Murphy A362 Voltage Query

[russell_w_b]

I'm on doing up a Murphy A362 for someone, and - three new valves (UCC85, low emission; EABC80, leaky; 10C1, seen off by my own hand ), some switch-cleaning, several new capacitors - including some tatty-looking H***'s brown moulded ones - and a couple of resistors later, I have a nice-sounding working set with plenty volume which - when connected to a long-wire aerial - has stations booming in on all bands, including FM.

However... On going through the voltages laid down in Pat Hawker's 'Radio Servicing' manual, I noticed that the measured volts (using an AVO 8 Mk III) differ significantly from those specified. I'm getting 245V AC at F1 (commensurate with the mains supply), but only 155V DC on the cathode of rectifier valve V6 (U404), hence only 150V on the anode of output valve V5 (10P14) and only 140V on the screen of output valve V5. The boxes on the drawing read 231V (or 222V) for the cathode of the rectifier valve V6, 210V (or 204V) at the anode of output valve V5 and 197V (or 170V) at the screen of output valve V5.

I've replaced R32 160R with two 330R resistors in parallel (the original had burned up, probably because the lump of gloop which was C78 had taken it out), replaced C78, and I've checked twin-capacitors Cs76 and 77; both
have in-spec ESR and hold a charge satisfactorily. The current drawn at the 250mA fuse F1 measured 37mA, so I don't think the power supply is being excessively loaded. I also noticed that V5 cathode resistor R28 measures 315 Ohms instead of the 270 Ohms it's meant to be. I haven't a spare 270R at the moment, and I don't know if it's worth replacing it for a 16.7% tolerance error.

Is it possible that rectifier valve V6 (U404) needs replacing? I'd hate to order one and find I don't need it! All other suggestions gratefully accepted. I haven't checked any other voltages yet, just having worked in from the power supply stage.

Just how critical are these voltages anyway? Yes, the set works, but will it work 'better' if I strive to have all voltages within spec (and so spend more time on the thing than I ought to)? Will damage or detriment occur if I don't?

[Steve_P]

Well if it's working as well as you say I'd leave it! I take it you've changed C73 ?

But, R32, C78, Check the voltages around V5. Then change V6 if needed.

Cheers,

Steve P

[paulsherwin]

It does sound as if the rectifier is on the way out given that the HT current isn't excessive.

The problem is complicated by the fact that you're restoring the set for somebody else. If the set was yours I'd suggest leaving things alone if the performance was adequate, maybe sticking a note on the back reminding yourself that the rectifier was weak. However, as this is being done for somebody else, it may be better to replace the rectifier now. It will only get worse with use.

You could always cheat and bypass the rectifier with a 1N4007 and 100 ohm resistor. You should be able to do this very unobtrusively. Leave a note explaining what you've done inside the case. You could do this temporarily to confirm or eliminate a rectifer problem before buying a new valve if you prefer.

The cathode resistor will be fine.

Paul

[russell_w_b]

Originally Posted by Steve_P

Well if it's working as well as you say I'd leave it! I take it you've changed C73 ?

But, R32, C78, Check the voltages around V5.

Steve P

Changed both R32 and C78! (Had to, really...). I haven't changed C73 yet, though. Ideally I'd like to swap out all the 'Hunt's' moulded capacitors, given the bad experiences I've read about on this forum; we'll see... The ones I did change crumbled readily when poked with a screwdriver.

[russell_w_b]

Originally Posted by paulsherwin

It does sound as if the rectifier is on the way out given that the HT current isn't excessive........... It will only get worse with use.

Thanks for the info, Paul. I shall order a new U404 and fit it. The person for whom I am restoring the set isn't too bothered about mods, new-style components, etc... as long as the casing looks smart, but I'd still like to keep a thermionic rectifier in the thing if I can. I'll try your suggestion (below) to substitute the rectifier with a diode-resistor and conduct further measurements prior to ordering a new U404!

Originally Posted by paulsherwin

You could always cheat and bypass the rectifier with a 1N4007 and 100 ohm resistor........... Leave a You could do this temporarily to confirm or eliminate a rectifer problem before buying a new valve if you prefer.

The cathode resistor will be fine.

Paul

[paulsherwin]

If I were you I'd change any remaining Mouldseals. It doesn't take long to do, and if you leave them in they're like a ticking timebomb The chances are the radio will sound better if you change them too.

Paul

[Tim]

A UY41 will work as a "quick and dirty" substitute, just for testing.

[flyingtech55]

Hi Russell

Rather than buy a new valve on spec. why not bridge it with a 1N4007 as a test?

If no (or little) improvement this can be caused by an O/C or low capacity smoothing capacitor. To check this, connect your Avo to the cathode of the main rectifier (valve or above semiconductor) switched to AC. If you get more than about 5V AC, then the smoothing block is faulty and needs to be renewed.

However, 230V DC is a bit optimistic for an AC/DC set. I would expect just under 200V at best HT across the main smoother when the set is running. My Murphy U502s or generally about 190V HT.

