Audio Research D-115 Mk II issue

[dlnolet]

Any thoughts would be appreciated. My D-115 is acting up in the following way. Within the first minute or two of turning it on, I get a wah wah
warbling sound coming through the speakers. This usually lasts for 30-60 seconds, after which time the amp performs flawlessly as long as I have it on. AR thought it might be the caps. I then had them changed locally. All tubes check out fine and biasing is correct but problem persists. I'd like to deal with this locally ( Metro Detroit ) if I could. Thanks in advance for any guidance.

Dave

 

[microstrip]

Did you try replacing the tubes in the stabilized power supply for driver and input tubes ? If this warble happens in both channels I would expect it to be due to this common section. See https://www.audioasylum.com/cgi/vt.mpl?f=tubediy&m=264600 for the schematic of the power supply.

The symptoms also suggest that one tube needs a long time to warm-up - that can be a signal of low cathode emission.

Great amplifier BTW!

 

[dlnolet]

Thanks for your response. I have not tried what you suggested as my tech said the tubes were good. However, I will. The warble does happen on both channels. Excuse my ignorance, but do you happen to know which are the tube positions V-? in the stabalized power supply for driver and input I should swap out?

 

[microstrip]

According to the schematic V16 and V17 - one 6550 and the 12AT7 . I would try exchanging them. I now remember that the input inverter V1 is also shared - next I would try it.
We can find the complete schematics here http://4tubes.com/SCHEMATICS/BY-BRAND/Audio-research/

 

[dlnolet]

Thanks. I'll trying report.

D

 

[Gregadd]

Seriously. Only 30-60 seconds. A vintage amp performs flawlessly. Turn the volume down until the unit stabilizes. Something that should be done anyway to protect the unit from the initial voltage surge.
At least you can relax until the culprit is determined

 

[microstrip]

Seriously. Only 30-60 seconds. A vintage amp performs flawlessly. Turn the volume down until the unit stabilizes. Something that should be done anyway to protect the unit from the initial voltage surge.
At least you can relax until the culprit is determined

Apologize me, but I fully disagree with this advice. This is a clear signal of something is not working properly, and considering the complex circuit of the Audio Research stabilized power supplies involving tubes, transistors and integrated circuits it should be solved as soon as possible - ignoring these symptoms can result in heavy damage to the amplifier.

 

[Gregadd]

You may be right. If you read my post you will see, "Then you can relax until the real culprit is determined"(quoting myself). That is hardly an invitation to "ignore the problem.
Given the fact that he has consulted ARC,had thr caps replaced and it plays flawlessly it hardly appears urgent and indeed maybe much ado about nothing

Just my lay opinion of couse.

 

[dlnolet]

I've swapped out V16, V17 and V1 . The problem persists. When I turn the amp on having been off for may hours, the noise is more pronounced. If the amp has been on and warmed up and I then turn it off and on, the noise is much less pronounced. Does it sound like it might be a loose solder joint?

 

[Gregadd]

Old age?

 

[Gregadd]

I suppose it might help to put some historical perspective here
It appears the d115 debuted on1983. I'm not sure if there is a mk1. Discontinued in'89 would make it around 32 years old +/- a few years. That would be about 100 in human years.
AR was one the better manufacturers of that era. It was not uncommone to encounter turn on transients, hum,buzzes,mechanical resonances and the like. It was a good idea to keep the volume mute when doing any kind of switching. Power on or changing input. Many amps came with soft start features. This allowed the amp to stabilize before it could be played.
If your amp had this soft start feature you would not be bothered by this problem.
I have solved some of these problems by merely changing electrical outlet. Sometimes an expensive repair is required.
I had a resonance in my sub once. My dealer located the problem. One of the kids had put a Lego inside. My brand new amp only played one channel. I thought I had blown it. Turns out the main wire had separated dueing shipment. He showed how to fix it o er the phone.
Don't despair.

 

[microstrip]

I've swapped out V16, V17 and V1 . The problem persists. When I turn the amp on having been off for may hours, the noise is more pronounced. If the amp has been on and warmed up and I then turn it off and on, the noise is much less pronounced. Does it sound like it might be a loose solder joint?

I would not expect a loose solder joint to modify its behavior systematically after such short time - it takes a longer time to raise the temperature.

The D115 has five different power supply sections, it will not be easy to diagnose just from the symptoms. I would however try all new tubes before taking any action - I have seen cases where we have crosstalk because of excessive current drain by one signal tube.

 

[dlnolet]

Thanks for your advice.

D