Leak Stereo 30 Overhaul... Worth it?

[bleachershane]

Originally Posted by Mooly

One thing concerns me on the circuit, the AF118 driver transistor. These have a peculiar failure mode, but if it is all working then leave well alone...

So just for completeness,
Anecdote: Tin Whiskers Inside of AF114 Transistors

Really funny you mention that particular transistor... I was looking at it and wondering "What is that forth lead for?"... Thank you for pointing me to that website, kinda worrying to think there may well be 'tin whiskers' growing inside that case! Does cutting the forth lead necessarily future proof it from any issues? Noticed that was a solution on that page...

[bleachershane]

Originally Posted by indianajo

Film caps don't deteriorate in my experience (SCR brand excepted as detailed in a post on here). Mallory & Sprague had their metal spray process down cold by 1966, don't know what brand these are.
Because the speaker cap does not cross through zero volts they shouldn't inhibit the sound (my experiments show), except 2200 or 3300 uf would give better bass response.

I'm also guessing that the film caps of that age may well contribute to the 'sound' of the tone circuit, something that may appeal to a future owner...
Perhaps I should've gone straight to 2200/3300uF... but I will audition it with the 1000uFs and see what it sounds like in terms of bass response...

[bleachershane]

Originally Posted by Mooly

You might be pleasantly surprised by just how good (musical) this amp could be given half a chance.

The circuit shows a test point to check the bias current in the output transistors. That's worth confirming... easy way, just measure the volt frop across either of the 1 ohms there and calculate current.

Some of the old carbon composition and "carbon deposited" tubular resistors (don't know the official name) can drift in value with age, so something else to be aware of, but again don't worry over it.

Will definitely have to check the bias current on the outputs next...
Wonder if it's worth checking the old carbon resistors for drift?

And now I'm terrified of tin whiskers lol

[indianajo]

Originally Posted by bleachershane

Wonder if it's worth checking the old carbon resistors for drift?

The carbon comp resistors that looked bubbled or dark brown in my ST70 and organ power supplies were definitely reading high, as much as 15% overrange. No other carbon comp resistors that I have pulled from the dynakit & hammond equipment (because metal film is quieter) have been off tolerance. I've been using $.07-.12 multicomp and vishay metal film resistors, and yes, they hiss less over 100000 ohms (plate resistors in the tube amps).
With a DVM you obviously don't have to pull the shunt to read the O.T. bias, just use clip leads to jumper the 0.47 ohm resistor. The 1000 ohm/volt meters your average hobbiest (like me) had in 1964 had would have changed the readings considerably.

[Mooly]

Originally Posted by bleachershane

Really funny you mention that particular transistor... I was looking at it and wondering "What is that forth lead for?"... Thank you for pointing me to that website, kinda worrying to think there may well be 'tin whiskers' growing inside that case! Does cutting the forth lead necessarily future proof it from any issues? Noticed that was a solution on that page...

I think I would leave well alone if it all works OK. Although a problem, it by no means suggests all will develop this problem... it was just something to be aware of.

The AFxxx transistors were often used in radio front ends and ran at very low voltages and currents where the problems would be very noticeable. It's possible (and I'm only guessing here) is that the currents and voltage swings when used as a driver transistor may well prevent these "whiskers" forming.

Enjoy the amp

[bleachershane]

Originally Posted by Mooly

I think I would leave well alone if it all works OK. Although a problem, it by no means suggests all will develop this problem... it was just something to be aware of.

The AFxxx transistors were often used in radio front ends and ran at very low voltages and currents where the problems would be very noticeable. It's possible (and I'm only guessing here) is that the currents and voltage swings when used as a driver transistor may well prevent these "whiskers" forming.

Enjoy the amp

I found the study on those transistors quite interesting (if not worrying!)
But yes, they seem to be working and so I will leave well alone... although germanium transistors are obsolete as betamax, I still find it a bit baffling that there aren't any on the (niche) market, and replacing them means finding originals from the early 60s! A shame!

[Mooly]

Originally Posted by bleachershane

I found the study on those transistors quite interesting (if not worrying!)
But yes, they seem to be working and so I will leave well alone... although germanium transistors are obsolete as betamax, I still find it a bit baffling that there aren't any on the (niche) market, and replacing them means finding originals from the early 60s! A shame!

Now then, I had two Sony SLC9 Beta machines in daily use until around 7 years ago. That's another story

You can still get germanium devices although the choice is limited. I built this from old parts I had,
GERMANIUM Single ended Class A Headphone Amp.

[bleachershane]

Originally Posted by Mooly

Now then, I had two Sony SLC9 Beta machines in daily use until around 7 years ago. That's another story

You can still get germanium devices although the choice is limited. I built this from old parts I had,
GERMANIUM Single ended Class A Headphone Amp.

Loving your putting to use of the germaniums! Why should they sit on the shelf when you can put something nice together out of them?!

Just finished the rest of the electrolytic recap... yet to test it as it's a bit late/ Also wired the voltage selector out and hard wired for 250V (I don't know why I have a habit of doing this, I just find them annoying!) and removed the absolutely pointless 'AC Outlet' socket... one less bunch of needless mains leads running through the loom! Starting to look neater... was going to tackle the inputs/switches but my mind exploded when I had a look and so I'll leave it for a day I actually have the mental energy



Here's how it looks now... fingers crossed it was worth it. A small amount of the old electrolytics had leaked/were looking corroded around their bases...

[indianajo]

Last year RS in UK had Germanium signal transistors at about L2 each. Or was it RB? whoever is #2 distributor over there, not Tandy corporation. Not that you know you need them yet. The only thing in the US (newark, mouser) was germanium doped silicon RF transistors, which is something else entirely.
FM radios used to run off the power outlet before they invented the power socket strip. You could turn off the whole system at your control hub (the amp). My PAS2 preamp had the power amp plugged into it for decades, one switch did it all. You don't need a preamp, it is all built into this unit. With the attendant selector switch problems.

[Mooly]

Originally Posted by bleachershane

Just finished the rest of the electrolytic recap... yet to test it as it's a bit late/ Also wired the voltage selector out and hard wired for 250V (I don't know why I have a habit of doing this, I just find them annoying!) and removed the absolutely pointless 'AC Outlet' socket... one less bunch of needless mains leads running through the loom! Starting to look neater... was going to tackle the inputs/switches but my mind exploded when I had a look and so I'll leave it for a day I actually have the mental energy

Here's how it looks now... fingers crossed it was worth it. A small amount of the old electrolytics had leaked/were looking corroded around their bases...

It's looking good. Sounds like you are going to keep it then