DH-220 Clone (SMART TA-242) can't adjust DC offset using offset bias pot? - diyAudio
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Old 20th February 2011, 03:09 PM   #1
beady is offline beady  United States
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Default DH-220 Clone (SMART TA-242) can't adjust DC offset using offset bias pot?

So Iím trying to figure out whatís going on with this Smart Devices TA-242 Iíve got. At first it had a crazy high bias current (in the Amps range) that seems to have been a faulty bias pot. The bias pot resistance didnít change (stayed ~10K) when the pot was rotated, except when on the CCW stop where it read .5 Ohms. I jumpered a 470 Ohm resistor across the pot with it on the CCW stop and got the bias current to ~235 mA (spec is 275 mA). Amp seemed pretty happy and neither channel got hot.

Then I decided to check the DC offset and found the good channel at less than 10 mV, and the channel that was acting up (but now stable with a 235 mA bias) showing ~600mV. When I rotate the offset pot, nothing happens to the offset.

I put a meter on the offset pot and it changed resistance as I rotated it, like it should. I noticed the offset had come down some and tried the pot again, and now was able to dial down the offset. I got the offset down to 4-5 mV and the other meter showed the pot at 600 Ohms. The amp seemed stable, and I left both meter on the amp, with the amp powered up for about half an hour.

I decided to call it a night, and turned off the meters. I turned off the meter monitoring the offset pot first, and as went to turn the DC offset monitoring meter off, saw the reading spike. I turned the offset pot back on and it was reading 600 mV again. Turned on the offset pot meter, and the offset dropped back to 5 mV. Turning off the offset pot meter caused the output offset to jump back to 600 mV.

I donít know enough to recognize what is going on here, can someone help?

Is this issue related to the bias issue I was having, or is it separate?

Can someone guide me on what to check next?

Thanks for any help,

Mike
--
My previous bias thread is linked here:

DH-220 Clone (SMART TA-242) WAY too high bias current issue?

Hereís an Audiogon post with pics of the amp:
Smart TA242 For Sale | AudiogoN

Hereís a link to the Halfer HD-220 manual:
http://www.hafler.com/techsupport/pd...20_amp_man.pdf

Hereís a link to the Smart Devices TA-242 manual:
http://www.smartdevicesinc.com/pdf/ta120-ta242.pdf
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Old 20th February 2011, 11:18 PM   #2
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So applying voltage caused by the ohmmeter into one of the pots (which one???) nulled the DC offset temporarily, until the meter was removed and then 600mv.

Well, almost everyone on this solid state forum has more experience than me, but I'd have to say that whichever pot that was is insufficient. It is beyond its range of adjustment.

If this is the type of control that is desensitized (so that its not twitchy) via series resistors so that the a 1k pot varies only 1k worth of what is an 11k divider, then you may be able to increase or decrease one of the fixed resistors by paralleling it with an additional resistor. But, I think you've got a defective pot as the unit may have been stored or operated in a humid environment some time in its past.

So, first, please clean or replace the pot and then report back.
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Old 21st February 2011, 01:18 AM   #3
beady is offline beady  United States
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If I have the Ohmmeter powered up across the DC offset pot, the DC offset can be adjusted via the offset pot to whatever value I want.

Without the meter in place, turning the pot through it's whole range of motion does nothing to the offset reading (stays about 600 mV), unless it is near the CW stop where the offset jumps all over the place, my meter showing "OL" before dropping to 12V, then back down. It will settle to about 330 mV on the stop, but it you move the pot at all, the offset leaps back up into the high single volt range, and dances around before setlling down closer to 600 mV again. Once off the stop just a little way, the offset pretty much stays steady at 600 mV and does not jump around at all when moving the pot.

With the amp powered off and the ohmmeter across the offset pot, the pot seems to read correctly across it's entire range, i.e. it starts at almost zero and runs up to about 1K Ohms.

I tried contact cleaner/lube spray with no effect other than to make the pot easier to turn.

