I replaced all the tantalum caps. Changed some resistors to 1%, used better electrolytics. The biggest change, replaced Q10/Q18 with 2SA968 and Q11/Q17 with 2SC2238, with a little twist of the leads, which these will be hard to find now. I have a list of changes somewhere. I changed the cap across the feedback resistor to 15 pf, from what, I don't remember, good luck.
MI
Thanks MadInventor for the schematics. My "basket case DG250 is back up and running! After the transistors and tantalums and the 3900 ohm one watt resistor things seemed fine. This morning it decided to dump 60 volts on the left channel. No big deal, A 1/2 amp fuse in the speaker line saved things. Q9 failed. I replaced both Q9 (left and right) and noticed the Q10 devices were hot! I went home and got my other DG250 and after a few minutes, I found I could cook on the Q10 devices (originals with the tiny heat sinks). I had some real heat sinks and changed all the little folded metal ones to the newer, put the little heat sink on Q9 and so far, so good! The devices run hot but my skin doesn't stick to them any more. Now to let it play!
ah, nuts!
You mean, if I didn't give you a schematic, I could of got my hands on a DG250, ahhh nuts!
MI
PS I need to check the forum, more than I have lately.
You mean, if I didn't give you a schematic, I could of got my hands on a DG250, ahhh nuts!
MI
PS I need to check the forum, more than I have lately.
DG250C
My experience with these amps (remember I worked for the company) was that they didn't like 4 ohm loads. That could get it to fail - and when it did, it would go into DC offset - and shoot DC straight out the speaker wires and lunch a woofer.
But avoid those low impedance loads and you're going to have a reasonably reliable amp.
The sounds.
The Audio Critic's first issue had a big amp comparo - including the best from Audio Research (both tube and solid state), Yamaha V FET amp (the B2), Threshold (a class A amp), and pretty much anything considered serious back then. The winner was an Electrocompaniet, but it was something like 20 watts per channel. So the real winner was the Quatre. Had a fairly "liquid" sound and insane "slam" to the bass. Especially with sealed cabinet speakers. The Dahlquist DQ10 was very popular back then - but it lacked bottom end. In the ads I would tell Dahlquist owners that if they weren't happy with their bass - before you get the Dahlquist subwoofer (the X-over was well built and designed, but the sub was kind of tubby-sounding) - try our amp. EVERYONE who tried it agreed - it was significantly deeper, faster and just ******* solid. Someone brought it some "antiques" - a pair of AR3a. And with the Gain Cell amp? AWESOME! I had heard one in 20 years, bought one with some old Avid 103s for a second home. Hooked it up and wow - the Gain Cell amp is a crazy-good bass amp. Still. (not sure about the 500 - Gary (the G in the GC) was our tech and took over the company when Guy Hickey gave up on it. ) Especially with an air suspension/sealed cabinet woofer. You'll thank me (but make it 8 ohm! Or you'll curse me!)
My experience with these amps (remember I worked for the company) was that they didn't like 4 ohm loads. That could get it to fail - and when it did, it would go into DC offset - and shoot DC straight out the speaker wires and lunch a woofer.
But avoid those low impedance loads and you're going to have a reasonably reliable amp.
The sounds.
The Audio Critic's first issue had a big amp comparo - including the best from Audio Research (both tube and solid state), Yamaha V FET amp (the B2), Threshold (a class A amp), and pretty much anything considered serious back then. The winner was an Electrocompaniet, but it was something like 20 watts per channel. So the real winner was the Quatre. Had a fairly "liquid" sound and insane "slam" to the bass. Especially with sealed cabinet speakers. The Dahlquist DQ10 was very popular back then - but it lacked bottom end. In the ads I would tell Dahlquist owners that if they weren't happy with their bass - before you get the Dahlquist subwoofer (the X-over was well built and designed, but the sub was kind of tubby-sounding) - try our amp. EVERYONE who tried it agreed - it was significantly deeper, faster and just ******* solid. Someone brought it some "antiques" - a pair of AR3a. And with the Gain Cell amp? AWESOME! I had heard one in 20 years, bought one with some old Avid 103s for a second home. Hooked it up and wow - the Gain Cell amp is a crazy-good bass amp. Still. (not sure about the 500 - Gary (the G in the GC) was our tech and took over the company when Guy Hickey gave up on it. ) Especially with an air suspension/sealed cabinet woofer. You'll thank me (but make it 8 ohm! Or you'll curse me!)
My QMI GC-200
I know this is a long shot and I maybe shouldn't even be posting here, because to be honest I am not an audio DIYer. But you all seem very knowledgeable, so I'm hoping you'll be willing to help a noob.
