failed to solder a joint or there is corrosion on valve terminals or sockets (if it is valve) or one is not in properly?

Thanks Marcc for replying, I do appreciate it.

good news to start with, I've got SSB back in the CW mode, more on that in moment.

The circuit can be found here via this link, www.vmarsmanuals.co.uk/new/cr300.htm.

The Marconi CR300/1 is an 8 valve set (9 if you count the rectifier of the separate PSU) it’s a LF/MF/HF from 15 kc to 25 kc, it has two IF’s of 570kc’s & 95 kc’s to allow coverage of the range without gaps.

The normal Valve lineup is

V1 – KTW61 signal freq amplifier

V2 – 6K8 frequency changer

V3 – KTW61 IF amplifiers

V4 – KTW61

V5 - DH63 second detector, AGC rectifier & LF amplifier

V6 - KTW61 BFO

V7 - 6V6 output

V8 - KTW61 calibration oscillator

However, I was surprised of the valve lineup in mine,

V1 – 6U7G

V2 – 6K8

V3 – KTW61

V4 – 6U7G

V5 – KTW61

V6 – 6B8G

V7 – 6V6

V8 – 6U7G


V1, V4, V6 & V8 are 6U7G’s V2 is a 6K8 with a 6B8G as the double diode in V5, upon checking the valve data sheets for the 6U7G’s, 6K8 & 6B8G are straight drop in replacements from what I can see in the data sheets.

The set requires a power supply of 250vdc at 60ma & 24v ac at 1amp for the heaters, at some stage in the past the set had its heater supply changed from ship voltage of 24v to 6.3, I have since converted this back to 24v, in the circuit the heaters are aligned in two rows of 4 valves each in a series parallel configuration.

Resistors – When I first started checking & replacing them, the vast majority were either out of spec or right on the limit, (I know resistors from back then had quite a wide tolerance) and they seem to be the original resistors from day 1 which was approximately 1942, I did measure all with one leg unsoldered.

Tuning gang/ Capacitor – I’ve not touched this, not even an attempt to clean it, it is in surprisingly good shape considering the circumstances and age.

Decoupling capacitors - The set has a series of 3 in 1 canister capacitors at 0.1 μF, (there’s approx. 11 of these canisters, 21 caps in total from menory) When I pulled these out I found them to be wax & paper, So I’ve restuffed these with modern equivalents.

Valve sockets – These have been cleaned as best I can as when I removed the valves silt from the flood waters had settled in and around the base and socket, I used a small stiff bristle brush with electronic contact cleaner, spraying the pin holes as well, One issue that concerns me is there may be a cracked valve socket of which there may be an intermittment spark which could cause issues?

Other Capacitors - I've changed all low value ceramic type caps in the Aerial, HF & Oscillator assemblies, there are a number wax covered large value caps in these assemblies, I've been told they are mica and are generally reliable so I haven't touched them at this point.

A lot of crackling static type noise I referred to in my first post seems to have gone, this could due to the number of capacitors I have changed so far although there is still some pops as I tune around a band but nowhere near what there was.

The set has 8 bands, band 1 & 2 don't appear to be alive, band 3 & 4, I can hear the aeronuatical beacons on the lower end of band 3 which is the broacast. band 4 has the 80mtr ham band which seems seems to be ok, band 5 is alive, 6 7 & 8 appear to be dead so some work need to happen on those.

The set has an AGC,In Phone mode the bands which I can receive well on at the moment only appear to be receiving quite well with the AGC on, when the AGC is switched off the gain drops considerably and its only the AM broadcast band that I can hear anything on, the same applird when the set is swtched to CW mode.

Now back to the good news for tonight, looking at the circuit I changed the coupling cap between pin 4 of V5 which couples pin 3 of V6, this cap was a 4.7pf, the manual has that particular cap as 5pf, it was after I changed this cap tonight that CW SSB came back, Whether that cap was leaky or simply the wrong value, I'm not sure but CW SSB has now returned.

So thats where I'm up to at this point in time, any advice you may offer is greatly appreciated.

cheers Ron

Hi all,

An Update on the Marconi, In one of my posts I wrote 2 errors with the valve types V5 & V6, V5 is a 6B6G second detector, AGC rectifier & LF amplifier while V6 is the KTW61 BFO.

In my earlier post I had regained SSB in the CW mode earlier in the week then I lost it again, this prompted me to look really closely at the circuit and specifically that of V5 & V6, V6 was fine although it took me ages to work out the physical layout of the components.

However, with V5 In the circuit there is a lead that connects pin 11 of switch 9 and was connected to pin 5 of V5, also the lead from the AGC switch through R45 and was connects to pin 4 of V5, according to the circuit, these
connections at pins 4 & 5 of V5 were in fact back to front, I resoldered the leads to the correct pins, when I switched the set on again this correction of wiring brought all bands to life, it also increased the overall gain of the receiver considerably, nearly all of the popping/crackling/static like noise I referred to in one of the earlier posts has virtually completely vanished.

I still have an issue with HF gain control with the gain only coming on at the 2/3 ‘s point when turning the knob, no problem with LF gain control it works its full range, I still have fair bit of tweaking, rechecking solder joints as per Marcc’s suggestion earlier.

Overall I am very very happy with the set at this stage, I was listening to the 80 mtr band late into the night and I am quite surprised at how stable the frequency is, it didn’t drift.

cheers Ron

These old communications receivers can be quite nice.

