PSU II heater supply cutting out. - World-Designs-Forum

The Shadow · #1 22nd April 2007, 06:56 PM

Hi Folks

Having given up on the preamp PSU I was building for my 5687 preamp due to intractable hum problems. I am currently powering it from the preamp output of the PSUII. This is great and I have no complaints at all.

The PSU was modded a long while back with a separate transformers for the heaters and the HT after the original toroid had burned out. While I was in there I had provided separate switching for the phono II output.

The problem I have now is that I cannot play records. After about 5 minutes the regulator overheats and shuts down the heaters in the phono stage when the phono and the preamp are on together.

I have an appropriate insulating kit and was wondering whether extending the leads and bolting the regulator to the case might keep in from cutting out. It is only running a 5687 and 3 ECC83s ferchrissakes!

If this does not work, is there a higher powered drop in replacement regulator I could use?

Steve

Andrew · #2 22nd April 2007, 08:35 PM

Hi Steve,

Doesn't 1 * 5687 = 3 * ECC83?

If so you're drawing something in the region of 1.8 amps, approx.

I would try the chassis method for the heatsink or separate regs for the 83s and 5687.

cheers,

-- Andrew

The Shadow · #3 22nd April 2007, 09:05 PM

Hi Andrew,

Yes a 5687 draws about 0.8A so it is about the equivalent of 3 x ecc83s which as you say adds up to approx 1.8 - 2A depending on the wind direction. Yes the PSUII reg would have been just about maxed out. Nevertheless it still ought to have have run properly given adequate heatsinking, which I would have thought WAD would surely have provided plus some margin of safety.

The regulator was a pig to desolder but it survived the indigity of being sucked at with the desoldering tool and wrenched from its former home on the PCB.

Whilst I was at it I replaced the buzzy 9V heater transformer with a far better one I had bought from Farnells to put in the the other (ultimately disastrous) PSU. I thought I might as well salvage something from the bloody thing.

Using the top hat washer and mica strip the offending regulator chip was fixed to the chassis and three flying leads taken back to the PCB. The phono and preamp have been running happily for around an hour. The bottom of the chassis near the regulator is warm to the touch but could hardly be described as hot.

Problem solved.
So why didn't WAD simply supply an insulating kit and an extra hole in the base of the chassis so that the chassis could have been used as a heatsink in the first place instead of the pathetic excuse for a heatsink they used as standard?

Steve.

The Shadow · #4 22nd April 2007, 09:53 PM

Here's a picture of the new location for the regulator.

Steve.

John Caswell · #5 22nd April 2007, 09:59 PM

Hi Steve,
Putting the 78S05 regulator on the chassis with a kit is something I have advocated elsewhere. It is all down to volts really and the heatsink is marginal for a curent draw of 1.8A it would only need the mains to rise 5% to cause it to go into thermal shutdown. After all you are looking at the total power dissipated by the reg, and a 3.8V drop (assuming 9V input) at 1.8A = 6.84 W which, without a heatsink, is pushing a TO220 device.

John aka Dr John

PS Did you use any thermal compound on the washer etc?

The Shadow · #6 22nd April 2007, 10:11 PM

Hi John,

I used a tiny bit of compound between the regulator and the mica plus another bit between the mica and the chassis. I did what MJ advocates and tried not to use too much.

Steve.

Andrew · #7 23rd April 2007, 08:34 PM

Nice job Steve.

I've done several of these ciruits for phono heaters and have found I like them best and they run coolest, and cool is good for solid state, with a simple CRC filter stage before the regulator to knock out some of the ripple and drop the voltage a bit. And, by giving the reg a bit less to do, it does a better job as well. The R before the reg spreads the heat around around a bit too.

cheers,

-- Andrew

The Shadow · #8 23rd April 2007, 09:05 PM

Hi Andrew

Yes I'd like to have another go with 4 DC supplies for each of the 76s and the 12E14s.
This time however I think I'll use the proven WAD circuit on each tube so at least I'll know it has a good chance of working silently. The circuit I used previously was one from MJs book but I just could not get it to function properly. The fact that it allowed me with help from Nick to diagnose an RF problem with the amp system was just a lucky side effect. Why it did not work when connected to the rest of the system I fail to understand even now.

Some people have commented on other forums that the LM317 regulator can go into oscillation given the right conditions ending up sounding like a wasp trapped in a coke can. My problem however was a steady state hum independent of volume that would not go away. I wasn't prepared to waste any more of my life on it.

Steve

Andrew · #9 24th April 2007, 10:34 AM

Hi Steve,

Which circuit was it? The one where he floats a 317 to give an HT series voltage reg?

cheers,

-- Andrew

BOONDI · #10 24th April 2007, 03:46 PM

What you can and do regularly get with a regulator is ringing of the supply caused by the transformer being maxed out. It can't quite give the juice on full current draw and the first cap in the filter is still charging....This sets the voltage see sawing up and down, hence the buzzzzzzz....

Or I could just be haeverrin again......