I also notice that when watching the DC offset and powering the amp on, the offset shoots up into the many volts range (10+) and sometimes will make the meter read "OL" (overload) before dropping back to 600 mV over the course of a couple seconds.

Mike

Last edited by beady; 21st February 2011 at 01:38 AM. Reason: Add more info
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Old 22nd February 2011, 11:21 AM   #4
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Okay, so the offset pot at full turn is insufficient.

As it is maxed out to get 600mv dc offset, one side of the pot is at dead short--wiper is all the way to one side, which is now 0 resistance.

You might could try adding either parallel or series resistance to the side that is 1k (same as switching to a 500R or a 2k pot). This doesn't fix the actual problem and it may not work for your offset.

However, its pretty likely that One of the three leads of your offset pot is pointing right at the actual problem--some component out of sorts.

As a last resort, you could use the "move the problem" method:
With the gel flux and the desoldering iron, drain the power, transplant one component at a time between the right and left channels, and continue until you have moved the problem from the bad channel to the good channel. If in the process, both channels go bad, there is more than one problem; so, check your work first, then make notes, and continue until the bad channel is now the good channel.
If you move the problem, you can replace the problem.
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Old 16th March 2011, 11:42 AM   #5
beady is offline beady  United States
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I think I have successfully repaired the amplifier.

I desoldered the faulty bias pot and Q1, as I believed Q1 was bad. However, testing Q1 out of the circuit using my DMM in diode mode showed that Q1 tested good. I had clipped the leads of Q1 removing it, so I had to wait for my replacement to arrive.

After soldering in a new bias pot I got to thinking about Q1 not being bad, and what the readings I had taken when it was installed meant. I went back and educated myself on how transistors operate (it’s been going on 20 years since I had a circuits class), and I realized that the zero voltage on the base likely meant that Q1 wasn’t getting a signal to that leg, not that it was toast. Now, I knew I had checked all the diodes and resistors on the board, and they checked good, but it seemed that an open diode or resistor between the input and Q1’s base was the most likely culprit.

I rechecked everything from the input to Q1 and all the components checked Sat. I sat there and stared at the schematic for a long while, trying to figure out what could be going on, when it struck me that the only “resistor” that I had not checked was the offset pot. I had checked it before, but apparently had only checked the leg that pointed away from Q1, because when I measured from the center leg to the leg pointing to Q1, I got a reading in the hundreds of Kilo-Ohms, no matter the position of the wiper.

After replacing Q1 and the offset pot, I was able to dial out the DC offset (to under 10 mV), and set the bias to 275 mA +/- 1, after the amp had warmed up for half an hour.
The amp has probably 8-10 hours of time playing music on it now, and it seems to be behaving.

I did build a dim-bulb tester to use when powering up the amp again.

Thanks for the help and comments to those who replied.

Here is the link to my other related post:

DH-220 Clone (SMART TA-242) WAY too high bias current issue?

Maybe this info will help someone in the future.

Mike

Last edited by beady; 16th March 2011 at 11:46 AM. Reason: Add more info
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Old 16th March 2011, 03:29 PM   #6
beady is offline beady  United States
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I meant to say since there was no signal at the collector, there must not have been a signal present at the base.
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Old 17th March 2011, 10:53 AM   #7
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Ah, I knew you could fix it.

Smallest can at the radio shack says "Deoxit" in extra tiny print on back of the can. This spray fixes potentiometers. Any amount of tinkering with older electronics definitely needs a can of Deoxit. Very dirty electronics needs the pots dusted with compressed air (to avoid mud) prior to using Deoxit.

A little white plastic tub of Gel Flux is located nearby. You can use this to do the precision soldering to add longer leads to clipped transistors and its also useful for SMD circuits. It makes the solder lock in place (in only the right place) cleanly and instantly, without excess heat. Its also pretty much vital for "sweating joints" which is renewing the soldering.
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