Looking for a friend with a variac

T

tigerdog

New Member
Hi All,

Long time stalker of the forum, mostly dabble in vintage speakers and tube headphone amps. Finally decided to recently take a plunge on a vintage Harman Kardon 930 receiver. The piece is almost in mint condition, the only issue being that it hasn't been powered on for potentially 20+ years. After reading a bit, it seems like a variac would be in order to get things moving again... Except that I live downtown Boston and don't have the required equipment and not really looking to buy it either (wasn't able to find anything on the cheap either). Does anyone live in the area that would be willing to help a friend out?
 
I suggest that you change your title to add "Boston". That way the AKers scanning the forum would know where you are and might be tempted to open your thread.
 
There are usually many used PowerStat variable transformers for sale on eBay - if you're a little lucky, you can get one for cheap - Chris
 
Variac is more for tube gear and capacitor reforming. Capacitor reforming is questionable since all tube gear should be recapped anyway.

A dim bulb tester is the way to go to fire up old solid state and that is sort of a safety measure test too. It is best for working on shorted gear.
But it can soften the start up process.

Solid state stuff can sit forever and fire up.
Even tube stuff can too.

Soft starting old solid state gear really isn’t that important. It doesn’t really avoid anything like a sudden melt down.

Build a simple dim bulb tester and fire it up.
 
blhagstrom said:
Variac is more for tube gear and capacitor reforming. Capacitor reforming is questionable since all tube gear should be recapped anyway.

A dim bulb tester is the way to go to fire up old solid state and that is sort of a safety measure test too. It is best for working on shorted gear.
But it can soften the start up process.

Solid state stuff can sit forever and fire up.
Even tube stuff can too.

Soft starting old solid state gear really isn’t that important. It doesn’t really avoid anything like a sudden melt down.

Build a simple dim bulb tester and fire it up.
Click to expand...


That's what I can't decide on. I've been trawling different threads and it seems like everyone is split on this. Since the HK930 is a solid state and I read that since it's been dormant for so long, the caps will need to be reformed and to take it slow. But then again the other half just says go for it since if something fails, it'll fail regardless. DBT might be a good option since it's cheap and relatively easy to make, just need to find the time to do it.
 
A simple Dim Bulb is just a light bulb in one line of the AC cord.
You can slap one together with and old lamp and an old extension cord.

Cut the end off the lamp cord, split and strip.
Cut ONE wire in the middle of an extension cord, strip.
Connect one lamp wire to each end in the middle of the extension cord.
A little electrical tape or a couple wire nuts and you're ready to rock.

Put a 60w bulb in the lamp.
Plug the unit into the extension cord.
Plug the extension cord into the wall.
Turn the unit and lamp on.

Watch the bulb "flash" and dim or stay bright if the unit is shorted.

Dim bulb supplies near full AC but acts as a current restrictor (avoids high current melt downs).

For the record, variacs supply full current but restrict the AC (voltage).

Reforming plates happens quickly as the power is applied but I doubt there is much of that, Caps are wound tight and goo'ed together.
Chemical reforming happens as the cap is used, over time, the "self healing" part of e-caps.

BUT, they are 40+ years old and do tend to work as is, even if they do a crap job.
The little ones are the ones that muck with the sound.
Running a unit for a while (weeks) after being stored a while seems to clean them up (the self healing process).
They won't get "all better" but can seem to sound better after a while.
 
Last edited:
blhagstrom said:
A simple Dim Bulb is just a light bulb in on line of the AC cord.
You can slap one together with and old lamp and an old extension cord.

Cut the end off the lamp cord, split and strip.
Cut ONE wire in the middle of an extension cord, strip.
Connect one lamp wire to each end in the middle of the extension cord.
A little electrical tape or a couple wire nuts and you're ready to rock.

Put a 60w bulb in the lamp.
Plug the unit into the extension cord.
Plug the extension cord into the wall.
Turn the unit and lamp on.

Watch the bulb "flash" and dim or stay bright if the unit is shorted.

Dim bulb supplies near full AC but acts as a current restrictor (avoids high current melt downs).

For the record, variacs supply full current but restrict the AC (voltage).

