GAS Grandson amplifier with distortion
Well I am glad not to be the only one. We have a GAS Grandson amp that has developed distortion in 1 channel. Not bad for having worked perfectly and been continuously powered ON for 28 years straight. I would like to have it repaired and keep it in use. Can someone help me or advise me? burnedfingers (love the name by the way), is this something you may be able to help me with? I have the schematic for it if that helps.
Brian
Well I am glad not to be the only one. We have a GAS Grandson amp that has developed distortion in 1 channel. Not bad for having worked perfectly and been continuously powered ON for 28 years straight. I would like to have it repaired and keep it in use. Can someone help me or advise me? burnedfingers (love the name by the way), is this something you may be able to help me with? I have the schematic for it if that helps.
Brian
Hi Brian,
I'm sure there are many of those amps around!
Your distortion could be a number of things. Firstly, what is the DC voltage across the output terminals?
My first thought are lots of old, tired capacitors. How knowledgeable are you with regard to amplifiers?
-Chris
I'm sure there are many of those amps around!
Your distortion could be a number of things. Firstly, what is the DC voltage across the output terminals?
My first thought are lots of old, tired capacitors. How knowledgeable are you with regard to amplifiers?
-Chris
Sorry to interrupt, but guess I'll chime in anyway. The search for a reliable repair/rebuild person has always been an issue and is not getting any better. This has been discussed here and there. I think we're running on fumes at this point and no wonder people don't know what to do at times.
The older amps can be a challenge replacing their obsolete parts, and most techs lack expertise to look at datasheets to determine a cross reference or upgrade.
I do a lot of design work and it's not difficult for me to repair/rebuild amps in general. I have had fantastic luck with rebuilding amps like Acoustat, Threshold, SAE, etc. Amps are actually pretty easy but then I've spent most my life in pursuit and have an engineering background, so it stands to reason.
If anyone is not a DIY-er and needs to get an amp to work again, feel free to hit me up. I'm semi-retired now, rebuild all the time, and work pretty cheap. Rebuilds/mods are my specialty, and most amps can be made to sound much, much better. Would be glad to assist. I even have a website coming online soon, but will forgo a shameless plug here...🙂
The older amps can be a challenge replacing their obsolete parts, and most techs lack expertise to look at datasheets to determine a cross reference or upgrade.
I do a lot of design work and it's not difficult for me to repair/rebuild amps in general. I have had fantastic luck with rebuilding amps like Acoustat, Threshold, SAE, etc. Amps are actually pretty easy but then I've spent most my life in pursuit and have an engineering background, so it stands to reason.
If anyone is not a DIY-er and needs to get an amp to work again, feel free to hit me up. I'm semi-retired now, rebuild all the time, and work pretty cheap. Rebuilds/mods are my specialty, and most amps can be made to sound much, much better. Would be glad to assist. I even have a website coming online soon, but will forgo a shameless plug here...🙂
Hi RetroAudio,
There seems to be a few good techs on this forum. Nice to see another one in the group.
What I don't want to see are any silly wars between them. So if a member would try the guys closest first. I really don't want to see a stunned price war. I will always advise people against getting work done if the price is far too cheap. The probability of getting a good job done drops with the prices quoted below what a fair price would be.
So, what is your professional repair experience?
-Chris
There seems to be a few good techs on this forum. Nice to see another one in the group.
What I don't want to see are any silly wars between them. So if a member would try the guys closest first. I really don't want to see a stunned price war. I will always advise people against getting work done if the price is far too cheap. The probability of getting a good job done drops with the prices quoted below what a fair price would be.
So, what is your professional repair experience?
-Chris
son of ampzilla repair
burned fingers,
I found my way here to the forum looking for help with my Son of Ampzilla. You have worked on an impressive number of these heavy rascals.
My unit works well, a little noisy, but the meters still work, too. Sounds good.
Older thread here, hope it pops up on your radar screen.
burned fingers,
I found my way here to the forum looking for help with my Son of Ampzilla. You have worked on an impressive number of these heavy rascals.
My unit works well, a little noisy, but the meters still work, too. Sounds good.
Older thread here, hope it pops up on your radar screen.
daddy dee
Nice to hear from you.
Yes, I manage to fix a few of the devils. If you need help send me an email and we can work something out.🙂 🙂
My bench usually looks like Gas Central with all the Ampzilla's, Son's, and Grandsons in various stages.
Nice to hear from you.
Yes, I manage to fix a few of the devils. If you need help send me an email and we can work something out.🙂 🙂
My bench usually looks like Gas Central with all the Ampzilla's, Son's, and Grandsons in various stages.
son of ampzilla
burnedfingers,
thanks. Since I'm a new member I don't yet have e-mail privileges. Would you mind sending to me?
brother.dee@sbcglobal.net
thanks.
burnedfingers,
thanks. Since I'm a new member I don't yet have e-mail privileges. Would you mind sending to me?
brother.dee@sbcglobal.net
thanks.
