Applying power first time

[Malcolm G6ANZ]

Hi all, I'm confused!
When switching on an old radio for the first time in many years everyone gives a warning to run it up slowly or use a lamp limiter. Also to change 'that cap'. All good sensible advice. Now in reading about TV sets of similar vintage I often read that they will just apply full power and see what happens. Given that the components will have aged at the same rate as those in a radio and that damage could result from leaking caps etc why the different switch on method?

Malcolm

[paulsherwin]

Originally Posted by Malcolm G6ANZ

reading about TV sets of similar vintage I often read that they will just apply full power and see what happens.

I don't know where you've got this idea from. It is more risky to do this with a TV than a radio because the currents and voltages are higher. I don't work on tellies myself, but everyone I know who does will start the TV on a variac after performing lots of component tests.

[Station X]

Some members of this forum say that you shouldn't do any work on a TV before applying power, because you may introduce more faults than were there to start with.

I disagree totally with this view, but must confess that I've never restored a TV.

[neon indicator]

I'd check any mains rectifiers are OK. Mains across electrolytic isn't fun.

Check for HT short
Check Heater chain is not partially short or open circuit (note it's lower resistance cold than at normal volts).

Remove dust and grime around EHT and Boost Volt / LOPT areas as this can cause failure due to arcing.

After removal of gunge visually inspect for any burnt, missing or exploded parts. Over heated resistors falling out or wires cooked at dropper corroded and snapped or not actually making connection is common. So wiggle, tug and meter those parts of circuit.

Carefully unplug and clean if corroded any valves inc tube base. Use hard ink rubber, not sandpaper to remove connector corrosion on plugs and pins. For sockets use switch cleaner or at a pinch w40 and "clean" by insertion & removal of plug/valve.

If a dual standard set, never ever turn on without checking the mechanical link from knob/lever is correct and that the switch sliders match up with switch pins in both positions. Never operate while set is on. Frankly I used to solder these in the 625 position and remove the mechanical linkage. However if you want "dual" standard operation double check this before power up. It may need extensive repair.

Before late 1970s most sets had no isolation. (If it has AV i/o or SCART in 1970s it's isolated). Use an isolation transformer.

I've "restored" (I called it repairs) 1950s and early 1960s sets in 1970s. I kept a Ferguson TX9 chassis I got for my Mum in 1976 I think going from 1991 when she gave it back till about 2005 when the mechanical issues of tuner made it too annoying to use any longer. In 1991 it was magenta from very poor green level. I redesigned the Green drive for more "Oomph" and that kept colour balance fine till it was scrapped.

The all valved 1950s sets and even Dual standard Hybrids (with no ICs or SMPSU) much simpler to fault-find and repair than a 1970s TX9 or TX10 domestic set never mind horrors like Barco Multistandard AV Monitor/TV that I repaired as part of my "real job" in 1978/1979.

Debugging anything with a SMPSU is a horror. Though I have repaired "newer" ones in Satellite receivers and DVD recorders, but only because they have used cheap capacitors that failed from heat. Replacing every bulging capacitor usually works if there is still intermittent operation.

[ppppenguin]

Just because some of the more experienced TV guys here (naming no names but will John Wakely please stand up) will apply full mains and hope for the best this doesn't mean that it's 100% recommended. Some of us (like me!) are devout cowards and will use a lamp limiter and/or variac as a matter of course.

The important point made by JW and others is not to start on mass component replacements before applying power. It's too easy to make a mistake for one thing and also once you get first light on the screen the set will tell you a lot about what's wrong. Then you can make a start on replacing caps etc. Most people (even JW!) would snip out the mains filter cap to avoid a bang. I err on the side of caution and would tend to replace "that" cap before applying power. I also believe in getting the heater chain working nicely before applying HT.

The above advice is mainly for traditional 1950s/60s valve TVs. For later sets, with transistors, switchmode PSUs etc then lamp limiters are usually a good thing.

