Re: Marconi NS702 / 2207E, SN 623 (Atalanta type) receiver
 

Hi all. There are two rectifiers shown on the bottom right of the picture how do you test them ? I have done a diode test with a multimeter they don't read anything .

Re: Marconi NS702 / 2207E, SN 623 (Atalanta type) receiver
 

I believe they are single plate Selenium rectifiers. There may be 7 of these in your receiver.

They are used for AGC, noise limiting and 'grid desensitising' functions.

The forward voltage drop may be about 5V and that would explain why you get no reading on the diode test. The reverse breakdown voltage may be as low as 20V.

If you desolder one end of each diode you could test them with a 9V PP3 battery wired in series with a 10kOhm resistor.

The AGC and noise limiter circuits may have been designed to make use of their characteristics.

If you replace them with a typical signal diode such as a 1N4148, you may alter the performance of the receiver.

Re: Marconi NS702 / 2207E, SN 623 (Atalanta type) receiver
 

Thanks for the info I had already ordered the 1N4148 diodes. I can't find any info on the rectifiers in the receiver would the cathode side be the one marked red on the rectifier or is that the anode ?

Re: Marconi NS702 / 2207E, SN 623 (Atalanta type) receiver
 

RED should be Cathode.

Re: Marconi NS702 / 2207E, SN 623 (Atalanta type) receiver
 

Thanks Hamish Boxer the receiver is working but the AGC is a bit hit and miss .

Re: Marconi NS702 / 2207E, SN 623 (Atlanta type) receiver
 

Just for general info on the diodes:

https://www.vintage-radio.net/forum/...4&d=1271872981

Lawrence.

Re: Marconi NS702 / 2207E, SN 623 (Atlanta type) receiver
 

Some info thanks Lawrence so the anode side is the disc side (red) .

Re: Marconi NS702 / 2207E, SN 623 (Atlanta type) receiver
 

Anode is the metal case.

Lawrence.

Re: Marconi NS702 / 2207E, SN 623 (Atlanta type) receiver
 

Cheers .

Re: Marconi NS702 / 2207E, SN 623 (Atlanta type) receiver
 

Hi all. Just to update on what's been done to the receiver the caps in the mains filter have been changed with y type capacitors and a new mains lead fitted . Quite a few 0.1uf capacitors were physically leaking and others measured high so i changed all of them on the various boards along with some others . The small value caps in the rf , oscillator, amp sections have not been touched . The caps on the output transformer and rf filter have been changed . Then checked the resistors again a fair few needed replacing . All the valve sockets had a good clean and the valves checked all okay except a low ECH 81 which I will replace . I had already changed some rectifier's . After this the receiver works quite well but not as it should the AGC was all over the place and the noise limiter didn't work so out came the AGC can I found it very fiddelly to remove but was careful on doing so . A couple of resistors were four fives times higher than than they should be so changed them also replaced two rectifiers with diodes and the refiitted the can . The set now works so much better the AGC is fine and the noise limiter works the rf gain is better as is the volume even with the small internal speaker i still have a few things to sort out but on the whole very happy with it .. The only problem I have is that band 6 is dead ! can anyone help with this as I am stumped cheers Chris .

Re: Marconi NS702 / 2207E, SN 623 (Atalanta type) receiver
 

Just to add the band switch has had a good clean .

Re: Marconi NS702 / 2207E, SN 623 (Atalanta type) receiver
 

Your "Band 6 dead" issue - can you confirm that the local oscillator is indeed oscillating on that band?

(Either use a 'scope, or - my preferred approach - use another receiver to listen for the local-oscillator; I prefer this method because sometimes the added capacitance of a scope-probe can be enough to stall a LO).

Another test-trick is to lift the grounded end of the LO grid-resistor and put a microammeter in there to measure the grid-current - this will also give you a feel for how the LO's 'perkiness' varies across the different bands/frequencies.

If the LO isn't oscillating, and 'band 6' is the highest frequency-range, first thing I'd do is swap the LO valve - a somewhat-tired valve may have enough gain at lower frequencies but not enough to sustain oscillation at the higher frequencies.

Re: Marconi NS702 / 2207E, SN 623 (Atalanta type) receiver
 

Hi Tanuki. There are 10 bands in total when you switch to cw on band six you can hear more but no signals .

Re: Marconi NS702 / 2207E, SN 623 (Atalanta type) receiver
 

Ah, that suggests the LO is working but you've got a gain-reduction issue in the front-end signal-frequency circuit of band-6.

One thing quick and easy to check is the continuity of the antenna-coupling winding on band 6; in the past I've seen receivers with this burned-out (after a lightning strike?) on one band, causing deafness.

If it's not that, then check the DC resistance of the various coil-windings on band-6 then compare the values with those for the windings on adjacent bands to see if they're similar. That your receiver works OK on other bands means the problem should be easier to identify than total-failure!

Re: Marconi NS702 / 2207E, SN 623 (Atalanta type) receiver
 

Following further investigation I checked continuity from the band switch to L7 the aerial coil that's fine a 220k resistor was reading 290k so I changed that on the coil the 150pf cap read okay. Next L16 in RF1 the switch to the coil is fine another 220k was reading high 270k so changed that. The coils L25 and L35 are ok as are the band switch to the coil there is improvement but no signals .. Band six works with a LF rejector that is only used on band six could the issue be that ?.

Re: Marconi NS702 / 2207E, SN 623 (Atalanta type) receiver
 

Any chance you could post a snapshot showing the LF-rejector circuit? I'm familiar with the classic "IF wavetraps" - series- or parallel-tuned L/C combinations fitted in the antenna leads of some receivers to prevent breakthrough, but not with the specific circuit in your receiver.

Re: Marconi NS702 / 2207E, SN 623 (Atalanta type) receiver
 

The 700kHz rejector is a parallel tuned job and it's in series with the primary of the RF coil that's connected to the anode of the 1st RF, it's connected between the "cold" end of the primary and HT, those two windings in series as a whole have a 680 ohm connected across the whole between the anode and HT, so far as I can make out.

Lawrence.

Re: Marconi NS702 / 2207E, SN 623 (Atalanta type) receiver
 

Hi Lawrence Is the RF coil L1 ?

Re: Marconi NS702 / 2207E, SN 623 (Atalanta type) receiver
 

L1 (plus C1) is the 700kHz rejector, I'm looking at the Atlanta 2207c schematic.

Lawrence.

Re: Marconi NS702 / 2207E, SN 623 (Atalanta type) receiver
 

Ah got it I will do further checks tomorrow eyes getting tired .