TSA8/erratic AVC

[e093116]

Hello,

I've noticed what I believe is erratic behavior of the AVC function in my LS3/TSA 8 amp.  The symptoms are very long delay at start of the record before the volume comes up, decreases in volume during the middle of the record (with a volume ramp back up after a variable period of low volume time during play - independent of record level), and so on.  When the problem occurs, what sound that is there is good quality, there is no distortion, and the "lowest" volume I get varies with the master volume setting and seems appropriate with the initial level/sound you'd get when the record comes on.

I have recapped this board and it was working properly a few months ago (August timeframe) or so I thought, the temperature in the shop is now an average of 50 degrees whereas before it was 75F.

My initial thoughts are that the temperature change could have cause an intermittent connection at the circuit board edge connector, I have a bad solder joint on the 200 uF cap (C5122), or a bad C5122 cap. 

One other thing is that this phonograph is mostly filled with records from the UK and Ireland (45 rpm small center hole), most of them made between 1972-1982.   I haven't checked if the problem is isolated to these records, but I did notice a "Europe" equalization setting on a TSA10 amp, could the records be the cause?  Back in August, I had US made records installed.  One UK record in particular (Sultans of Swing), seems to always have the volume drop when playing.  Would shorting across the 200 uF cap negate the AVC function and keep the volume level "high"?  I suppose I could just move the records to a different phonograph and see what happens...

Any thoughts on other things to check or troubleshooting advise would be appreciated.

Thanks as always.

Bill

 

[Ron Rich]

Hi Bill,
Well, without a schematic, I have no idea which circuit"C5122" is in, but my first thought is "fix-it" ! After re-seat the AVC board.
Have you determined if this happens in both channels ? When you re-capped it did you clean the mute relay ? Did you clean the volume control--is the problem independent of volume setting ? I doubt it record related-- Ron Rich

[robnyc]

Bill, the first thing to examine is the action of the AGC squelch. When the amp in in either standby or trip mode the mute relay is pulled-in. A set of contacts open and allows a squelch voltage of approx 5 VDC to be applied to the AGC time constant capacitor.

See it here: www.flickr.com/photos/90641375@N06/38387854546/in/dateposted-public/ The T-C is the blue 'lytic near the top. It is normally a 100mfd. I use 47mfd as part of a set of mods. If the squelch contacts on the relay are making poor contact it will allow buildup of the squelch voltage when it should not be there.

Connect a meter across the time constant cap. Plus probe to cap and neg to chassis. Select a record and measure the voltage as the record plays. I can't recall the normal range for an un-modded amp at this point, but approx 2 vdc will be for a "loud" record on-average. If the volume changes in error lift the tonearm and watch the AGC voltage. It should slowly decline to zero or nearly so.

Anything outside these actions indicates a fault in either the relay contacts or a possible problem in one of the AGC driver transistors.

The 200mfd caps on that same board are the hold-time constants for the protection circuit. They should also be replaced but are not the issue here.

RobNYC

[Ron Rich]

Rob, Bill,
There also could be an intermittent stabistor--these were famous for that problem--, but I would bet on the mute relay---Ron Rich

[e093116]

Ron, Rob,

Thanks for your comments.

I did the test as Rob described last night and found that the voltage would drift high (3-5 VDC range) while the record was playing, the volume would decrease when the voltage went up. I found that if I gave the top of the amp a light "Benny Hill" type head tap, it would induce the problem.

I cleaned the relay contacts with contact cleaner, but this didn't do much. I took it back apart and mechanically cleaned the contacts...this took care of the problem, I could "bitch slap" the amp and the voltage wouldn't change.  I consider this problem solved.

Thanks again for your guidance on this, I probably would have spent a lot of time and $$$ trying to figure this out on my own.  I like $0 fixes!

Regards,

Bill

[Ron Rich]

Hi Bill,
Glad itz fizzed !
Just curious---Which contacts on which board(s), did you "spray", with what ?
Which contacts did you take apart , and how did you clean them ?
Thanks, Ron Rich

[e093116]

Hi Ron,

I sprayed the contacts inside the mute relay (after taking the can off) with aerosol contact cleaner that you can find in most big box store electrical departments.

When that didn't work, I took the little three pronged guy off of the mute relay coil (disconnect the spring and release the little locking clip), the contacts were not burned or pitted, just oxidized, I polished them lightly with a small piece of 3M 216U P400 sandpaper (folded in half) torn from a 2.75" wide PSA roll (typical autobody item).

I have a couple burnishing files but find that I like to use sandpaper in most instances for size/flexibility, greater control of the amount of material removed, and surface finish. 

I found I didn't need to do any reforming/bending in this case.

Have a nice weekend.

Bill

[Ron Rich]

Thanks, Bill,
BTW, IMHO, NO "sandpaper" of any kind belongs anywhere near a silver/gold contact point. Sand embeds itself into the soft material, to later cause problems. Also, no "spray cleaner" that leaves any residue should be used-- That relay used "cross-bar" contacts, which require re-dressing or replacement, if gouged out. For cleaning, a piece of stock paper, saturated with a non residue cleaner ( such as 91 % rubbing al-key haul) works fine. For other contacts see the FAQ's above) Ron Rich