Heads Up! Van Alstine Model One preamp and Model 2 poweramp

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Toni Rambold

Re: Heads Up! Van Alstine Model One preamp and Model 2 poweramp
« Reply #20 on: 14 Sep 2018, 12:05 pm »
… These are the Biasing Instructions:






the bias adjust potis on the boards are the outer ones. the inner ones
are called offset trim - you have to ask my old master of electrical engineering
what that means.


hope i could help - Regards Toni

avahifi

Re: Heads Up! Van Alstine Model One preamp and Model 2 poweramp
« Reply #21 on: 14 Sep 2018, 03:52 pm »
When you replace those old power supply electrolytic capacitors, you are going to have to be creative.

The problem is that new electrolyics have gained vastly in volumetric efficiency over the years.  A capacitor of the same capacitance and voltage rating is now MUCH smaller physically than the ones made 30+ years ago.  This is a good thing, except in your case.

The Model 2 was designed for specific physical size capacitors to provide support for the audio circuit boards.  These capacitors are now long gone.  You will find the right value and voltage ratings at DigiKey or Mouser, but you will need to add connection wires to get their terminals to the audio boards as they will be much smaller.

Frank

dB Cooper

Re: Heads Up! Van Alstine Model One preamp and Model 2 poweramp
« Reply #22 on: 15 Sep 2018, 03:20 am »
Wow, this thread was started almost ten years ago....

Toni Rambold

Re: Heads Up! Van Alstine Model One preamp and Model 2 poweramp
« Reply #23 on: 15 Sep 2018, 11:45 am »
 :lol: ... no, it started almost 40 years ago, when a magazine in his January 1979 issue

introduced the three van alstine components:



... sorry for the cutout !

Remember the good old days:

A pair of Magnepan Tympani 1D for $1395, a Model Two for $800 and a Model One for $600 -

and you could live happily ever after !


depaj

  • Jr. Member
  • Posts: 6
Re: Heads Up! Van Alstine Model One preamp and Model 2 poweramp
« Reply #24 on: 25 Sep 2018, 08:17 am »
Hi guys,

Sorry for the delay, I didn't get a notification for your replies (strange because I got them for the first ones).

Thank you Toni for the info on the biasing procedure  :D

Anyway, I bought the new caps, 100v 22000µF Nippon Chemi-con with exactly the same dimensions except for a little less high. I will place something underneath them to get them to the right height.

Is there something I should absolutely check before turning on the amp with the new caps, waht they call the "monistor" for example, or the triac ? Are those prone to failure ? I ask this because when I first turned on the amp without the dim bulb the circuit breaker was "cracking like hell before stabilizing after a few seconds and well next of course the cap blew so... (again voltage across the cap was 76V.

Also I noticed the thermal sensor that is in parallel with one of the resistors in the fan circuit is reading open circuit, which means I guess that the fan is always going to be in the high position right ?


depaj

  • Jr. Member
  • Posts: 6
Re: Heads Up! Van Alstine Model One preamp and Model 2 poweramp
« Reply #25 on: 25 Sep 2018, 08:40 am »
Well, I answered my own question partially, the triac is dead, any advice on what part to replace it with ?

depaj

  • Jr. Member
  • Posts: 6
Re: Heads Up! Van Alstine Model One preamp and Model 2 poweramp
« Reply #26 on: 25 Sep 2018, 03:19 pm »
I was wrong about the thermal sensor for the fan speed control. Its open below 63°C and when heating it above that point (with a hair-dryer) it closes, makes perfect sense now that I look at the circuit again.

So only thing to replace would be the triac, I saw a previous post mentioning the Q6015L5 as a replacement that has now been replaced by the Q6015L5TP from littlefuse.
I will order one and keep you posted.

Toni Rambold

Re: Heads Up! Van Alstine Model One preamp and Model 2 poweramp
« Reply #27 on: 25 Sep 2018, 04:20 pm »
Triac and Movistor work as a switch-on current limiter.






part no. triac: Q6015L5

part no. movistor: 2502 GEKB



There are thermal sensors on both boards, e.g.:



so the fan operates in two speeds:

under 63°C low speed - over 63°C high speed

thermal cutoff is at 85°C:




charmerci

Re: Heads Up! Van Alstine Model One preamp and Model 2 poweramp
« Reply #28 on: 25 Sep 2018, 04:24 pm »
Remember the good old days:

A pair of Magnepan Tympani 1D for $1395, a Model Two for $800 and a Model One for $600 -

and you could live happily ever after !


There are inflation calculators - they really aren't such bargains when you add that.