I recently picked up a DAC10. It had a UK plug and looks to be in original condition inside. I don't know if it works and I don't have a variac, but I was told that if I power it up in series with a 100W bulb to limit the current it should be OK to power up at least to see if it has life.

There seem to be lots of conflicting ideas though, anything from just going for it to recapping being a must before you even attempt to turn it on. What do people here do in the real world and is there is anything specific to this model I should check?

TIA.

The Dim bulb test is essential . It would be best to use it through a variac and isolation transformer if you can. Slowly bring up the voltage through the variac.

If you don't have a variac, start with a lower wattage (always tungsten!) bulb (say 25 or 40 watt) and let it soak on that for a while. Any fault that happens to be in the radio will be current limited and you won't let the smoke out.

(Electronics runs on smoke. You let it out, it's broken!)

Using a gentle startup like this can often (but not always) re-form electrolytics so you may not need to replace them. Wax paper capacitors always need to be replaced if you want long-term reliability and good performance but a "protected" radio will often run (sometimes poorly) with lots of leaky caps in it. You can then determine what you will need to replace.

There are some paper caps (especially in TVs) that I will always replace even before a power up attempt. Output stage grid couplers and boost caps in TVs are top of this list. That's because a bad cap in these positions can cause collateral damage.

Having it run first, safe and protected by your dim bulb, before you change caps 2 or 3 at a time makes it easier to find out where you went wrong when it doesn't work.

I am one that never uses a "dim bulb tester": Different ideas (have Variac). I will not power old electrolytic's from the set.: Transformers are expensive. I will power them from the reformer, it can handle a short. Sometimes on a valve set, where I think I have a dud electrolytic etc. I will leave the heaters / filaments off & power the "B" circuit with the reformer and gradually step up the voltage

If a wax paper coupling cap leaks below 200Meg & 250VDC its a dud as that will seriously impinge on bias.

Most of the valve grids in a radio draw no significant current and that why the leakage is significant: It is rare to find a wax paper cap over 100K. so close to 50 years of experience says toss them. Never forget all electrolytic caps, the ones on cathodes are just as bad, sometimes worse than, the filter caps.

NB "IC" clips are handy for meter connections that are staying on a wire or component for a while (monitoring).

A transformer will not be damaged, even by a direct short across its output, with an appropriate dim bulb in series. Period.

If you doubt me about this, try it for yourself. If you manage to destroy a transformer, I'll get it rewound for you!

Another thing you can try if you doubt the efficacy of the dim-bulb method. With a radio running on the dim bulb, get a clip wire and short out the G1 coupling cap to the output stage. The bulb lights up immediately and reduces the current to a safe level so nothing is damaged. Short out the B+. or the plate of the output valve to ground. Same story. Short the cathode of the output valve to ground. Same.

Whereas a variac does NOT limit fault current, it relies on you to do that. Turn away for a minute and you can let the smoke out, you need to monitor the current closely all the time.

The GREAT thing about the dim bulb is a tungsten lamp has a very strong positive temperature co-efficient. This means, as the filament heats, its resistance increases. So it becomes a very efficient current limiter.

My reason for not replacing all the paper caps at once should be self-evident. Sure, replace 'em all, but not without checking in between. However, if you are sure, really sure, you will never make a mistake, go ahead and do them all at once!

Re-forming B+ electros with the dim bulb - assuming its a conventional radio and not series heater - use a 25 or 40 watt bulb, remove all the valves except for the rectifier, plug it in and let it run for up to a few days or while you do something else. Eventually the bulb will go dim and, when it stops going dimmer, you are done. Easy! No need to stress with any other method.

According to the information at hand, it's a AC/DC transformerless set, which means there's a possibility of getting a severe shock just by touching the metalwork.

Valves: UCH42, UF41, UBC41, UL41, UY41

It should always receive power via an isolation transformer while being worked on.

The main electros are C25 (32uF) and C26 (16uF).

The capacitor at C23 (0.01uF) must be replaced if there's any sign of leakage. Also check that R15 (470k) hasn't drifted.

