HCA-1000 Repairing Left Channel - diyAudio
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Old 6th October 2013, 05:01 PM   #1
MrKay is offline MrKay  United States
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Default HCA-1000 Repairing Left Channel

Greetings DIYers. I am hoping to find some direction on what to do next.

I have a HCA-1000 that is on my bench a second time. The problem that started this is both fuses on the Left channel were blown due to Q117 and Q120 being dead shorted. Upon finding this I have replaced Q117-119 and Q120-122. After the replacing these (and of course the fuses) I powered the amp on and the Left channel with no input gets hot quickly. I have been working my way "back" with Q109 and Q110 removed the unit does not heat up so the problem I am guessing is "upstream" of that location. Any suggestions of common failiures to look for or suggestions on what to test would be appreciated.

M@
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Old 6th October 2013, 11:49 PM   #2
nattawa is online now nattawa  Canada
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Quote:
Originally Posted by MrKay View Post
........I have been working my way "back" with Q109 and Q110 removed the unit does not heat up so the problem I am guessing is "upstream" of that location.
M@
Not necessarily upstream. If the bias control circuit/components were shot you can have an out of control bias current problem.

Perhaps you want to first give the bias control transistors a health check, in my HCA-1000A schematic they are both Q111, one is PNP the other one is NPN. They are bolted on the heat sink. When these check out okay, you want to turn the bias pot to its maximum resistance value.

Then, put back the Q109 and Q110 you've just removed, but don't do it without giving them a health check first.

Next, lift the six 10-ohm resistors (R121~R126) at the base of the power transistors. Health check Q115, Q116, R119, and R120. Make sure they are all good.

Now, with a multimeter monitoring the DC voltage between the base of Q115 and Q116, turn on the power......wait, first make sure there is no speaker or other loads hooked up to the output of the amp, oops!

You should see a DC reading of about 2V on the meter. Turn the bias pot and you should be able to smoothly increase it to about 2.5V or so. It's also a good chance to make sure the DC offset at the output (the joint of the 0.33-ohm resistors) is under control - no more than a few mV. Turn the pot back to the minimum DC reading position. Turn off the power. Remove the multimeter test leads.

Health check the six power transistors, put back the six resistors (R121~r126). With multimeter monitoring the DC voltage between the bias test points, turn on the power. Turn the bias pot to set the bias. You can use the good channel as a reference.

One more words but could be irrelevant, my HCA-1000 has +/-69V on the rails at idle yet all the electrolytic caps were 63V rated, including all the 47uF e-caps. I bought mine used but I'm sure they were factory caps. I replaced them all with 80v and 100v rated ones. You probably want to check yours too.
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Old 7th October 2013, 05:02 AM   #3
MrKay is offline MrKay  United States
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Nattawa, Thank you for your quick and detailed reply.
Here is what I have found following your directions:
The Q111- My schematic labels both as Q111 but the board shows Q111 and Q112 I just asumed a typo in the schematic Q111 .64 and infinate Q112 .64 and infinite using the diode test on a Fluke 189

I re-installed after checking Q109 and Q110 (both showed .69 and infinite).

Checked Q115, Q116, Q119, Q120 (between .56-.58 and infinite)

Checking the DC voltage between the bases of Q115 and Q116 from on the low end was 2.15V. I set it to 2.5 as you directed and checked the voltage between the .33ohm resistors and ground was .001V. Set the pot to back to lowest setting. Checked Q117-122 all tested good .57 and infinite.

I checked the Right channel and the bias was set at 2.33V so I set the Left channel to this everything looking good so far I went to grab a signal generator to put a test signal on the channel and when I cam back to the bench, maybe 2 minutes the Left channel was hot (130 degrees F). The bias voltage was still showing the same.

Any other insights or things that come to mind to check would be appreciated. Thank you again for your help so far.

M@
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Old 7th October 2013, 01:02 PM   #4
nattawa is online now nattawa  Canada
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I assume by the "bias voltage was still showing the same" you were referring to the voltage between the base of Q115 and Q116 (that 2.33V)? What is the reading on the other channel when warm? Can you make sure Q115 and Q116 are securely fastened onto the heat sink (good thermal coupling)? Are you able to turn back the bias pot and lower the temperature? I would now probe with an oscilloscope at the output to see if there is oscillation.

Did you look at the rail voltages and the voltage rating of the electrolytic caps? If you have failed or failing caps they may contribute an oscillation.
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Old 10th October 2013, 09:55 PM   #5
MrKay is offline MrKay  United States
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Sorry for the delayed reply, I had some car troubles that demanded immediate attention.

In response to your questions. Yes I was referring to the to the bias reading between Q115 and Q116 when I stated that "bias voltage was still showing the same". The reading on the other channel when warm was 2.33V.

I checked and both Q115 and Q116 are securely fastened to the heat sink.
Turning the pot did not have an discernible effect on the temperature.

I did probe the output. I was not sure what output you were referring to Q115, Q116 or the channel output on the rear of the Amp, so I did all 3. When the amp is first powered on the signal is flat but after a short period of time less then 1.5 minutes they all begin to oscillate at Q115 and Q116 my scope shows an amplitude of 1.58 volts and a frequency of 4.75 Mhz the channel output (at rear of amp) shows 3.8 volts at the same frequency.

The rail voltages are at 66 and 64 volts on both channels and the caps are 63 volt rated like you said.

Do you have suggestions on where to start in looking for the failed capacitors?

