Jim Marantz
Active Member
This thread was originally posted in "General Audio Discussion" by mistake. Moderators please remove that thread.
OK. I picked up another ST-400 from a local dealer. I paid probably too much but I liked the fact that I could see it in person before paying for it. The amp does work 100% but I could tell it needed a lot of work before I even opened it up. The DC offset was checked before I put speakers to it and it measured under .2mV on both channels, so I assumed it would be safe to listen to.
The sound is very distorted especially on the top end frequencies. The Low end kicks like a mule and the mids are a little dry. Plus, it has a nice hum, seems to be 120Hz so I'm thinking power supply problems.
Here's her pretty (subjective) face. Not perfect, but a good polishing will help a lot.
As a note, I'm approaching this "restoration" as if I'm dealing with an antique, more than a piece of "vintage" electronics. This amp has the possibility to produce 600W @ 8ohm in mono-block configuration. The transformer is 1,000 VA and is not to be toyed with for your own safety and it's complimentary equipment.
My other restored ST-400 puts out over 285 WPC @ 8ohm and 340 WPC @ 4ohm before any clipping can be detected, so it's a worthwhile endeavor.
Opened up, it's very dirty inside. I don't think any service was ever done to this amp.
The chokes on the fuse plate are black, meaning this amp was produced after 1975. The resistors in the choke assembly were originally 10 ohms in the first versions, but changed to 1.8ohms in response to some reviewers of the day not liking the 400's top end frequencies. To delineate the change, Dynaco covered the coils in black tubing.
Owners could bring their first generation models up to spec. by piggybacking a 2.2 ohm 2W 5% resistor to each coil.
Also, this unit has first generation output boards PC-28 which is a single sided board. Later boards were double sided to save a lot of jumpering. I would approximate a manufacture date of 1976.
Taking a measurement at the positive fuse holders I get a reading of 93V. I should be getting about 74V. My power supply suspicions are correct so far.
Here, the fascia has been removed and the cabinet face folded forward. I've removed the fuse plate with speaker relay and the main filter caps. We can see a little discoloration on the power supply board PC-30. (This is common on the ST-400)
Here's the other side of PC-30. One zenner is blown, the 850ohm resistor is cooked, the capacitor is dried out and the bridge rectifier gets flakey at higher currents. All component were checked "out of circuit". I will be changing every component on PC-30 and installing a new bridge rectifier of 35A 800V.
I tested the old filter caps (silver) and you can see the results above. The new caps should be a nice upgrade in performance.
As of today, I have some large orders coming in from Newark and Mouser with tons of components to rebuild the power section and also all new components for the (2) PC-28 output boards. I'm wanting to square away the power regulation before I attack the audio section. I'm still under $200.00 in on this restoration, not counting the purchase price. The Dynaguard / "Preamp" board PC-29 still has to be looked at, but I'm trying to not overwhelm myself at this point.
Jim
OK. I picked up another ST-400 from a local dealer. I paid probably too much but I liked the fact that I could see it in person before paying for it. The amp does work 100% but I could tell it needed a lot of work before I even opened it up. The DC offset was checked before I put speakers to it and it measured under .2mV on both channels, so I assumed it would be safe to listen to.
The sound is very distorted especially on the top end frequencies. The Low end kicks like a mule and the mids are a little dry. Plus, it has a nice hum, seems to be 120Hz so I'm thinking power supply problems.
Here's her pretty (subjective) face. Not perfect, but a good polishing will help a lot.
As a note, I'm approaching this "restoration" as if I'm dealing with an antique, more than a piece of "vintage" electronics. This amp has the possibility to produce 600W @ 8ohm in mono-block configuration. The transformer is 1,000 VA and is not to be toyed with for your own safety and it's complimentary equipment.
My other restored ST-400 puts out over 285 WPC @ 8ohm and 340 WPC @ 4ohm before any clipping can be detected, so it's a worthwhile endeavor.
Opened up, it's very dirty inside. I don't think any service was ever done to this amp.
The chokes on the fuse plate are black, meaning this amp was produced after 1975. The resistors in the choke assembly were originally 10 ohms in the first versions, but changed to 1.8ohms in response to some reviewers of the day not liking the 400's top end frequencies. To delineate the change, Dynaco covered the coils in black tubing.
Owners could bring their first generation models up to spec. by piggybacking a 2.2 ohm 2W 5% resistor to each coil.
Also, this unit has first generation output boards PC-28 which is a single sided board. Later boards were double sided to save a lot of jumpering. I would approximate a manufacture date of 1976.
Taking a measurement at the positive fuse holders I get a reading of 93V. I should be getting about 74V. My power supply suspicions are correct so far.
Here, the fascia has been removed and the cabinet face folded forward. I've removed the fuse plate with speaker relay and the main filter caps. We can see a little discoloration on the power supply board PC-30. (This is common on the ST-400)
Here's the other side of PC-30. One zenner is blown, the 850ohm resistor is cooked, the capacitor is dried out and the bridge rectifier gets flakey at higher currents. All component were checked "out of circuit". I will be changing every component on PC-30 and installing a new bridge rectifier of 35A 800V.
I tested the old filter caps (silver) and you can see the results above. The new caps should be a nice upgrade in performance.
As of today, I have some large orders coming in from Newark and Mouser with tons of components to rebuild the power section and also all new components for the (2) PC-28 output boards. I'm wanting to square away the power regulation before I attack the audio section. I'm still under $200.00 in on this restoration, not counting the purchase price. The Dynaguard / "Preamp" board PC-29 still has to be looked at, but I'm trying to not overwhelm myself at this point.
Jim