That old 1S970 again...
If you google for this diode,a big 1000V power diode comes up,so something is obviously wrong.
http://www.vintage-radio.net/forum/showthread.php?t=96571
The diode in module 80 probably(I can't see any other explanation)works as a stabilizing diode since it draws around 12mA of extra current from the midpoint voltage.Now,since the current drawn by the transistor is far lower than that,the voltage across the diode is more or less constant in spite of the transistor's current varying.It stabilizes the amplifier is my theory.
Normally there will be a resistor and a capacitor here.
Try to find a diode that develops a voltage of 0.75V when 12-15mA flows through it.
Have you measured this voltage on your's?

That's what my guess would have been. Old amplifier designs often used some diode with bizarre characteristics to track the output stage and control idling current. If I recall, some of them were "selected" by the maker. It worked, sort of, but you find so many of these old dogs with blown output stages I wouldn't build one for a new design. Much better options available at lower cost.

 

It's probably not that important to find an exact replacement since R105 has a wide range of adjustment.
Another thing to keep an eye on is the quiescent current through the output transistors since different transistors and diodes are used.I would short R114(33ohms) to begin with,and very slowly increase it's value until the correct current is reached.

 

That's what my guess would have been. Old amplifier designs often used some diode with bizarre characteristics to track the output stage and control idling current. If I recall, some of them were "selected" by the maker. It worked, sort of, but you find so many of these old dogs with blown output stages I wouldn't build one for a new design. Much better options available at lower cost.

Actually is this one quite normal also in later designs.

The diode is only biased via that R110.
Take in mind that the power is "upsideDown".
Ground is the positive, and the voltage on "top" is negative 56V.

Having this in mind, it is quite easy to see that the W11-diode, the potmeter R105 and the transistor TR18 i no other than a constantcurrent-generator to achieve a well centered voltage at the output capasitor.
In worst case one could use a 1N4007, but a 1N4148 will probably do the work. 12mA is right in the middle of optimum range for that one.

 

Wow did not expect 3 pages thank you for the help!

TANDBERGEREN ....The diode is only biased via that R110.
Take in mind that the power is "upsideDown".
Ground is the positive, and the voltage on "top" is negative 56V.

This baffled me when i first took one of these apart. (I'm only 26) First time I've seen a positive ground circuit.

The diode in question isn't faulty or anything i just wanted to know for certain what type it was and because of you guys helping me that "1N4148" will work out just fine.

Just to explain myself a bit more here's a couple of images of the previous goodmans 80 innards that went into a box i whipped up. Looks fairly grotesque inside but does the job.

Inside

The front is just a piece of hardwood flooring i cut and shaped.

Front

It sounds ridiculously awesome combined with a DAC i call the "plank" (Bought the dac as PCB kit from china.).

Anyhoo,

My plan now and reason for needing to know about that diode; I'm having a go at making a prototype. A prototype amplifier Inspired by the goodman's with brand spanking new components. It probably won't work BUT i'll have fun trying.

P.S I have 2 more of these goodman's 80s stripped down. I'll probably try and strap the two left and right channels together and make two monos next.

Hopefully it won't all end in tears!

 

...
It worked, sort of, but you find so many of these old dogs with blown output stages I wouldn't build one for a new design. Much better options available at lower cost.

And modern amp chips have much better sound. That PSU will drive something like a "gainclone" amp and give incredible sound quality, and fantastic reliability as the amp chips have sophisticated overheat and overcurrent protections.

I would keep the nice old cabinet and put a couple of world class modern amp chips in it.

I did a similar one here;
http://www.romanblack.com/gainamp.htm

 

It's probably not that important to find an exact replacement since R105 has a wide range of adjustment.
Another thing to keep an eye on is the quiescent current through the output transistors since different transistors and diodes are used.I would short R114(33ohms) to begin with,and very slowly increase it's value until the correct current is reached.

Yeah, but the output current doesn't track because the temps of the power devices climbs and the bias transistors stay cool. That means the gain of the power transistors change with temp, even though the bias currents may be "constant".That's how they go into thermal runaway and smoke.

 

And modern amp chips have much better sound. That PSU will drive something like a "gainclone" amp and give incredible sound quality, and fantastic reliability as the amp chips have sophisticated overheat and overcurrent protections.