Back in the early 1980's I was a wannabe audiophile and acquired the following:
Last week I unboxed it all and started hooking things up. I started with a tuner, pre-amp, amp and speakers. Lo and behold it worked! I then tried adding the sub-woofer between the pre-amp and amp and got silence. At one point I did hear the characteristic hum and managed to turn off the amp and pre-amp before hearing the pop. I do get sound when I plug headphones into the pre-amp. I get no sound from the sub-woofer when I connect it to the pre-amp. The speaker line fuses didn't blow and the speaker cones move freely. None of the amp's fuses blew.
So what to do? I'm a decent troubleshooter (40+ years as a network engineer) with a multimeter. I also have a Pioneer SP-SB32W sound bar with RCA inputs. Should I even try troubleshooting and, if so, what should the first steps be? Or is everything so old that it would need to be upgraded even if I wasn't having problems? I'm assuming the cost for that would be significant.
Thanks in advance for any advice.
Bob
I know this is a long shot and I maybe shouldn't even be posting here, because to be honest I am not an audio DIYer. But you all seem very knowledgeable, so I'm hoping you'll be willing to help a noob.
Back in the early 1980's I was a wannabe audiophile and acquired the following:
- GAS Thalia II pre-amp
- QMI GC-200 amp
- ace-BASS B2-50 sub-woofer
- KEF Corelli speakers
- Various input devices
Last week I unboxed it all and started hooking things up. I started with a tuner, pre-amp, amp and speakers. Lo and behold it worked! I then tried adding the sub-woofer between the pre-amp and amp and got silence. At one point I did hear the characteristic hum and managed to turn off the amp and pre-amp before hearing the pop. I do get sound when I plug headphones into the pre-amp. I get no sound from the sub-woofer when I connect it to the pre-amp. The speaker line fuses didn't blow and the speaker cones move freely. None of the amp's fuses blew.
So what to do? I'm a decent troubleshooter (40+ years as a network engineer) with a multimeter. I also have a Pioneer SP-SB32W sound bar with RCA inputs. Should I even try troubleshooting and, if so, what should the first steps be? Or is everything so old that it would need to be upgraded even if I wasn't having problems? I'm assuming the cost for that would be significant.
Thanks in advance for any advice.
Bob
Had a customer bring in a DG250 a couple of weeks ago. He said he bought it broken. And, it was BROKEN. The upper channel circuit board bottom was covered in dark brown soot from a previous failure. It was a job cleaning it. The channel had been worked on by someone who put NTE191 and NTE240 devices in place of all the U10 and U60 devices. The outputs were dead and R33 and R34 as well as R21 and R23 also. I changed the outputs to MJ15003 ON transistors and replaced the resistors and the NTE240 and NTE191 devices with NTE375 and NTE398. Dim bulb on a variac resulted in a dead pair, Q17and Q18. More work and two new devices and a bit more testing showed the 2n2222 and 2n1305 (Q15, Q16) open. replacing them gave a pitiful sine wave. The diode across R26 was shorted. Replacing this diode gave a sine wave with parasitic oscillations on the negative portion of the sine wave with bad crossover distortion. I played a hunch the previous person working on this amp turned everything. I was right, Bias was at it's lowest setting, the gain adjust got rid of the parasitic. Now the channel looks great on the scope!
Then, oh brother! The lower channel appears to be a early version with notes on a piece of paper from 1976 stuck on it. The underneath is covered with jumpers (factory mods?) It has terrible crossover distortion. Bias control does nothing. The 2n1305 is shorted. More to come when that device comes in!
Then, oh brother! The lower channel appears to be a early version with notes on a piece of paper from 1976 stuck on it. The underneath is covered with jumpers (factory mods?) It has terrible crossover distortion. Bias control does nothing. The 2n1305 is shorted. More to come when that device comes in!
Had a DG250. best thing you can do with it is strip out the boards put new caps in if needed and then build the honey badger in the chassis. that is what I did and it sounds wonderful. That HB is a great amp. Even on tough loads. Used mine for years on older maggies. Boards fit inside perfect.
Understand. I enjoy working on the audio oddities from the 70's. The only ones that are unfixable so far are the amplifiers that have potted modules and special transistors that have been unavailable (V-FETS, some hitachi Mosfets) for some years now. My GC500 amp, once gone thru and the heat sinks replaced on the VAS transistors is capable of playing my Magneplanar IV speakers to unreasonable levels. While I am not a headbanger, my girlfriend is. Loud is an understatement to her listening.