I looked up the specs of the KTW61 and 6U7G and while very similar they are not electrically identical. The KTW61 operates over a much larger range of grid bias (approx -1 to -30 volts) versus the 6U7G (approx -1 to -10 volts). This would explain the lack of RF gain control on the receiver as the 6U7G is fully cut off at an RF gain setting that would still have the KTW61 still operating.

As a temporary measure swap the BFO and RF amp valves, this at least will restore some (or all) of the lost control of the receiver gain.

6U7 is a horrid tube & its 6pin predecessor (same tube different base) was just as cranky. It just loves picking up as much stray RF as is possible & using it to destabilise everything.

The American numbered valve you need is 6J7 sharp cutoff Pentode 6U7 is a remote cutoff Pentode enough difference to get you into trouble.

Scratchy noises when tuning are more often than not dirty plates, or bad wire connections.

Beware of pin1's on most of the octals (not all.. check) the valves without a "G" suffix will have shields & that, like most metal octals ties to the chassis via pin 1.

Marc

€�As a temporary measure swap the BFO and RF amp valves, this at least will restore some (or all) of the lost control of the receiver gain.”

TV collector – I’ll try this tip later on tonight


“Beware of pin1's on most of the octals (not all.. check) the valves without a "G" suffix will have shields & that, like most metal octals ties to the chassis via pin 1.”

Marc – this is one that I’ve been especially aware of from most of the reading & research I’ve come across for this receiver, Particularly when I re-stuffed the 3 in 1 canister de-coupling type caps, I made sure the earth went to where it came from, as for the 6J7 you mention I think have two of these at home which I’ll check out.


So I spent last night again attempting to correct the RF gain pot, I swapped the leads of pins 1 & 3 on the pot, I found this did restore the HF gain to almost the entire rotation of the pot albeit in the wrong direction, which leads me to believe the original pot may have been an RA / C type pot ??? I had discarded the original pot in the set as it was completely rusted out and assumed it was an Log pot, the other point could be changing the capacitor C102 across the output transformer primary, I changed this to a .0047μF, the original was .005 although it probably wouldn’t make that much difference?

Anyway I’ll revert the leads back as per circuit diagram and try TV collectors suggestion, However, before I can do that, I think I may have a big problem on hand?, last night I noticed at the bottom of the HF assembly some small writing in pencil with the word “short and an arrow” the writing was really hard to see, partially covered by the leads to the other 7 coils in the assembly, pointing to RF coil L6. (in different sections of the service manual L6 & L16 are the same unit, just to confuse anyone).

If indeed L6 does have a short it may explain why 1 or 2 of the bands have been coming & going, It could also explain an anomaly when I tune to the top of band 6 which covers 6.5 MHz to 16 MHz, when I reach the 14 MHz point a very strong electrical type noise is heard, it’s like a flashing/buzzing sound of a live wire that is striking the chassis, when this happens the gain drops off dramatically then returns to normal after a short period, this noise doesn’t sound good at all.

Here is a link to some data sheets for KTW61 & DH63:-

Osram KTW61 Valve
Osram DH63 Valve

The data sheets describes the KTW61 as a variable mu screened tetrode. Having a suppressor grid means it is really a pentode.

You'll also see that the DH63, though very similar, is not exactly the same as the 6B6G. A 6Q7G would be a much better choice as a replacement for a DH63 as its specs are virtually identical.

I Played around again last night with the HF gain, firstly I tried an 10k Reverse Audio or C type pot which didn’t make any difference at all, I then refitted the type A log with no difference, I then changed the pot to a 10K linear and also implemented TV Collector’s suggestion of placing an KTW61 in the RF amp socket, this certainly made an improvement!

With the set switched on in the PHONE position (AM) it’s now giving me close to the full rotation of the HF gain control not quite full but close enough that I’m happy with, I can now use LF & HF gain in conjunction with each other.

Now, with the set switched in the CW (ssb) position, this is where the difference is noticeable, although I haven’t gained the full rotation of the RF gain I’m now getting at least half the rotation of the HF gain which is way more than what is was, I’m happy with this now that the gain controls are functioning close to what it should be, Hopefully I can sort out the other issues which may return HF gain to its full.

As per Monochrome’s suggestion of the 6Q7G I can get my hands on one of these, so I’ll give that a go as well.

While tonight, I’ll have a closer look at the coil in the HF assembly I mentioned in my last post, I know there is no sure fire way of determining if a coil has a short.

Sometimes you can be lucky & the circuit diagram has the resistance of the coils. More often than not, if there is a trimmer & a Monkey involved, they wreck the trimmer. There is always the risk of a corrosive break on terminals, or wire breakage on coils when parts are changed.

Marc

Hi all,
Here is an update on the progress for Marconi, I really 2 or 3 issues left to deal with which seem to have been resolved.
I mentioned in one of my previous posts of a really bad noise when tuning up and just past the 14MHz mark, I think I have resolved this issue as when I swapped the RF amp valve, that KTW61 that I placed in the RF amp as per monochromes suggestion, the valve’s glass envelope had come loose from its base, So I placed the other KTW61 I had in its place with the result of that noise now gone.
I did also mention in my last post that I thought I may have a short in one of the coils in the HF assembly which I thought was related to the bad noise at 14MHz mark, However for the life of me, there doesn’t seem to be any other issues with the set now.
The only other issue is I have a really noisy LF1 transformer, the hum coming from that unit is quite noticeable, I don’t what to do about it? Knowing the circumstances of this set I’m pretty sure the floodwater has either loosened the lacquer or laminations and they are vibrating, Would it worth trying to re-lacquer the coil windings? If I have to can live with it.

Apart from this, the set seems to be running really well, I think I may be ready to try a re-alignment.