[Hybrid tellies]

Interesting topic this. I would advise anyone with a pre 1960's tv radio or amplifier not to apply full power as electrolytic smoothing caps from this age can and do explode after years of non use if full power is applies. From the 1960's onwards different electrolytic materials were used which did not have such explosive properties.
I mostly look at tellies from the 1960's and 1970's most of them hybrid sets. If they have not been used for some time I will leave them standing in a warm dry room for perhaps a month. Then before switching on I will do some cold checks checking for shorts on the HT and LT lines then checking the line output stage for short circuit capacitors etc. I do other checks as well for eg if the tv uses a valved sound output stage I will check that capacitor for leakage and a good visual check looking for evidence of overheating or signs of spillage etc.
This proceedure works well for me and so far I have avoided any spectacular but damageing firework displays. I also apply these same rules for vcr's audio amps tuners etc or any mains powered device that has sat idle for some time.

[murphyv310]

Hi.
The very minimum you should do with any Radio & TV is to have a good look around firstly for obvious faults, Valves down to air, missing fuses, horrible looking work and bodges, snip out the mains filter and check for shorts on the HT line. On an AC radio remove the HT rectifier valve and power the set up on full mains this will tell you if the mains transformer is OK and that the rest of the valves heat up, I then would say run the set at say 150v on the variac and put the rectifier back in, this will then give a slow rise in HT and the other valves will still be warm, a gentle increase in variac output should be done within the course of half an hour, this should show up any faults.
AC/DC sets respond well to the lamp limiter.
I rarely just plug in a TV unless I know it has been run fairly recently.
The variac is best on TV 's and after my cursory look over and the snipping out of the mains filter cap I run the variac up to 40 volts and monitor the HT, usually the HT will go to 45-50v at this setting, and the voltage will come up slowly to about 55-57v if the smoothers are good, a low voltage could mean either faulty smoothers or a duff rectifier, If the set uses a valve rectifier a different approach is needed. I would then up the variac to 75v this should give an HT heading nearer to 80-95v and should still rise a little after a few minutes, upping the volts to say 150v should now result in dull heaters appearing after a few minutes and the HT will begin to drop, keep your eye open for leaking wax caps or a big drop in HT as this could mean that an electrolytic is in distress, if so it will get warm at this voltage on the variac but is very unlikely to go bang, if the HT has dropped but not alarmingly go to 180-200v by this time the set should be doing something, using your eyes ears and nose will tell you a lot of what's going on and what needs to be replaced.

TV's with a valve HT rectifer will need at least 120v mains before any glimmer of conduction will occur in the valve, so again monitor the HT while slowly upping the volts from the variac, again use eyes ears and nose.
Good Luck!

[Malcolm G6ANZ]

Hi Many thanks for the detailed replies. All makes sense now. Maybe I'm misinterpreting some of the TV posts as it seemed that it was a case of power on and see what happens obviously there is lot of work done on a set before it has power applied.
As people have said replace 'that cap' (the one in the audio chain) would there be any others you replace as well? By that I mean ones that could cause damage if leaky. Mains electrolyitics are to be watched as MurphyV310 said.

Malcolm

[mark pirate]

I always use a variac for "waking up" radios and TV's, keeping a close eye out for odd smells and smoke!
As a rough guide, winding up the volts over 20 mins to half an hour before reaching full mains.
I learnt my lesson many years ago, when a cap exploded and shot past my face
Better to be safe than sorry....

Mark

[neon indicator]

Low voltage though is to be avoided on SMPSU (Mid 1970s and later). Because the feedback loop makes the primary take more current. Some designs without adequate safety features may blow thyristors, fuses, fets, transistors or rectifiers at somewhat less than 200V if a Europe only model or less than 75V if automatic 110/220V model. (some have a switch and use the input rectifiers as voltage doubler when on 110V).

Thus very important to have correct fuse on SMPSU. You don't see many without their own local fuse.

[murphyv310]

I echo the above posting. Many engineers would use a variac when running up a repair on a switch mode PSU and then suffer a blow up, I had many arguements with "engineers" who "knew better". I always just hit them with mains as I was confident of my repair work and only very rarely did I have problems.

[neon indicator]

Yes. I think the lamp limiter and/or Variac only applies on SIMPLE PSUs that have no regulators. Pre 1970 radio and Tvs and not test gear or professional gear.

There are some high voltage linear regulators and circuits (e.g. HP141S and HP141T) that will do themselves ill, or the valuable plug-ins get damaged if any significant duration of undervoltage supply.

[Hybrid tellies]

The trouble with running a valved telly up with a variac is that the reduced input voltage may cause the line output to overheat if the line oscilator valve is lazy. The only way round this would be to disconnect the HT to the line output stage or disconnect the screen feed resister to the line output valve.