Also, before applying power, make sure the speaker transformer is conducting.

Ian, you get 100 percent marks for your comments.
I have been using what is called the dim bulb for over 40 years, although never called it that.
Just a 240 incandescent in series with the item for repair.
I use two globe sockets so can vary the series protection from 15/25 watts to over 200 watts.
Once you get used to using this it becomes a permanent bench device, and in my case in series with an isolation transformer.
As a matter of fact, today was asked to look at a dead DVD player. Plugged it in and with a 40 Watt globe in my tester it gave a brief
Bright flash then settled into a low glow.
Immediately I knew the power supply was operating normally. I had not even pulled the covers off.
JJ

I support you on that JJ

I use my "series load lamps" on everything. Two lamps is all I use----40W and 100W.

Everything is started on 40W and ran between 20 to 60 minutes, depending how yucky everything looks.
After that, a look, feel and smell exercise is carried out, and if all looks good the 100W runs the show for a few hours. If nothing goes
smokey, smelly or hot during the soak test, then straight into the mains.

I learnt this technique from the K9 courses, and began applying it to all those early SMPS boards, some which could protect the rest of
the TV but not themselves---spared me (and our customers) a lot of angst.

And if you happen to have a dead short hiding somewhere, no worries.The lamp just runs brighter (in fact, after some experience one gets to see shorts and overloads simply by observing the lamp's brightness)

I highly recommend this technique to everyone; it's simple, uncomplicated but highly effective. (And cheap)

I use a dim bulb tester now, even though I also use a variable transformer. Years ago, I was slowly applying power to an old TVset when a filter capacitor suddenly went dead short. Before I could react, the 5U4G rectifier was destroyed. If I'd been using a dim bulb tester, the valve would have been saved.

One thing though, some SMPSs need a low impedance power source to start, and won't start up unless switched on directly to the mains.

Cheers.

OK, finally got the time to check the radio. I decided to bite the bullet and rewired an old desk lamp into a dim bulb tester with a 40W bulb.
News is good, the DAC10 powered up and the globe started and remained dim for approximately 15 minutes and I could tune into a station. Volume was low, but it worked.

After 20 minutes I replaced the 40W bulb with a 100W and the radio appeared to work quite well with good volume during the brief test today. There was no hum either. The bulb barely lit at all so it looks like a winner. I'll replace the paper caps in due course.

Thanks everyone.

Excellent news - your radio lives again.

Just take note of what I wrote earlier, replace any paper caps, and you should be good to go.

Great news.

The only thing with the dim bulb method is you never get to find out what dramas you avoided!
Exploding electros, smoking transformers etc. etc.

Remember with the paper caps, one at a time!

I don't believe in just replacing paper caps, electrolytic caps left unused for ages, have by far the greatest likelihood of a catastrophic fail.

I have not used a dim bulb in decades. I have a "Variac" / Slide transformer, should I need it. on the odd occasion where I need to prove a cap or its been awhile in storage, that's what the reformer on the bench PSU is for (25 - 400v). Some times I will leave the heaters off & use it to power the "B" rail if there is something I am suspicious of.

Powering: With experience one has a good idea as to how the "B" voltage behaves with different rectifiers. So I fire up a refurbished radio (/ test equipment) with an analogue voltmeter on "B" (IC clips are good). There will be no drama with mains, as that side will have been well tested & made safe: I will power it up with my isolation transformer and perhaps the Variac. On the Isolation transformer's box (Lighting type) is a "Kill Switch" (Jaycar SP0786). If things look to be going awry and you have your "finger on the button" you can kill the circuit swiftly.

Every one has their own ideas, but I work on "Never power" without checking things, what you find that way can be scary. Many things can & should be checked, before entertaining the thought of powering. Methodology should eliminate most things liable to cause disaster..

It is a pity sometimes with the tyranny of distance, I have trained some, in the Radio Club, one on one, on the practical parts of refurbishing. I am in the ideal position at the moment, as there are eight chasses "in the line".