Using A Variac?

[manhattanman]

I've got 3 old pieces that haven't been run for some time. I don't know their operating condition. They are all circa 1990. The components are as follows: Tandberg 3006A amplifier, Meridian CD player and pre amp 208, Denon AM/FM tuner TU-800.

Should these components be brought up on a variac? Opinions on variac use seem to be all over the board. Should a variac be used on digital equipment, and if so, how should it be used? For how long and at what volt intervals?

In their day, these pieces were top of the line and I don't want to damage them.

Need some knowledgeable opinions here.

 

[Silversleep]

Does not sound like very old gear. Dim Bulb Tester will save the outputs on your amps if there is a problem somewhere. Variac will not.

 

[Audiovet]

Ideally also, only the power supply caps should be brought up like this. In high-feedback amplifiers those are not guaranteed to be stable under all lower voltages.

Simply put, I would prefer the amplifier circuit to be disconnected if convenient. In tube equipment the tubes can be drawn - but slow power-up might then involve an under-heated rectifier. Such a 'power up' needs sometimes be done slowly. The right way is through an ampmeter, watching out for too high a charging current. Should that be evident, power supply caps should be individually tested.

All in all often not very convenient. Visual inspection of particularly electrolytic caps can give clues. I agree that in his case the age is not particularly high; one might get away with a variac power-up plus voltmeter to show that the rail voltage rises accordingly. A series bulb - mmmm, if there are no other ways; better than immediate power-on.

Variacs on digital equipment? Far to open a field to give an answer. For digital (switching) power supplies e.g. I would not do it but have no experience. For digital loads? Perhaps to hear from others.

 

[Westy56]

How does a "Dim Bulb Tester" save the outputs?

 

[jdurbin1]

By current limiting the amp if there's a shorted component in it.

You can use a DBT on the 3006A but IME it'll need to have roughly 100 watt lamp in it, otherwise the amp will not come up to normal operating voltages.

That amp has internal adjustments for bias, offset, distortion, and clip indicator - all should be checked following the factory manual. Also worth it (if it has never been serviced) to remove the output MOSFET devices and reinstall them with fresh thermal grease. The old grease will likely be dry and powdery at this point and those Hitachi MOSFET parts are very hard to come by these days, esp. in performance-matched sets which this application requires.

If you do service it, make sure to get the right manual - there's an early version that used some negative feedback and a later version that did not, and the amp front ends differ in those two versions.

John

 

[manhattanman]

I appreciate the input, folks. But these responses don't answer my initial question: Should this digital equipment be brought up slowly on a variac, DBT, or what?

The variac issue seems to be highly subjective.

Need to get some answers from smart people familiar with restoring 25 year old digital stuff.

 

[fred soop]

.....
Variacs on digital equipment? Far to open a field to give an answer. For digital (switching) power supplies e.g. I would not do it but have no experience. For digital loads? Perhaps to hear from others.

This would vary with the specific equipment. TVs (the more modern but now obsolete CRT type) were using horizontal output stages that were driven directly from the AC line with no isolation. Then all internal operating voltages outside that stage are taken from the high voltage transformer and the horizontal stage itself is also driven from an isolating interstage transformer.

So, doing a repair, I'm trying to bring it up on a variac. Will not work. Turns out it has to be "hit" hard enough to get a large enough pulse out of the horizontal output to kick start the horizontal oscillator in order to sustain output from the horizontal output. It had to be "hit" directly with full line voltage to start.

A similar situation could exist with any digital equipment and any switching power supplies.

 

[Hyperion]

Overall, a DBT is probably best to test an SS amplifier, (IF it didn't have MOSFET outputs - this one does). If there is a fault, it will be indicated by the brightness of the bulb after the initial power on flash (capacitors charging). If the bulb stays bright - or gets steadily brighter after this - time to turn off fast. (I am assuming here that the bulb wattage is correct for the equipment).

A variac is a useful tool, but only really useful for Valve (tube) equipment in my opinion. Using one on SS equipment does not give you automatic instant current limiting that the DBT has. I don't know anyone who can react fast enough on a variac control knob to prevent output transistor meltdown, the DBT does this automatically - providing the bulb wattage has been selected carefully.

Whether using a variac or a DBT do not expect any kind of normal operation from the electronics being powered up - it's only an indication to save the gear from self immolation in the case of a 'short' or serious PSU fault being present - that's all. And also never 'use' the equipment while connected to a DBT, or a variac at less than 100% output, (matching the mains input voltage, or the voltage setting of the equipment).

Missed the fact that the amp has MOSFET outputs - Thx jdurbin1

 

[jdurbin1]

The amp should be brought up on a DBT because if there ARE problems in that amp, you risk destroying some very hard to replace Hitachi MOSFET output devices. There are no digital circuits in this component, obviously. You don't typically use a variac for solid state amps like that, frankly if it does have caps that need reforming I'd rather see them cause problems and get replaced rather than hope they reform via the slow voltage increase. Caps cost next to nothing and are readily available, the transistors in these not so much. If you want to isolate the two large Elna PS filter caps and reform those fine, but I have never seen a bad one in any Tandberg 3000-series amp at this point. I took a pair out of a 3026A recently that was described by the seller as "sparks flew out of it when I powered it up" and that had been swimming, and both caps tested to spec on my Sencore LCR meter.

I'd power up the rest of that stuff that does have digital circuitry in it (not sure the preamp does but the CD and tuner probably do) on line voltage without any concerns. Using DBT or variac on those is a waste of time IME.

John

 

[fred soop]

In some cases, it depends on the actual design. In the Douglas Self designs, there are no zener diodes with a minimum threshold and the power amplifiers WILL operate on 5 volt rails, though obviously at greatly reduced power. The Marshall Leach designs did have zener diodes but can still be checked for faults at low rail voltages. I was able to do the same with a Fisher 600-T power amplifier section though you will only find a few examples of that design.

 

[manhattanman]

Thanks, Judrbin1

You've apparently worked on Tandberg stuff. Do I start the DBT with a 100 watt bulb? For how long? Is it the only bulb needed?

The Meridian has both preamp and CD player. (1989 MSRP was $4,500.) It was originally married to the Tandberg (1990 MSRP was $900.00) I found both pieces in the same trash but they're excellent cosmetically and I don't want to damage them.

The Denon (also found in different trash) '89 MSRP was $500.00, and has been reviewed as one of the best digital tuners made.

Any further suggestions on the other two pieces?

I normally collect tube and SS stuff, so this is a little new to me.

Thanks, again.

 

[jdurbin1]

Start it with 100W bulb, it should come on super bright initially and fade out partway. If it stays bright, shut off the amp and start looking for shorted or damaged components.

Once it dims, see if the protection relay clicks on. If it does, check the DC offset at the speaker terminals (no speakers connected yet). It's normal for that to lope up and down about +/- 20mV or so when there are no speakers connected. If you see DC higher than say 100mV either way, it probably needs to be aligned.

These are not too bad to align electrically and the manual is available but start with the above and if things check out ok you may be able to just start using it.

While you're at it, check the serial # - can tell you which version you have based on that. If the one on the bottom rear has fallen off, there's usually a second one stuck to an inside side chassis member.

I don't have anything else specific to offer on the other gear, other than to power it up and see what you get for functionality.


John