Audio Research Stereo Amplifier HELP

valco

Active Member
I'm wondering if you guys could help me out. My father's AR amplifier is not working properly, all of a sudden. The amp will work for about 20 min just fine when first turned on. After that, the volume drops significantly and the sound becomes distorted and staticky. This all happens much sooner if you turn the volume up high.

I don't live near my father, and he does not have any decent techs near him. So I can't take a look at the amp myself. Any ideas as to what could be wrong with the amp?

Also, I can't seem to find a schematic for this amp. Anyone have one, or know here I can get it?

I'd greatly appreciated the help!

I've attached a photo of what the amp looks like. Not my father's. Just some pics I found online. Seems there is no model number
DSC_1687.jpg DSC_34751.jpg
 
That would be an Acoustic Research integrated amplifier. If both channels are acting up and it happens sooner when played loud it sounds like it's power supply and heat related. Part of the power supply is regulated and some tend to run warm, so that would be the first place to look, check all power supply voltages. The regulator supplies voltage to the preamp and the input stage of the power amp. The Service Manual is available at HiFi Engine, it's free just need to register.

Craig

Brian types faster than me!
 
Does the volume drop significantly and become staticky in both channels, or just one?

Does this happen on all inputs, or just a particular one?

If this issue happens on both channels, my guess is I would say first look at the power supply. Once a transistor gets hot, it is getting faulty, it reduces power to both channels, significantly reducing the output volume.
 
Does the volume drop significantly and become staticky in both channels, or just one?

Does this happen on all inputs, or just a particular one?

If this issue happens on both channels, my guess is I would say first look at the power supply. Once a transistor gets hot, it is getting faulty, it reduces power to both channels, significantly reducing the output volume.


He said the stereo is distorting on both channels. It also distorts if he switches to mono. With the volume backed down, the distortion is gone. So something in the power supply, perhaps?
 
Unfortunately, that appears to be a double filter cap that is two caps in one. Onkyo 4500s used to have those, but at least Onkyo put in two spaces for capacitors; so they could be replaced with two caps. Part of the issue could be the filter cap is getting weak.
 
Hmmmm, okay. Perhaps I'll just replace the caps when he visits in a few weeks.

Is there somewhere I can get replacement multi-caps? Or should I just use individual caps?
 
Is there somewhere I can get replacement multi-caps? Or should I just use individual caps?
See if you can read the readings on the cap you have in there now - take a couple of pics of it and post it here.

See post #3 above - we all reckon this is a powersupply issue. You could take those measurements or just change the cap to see if that does it. Doe you have a digital meter?

Potentially, change the cap first, and then if that doesnt do it, we will look at other components in there next.
 
Hi, sadly I don't have the amp. My father has it. He is gonna bring it up sometime in the next few days or weeks. I'll take a look inside then and see what the values of the multi-caps are. In the photo here, it looks like there are 3 multi-caps. Perhaps I should just replace all 3 of them?

AR_AMP__CAL5855.jpg
 
I don't think you are going to find any new multicaps in solid state values, low voltage/high uf. They are still made for vacuum tube equipment though, high voltage/low uf. Before throwing new caps in it in hopes of repairing it you troubleshoot and if a capacitor is the problem great. Capacitors can be checked in circuit with an ESR meter.
 
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Okay, I finally got the amp in my hands here. I'm getting very different issues than my father described. As far as I can tell, it seems that one channel is dead. The other one sounds just fine. Any ideas what should be looked at or replaced?
 
Hmmmm, okay, right after I made my last post. The whole amp went silent. So yeah, something is up here. Ive got it here opened up. Any help would be appreciated!
 
Im seeing:
about 7.8vdc on the tone control board. Should be 20vdc.
about 7.8vdc on the driver board. Should be 20vdc.
about 18vdc on the phono board. Should be 20vdc.
 
Everything but the phono, driver/output stage runs off of the 20VDC regulator circuit. The phono and output stage driver run off of T4 power supply. Looks like there are some Service Bulletins pertaining to the regulator. If you don't have the Service Manual or Bulletins they are available at HiFi Engine.
 
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40311 Regulator is reading:

C 28.8vdc
B 54vdc
E jumps around a bit but around 0.4vdc.

Also, seems the places where i was reading a low 7vdc now match the E vdc of 0.4vdc.

So, bad regulator?
 
Are you sure of your measurements, the regulator circuit is fed with +35VDC, not sure where 54VDC could come from. The base of Q28 has a 20V Zener on it so the base should read +20V or so. The collector of Q28 should have approx. +35V on. The emitter should have a bit less than the base if working correctly.
 
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