Marconi 2024 not powering up

[stahodad]

Hello from Canada. I am working on a Marconi 2024 with power supply issues. Power is getting to the PS board, and the fuse and switch are good. There is power getting about half way through, (some of the big caps have 162 volts) but I haven't been able to trouble shoot further. (even after being unplugged for 20 minutes I got some noticeable shocks).

I have the manual but it does not seem to have a schematic or diagram of this type of power supply. The power supply does not have any noticeable output when plugged into the wiring harness or unplugged, (so It is not a loading issue) .

The puzzling part is I measure the same small voltage (30 millivolts. sometimes 300 millivolts) on each pin, even the pins for the black wires.

I don't see any burned components or other obvious damage. Any ideas would be a great help. Thank you, Have a good weekend. Cheers Chris.

[jjl]

These are classic symptoms of a switch mode power supply that is not "switching". Assuming you have around a 110V mains supply in your location, the large caps that are charged to 162V will be on the primary side of the supply and are supplied with full wave rectified mains.

Often in supplies such as these, there is a high value resistor on the primary side that provides a "startup" supply to the switching circuitry. Such resistors which are often around 120K in value have the habit of going open circuit thus disabling the switching circuitry. I'd (safely) discharge the large capacitors then have a look for such a resistor and measure its value.

It would be a good idea to obtain the service manual for your signal generator.

Please be careful - mains powered equipment and switch mode supplies contain hazardous voltages.

John

[chriswood1900]

The only Service Manual I have ever seen for these treat the PSU as a replaceable part and include no information or circuit diagrams. So I suspect it will have to be basic fault finding of something that was probably bought in..
Chris

[chriswood1900]

BTW did you find this thread
https://www.vintage-radio.net/forum/...d.php?t=125027
It may be a bit of help.
Chris

[G0HZU_JMR]

Please be careful - mains powered equipment and switch mode supplies contain hazardous voltages.

Agreed. I think some of the exposed metal chassis parts are live and this includes at least one of the heatsinks.

I'd also recommend wearing safety glasses when working on this PSU if it is live. I've seen what happens inside these generators when one of the big 100uF 250V capacitors explodes. I wouldn't want to be up close to one of these capacitors if/when this happens.

[ekjdm14]

I'm afraid I can't offer any further advice to what's posted above, but have to ask is the PSU a genuine/original item or a replacement?

Reason I ask is, I noticed on the PCB in photo 4 the silkscreen reads "WARING" as opposed to (presumably) "WARNING"... Is there any possibility this is a cheap knock-off supply, or just careless manufacturer?

[turretslug]

I'd wondered if a former rugby commentator had had another string to his bow, too, but I think it's yet another harmless and inconsequential piece of Far Eastern English (aka Chinglish, though not restricted to Chinese-made stuff). Even highly-respected makers came up with the occasional howler.

It looks like the sort of thing that Astec made, Marconi would have taken a bit of care over something like this, especially if they hoped to supply the MoD as they did with so much stuff.

[G0HZU_JMR]

My Marconi 2024 PSU also had the same (Eddie) 'WARING' typo on the main PCB so I think it is just a mistake on the silk screen.

[stahodad]

Hello and thank you for all the advice with this Generator and the warnings about safety.

That makes sense that something is just preventing the switching from taking place but everything "looks" ok. There are some big resistors after those big capacitors that I will check out.

I have located a maintenance manual PDF (184 pages) from 1998 and it looks promising. The last 7 schematics are of the different power supply modules, and it has some block diagrams. They are for a Powertron LTD. power supply (address Ainsworth Place. Cambridge. ) I will see if it matches this power supply and go from there. I will let you know how it goes. Cheers Chris.

[G0HZU_JMR]

If you look at your first photo image you can see the manufacturer's name and model number and serial number on the side of one of the transformers. It shows this is a Coutant HVI60-47 PSU made in 9637 which is probably week 37 in the year 1996. The serial number is 0145.

