Hello!
I've recently 'repaired' an ARC D70. It was victim to a catastrophic failure, with the filter caps shorted with signs of arcing, a bias cap bulging and a blown 6550 tube. After replacing all the electrolytic's, a couple tubes (and re-biasing, of course) and some burnt out resistors, it sounds amazing!
The only trouble is that it makes a short zappy 'hum' on startup while it warms up (faulty thermister perhaps?), and every time you turn it off, there is a loud thump/pop in the speakers. I would assume since this beast has very complex (at least for me) power supply design, that this might not be normal operation- especially the shut down pop.
I'm running an attenuator from my source to the inputs, so no preamp to blame.
Any suggestions?
Schematic attached.
I've recently 'repaired' an ARC D70. It was victim to a catastrophic failure, with the filter caps shorted with signs of arcing, a bias cap bulging and a blown 6550 tube. After replacing all the electrolytic's, a couple tubes (and re-biasing, of course) and some burnt out resistors, it sounds amazing!
The only trouble is that it makes a short zappy 'hum' on startup while it warms up (faulty thermister perhaps?), and every time you turn it off, there is a loud thump/pop in the speakers. I would assume since this beast has very complex (at least for me) power supply design, that this might not be normal operation- especially the shut down pop.
I'm running an attenuator from my source to the inputs, so no preamp to blame.
Any suggestions?
Schematic attached.
All, or just the ones shorted/arc'd? Sounds like there's others that are bad in there, throwing the turn on/off sequencing off kilter. Go deeper in replacement of electrolytic caps. Can you measure them in circuit? Doing so might be a place to start.
Thanks for your prompt response!
Yes, ALL the electrolytics have been replaced- even the hard to reach ones!
By measure 'in circuit', do you mean for correct value and/or leakage, or do you mean measure to see if the voltage at the various caps is correct?
Yes, ALL the electrolytics have been replaced- even the hard to reach ones!
By measure 'in circuit', do you mean for correct value and/or leakage, or do you mean measure to see if the voltage at the various caps is correct?
You're welcome. If they've all been replaced, no need to measure 'em. Something else may have got taken out during the arcing; perhaps one of those (big) pass transistors?
Be careful, I'd look what's happening voltage wise with a scope during power on/off. May take many cycles with a long wait between, just to move the probe about that circuit to see. Plus something that happens first to trigger the sweep. Connect the probe and move away when powering on. Maybe solder on probe hook when it's been off for a long time, after checking voltages for safety with a DMM.
Be careful, I'd look what's happening voltage wise with a scope during power on/off. May take many cycles with a long wait between, just to move the probe about that circuit to see. Plus something that happens first to trigger the sweep. Connect the probe and move away when powering on. Maybe solder on probe hook when it's been off for a long time, after checking voltages for safety with a DMM.
TRANSISTORS! Oh no!
I'll go through them all to see if the voltages are close to expected. As far as scope testing, I'd be trying to trace where the pop is coming from? I apologize, I'm self taught and not up on all the lingo and theory. I'd assume a dummy load and testing backwards from output...
I'll go through them all to see if the voltages are close to expected. As far as scope testing, I'd be trying to trace where the pop is coming from? I apologize, I'm self taught and not up on all the lingo and theory. I'd assume a dummy load and testing backwards from output...
Happy hum? As in transformer inrush hum? If so that sounds like a short somewhere, an alternative could be that the muting circuit died and you're simply hearing the start up.
My old (and certainly cheap) A220 will move the cones on start up and shutdown. It doesn't have a output mute and only uses a grounding of the pre section to mute during startup hence the power section moves the cones.
I've spent some happy times with a D70 in the eighties, and I don't recollect any of the issues you mentioned. While ARC gears have a strong tendency to self-destruct, their intrinsic audio quality makes them totally worthwhile to spend time and effort to make them right, even improving on the 40 y. o. original design.
In your case, I suspect there could be others parts that are faulty beside those you've replaced. It could be as simple as your external power switch: you may want to add a spark suppressor to it. Else, you'll have to take your time and understand the (fairly complex) power supply:
In your case, I suspect there could be others parts that are faulty beside those you've replaced. It could be as simple as your external power switch: you may want to add a spark suppressor to it. Else, you'll have to take your time and understand the (fairly complex) power supply:
- The main beef (+430V, tags 22-23-24) is provided by the section comprising D5-6-7-8, of which C15-16 are usually cooked to an early death.
- A supplementary section, D1-2-3-4, rides on top of the main section and provides +570V for:
- The screen grid regulator, V12-13, which provide +335V to the screens
- The driver regulator, Q9, which provides +430V to the drivers
- The low level regulator, Q1-2 & U2, which provides +327V to the input stages
- An incredibly convoluted scheme based on U1 is used to provide the -42V bias and the voltage reference to the regulators above. I'm pretty certain that's from excessive doses of LSD.
Bah-Ha! I can only imagine the spaces some designer gets into, when deep on something like that power supply.
As mentioned by other members, yes I would suspect solid-state sections everywhere in your D70... Much numerous than on my D76 when comparing the schematics !