[murphyv310]

I have never had a problem with running up a valve set with low voltage even with no line drive, remember that on low volts the heater emission is low and the HT is low too so the line output valve won't be passing huge currents anyway.

[neon indicator]

Valve LOPT isn't problem for a short while. The Transistor based LOPT won't like it.

[Sideband]

Originally Posted by murphyv310

I had many arguements with "engineers" who "knew better". I always just hit them with mains

...the engineers or the power supplies??


It think it is necessary to treat telly's slightly differently from radio's. In many cases, the fate of the set depends on the CRT and/or LOPT so it seems sensible to get the set into a condition where it's reasonably safe to apply mains and assess these items as soon as possible. It would be silly to change 30 odd capacitors and some out-of-spec resistors only to find on powering up, you find the CRT is duff or the LOPT is arcing. Whilst these may not be impossible to source, sometimes it can take months or years to find replacements.

Personally before applying mains for the first time to a TV I like to reform smoothing caps and any electrolytic decouplers, replace the boost capacitor and snip the mains filter. I leave the boost diode disconnected (top cap) so that the EHT does not appear at first and just concentrate on getting the HT up and checking for life in the audio/sound IF at first. If nothing nasty happens, I then connect the boost diode and wait for the fireworks. So far I've not had any problems by using this method. Jeffs idea of getting the heater chain up first is also a good idea

With radio's of course the problem is much simplified. Any with a mains transformer, I like to test this as soon as possible and most times you can just remove the rectifier valve, do a few quick resistance checks and then apply mains. Check that the valve heaters are OK and once the transformer is proved, the rest is fairly plain sailing.


SB

[mrmagnetophon]

I dont believe in variac because i deal alot with recorders and hate to under power the ac motors on them, i stick with lamp limiter if necessary, but mainly just a GFCI outlet.
-Chris

[Malcolm G6ANZ]

I never expected this amount of interest and replies. As I said earlier I was wrong to think that TV were different. They arn't just more of it. The general principle seems to be; change the obvious and little else in case extra faults are added, then power up slowly.
If I'm ever allowed to try a TV I will certainly take things very slowly and carefully bearing in mind all the words of wisdom written here. Thanks all again, Im no longer confused!
Malcolm

[murphyv310]

Originally Posted by mrmagnetophon

I dont believe in variac because i deal alot with recorders and hate to under power the ac motors on them, i stick with lamp limiter if necessary, but mainly just a GFCI outlet.
-Chris

A lamp limiter will do the same as a variac and run motors slowly. Its never a good idea to underrun motors either with a variac or lamp limiter, especially ones with start/run capacitors.

[Billy T]

Originally Posted by murphyv310

I echo the above posting. Many engineers would use a variac when running up a repair on a switch mode PSU and then suffer a blow up, I had many arguements with "engineers" who "knew better". I always just hit them with mains as I was confident of my repair work and only very rarely did I have problems.

I guess I would have argued with you too. Back when I was a TV Dept SM I took on all power supply repairs for an extended period as a quality control measure, because our out-sourced PSU service was delivering very unreliable results, and even with good techs, our own workshop service was very little better. I wanted to develop a better system that gave reliability for customers' and company's benefit alike.

I have never subscribed to the full power test idea at all, and built a simple test bench with variac, lamp limiter and output voltage meter built in. All functions could be switched in or out and LV rails were monitored by LEDs

Blown fuses meant a cold repair first, but then all supplies were first slowly powered up via the variac to see if they were capable of producing power, and if they were, they were run up to full input volts via the variac with the lamp limiter in circuit and a suitable light bulb as an HT load.

Once running they were left on the lamp limiter and 'impact tested' for dry joints, and connectors wriggled for loose pins and sockets etc, then the variac and limiter were switched out and a full load test run. With this system in operation we very rarely lost a patient on the operating table, and the reliability stats over 1000's of supply repairs amply justified the effort.

It wasn't possible to use with the test bench for integrated power supplies or single board chassis when they became the norm but the lamp limiter and variac were still used with good results.

Pre-colour we had a final test bench for mono sets and all had to pass geometry (pattern generator), low signal, and low mains voltage checks before being signed out. We had the happiest delivery crews in the business, they were spared the wrath of customers whose sets were DOA following workshop service. I set up a similar system for colour sets with reduced lighting so that greyscale colour tints didn't get missed.

Cheers

Billy