Strange hum when powering up Tascam deck

[Bargainhunter]

There is probably a simple explanation for what I'm experiencing with this repair.  I am currently repairing a Tascam 122 MKII.  I recapped the motor board and put the deck back together.  I connected the Tascam to my amplifier to test it, I turned the deck on and gradually brought up the power on my variac.  When I got to 20v a/c, I got a loud hum in my speakers.  I turned the power off a double checked all my connections and my solder connections for any bridges.  When I went to test the deck again, the same thing happened.  I then removed all the boards and disconnected the cables from board to board.  I connected only the transformer to the main board and connected the amp to the deck output.  The same thing happened.  I got a hum when I got to 20volts.  After checking the rectifier diodes and filter capacitors, I tried it again and got a hum at 20volts.  I then decided to try again and keep increasing the voltage until the hum got loud or I saw smoke.  Well when I got to 25volts the hum stopped.  I could go all the way to 110volts with no hum.
Why am I getting a hum only at 20 to 25 volts?  Can anyone tell me what I am overlooking?  Could it be the voltage regulators causing the hum at the low voltage?   :phew:I've attached a schematic diagram.
Thank you for your input.

[Runco990]

Well, yes.  At low voltages you have all ripple and no regulation.  The variac method works on tube gear and OLD solid state gear.  The stuff with IC's in it doesn't really like it.  I'd be especially careful with microprocessor controlled gear.  You can actually damage it.

Tascam 122MKII is a good deck! 

[Bargainhunter]

Thank you for your input.  I'm glad you told me that I could damage the unit with a variac.
Do you think a dim bulb tester would be better or is there another way to safely test microprocessor controlled or switch mode power supplied audio equipment?

Thank you.

[Runco990]

Dim bulb is likely safer as it limits current but not voltage.  I do use a Variac but switch in a bulb when needed.  It's a judgement call on when to use what.

[Audiorepair]

Dim bulb is likely safer as it limits current but not voltage.  I do use a Variac but switch in a bulb when needed.  It's a judgement call on when to use what.

Using a lightbulb in series with the mains does significantly reduce the secondary voltages.
Often equipment will not operate correctly with the bulb in place.


It is there to limit current should something go horribly wrong during testing.

One should always bear in mind that this is not normal operating conditions, the secondary voltages are reduced considerably, and until you remove the lightbulb, you cannot be sure the equipment is actually working properly.
Relays may not click in, fans may not run etc.

[Runco990]

Well, that's true.  Using ohm's law you can figure that out.  I think I was being TOO general with the dim bulb.  The idea is to use a lamp of high enough wattage to power up a device but avoid catastrophic failure.  Yes, it will reduce voltage depending on current drawn, but choose enough wattage it's within reason.

In general, devices that are small current draw and transformer isolated like a tape deck.... I use my metered variac at 115 v and check for abnormal current draw. 

Point is, some things do not like being powered up this way, as the IC's can lock up in a bad state, drawing far more current than normal because they don't power up in a "reset" state.

It's a judgement call.  Tube gear?  Bulb or variac is totally fine, as is most old transistor gear.
But be careful with Microprocessor stuff.

[Audiorepair]

Many systems just will not power up corrrectly with a lamp in series.
This can lead to a false positive fail.

I use a lamp in series regularly when fixing stuff, mostly just to make sure nothing is going to go bang.

I reckon 20 - 30% of repairs don't run properly with the lamp, but it saves blowing stuff up.
That's all the lamp is intended to do.



One obvious example would be a power amp.
Normal behaviour would be switch on, fans blow full on for 2 seconds, fans turn off, speaker relays engage, amp is ready to go.

Stick a bulb in there, and none of the above happens, and you think the amp is faulty.

[Ed.Kloonk]