Transistor Beta Numbers & Matching Q?

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I am working on a 1976 vintage Audio Research D100A power amp and i have some questions about Beta matching output devices.

This amp uses 6 pairs of output devices plus a pair of drivers and another pair of transistors for Biasing. (wierd amp design).

Each transistor has a Beta number marked on it. The negative half has 3 output's marked "14" and 3 marked "39"

The positive half has 2 marked "17", 2 marked "43" and 2 marked "19"

The drivers and the Bias transistors are all marked "72" and are noted that they must be matched.

But for the output transistors. How close is close enough? all of the transistors are factory parts, so i can only assume that they were originally installed that way, or were installed by the factory that way during a repair of some sort.

17 & 19 i would imagine are close enough, but is 17/19 & 43 close enough??

How about the half with 14 & 39?

And what about the differences between each side?

I really dont want to have to replace all the transistors at $18.00 each!

This thing is such a PITA to work on that i only want to do this once!


Zc
 
Most people has been happily listening to class B amplifiers with over 2:1 gain mismatches between output stage halves for ages, and they don't complain at all, so who cares :D It seems quite strange to see such advanced matching practices applied to a 1976 product.

I would try to get close to 25, that seems to be the target value, together witn 72 for the drivers for a total gain of 1800.

Which are the gain figures of broken transistors and available replacements?
 
One Beta 17 output device failed. I was able to buy a replacement Beta 17 device directly from A.R. and One Driver failed. I bought a matched pair of drivers from A.R. so i should be ok.

I was just concerned that i would have to replace ALL the transistors because the numbers were all different. I missunderstood how the numbers worked.

Zc

P.S. one capacitor in the V-reg for the front end shorted out sending Un-reg to the driver & first output device in line before the fuses blew.
 
Here are the schematics. Note these is not standard BIAS arraingment. isntead they use 2 constant current sources to bias the output stage.
 

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Hi,
that DC servo seems somewhat unusual.
Connected to two compensation pins to pull the amp1 level up or down.

Is amp1 an IC opamp (+-55V regulated) or a potted discrete?

Amp2 is also high voltage.

I don't understand the biasing. Any help coming?

1976 and a snubber on the PSU. Who says these are new?

What are all those diodes and Zener on the feedback loop into amp2? Bridged limiter for protection?
 
AndrewT said:
Hi,
that DC servo seems somewhat unusual.
Connected to two compensation pins to pull the amp1 level up or down.

Is amp1 an IC opamp (+-55V regulated) or a potted discrete?

Amp2 is also high voltage.

I don't understand the biasing. Any help coming?

1976 and a snubber on the PSU. Who says these are new?

What are all those diodes and Zener on the feedback loop into amp2? Bridged limiter for protection?

Amp 1 & 2 are potted "Magic" modules. have no idea whats in them. the cap cam off of one and inside is a black potting material filled with small cork bits so there is no way to see inside.

I got the bad channel repaired and back up and running last night. I just need to fix the fuse board and its done! there is a 1.8K 5 watt carbon comp resistor for the LED power light that was sitting right noext to a fuse and it ROASTED the board and the fuse to the point the solder leaked out of the fuse. My cals say that resistor is dissapating about 4 watts so i am going to move the resistor to a new location where it can get some air around it. Plus the board is burnt beyond use in that spot again.

It is a weird amp thats for sure. even A.R. claims it is not one of there best. the parts guy told me they have hundereds of them in the warehouse that will never see the light of day again. I tried to talk him into selling me some, but he said no way.

BUT, side note. WZJ was there! didnt get to meet him but he dark green Caddy was parked in his assigned parking spot! He isnt there much these days.


Zc
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.