Reforming plates happens quickly as the power is applied but I doubt there is much of that, Caps are would tight and goo'ed together.
Chemical reforming happens as the cap is used, over time, the "self healing" part of e-caps.

BUT, they are 40+ years old and do tend to work as is, even if they do a crap job.
The little ones are the ones that muck with the sound.
Running a unit for a while (weeks) after being stored a while seems to clean them up (the self healing process).
They won't get "all better" but can seem to sound better after a while.
Click to expand...

BRAVO! This needs to be a sticky! Your post is the most concise explanation of several essential theories and practices that anyone that works with vintage equipment needs to know and understand. Thank you!
 
And about that needs to be a sticky thing, how about starting a thread in this forum, under your user name and signature blhagstrom, titled something like “Dim Bulb Tester Vs Variac — theories, practices and applications explained”. Copy and paste exactly what you posted above. I think it is perfect exactly as it is. OK, others can, and should, feel free to chime in and post some value added content if they wish to. That will make it easy for folks present and future to find and apply properly.

At the risk of repeating myself, I really like what you did there.
 
Last edited:
I wish I had read this two weeks ago. I just bought a new Variac off of Amazon because I have a couple of long sitting receivers I want to fire up. I had just read a thread where the importance of Variac was mentioned. Blhagstrom's comments make a lot of sense. Oh, well. I'm sure I will find a use for the Variac sooner or later. Never hurts to have another tool. :D
 
Steven Tate said:
Oh, well. I'm sure I will find a use for the Variac sooner or later. Never hurts to have another tool. :D
Click to expand...

A DBT and a variac can be used together, they are not mutually exclusive.

The magic sauce of the DBT is automatic current limiting, but it comes with a price, there will always be few volts of voltage drop due to the bulb.

The variac will allow one to bring up the voltage slowly (reforming caps for example) while the DBT will provide protection by providing automatic current limiting, should there be some type of failure. The variac can be used to compensate for the inherent voltage drop of the DBT.

It is not necessarily one versus the other, it is one compliments the other.
 
blhagstrom said:
A simple Dim Bulb is just a light bulb in on line of the AC cord.
You can slap one together with and old lamp and an old extension cord.

Cut the end off the lamp cord, split and strip.
Cut ONE wire in the middle of an extension cord, strip.
Connect one lamp wire to each end in the middle of the extension cord.
A little electrical tape or a couple wire nuts and you're ready to rock.

Put a 60w bulb in the lamp.
Plug the unit into the extension cord.
Plug the extension cord into the wall.
Turn the unit and lamp on.

Watch the bulb "flash" and dim or stay bright if the unit is shorted.

Dim bulb supplies near full AC but acts as a current restrictor (avoids high current melt downs).

For the record, variacs supply full current but restrict the AC (voltage).

Reforming plates happens quickly as the power is applied but I doubt there is much of that, Caps are would tight and goo'ed together.
Chemical reforming happens as the cap is used, over time, the "self healing" part of e-caps.

BUT, they are 40+ years old and do tend to work as is, even if they do a crap job.
The little ones are the ones that muck with the sound.
Running a unit for a while (weeks) after being stored a while seems to clean them up (the self healing process).
They won't get "all better" but can seem to sound better after a while.
Click to expand...


I will echo everyone in saying thanks for the great write up, the idea for a thread on DBT vs Variac is great since I had trouble finding that answer specifically. Tried scrounging around the house for a spare lamp but couldnt turn anything up (girlfriend wasnt ok with me making a small "modification" to the bedroom lamp). Decided to take the plunge and build a full DBT using a great how to found here on the forums http://audiokarma.org/forums/index.php?threads/dim-bulb-tester-build-and-how-to.808399/ (personally my favorite because the pictures and explanation are really clear). Plan on putting this together in the next few days, wish me luck.
 
tigerdog said:
Not sure if anyone cares, managed to put together the DBT and fire up my "new" amp (Harman Kardon 930). Everything worked perfect and the DBT took no longer than an hour to put together. The amp is firing like new and I couldn't be happier, thanks for the recommendations folks.
Click to expand...
That’s great! If you work on these units, you will use that DBT often — I hope. :D
 
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