Help with Dynaco 120 (SS)

[ilanco]

Hello,

I'm slowly introducing myself to the world of diy audio and so far it's been great ... untill i met the dynaco 120
After an easy repair (2 transistors and a capacitor) it played really good for a while.
Then one day it blew up
After trying to repair it like i did last time, it failed again with more smoke ...
I decided to change all the electronics on one channel and still nothing

I am using MJ802 as replacement for 2N3772 as that part is not available here.
In fact the amp used to work with MJ802 for a while so I guess they should do the job.

The power supply gives a steady 71.6V when the B+ of both channels are disconnected.
When I connect the right channel I get 71.6V across both collectors of Q5 and Q6. I know that 1 should be 71.6 and the other around 36V ... half the supply voltage.

With the left channel connected, the supply outputs around 10V and readings on the pins of the PCB get really weird -72 and such.

Hope someone can shed some light on whats going on.

Thanks for your help,

Ilan

[ilanco]

Update:

I desoldered all class B transistors (again) and checked them.
TIP31C went bad and is shorted. The strange thing is that it's brand new and i got shorted after the first time I turned on the amp.

Any idea what can cause this? I keep changing transistors and every time one or more of the 4 class B transistor fails.


Regards,

ilan

[Conrad Hoffman]

Can you bring it up to a lower voltage slowly on a Variac and look for unusual voltages? Do you check for shorts between device cases and the the heat sink? Also, check the connections of whatever the bias scheme is. If a bias pot or transistor goes open, outputs are sure to fail.

[slbender]

Hi ilan

The Stereo 120 despite its simple circuit path is actually quite difficult to fix when broken.. Randomly changing parts might make it unstable and then at some point while playing....pooff!

The TIP-31c/32c are sometimes too fast and the circuit needs some small value ceramic caps (or polystyrene) placed to limit the bandwidth, if you didn't fully install the "TIP" Retrofit, put those caps in now. Also check the 5.1 Volt zener. its probably blown. When any transistor in the 4 class B goes, that zener usually blows, replace with a 5.1V 1Watt zener. And maybe buy a hundred of those zeners.... you may need them!@! The worst part of the Stereo 120 is the people who soldered them. Over 90% of the people who built these kits couldn't solder right, and used really crummy soldering irons.... so the circuit board parts and wire connections, every one of them, is a failure waiting to happen...

I've got maybe 12 or 13 Stereo 120's in my closet which were the prototypes for my Bender-120.ULT Rebuild Kit (at Levels 1, 2, 3, 3A) which uses modern parts to bring the 1969 amp forty years into the future. Still if the soldering is bad, the modernized amp will also blow up.


-Steven L. Bender, Designer of Vintage Audio Equipment

Originally posted by ilanco
Update:

I desoldered all class B transistors (again) and checked them.
TIP31C went bad and is shorted. The strange thing is that it's brand new and i got shorted after the first time I turned on the amp.

Any idea what can cause this? I keep changing transistors and every time one or more of the 4 class B transistor fails.


Regards,

ilan

[llwhtt]

Years ago there was an article about the old ST120 in Audio Basics. Just search for Audio Basics and you will learn some of the problems and how to repair them. If you keep throwing parts at it without actually fixing the problems you'll get discouraged and give up. They CAN be reliable.

Craig

[ilanco]

Hi,

thanks for all your kind help.
I have one question thats keeps bugging be.
The supple has 2x72V output leads

One goes to the caps and one to pins 9-10 on the left channel.
Do both channels have to be in place to test the one of the channels or can I just connect the second 72V lead to only 1 of the channels ?


Thanks,

ilan

[TheLazyComic]

Hi ilan... I know this is an old post, but was wondering if you ever figured this problem out. If not, get back to me. thanks.

Randy