Transistor variants - interchangeable?

[tigger01]

Thanks everyone for your replies, I now understand a lot more about the specs of transistors. I had no idea that the gains can vary so much!

I am about to rebuild one channel of an amplifier (left side works but the right doesn't). Given the inaccuracy of transistor gains, and the fact that I am changing about 10 of them, am I likely to end up with anything useable let alone of equal volume both sides?

Thanks,

Neil

[anatech]

Hi Neil,

I had no idea that the gains can vary so much!

They sure do ......

Given the inaccuracy of transistor gains, and the fact that I am changing about 10 of them, am I likely to end up with anything useable let alone of equal volume both sides?

No, you should be fine. You really should match the differential pair. The overall gain is set by the feedback network - no worries there.

What is the make and model?

-Chris

[tigger01]

Hi Chris,

Thanks for the reassurance! It's a marantz PM66 KI Signature amplifier.

I'm afraid I'm a rank amateur, which ones are the differential pair, and how do I match them? So far I have simply obtained a schematic of the amplifier and ordered the necessary transistors for the power amplification. I have just received the parts today, so can now start playing!

Neil

[AndrewT]

Hi,
if you have just bought enough parts to repair the amp, then you won't have sufficient to select matching pairs.

Out of a batch of 100 small signal transistors, your work will get about 10 well matched pairs, 20 fairly close pairs and the rest random that have to be used where matching is unimportant.

To build a stereo amp you probably need about 50 to select for a scratch build.

Read up on matching and decide whether you want to buy the equipment you will need, what parts of the circuit benefit from matching, what tolerance you should work to and finally what parameters you need to match.

[anatech]

Hi Andrew,
No worries.
We found another AN7062! This is the front end, so matching is no possible.

Hi tigger01,
Once you have this together, start it slowly. DC offset without high current draw means you need one of these IC's. I have attached the datasheet for this IC.

-Chris

[tigger01]

Hi Andrew,

So to match the transistors I am going to need to buy / build a suitable power supply circuit and a test circuit. I will also need to buy something like 10 of each transistor I need as far as I can see. (found this useful site)

To be honest, this looks like it will start to get quite expensive, and I am trying to repair it in order to save the cost of replacing it.

Hi Chris,

Are you saying that matching is not possible for what I am doing?

I can see that my amp contains an AN7062, but why do i need to buy one?

Neil

[anatech]

Hi Neil,
For a straight repair, you are doing better than most shops. For a really good repair, you need to match the parts. From what I'm reading, just install the new ones. But you are now beginning to see what value a good technician is. They also have all teh equipment needed to do this job safely and properly without any further damage.

The AN7062 is the front end of the amplifier section in your amp. This is where the differential pair is located. Look at the data sheet I posted for you. I explained why you might need to replace it.

This amp is worth rebuilding properly. If you are only slapping it together against the cost of buying a new one, your reasoning is suspect. Go for the experience, but in all honesty you have much to learn. I feel your sense of values is horribly skewed.

Any job you do is worth doing right. If you happen to be successful in this attempt, understand that it was luck. Do not work on any friend's equipment, you are not qualified to do so. If you are serious about electronics as a hobby, anything you buy will be of continued use in the future - so you can't "charge" that against this one amp repair.

I have a feeling you do not appreciate all the things you don't know and have an under appreciation for the people who are trained and committed to repair these products properly.

Said my piece.

-Chris

[tigger01]

Chris,

To be honest I don't consider electronics a hobby and my knowledge and skills are minimal!

I know how good this amp is, I've had many years thoroughly enjoyable use from it, but I can't see myself getting "in to" testing or repairing amps for myself or anyone else. I embarqued down the road of trying to repair this one pretty much just to see if I could, and I didn't anticipate having to change all the transistors in one channel when I started!

My comment about cost stems purely from the fact that I wouldn't be likely to get much use from any power supplies and other equipment I bought, let alone all the extra transistors which I wouldn't use. I could replace this amp like for like on ebay for about £100 so I guess that is my yard stick.

So far I have spent over £40 on transistors for roughly twice as many as I need. I did this because I (wrongly) thought that if the spec of the replacement transistor did not closely enough match the original, that I may be better off changing both channels to keep things even. (Yes I know this sounds stupid now..) To have ten of each to choose from however would cost £200 and most would never be used!

If I am wasting my time, carrying on without matching then tell me and I will have to buy another. I'm not prepared to spend the price of a reasonable new amp on parts and equipment to repair this one, especially since I can't even guarentee I'll get it working without frying something else!

Hope I've cleared up my motives.

DC offset without high current draw means you need one of these IC's

Sorry but this really doesn't mean much to me. Can you elaborate?

Neil

[tigger01]

Hmmm...

Could I use the 12V rail from a PC power supply instead of the circuit shown in the site I linked to above?

If the differential pair are the ones I think they are (Q702 and Q704 from the schematic) they are quite cheap (2SA970GR and 2SC2240GR) although I can't find the GR flavour. Maybe I'll have a go at matching the transistors.

Are the transistors usually well matched originally or can they be improved upon?

What other transistors in the schematic would benefit from matching. I plan to replace Q702, Q704, Q752, Q754, Q756, Q758, Q760, Q762, Q764, QN02 & QN03.

Curiosity aroused!

Neil

[anatech]

Hi Neil,
Q705 contains your differential pair. The process used in making an IC normally guaranties extremely tight matching between transistors.

There is really no need to worry about the gain ranges in the parts you have bought, just replace the parts in the defective channel.

What exactly is the problem here? I fear we may have put the cart before the horse. Let's find out what is going on before touching this thing.

-Chris