I would keep the nice old cabinet and put a couple of world class modern amp chips in it.

I did a similar one here;
http://www.romanblack.com/gainamp.htm

That's a neat job you did on rebuilding that amp.

In your write-up you noted it was made with a regulated supply. Have you ever seen an old Dynakit Stereo 120? It was one of the first solid-state amps with 60W channel into 8 ohms and it had a regulated supply. I built one in the 70's (man that's a long time back) and I'm still using it, now to power my old JBL bookshelf speakers on each side of my computer (kind of overkill). It still works and sounds very good (listening to Bluegrass on Pandora right now). Has a very transparent clean sound, no sign of crossover distortion. Only had to replace one electrolytic in all that time.

 

Tandbergeren-I wouldn't call it a constant current generator,but rather an amplifier with constant current.
In my world a constant current generator FEEDS current into an amplifier.
It doesn't amplify.

bountyhunter-yes,the diodes aren't physically connected to the output transistors,but that's how they were made these days.
Amplifiers with bias diodes or transistors connected thermally still blow up in thousands in spite of being designed that way.That's why there are so many forums around the world discussing broken amplifiers.
I wouldn't call a VW Beetle from 1952 a bad design since it only develops 25hp.It was just how they were made at the time...

Amplifiers with a stabilized power supply and positive ground and such don't surprise me one bit.They were common in early stereo amplifiers.But then I've had this hobby of electronics since the 50's..

 

Amplifiers with bias diodes or transistors connected thermally still blow up in thousands in spite of being designed that way.That's why there are so many forums around the world discussing broken amplifiers.

Hence the suggestion to use an integrated modern IC amp. They have inherent thermal tracking because it's all on the same substrate. They have built in current limiting and SOA protection as well as soft start (turn on) and turn off protection to eliminate speaker thumps. They really are a better way to go.

It was just how they were made at the time...

And they make them a lot better now. I have built dozens of projects. If I can make it better building it out of components, I do it that way. I don't think discrete design is better here. Just my opinion.

 

That's a neat job you did on rebuilding that amp.

In your write-up you noted it was made with a regulated supply. Have you ever seen an old Dynakit Stereo 120? It was one of the first solid-state amps with 60W channel into 8 ohms and it had a regulated supply.
...

Thanks for the nice words.

It's great to hear of well designed old amps and things still running well after decades.

And I love the idea of a voltage regulated PSU for audio amps, not just for the (potential) audio benefits but even for peace of mind, knowing that amp is never going to suffer an overvoltage situation or weird dropouts/glitches etc from mains supply issues.

Hence the suggestion to use an integrated modern IC amp. They have inherent thermal tracking because it's all on the same substrate. They have built in current limiting and SOA protection as well as soft start (turn on) and turn off protection to eliminate speaker thumps. They really are a better way to go.
...

And don't forget the incredible audio performance of modern amp chips! Really low THD figures and excellent transient response etc. Because everything is on the one silicon chip and is really well designed with modern technology they outperform old transistor amps by a huge margin.

(and apologoes to the OP for going a bit off-topic.)

 

It's great to hear of well designed old amps and things still running well after decades.

I have a 1974 Akai Tuner/Amp (60W/channel) that still lives. Never worked on it. Always had good sound. have no idea what's inside it. back in those days, you measured the quality of an amp by dead lifting it........ that told you how much money went into the heatsink and power transformer.

Took a real man to lift an amplifier back in the old days.

 

I fixed a ton of 70's Akai amps in the mid to late 80's, lots of gold coloured ones, and silver coloured too.

Yours is probably a discrete amp. Back when "60W" meant it could make a sinewave at 60W into a load. These days "60W" means it has a 5W per channel amp chip and a 7W (total) power transformer...

 

I fixed a ton of 70's Akai amps in the mid to late 80's, lots of gold coloured ones, and silver coloured too.

Yours is probably a discrete amp. Back when "60W" meant it could make a sinewave at 60W into a load. These days "60W" means it has a 5W per channel amp chip and a 7W (total) power transformer...

I have also seen this quite often. Like PC speakers with a max power rate. Several times higher than the maximum rated output for the accompanying wall-wart