Replacement Transistor advice please

[Linnovice]

Hi all, I have a problem developed in my Revox A77 MkIII 4 track, I've had one of the input channels go down. I've traced it back to a faulty transistor on the input board 1.077.700 (No. Q401 on the schematic). I would like to change both transistors (channel I and II) and am unsure what alternative to use.

The faulty one in the board is marked MBC 3228 (or maybe 322B) EBC (which I presume means Emitor, Base, Collector).

According to the manual it should be BC 179 B & BC159 B.

Would any of you know a) why there is a difference in the board to the schematic (is it a later board?) and b) what would be a suitable replacement for either. I've looked on the internet for an alternative but, in truth, do not understand what alternatives are being offered.

Please excuse my ignorance, this is the first time I've had to change a transistor on an A77 that I couldn't get a straight replacement for.

[paulsherwin]

The BC159 and BC179 are basically the same transistor. A BC559 should have similar characteristics, though you might want to consider an upgrade to a lower noise transistor. I'm not familiar with the circuit in question.

[Linnovice]

Hi Paul, thanks for responding.
I have a sneaky suspicion that this board is from a later machine. All the transistors on the board have the same physical appearance, ie a semi circular black plastic material. (This is the one marked 3228). The other A77s I have are fitted with transistors that look like metal top hats (these are the BC 179 B type), hence my confusion.

I am with Paul on this, you won't go wrong with a better transistor (they get better all the time) a good circuit design (I don't think Revox ever did bad designs) has to cope with the very variable characteristics that all transistors have.

[paulsherwin]

The BC159 is a Lockfit. You definitely don't want one of those

The BC559 is the standard modern equivalent. A 2N3906 will be OK, but might be a bit noisy depending on the circuit. Check the leadout carefully.

See https://en.wikipedia.org/wiki/BC108_family

[G8UWM-MildMartin]

Might the original transistor be a Motorola BC322B?

[G6Tanuki]

BC179B - the 'top hat' metal-can transistors you mention - are industry-workhorses; the 'B' on the end indicatses it's a middle-of-the-road one as far as gain is concerned [there were A and C variants too - A having the lowest average gain and C the highest].

The suggested 2N3906 is also a typical workhorse - their gain-groupings were indicated coloured blob [orange, green, yellow].

Cricklewood have BC179B transistors at £1.50 each. See https://www.cricklewoodelectronics.com/BC179B.html

[Linnovice]

Originally Posted by G8UWM-MildMartin

Might the original transistor be a Motorola BC322B?

Yes, I think that may well be the case MBC 322B.

What would be the difference between this and the BC 179 B ?

[paulsherwin]

Looking at the specs, they are very similar to all the BC179 variants which have been mentioned. The BC179 is the original PNP complement to the classic NPN BC109.

The 'B' is the gain grouping and indicates a medium gain transistor. More gain is unlikely to matter - industrial designers generally used the lowest gain grouping that they could get away with, because these were normally a bit cheaper.

[Radio Wrangler]

Stick a 2N3906 in and ask yourself if the noise is any worse on that channel compared to the other. If OK, job done.

If not OK go looking for a transistor with lower noise specs. You'll need to know the bias current and the effective source impedance it is fed from to go down this route.

The thing about transistors is that there are so many to choose from.

The second thing about transistors is that there is so much overlap between types.

David

[toshiba tony]

Sorry to butt in on this post. Was in electronics all my adult life and worked for Sony and LG and loved every minute. Forgive my thickness, what was the bad issue with lockfit transistors? Are they they the plastic ones with a black plastic encapsulation, left the trade three years ago with a dementia issue but this lockfit issue is bugging me. Many thanks

[Cobaltblue]

Hi Tony

There have been a few threads on this but check this one:

https://www.vintage-radio.net/forum/...d.php?t=144412

I have added a picture grabbed off the net as an aid memoir.

If you want to explore this further please feel free to start a new thread if applicable the Mods can always merge it with an existing thread.

Cheers

Mike T

[Radio Wrangler]

Quite a large black plastic body, with legs pressed form flat sheet, having barbs and stops in their profiles so that they clicked into PCB mounting holes, s the transistors were secured for wave soldering.

Very high rate of failure in later life. I'm not sure whether it was stress on bondwires or corrosion, probably both! File under A-for-Avoid, with a capital 'A'.

David

[Michael Maurice]

I don’t think Revox ever used lockfit transistors in any of their machines. I’ve used BC327, 328 and 338 in the past with success.

[DMcMahon]

Originally Posted by Linnovice

Hi all, I have a problem developed in my Revox A77 MkIII 4 track, I've had one of the input channels go down. I've traced it back to a faulty transistor on the input board 1.077.700 (No. Q401 on the schematic).

According to the manual it should be BC 179 B & BC159 B.

Would any of you know why there is a difference in the board to the schematic (is it a later board?).

Looking at the various different documentation for the A77 Mk. variants I cannot easily see any other Input Amplifier Board other than the stated 1.077.700

All the various schematics shows the transistor to be a BC 179 as you say and listed in the parts lists as BC 179B, BC 159B as you say.

All the Input Board photos show top hat metal cased transistors (so BC 179 type rather than the Lockfit BC 159 type), so do not know how your board came to have the black plastic transistors.

Note that if the suspect transistor is a BC 322 it looks like the emitter and collector leads maybe physically swapped over wrt the BC 179, so carefully check the pin out of the replacement transistor legs.

David