Murphy 310A

[BGmidsUK]

Some progress ...

Having checked the voltages on the CRT cathode, grid and A1 I found these to be way out according to the service data so went on a "find the concealed cap" mission. Having spent some time tracing the circuit to find them I practiced contortionism to replace the two hidden under the chassis next to the brightness and contrast controls (actually I removed a couple of screws and rotated the frame to make the job easier). This then resulted in some nice sparks as I'd bent the earthing wire creating a gap between this and the aquadag, so this was removed and re-shaped.

On switch-on (again) and after some more fiddling with the magnet I was rewarded with a nice bright line across most of the screen, rather skewed and as I found when I attempted to adjust the focus, this was due to the scan coils being rather loose.

More time spent trying to find the prime suspect for absence of frame scan (the controls nudged the line and measured OK) and I decided that the plastic thing in the picture below (C116) must be a Waxy in disguise and, despite trying to elude me by not being the same value as on the service sheet, I checked it and found it to be faulty. A replacement resulted in full scan .. and a pretty good video signal which could I could just about get to lock with careful adjustment! The video is rather weak and the user contrast control has no effect (the preset one does) however I've done nothing to the receiver section so this is to be expected.

The tube looks to be in good shape, there is plenty of available brightness.


Brian

[BGmidsUK]

More progress!

First job was to tackle a couple of the waxies in the sync stage, this produced a big improvement. The line hold is rather sensitive but does lock in nicely and stays locked! Only problem is the interference (which can be seen on the photos) and I won't be able to do anything about this until I get the set working properly on Band III. The first photo shows the result.

However, I observed that the picture was fading and expanding after a few minutes (there appeared to be insufficient line drive anyway with lack of width). The LOPT was slightly warm and I started to think that the oil had indeed leaked out. Maybe it has? I'll get to that later.

Replacing C129 appears to have fixed this with an improvement in brightness and the picture now just about fills the screen. The flyback lines have also become less noticeable. The second photo shows this.

Another fiddle with the focus knob to move the obviously loose magnet gave a nicely centered picture, see photo 3. The set was left running for a good 10-15 minutes with no noticeable reduction in brightness or change in picture size and the LOPT also remained cold.

The focus is pretty much spot on whatever I do to the control so I assume the thing is stuck in the assembly? It does rotate and seemingly hits end stops.

There is noticeable horizontal "breathing" of the picture (but not the raster!) I thought maybe this would be down to smoothing but I suspect it is one or both of the following: The patterning does affect the sync and the V19/V20 cage cover is off.


Brian

[ppppenguin]

That interference looks nasty. Suggests that some high voltages are getting where they shouldn't. Might be a good idea to have a careful listen for any rustling or hissing noises. Don't put your ear too close to any high voltages Another approach is to look at the set in a darkened room. Turn down the brightness to make any faint corona or other discharges more visible.

[murphyv310]

That doesn't look like corona discharge to me, perhaps some external noise coming in possibly a CFL or other SMPS noise, The aerial sockets are not the best on these sets and the short coax from there to the tuner often has an o/c on the screening via the isolator caps the aerial socket.

[ppppenguin]

It does have a strong 100Hz element which suggests that something else mains powered is causing it. Also the white "dashes" are a bit too regular for discharges in the set. Doesn't totally eliminate discharge within the set as a cause, just makes it much less likely. Sorry if I was scaremongering. If it's getting in at RF then that's a pretty high frequency for a SMPS or CFL. They usually give more grief on LW or MW.

[radioredcat]

I see the same interference when DX-ing (625 obviously) in band 1 so it probably is SMPS or PC related.

Andy

[Mr Hoover]

Hi

From bitter experience some SMPS radiate very badly in band 1,.

As Trevor says check the coupling from the aerial socket to the tuner,almost certainly something not quite right there with the coax screening.

Cheers

Hugh

[BGmidsUK]

The interference is external and pretty much covers Band 1, it's the first tine I've observed it as definitely being mains related though. It's not likely to be a CFL as it's there all the time. I'm keeping my fingers crossed that it won't completely ruin DX reception as I still haven't put my aerial up! Medium and Long Wave are nice and clear these days with good reception. My Commodore 64 puts paid to this ...

I'll try running the set with the covers on (including the tuner which is also open) and see what difference this makes. Also I haven't yet replaced the caps in the aerial socket. Band III reception on Ch8 is very weak but free from this patterning.


Brian

[Mr Hoover]

Originally Posted by BGmidsUK

The interference is external and pretty much covers Band 1, it's the first tine I've observed it as definitely being mains related though. Brian

Sounds 'orrible though all too common these days....You could try switching
any pc's/routers nearby off and see if it goes.If you actually listen to this "shash" on a Vhf communications receiver there's usually 50/100 Hz modulation/noise being repeated every few hundred KHz,gradually fading away as you get higher up the Vhf spectrum.

Hugh

[Mr Hoover]

I've found that good way to trace the source is to use a small
battery powered shortwave radio if you have/can borrow one.

The interference will be there at the top end of the SW bands for sure,Once you've tuned it in,switch your house power off,if it goes away it's in your house somewhere.If not it's probably coming from a neighbour.

