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Author Topic: AMI CONTINENTAL 2 INSTABILITY PROBLEM (Motorboating)  (Read 3729 times)

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Squaddieskin

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AMI CONTINENTAL 2 INSTABILITY PROBLEM (Motorboating)
« on: March 29, 2009, 05:04:08 pm »
As a newbie on here could Alan or anyone else please advise on the following problem.

I have an annoying and apparently untraceable instability problem with my AMI continental 2.

When I first bought it it had terrible instability (typical screeching and banging) and I naturally assumed it was a decoupling problem.  The low emission valves didn't help either, so when it came to the rebuild I opted to change the basic amp circuit from the old 6973's for more easily available EL84s which many site operators did back in the 1970's.

The 2020-A pre-amp was thoroughly gone through at the same time.  As many of the resistor values were also way out these were all replaced and tightened up to 5% tolerance and the old .02 capacitors relaced with .022

New valves were replaced throughout.

The mute was shot but this was just a general contact clean and overhaul,

So far so good, but all of this hasn't entirely cured the problem with the motor boating which although a good 80% better is still there just as the AVC kicks in.  There is also a loud electrical pop when selecting records with the button bank or when adding credits manually. Also when the selector pins are cancelled.

Instability also occurs when the volume control is activated manually.

The bias line on the R-2017 basic amplifier splits through two 470k resistors to 0.47 capacitors ultimately going to the anodes of the 12AU7. 

I'm beginning to wonder if perhaps these really ought to be replaced with .01 instead but is this annoying problem in the amplifier itself or in the preamp - that's the question so hopefully someone on here can point me in the right direction

Plugging an external source into the amplifier alone its as clear as a bell, with no problem but of course its a fixed input load and doesn't vary with the AVC.

Many thanks in anticipation

John



« Last Edit: March 30, 2009, 07:58:53 am by Squaddieskin »

ami-man

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Re: AMI CONTINENTAL 2 INSTABILITY PROBLEM (Motorboating)
« Reply #1 on: March 31, 2009, 11:50:45 am »
Hi John,

I would post this problem on the www.phonoland.co/forum to get some info on the people who may have operated the continental.
I think that you need to get back to the original values or gain some insight into any mods that have been performed on the amplifier.

I have not done any work on that amplifier as of yet.

Regards
Alan Hood
ami-man
UK