Hello everyone. While visiting a local audio store, I spotted a Dynaco Stereo 400 in their used gear section. I'd never seen one 'in the flesh', but had heard a fair number of good things about it and the rest of the series over the years. The amp had a mystery wire sticking out of the side, which I later realized had once been connected to a fan of some sort. After some discussion with the store owners, and after a listening test that showed some distortion in the right channel, they sold it to me for $150, which I think was a pretty good deal.
I've owned numerous Dynaco equipment over the years, but other than a couple of PAT-4 preamps, all of it has been tubed (ST-70, Mark IIIs, ST-35). Also, the vast majority of the component-level work I've done on vintage gear has involved tube equipment. Additionally, this ranks as the largest amplifier I've owned thus far, displacing a Pioneer SPEC-4 in that position. Regardless, I feel good about my prospects of being able to fix this ST-400, since it was designed to be built as a kit, and lots of documentation is available about it. Also, I have a fair amount of test equipment around here, including a transistor checker.
Anyway, as mentioned earlier, I was able to give it a listening test before purchase, which showed some (hopefully) minor faults. When powered up for the first time, the pilot lamp came on, as did both of the lamps behind the smoked plastic window, which I later learned are the DynaGuard indicators (the switch was turned to OFF, and cycling the switch through the settings has no effect on their illumination), and the relay clicked. Once fed with a signal, I got audio from both channels, though some distortion could be heard in the right channel. When I checked the DC offset, the right channel showed -22mV when first powered up, which later fell below -1mV, while the left channel shows a pretty steady 18-19mV. Granted, I didn't give it a full 15 minutes to warm up, but things seemed to get settled in fairly quickly.
In summary, my question is as quoted in the title. As mentioned before, I have little experience working on solid-state equipment in general, though I think it's time I learned how. My guess is that new capacitors are needed, along with whatever drifted resistors there may be, but I'd rather not just shotgun everything or replace all the boards with new ones (as shown in my ST-70 restoration article, I like to give the stock circuitry in these things a fair shake if at all possible). Anyway, here are some pictures of the amplifier:
The front panel. Seems to be in pretty good shape overall.
The rear panel. The presence of a serial number sticker makes me think that it was factory-wired, as was the case with Dynaco's tube equipment, but I don't know if that's the case for the ST-400.
The inside of the cage. Seems decently clean overall, and I didn't see any obviously-burnt components or whatnot. The copper-colored coils near the fuse holders make me think it's an early version, as discussed in this article about a ST-400 modification (unless it was referring to a different coil-wrapped resistor).
The 'mystery wire' mentioned above. Both leads of the zip cord are connected to a terminal strip near the power switch; one to a primary lead of the power transformer, and the other to one of the leads from the power switch. I removed some of the electrical tape, and the two bare ends are isolated from one another.
Anyway, thanks in advance!
-Adam
I've owned numerous Dynaco equipment over the years, but other than a couple of PAT-4 preamps, all of it has been tubed (ST-70, Mark IIIs, ST-35). Also, the vast majority of the component-level work I've done on vintage gear has involved tube equipment. Additionally, this ranks as the largest amplifier I've owned thus far, displacing a Pioneer SPEC-4 in that position. Regardless, I feel good about my prospects of being able to fix this ST-400, since it was designed to be built as a kit, and lots of documentation is available about it. Also, I have a fair amount of test equipment around here, including a transistor checker.
Anyway, as mentioned earlier, I was able to give it a listening test before purchase, which showed some (hopefully) minor faults. When powered up for the first time, the pilot lamp came on, as did both of the lamps behind the smoked plastic window, which I later learned are the DynaGuard indicators (the switch was turned to OFF, and cycling the switch through the settings has no effect on their illumination), and the relay clicked. Once fed with a signal, I got audio from both channels, though some distortion could be heard in the right channel. When I checked the DC offset, the right channel showed -22mV when first powered up, which later fell below -1mV, while the left channel shows a pretty steady 18-19mV. Granted, I didn't give it a full 15 minutes to warm up, but things seemed to get settled in fairly quickly.
In summary, my question is as quoted in the title. As mentioned before, I have little experience working on solid-state equipment in general, though I think it's time I learned how. My guess is that new capacitors are needed, along with whatever drifted resistors there may be, but I'd rather not just shotgun everything or replace all the boards with new ones (as shown in my ST-70 restoration article, I like to give the stock circuitry in these things a fair shake if at all possible). Anyway, here are some pictures of the amplifier:
The front panel. Seems to be in pretty good shape overall.
The rear panel. The presence of a serial number sticker makes me think that it was factory-wired, as was the case with Dynaco's tube equipment, but I don't know if that's the case for the ST-400.
The inside of the cage. Seems decently clean overall, and I didn't see any obviously-burnt components or whatnot. The copper-colored coils near the fuse holders make me think it's an early version, as discussed in this article about a ST-400 modification (unless it was referring to a different coil-wrapped resistor).
The 'mystery wire' mentioned above. Both leads of the zip cord are connected to a terminal strip near the power switch; one to a primary lead of the power transformer, and the other to one of the leads from the power switch. I removed some of the electrical tape, and the two bare ends are isolated from one another.
Anyway, thanks in advance!
-Adam