If it is in your house then retracting the radio telescopic aerial to attenuate the interference and walking round the house with the radio should locate the source.

Good Luck

Hugh

[murphyv310]

Brian
Remember this is a fringe set and quite sensitive. Both my 310 & 320A have had problems in the coax socket area! I am sure once you have it all sorted out you will be impressed.
If the line whistle is loud you may be short of oil in the LOPT, they usually run really quiet, one reason they were liked in the 405 line days.

I take it you are going to try for the BHTG transmissions on it? Have you got a channel 4 or 5 biscuit?
Want more gain? Well fit a 30L17 frame grid valve to replace the 30L1 in the tuner it will up the sensitivity quite a bit and the Murphy tuner will be happy with it, cracking tuners by the way, 100% drift free, that's why there is no fine tuner. Great sets and very much maligned by those that have never worked on them!

[BGmidsUK]

Hi Hugh, the interference is definitely not coming from anything of mine. The Roberts R809 (which I see is in a few of the photos) will be an ideal radio to try and find the interference source. The same sort of interference is present at my mate's flat in Oadby (we're doing a bit of experimentation with PIC-based pattern generators so one of my TV-990s is presently residing at his flat).

I wasn't initially planning on using this set for BHTG reception attempts but it does look like a good candidate! I'll also be posting a request in the Parts Wanted for coils which can be used/modified accordingly for Channels 6 and 13, which are in use here. The set has coils for 3(? - Channel 10 coil is fitted) and 4 but 3 doesn't work yet on the other Band 1 channel apart from audio, so it's probably down to dodgy fingers.

Thanks Trevor for the tips on the 30L17, I'll see if I've got one in my valve box.

Also thanks for the pointers on potential sources of interference


Brian

[BGmidsUK]

I'll also add at this point that the (user) contrast control has no effect, so I'll Investigate this


Brian

[Mr Hoover]

This is a useful site about interference.

http://www.mikeandsniffy.co.uk/UKQRM/


Hugh

[BGmidsUK]

Slight problem looks like I've got a short in the LOPT.

Changed the aerial isolator caps last night and checked the co-ax etc. - all OK there but at switch-on, the line frequency increased followed by a screech from the LOPT and loss of line scan & EHT. I thought I'd heard a couple of "clicks" from the LOPT when doing initial full mains tests.

Hopefully this will "just" be the U26 heater supply but I don't have a stick to substitute it with. I'm wondering if the lack of oil has caused this. Any tips on getting the LOPT open without causing too much damage will be appreciated!

I'm about to give it another test but with the anode cap off to see if it makes any difference. Maybe it's time to put a request in for a TV20 stick ...


Brian

[BGmidsUK]

Looks like there's oil everywhere EXCEPT in the transformer ... the U26 base is coated would I be OK to use bog-standard veggie or sunflower oil?

>EDIT< Just found the "filler" cap


Brian

[murphyv310]

They can spew oil, my two here are dry fortunately, any sunflower oil will do fine. The can can be un-peened and you can use a beans tin as a replacement, Label and all! The oil itself was used as a coolant and sound absorber, the transformers are actually well insulated, it's well known to have a fair bit of water in the can though.
I must admit the ones that have leaked it is usually the line drive or wrong DC conditions in the line stage that cause them to get hot, then the oil expands causing leaks, also coal fires and dirt in the set attacks the neoprene seal that exacerbates the problem.
I would dismantle the lopt and clean it out, run the set with it out of the can and see if the lopt is arcing at all, then go from there.

[BGmidsUK]

Thanks Trevor - I have opened the can and should be able to re-use this as it can hopefully be peened shut again. There's literally only about 1½ fl oz left of oil in there, not even enough to cover the heater winding and also some water droplets in the bottom (plus the oil stinks!) - so I'll sit it in oil and "cook" it for a bit to drive any moisture out. It looks like the set was stored on its back at some point as well as being damp.

The set seemed to be OK when run up on the lamp limiter first but evidently didn't like full mains after being allowed to sleep overnight.

There appears to be decent clearance on the U26 heater winding so I hope a repair, if necessary, won't prove too difficult.


Cheers,
Brian

[BGmidsUK]

I finally got round to connecting the LOPT up now it's had a chance to dry out (suspended in a jar to make sure it didn't touch anything) and warmed the set up for a bit on the limiter with a 200W lamp, I want to be kind to the electrolytics for now, then tried giving it more juice - line frequency correct but as soon as the EHT came up it sqwawked loudly. Couldn't see any arcing though.

Next job was to isolate the heater winding (I may remove it altogether, I might be able to see where it's been shorting). Hope it's not the main winding otherwise it'll be a rewind job .. assuming it can be done. I have a couple of TV20 sticks thanks to a very generous Forum member so I'll try one. I've prepared the valve base to accept it.

Just one rather dumb question .. I presume the marking on the stick is the same convention as a normal diode?


Cheers

Brian

[BGmidsUK]

Unfortunately having tried it with the stuck in place the short is still present. However, when I removed power the screen lit up ...


Brian