HTH

TimR

[russell_w_b]

I think I'll do this, make further measurements, and - if it improves things - leave the 1N4007 in cct with the fils connected on the original U404. I shall tell the chap for whom I'm doing the set up that if he wants a thermionic rectifier, it'll be an extra £6.40 (just got a quote back this morning)! Regarding the then-instantly-applied HT to the other valves, though, will the existing 160R in series with the rectifier valve suffice to limit the current, or would it be better to wire a further 100R in series with the 1N4007?

I've checked smoothing-cap C77 for ESR and capability of holding a charge (OK on both counts) but I haven't checked yet for AC on it, or measured its value. I shall do.

As I say, the set works fine, with the HT as measured. I'll just have to learn when to stop fiddling and leave it alone!

[paulsherwin]

Originally Posted by russell_w_b

Regarding the then-instantly-applied HT to the other valves, though, will the existing 160R in series with the rectifier valve suffice to limit the current, or would it be better to wire a further 100R in series with the 1N4007?

It will probably be OK. Check the on-load HT voltage afterwards and add some more resistance if it is too high.

Check the voltage rating of the smoothing electrolytics is OK for the off load voltage. It will probably be about 280V until the valves warm up.

Paul

[flyingtech55]

Hi Russell

If you're repairing the set for someone else (other than yourself) I wouldn't advise leaving the 1N4007 in place. I meant to fit it temporarily as a test only. This means you don't buy a valve which you don't need if the test does not provide the expected result.

TimR

[paulsherwin]

Originally Posted by flyingtech55

If you're repairing the set for someone else (other than yourself) I wouldn't advise leaving the 1N4007 in place. I meant to fit it temporarily as a test only. This means you don't buy a valve which you don't need if the test does not provide the expected result.

It won't do any harm to leave it in place though. It's an easily reversible mod. I would disconnect the rectifier leaving just the heater connected rather than leaving it permanently in circuit paralleled by the diode, though it probably won't do any harm if you don't bother. Document your mod on a bit of paper inside the set so anybody working on it in the future will understand what you've done.

Paul

[russell_w_b]

Originally Posted by paulsherwin

...Document your mod on a bit of paper inside the set so anybody working on it in the future will understand what you've done.

Paul

I certainly shall. There's nothing worse than trying to fix something that has been modded out of all recognition; I'm not Sherlock Holmes! I shall pretend I'm at work (OK, I'll admit it... Most of the fixing-up was done there anyway ) and slip full documentation inside the case.

[Steve_P]

I've said it before and I'll say it again - C73. Otherwise the HT will find it's way onto the grid of the Output Valve. Not a good idea, and it can pull the HT down a few volts too. I had a HMV in here a while back which had the HT pulled down by about 10v by this. And it sounded a whole lot better when changed.

Anyway, I am sorry, but I would not do any mod with a diode. For the price of a valve it isn't worth it. Get a replacement valve and fit it. The radio stays original and the HT will come up gradually rather than all at once. Much better that way.

Cheers,

Steve P.

[Phil G4SPZ]

Originally Posted by Steve_P

For the price of a valve it isn't worth it. Get a replacement valve and fit it.

I agree, especially as the customer is paying... And if the original valve wasn't faulty, you can add it to your own valve collection ready for future use! Or is that unethical?

Phil

[russell_w_b]

Originally Posted by paulsherwin

It won't do any harm to leave it in place though. It's an easily reversible mod. I would disconnect the rectifier leaving just the heater connected rather than leaving it permanently in circuit paralleled by the diode, though it probably won't do any harm if you don't bother. Document your mod on a bit of paper inside the set so anybody working on it in the future will understand what you've done.

Paul

Well, I fitted a 1N4007 diode across the U404 rectifier valve and - intially - the HT rail went to 265V! Little bit high, I thought, so I replaced R33 (150R) with 340R and the HT rail now sits at 240V, with 88mA drawn from the mains when measured at the fuse F1.

A quick check revealed the smoothing capacitor to be rated at 350V DC, so should be OK.

[Chris55000]

Hi!

A quickie point - many UK-made can-type revervoir/smoothing capacitor units had a "Surge" voltage rating annotated on the can as well - if the capacitors in your Murphy A362 have this it would probably be 400/425V DC for a nominal 350V wkg or 350/375V DC for a 275V working unit. If a "surge" rating is indicated there is *certainly* no harm in using a silicon diode or contact-cooled metal rectifier as a temporary test for a suspect valve, however I once had an AC/DC British Rentals AM/FM set modded with a '4007 and a 100 ohm wirewound and things kept going west in it every time I used it, so I would *also* revert to the valve rectifier before you return the set to your friend!

Chris Williams

[russell_w_b]

Arghhh; too late! I shall bear this in mind if he fetches it back, though . I had the set on soak in the workshop on several occasions prior to handing it over, and it worked fine. I replaced the existing 150R in the HT line with two ceramic W/W 680R resistors in parallel, and - as I mentioned - the voltage is a leeeetle bit higher than normal, but within anode spec. Less hum as well, but - that said - a new rectifier valve would probably be less hummy too.

We'll see how it goes...

[paulsherwin]

The whole point of fitting a resistor in series with the diodes is to limit the surge current. I'd be very surprised if this set comes back (with this fault anyway )

Paul