Again thank you for your help.
M@
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Old 11th October 2013, 09:14 PM   #6
nattawa is online now nattawa  Canada
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I meant to say output node, which is all the 0.33ohm resistors are tied together onto. But it was not important precisely where you probed at, once you spot an oscillation it explains the overheating.

There are two types, total 16 caps in this amp that can be operating at voltages above their rating and you probably want to change them all. They are:

6800uF, (could be 5600uf in HCA-1000) C001, C006, C011, C016, in the middle of the PCB, big and tall, you can't miss them. In my amp they were glued down to the PCB by some strong adhesive to protect against G forces. You may want to de-solder and free up the pins first with the help of a suction tool or de-solder wick, before working on breaking up the glue. For the diameter and the pin spacing you probably will be using higher capacitance caps as replacement. But there is no need to go further higher than that.

47uF, C109, C111, C113, C110, C112, C114 in left channel, another 6 at same locations in right channel that the schematic does not show their labeling, but they are the only and all 47uf 63v electrolytic caps, fairly easy to locate.

When you're at it, inspect the solder joints of the small caps, C106, C107, C108, and C127, or simply all solder joints for suspected cracked joint. It's not a bad idea to give them a quick touch up burn with your solder gun. Do use flux.
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Old 14th October 2013, 02:23 AM   #7
MrKay is offline MrKay  United States
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I do not have replacements for all the caps that were listed on hand so I removed and checked there capacitance, all checked out. I decided that I would swap with the corresponding component on the Right (fully functional) channel.

I switched the following caps C001, C006 with C011, C016
C109, C111, C113, C110, C112, C114 with the C209, C211, C213, C210, C212, C214
C106, C107, C108, and C127 with the C206, C207, C208, C227

checking both channels on the oscilloscope the Left continues to show the oscillation and the Right remains the same.

Not the news I was hoping to report.
Any further suggestions?

M@
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Old 14th October 2013, 07:14 AM   #8
Ektalog is offline Ektalog  United States
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Well well! I just dropped by the forum to ask Nattawa if he may have further thoughts on my own HCA 1000a behavior. He helped me months ago on my haltingly slow but ongoing quest to fix my unit's bias issue. Yet, it was only last weekend that I had time to do some extra work. I am not there yet. The lack of proper diagnostic tools does hamper my progress, which is why I am very patient about it.

Rather than risk thread hijacking, I'll just wait on the sidelines and see what I can contribute in the meantime.

In case it helps, so far I have changed the trim pots (right channel) only, I refreshed suspicious-looking solder points, upgraded most electrolytic caps to 80V (not the power ones) and checked the c215/c216 caps (which is really two each in bypass mode). None of these seemed obviously bad though the exercise did yield a less "jumpy" bias adjustment sensitivity. The right channel's bias is still the issue but it now is more like "predictably bad" as opposed to "erratically bad". [I am now thinking that bad trannies may be at work].

My unit seems to have been adjusted (in later production?) such that I measure power rails of no more than 55V max on the DMM...or am I reading average and not peak?

Factory bias specs calls for 10mv measured across the .33/3w emitter resistors. The left side is rock stable at this value. OTOH, the right channel ain't. On this channel, turning on the unit and waiting for warm up can only be expected to adjust up to about 5-6mV stably. As one adjusts the pots and approaches 10mV, it suddenly starts climbing into the upper 30mV at best, or lower 40s at one recent time. This happens gradually over minutes, not all at once (unless one cranks up the pot, of course).

On the stable left side, the 10mV corresponds to around .75V across both R105/R106. At the adjusted starting value of 5mV on the right channel, R205/R206 are around .64V.

On no input conditions, the above state will all sit fat and happy for hours if untouched. However, like mentioned before, attempting to raise bias at the pots on the right channel will launch an auto move towards the high end. The highest voltage measured across R205/R206 during this climb was .74V (but I got interrupted by visitors and dropped the matter just then).

Oh, if the bias goes up on its own, a slight trim pot nudge to bring it down will THEN result in a gradual auto descent ALWAYS to below 10mV. This value is the obvious threshold.

Like I said, this is info in case it helps with the original post. We can take on my issue later, in a new thread.
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Old 14th October 2013, 05:02 PM   #9
nattawa is online now nattawa  Canada
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Hi Ektalog, when you have a "threshold" in the bias pot adjustment you almost definitely have oscillation, which is most likely the same stability issue as MrKay is having with his amp. Unfortunately, or fortunately rather, I have never had to go through it with mine so I have little more to offer in terms of troubleshooting ideas than what you both have done. One more easy thing that you can check is the connection and parts of the Zobel network at the output node, R143/R243, C126/C226, R145/R245, C133/C233.

Other than that, I hope other experienced can chime in and help. Maybe Mr. John Curl, the original designer himself could offer some insight as well.

As far as I'm aware of, one of the design target of HCA-1000 was ridding of the inductor at the output node, so that the amp establishes a more "direct" connection with the speakers hence a better control over them. This inductor is required in majority more "conventional" amp designs serving for good stability margins at high frequencies. HCA-1000 obviously did that successfully, but not without employing more complicated compensation techniques than conventional designs. It is also my suspicion that such design can be more sensitive than others to component aging/deterioration in terms of stability.

Last edited by nattawa; 14th October 2013 at 05:19 PM.
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Old 14th October 2013, 05:55 PM   #10
Ektalog is offline Ektalog  United States
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Thanks, Nattawa. I'll look into it...one never knows!

You just reminded me of something that may be useful to MrKay.
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