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
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
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
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
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.
Help with Dynaco 120 (ss)
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
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
ilanco said:
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
Craig
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
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
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
Randy
Hi Steven I hope you can help me. Many years ago I upgraded my Stereo 120 with your Bender 120 ULT level 2 kit. The amp has been working fine until recently. The right channel now has become very raspy sounding with diminished volume and lots of distortion. The left channel is fine. The PAS3 preamp I'm using to drive it is fine (I checked it out by connecting it to a different amp). Thinking the C7 capacitor for the right channel may be going bad I removed it and it seemed to test fine. Next I swapped the Left channel C7 cap to the right channel and installed the right C7 in the left channel. The results were the same distorted sound on the right channel while the left channel sounded fine.Help with Dynaco 120 (ss)
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
Do you have any ideas as to what could be the problem? Thanks Carl