I don't think this is the same as the PSU in the older 2024 service manual.

[Dickie]

The Powertron power supply was a different beast, and only fitted if the customer ordered the option that allowed for low voltage DC operation.

[G0HZU_JMR]

Yes, I think the regular PSU is a Coutant model similar to this:

https://teamequip.com/lambda-coutant...upply-t121840/

I don't know what voltage range these things work over so this one might not be quite the same.

[stahodad]

Thank you for the correction and the additional information. Yes, This one does not have the DC power option. The one in the link looks just like it. Cheers Chris.

[stahodad]

Hello again. I got back to work on this power supply, after thinking over all of your advice for a few days. The capacitors seemed ok (none were shorted), so I started checking the resistors in the area after the 4 larger filter capacitors.

I tried measuring resistors in circuit, but the results were inaccurate. I had better luck unsoldering each of them and testing one by one. As luck would have it I found both R3 and R2 100 k ohm resistors were open.

The colour code was odd with the reddish brown resistor body made the Brown stripe look pink to me, but I have installed 2 new 100 k ohm resistors and the generator powered up just fine.

I tried measuring power supply output pins without the wiring harness installed, but the outputs were very low. I think that is because the rest of the generator circuity loads the power supply.

Once the new resistors were installed I also noticed the large capacitors no longer stored a charge after power was removed, and that would partly explain why I was getting shocks when I worked on it the first time.

I am not really sure why the resistors failed, hopefully it was just age related on this 20 plus year old power supply.

I did not locate the schematic for this power supply but it was nicely labelled, on the non soldered side, so that did help identify components like L4, a small inductor.

Once again, I would like to thank everyone for their help with this generator.

My confidence in troubleshooting has been given a little boost!. Cheers Chris.

[jjl]

Well done on repairing your PSU Chris. This a very common fault with switch mode supplies of a certain age.

I think that it's most likely that these resistors fail because they are presented with more or less the full peak mains voltage until the small capacitor downstream of them charges up, their dissipation limit may not be exceeded but they experience unusual stress for a few tens of milliseconds. Better quality PSUs seem to use special high voltage rated resistors for this function, but they still seem to fail.

I replace failed resistors in this position with 2W rated metal film types.

John

[G0HZU_JMR]

Yes, a great result... well done!

This failure mode is well worth knowing for the future and I suspect quite a few of these PSUs will fail like this.

I don't have images of the colour codes for these resistors for my Marconi 2024 PSU but my PSU failed because the big 100uF 250V cap (with the red caution sticker on it next to your failed resistors in your image above) decided to vent itself quite spectacularly. I was surprised just how much white steam was then vented out of the back of the sig gen by the internal fan! This went on like a mini smoke grenade for many seconds. Amazingly, the sig gen (display) was still working when this was happening and also it was fortunate I was in the room when it happened so I could cut the power.
Inside the sig gen the cap had burped out a lot of hot oily stuff and it was all over the insides of the PSU cover.

I replaced all 4 caps and I keep meaning to measure the ESR of the three (healthy?) survivors because I think this may be important when looking for replacements. I plan to measure at 100Hz, 1kHz, 10kHz and 100kHz but I guess the 100Hz value is the most important. I did try and find low ESR replacements but I fitted them in haste and forgot to measure them first!

Note that the clear gloop that holds these caps in place on the PCB is very difficult to remove. I had to slowly and carefully cut it away with sidecutters and it took a long time.

[Pamphonica]

For longer-term reliability I would replace all the electrolytics in the power supply if it's currently "on the bench". The localised heat in those switchmode power supplies dries the caps out and shortens their lives.
I have found that caps from reputable suppliers (RS, Farnell etc) rated at 105 degrees C seem to work very well. All have good low ESRs.
I have even had to replace 6 surface-mount tantalums recently on a single-board 500MHz oscilloscope (Tektronix) to get it working agains. Caps, caps, caps....!
The Marconi 2024/2025 sig-gens are actually very good workhorses, as the military found out. They bought them in large numbers.