T
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The turnoff noise you are referring to (thump, pop, whatever) is present in all D70 & D115 amplifiers. There are ways to eliminate it, but they require a fair amount of effort and technical savvy. Most of us just live with it.
Over at Audiokarma, Dave Gillespie wrote a fascinating thread about restoring a "mystery" D115. Resolving the turn-off thump occupies a significant portion of the thread. As audio king says, this is not uncommon and requires some expertise to eliminate.
https://audiokarma.org/forums/index...-true-arc-mystery.927813/page-9#post-14177399
https://audiokarma.org/forums/index...-true-arc-mystery.927813/page-9#post-14177399
I had completely forgotten about that thread! I have been working on Audio Research equipment since the 1970's, and I found that project fascinating.
On AK I go by "arts". As that name was already taken here, I used my old shop name instead.
These are what I refer to as ARC's covering all the bases models: Vacuum tubes, discrete semiconductors and integrated circuits, all working together on the same board. The power supply, for those who can actually understand it, is a magnificent piece of engineering.
Dave's efforts to identify and resurrect what was essentially a pile of unknown bits and pieces, achieve success, and then resolve the "thump" issue was a credit to his skill, experience and perseverance. One of the few guys (and I deal with engineering staff every day of the week..) I have ever known who truly understands circuit design, and can "do the math". And a darned good technical writer as well.
My old D70 MKII is still sitting in my to do pile. I have been avoiding it as they are such a PITA (physically) to work on. Some day🙂
On AK I go by "arts". As that name was already taken here, I used my old shop name instead.
These are what I refer to as ARC's covering all the bases models: Vacuum tubes, discrete semiconductors and integrated circuits, all working together on the same board. The power supply, for those who can actually understand it, is a magnificent piece of engineering.
Dave's efforts to identify and resurrect what was essentially a pile of unknown bits and pieces, achieve success, and then resolve the "thump" issue was a credit to his skill, experience and perseverance. One of the few guys (and I deal with engineering staff every day of the week..) I have ever known who truly understands circuit design, and can "do the math". And a darned good technical writer as well.
My old D70 MKII is still sitting in my to do pile. I have been avoiding it as they are such a PITA (physically) to work on. Some day🙂
I once had an amp play up and I tested all the transistors with a diode check.
Every transistor was fine but amp didnt play well at all.
In the end I took out each transistor and checked hfe.
One of the input long tailed pair transistors had an hfe of 1.
Changed it and all was well again.
I had another that was highly distorted.
Again checked all transistor with diode check and all were good.
I kept going back to one transistor where voltages didnt seem right.
Then it suddenly hit me, while diode check was good the polarity of the transistor was wrong.
Some kind previous owner had tried to repair it and put in a pnp instead of a npn.
The number had rubbed off.
Found the obsolete transistor on ebay and amp burst into life and no distortion.
Never take anything for granted.
Testing transistors is no big deal, you can buy a Mega328 based tester for 10 bucks. Plus it tests LCR with better than 1% accuracy. It's all over Aliexpress, a must have.
Getting the transistors out is more trouble. I recently fixed my Valhalla board for the Linn LP12 by clipping ALL the IC, desoldering the pins, and replace with new ones. For ordinary components, the cost is negligible, and it's FAST!
Getting the transistors out is more trouble. I recently fixed my Valhalla board for the Linn LP12 by clipping ALL the IC, desoldering the pins, and replace with new ones. For ordinary components, the cost is negligible, and it's FAST!
Those things are great! I hope they keep iterating on that until...we get space odyssey 2001 level, where it creates the schematic and shows you the highlighted connection trace - with no prior knowledge of the device you're probing. Am I asking too much again?
Thank you all so much! I unexpectedly got crazy busy and haven't had a chance to get it back on the bench yet, but at least now I've got some things to try. I'll report back soon.
Oh! I'm not sure if this is a clue or not, but immediately after turning the unit off, that beautiful power tube blue-glow blinks off for a half second, then briefly flashes blue again in perfect time with the 'pop' before going dark. I've also left the power switch in the on position (in case the contacts were corroded) and pluged and unplugged the power cord with no change, as well as by-passing the relay/thermistors (also with no change).
Oh! I'm not sure if this is a clue or not, but immediately after turning the unit off, that beautiful power tube blue-glow blinks off for a half second, then briefly flashes blue again in perfect time with the 'pop' before going dark. I've also left the power switch in the on position (in case the contacts were corroded) and pluged and unplugged the power cord with no change, as well as by-passing the relay/thermistors (also with no change).
The power tube pulse is the symptom, not the cause. The turn-off surge is sending an enormous amount of power throught the output tubes. That's why is important to address the cause, if possible.
Hi Smoth Check R80 27k 7-watt wire wound is not open as I had exactly the same problem on my D70 and have replaced most of the resistors as some had drifted 20% outside tolerance. The problem was quite a large pop when the amp was turned off with some visual kick back in the output valves. You will need to lift one leg of the R80 resistor as mine looked ok but measured high in circuit but was OPEN when tested. All the best