[G0HZU_JMR]

Yes, the 2024 can be a very good signal generator for certain tasks. In terms of phase noise it is at its best across carrier frequencies of 10MHz-20MHz. This is because it uses division to get down to these frequencies. This puts it in the same ballpark as the very best lab sig gens for close to carrier phase noise in this frequency range. Up at higher frequencies towards UHF the close in phase noise performance of this sig gen is only average.

The other key feature is that the on board DSP based modulation is referenced to the main 10MHz reference so if you buy a 2024 with the accurate/expensive OCXO option the onboard modulation is extremely accurate in terms of the modulation frequency. A 1kHz modulation tone should be accurate to within a few uHz for example. This can be very important in some cases where you want to lock everything (including the modulation) to a common frequency reference. It also has an option for a high performance pulse modulator and this can be very useful if you want to generate pulsed signals that are 'blanked' to a very low level with accurate timing. Back in the day, these features set this sig gen apart from a lot of the competition in this (medium) class of signal generator. Even today this sig gen will outclass a lot of modern alternatives in these areas.

The weak points in terms of performance are in the ALC/levelling because the amplitude accuracy (vs frequency) for this sig gen is fairly mediocre by todays standards. It wasn't that great 20 years ago and I think the designers of this sig gen got this bit wrong. Also, the ultimate noise floor is average at best and I think it is not as good if you buy the high power option that goes up to +25dBm. I'd recommend using the attenuator lock feature to try and squeeze the best out of it here. Otherwise, the wideband noise floor performance is less than impressive on the +25dBm version of this sig gen. I think this is because they compromised the gain distribution in the signal path to accommodate an ALC system that supported up to a +25dBm capability.

[stahodad]

Hi Jeremy. That is some good information on these generators. Yes, Caps caps caps, indeed!.

On another power supply repair (Blowing fuses) I had a diode that was completely unsoldered and dropped free from the power supply board, (PS Board was mounted upside down) which led me to a shorted cap. The diode was still good, and went back in. That was a fun one. Thanks to you, John, and everyone else for their help. Cheers Chris.

[Radio Wrangler]

The military bought lots of the 2024 sig gens and then released them onto the surplus market. Many of these have an option set including:

Ovened reference oscillator
Fast pulse modulator
+25dBm output amplifier
2.4GHz coverage.

You can spot them from the front panel because the bottom left BNC is the input for the fast pulse modulator, and the AM/FM mod input BNC is at top right.

The base model 2024 has the AM/FM mod input BNC at bottom left.

So if you see 'pulse' written above the bottom left BNC on a 2024 in the UK, you probably have an ex-military unit with the option pack they had in their contract.

As Jeremy says, these are good, very versatile sig gens. We have a lot of them at work. Calibration fails are mostly OCXOs ageing a little out of tolerance. They aren't the best for adjacent-channel measurements, though, and I break out the HP8662s.

High broadband noise floors are a failing of most modern signal generators, especially those with complex modulation capabilities (I&Q) because of the level the modulators had to run for linearity and the sheer amount of gain needed after them. This limits dynamic range measurements on broadband receivers via the noise versus noise reciprocal mixing process which is quite nasty and not common knowledge.

250v reservoir capacitors are too low a rating for use in a PSU with a 240v input range. I suspect they are used in series pairs to get 500v rating and also the rectifier might be switched to bridge for 240v and voltage doubler for 120v ranges. This is common for higher power supplie as it makes better use of the energy storage capacity of the reservoirs. Low power supplies just bridge rectify the mains and the switcher takes care of mains voltage differences, there is no range switch.

I suspect the open circuit 100k resistors were the bleeders/balancing resistors for the capacitor bank, and that one of them doubled up as the start-up supply.

David