Gas Grandson Meters
Hello Everyone, New to the Group. I am restoring a Grandson but the meters have me stumped. The movements are fine and I replaced I.C. 301 opamp, still no movement. On the bad copy of my schematic, it appears the bias I.C.'s have something to do with the meters? The amp plays fine, just no meter action? Any ideas? Thanks, Fred
Hello Everyone, New to the Group. I am restoring a Grandson but the meters have me stumped. The movements are fine and I replaced I.C. 301 opamp, still no movement. On the bad copy of my schematic, it appears the bias I.C.'s have something to do with the meters? The amp plays fine, just no meter action? Any ideas? Thanks, Fred
IC 301 is the DC servo.
The bias IC's have nothing to do with the meters.
Check the 10K pots in the meter circuits they could be open.
Check the meter with a VOM meter. Have the amp powered off and disconnect the meter leads from the meter. On the ohm scale you should see some deflection when you put the meter leads across the meter. If you don't then they are toast.
The bias IC's have nothing to do with the meters.
Check the 10K pots in the meter circuits they could be open.
Check the meter with a VOM meter. Have the amp powered off and disconnect the meter leads from the meter. On the ohm scale you should see some deflection when you put the meter leads across the meter. If you don't then they are toast.
Tried to post the Grandson schematic file too big
marrscom
Feel free to email me and I will send you a readable copy.
marrscom
Feel free to email me and I will send you a readable copy.
Grandson
Thanks burnedfingers! I tried to email you but I am still under moderation. I had already confirmed the meter movements were good prior to my first post. From your suggestion, I did check the 5K pots and 1 was indeed open. I replaced both, but still have nothing at the meter outputs on the board. My next move would be to replace C-114 and C-115 but I don't really think they are the problem. I have checked every diode on the board, all ok. R-125, 225, 122, 222, 123, 223, 126, 226 are all within spec. I am also have trouble finding a GOOD replacement for Q205, Q206, which are Motorola MPS-U07 & MPS-U57, which have been discontinued. I did find some surplus and ask for a quote. The NTE cross says use NTE-128P & 129P but they only have 1/2 of the collector current rating. Feel free to contact me directly. marrscommunications@marrscommunications.com Thanks, Fred
Thanks burnedfingers! I tried to email you but I am still under moderation. I had already confirmed the meter movements were good prior to my first post. From your suggestion, I did check the 5K pots and 1 was indeed open. I replaced both, but still have nothing at the meter outputs on the board. My next move would be to replace C-114 and C-115 but I don't really think they are the problem. I have checked every diode on the board, all ok. R-125, 225, 122, 222, 123, 223, 126, 226 are all within spec. I am also have trouble finding a GOOD replacement for Q205, Q206, which are Motorola MPS-U07 & MPS-U57, which have been discontinued. I did find some surplus and ask for a quote. The NTE cross says use NTE-128P & 129P but they only have 1/2 of the collector current rating. Feel free to contact me directly. marrscommunications@marrscommunications.com Thanks, Fred
Re: Grandson
Hi Fred,
If the amp is working the MPS U07/57 are fine and I would not suggest changing them. They are the voltage amps and are perfectly selected for the application. Changing them, especially with NTE or most other options might change the characture or stability of the amp.
In my experience with the GAS amps they rarely had problems that weren't caused by something else.
Regards, Mike.
(Hi Joe, I hope your enjoying the holidays.)
marrscom said:
Hi Fred,
If the amp is working the MPS U07/57 are fine and I would not suggest changing them. They are the voltage amps and are perfectly selected for the application. Changing them, especially with NTE or most other options might change the characture or stability of the amp.
In my experience with the GAS amps they rarely had problems that weren't caused by something else.
Regards, Mike.
(Hi Joe, I hope your enjoying the holidays.)
Hi Mike,
How's the memory today?
Is a GAS 107 the same as a MPSu07?
I can't find my cheat sheet and my memory isn't working too well today.
How's the memory today?
Is a GAS 107 the same as a MPSu07?
I can't find my cheat sheet and my memory isn't working too well today.
Hello Joe, Mike thank you for your input! Joe I checked my meters and got 12 ohms and they also read 0 db. All resistors are within spec so It has to be a cap. I will take you up on the GAS 107's and will send you my address offline. This unit, has 2N5629's and 2N6029's Motorola devices in both sides for outputs. I though GAS used Sanken outputs BUT, the schematic shows 2SD426 amd 2SB556 ?? Fred
Fred,
The 5629's and the 6029's are common in the Grandsons for outputs. The SAnkens were common for the "Son's". The 2sB's and D's were common in the Ampzilla's depending on age.
After looking at my Simpson meter with my glasses on I also had 12 ohms on the R times 100 scale.
I am trying to confirm that the GAS 107's are identical to the MPSU07's.
The 5629's and the 6029's are common in the Grandsons for outputs. The SAnkens were common for the "Son's". The 2sB's and D's were common in the Ampzilla's depending on age.
After looking at my Simpson meter with my glasses on I also had 12 ohms on the R times 100 scale.
I am trying to confirm that the GAS 107's are identical to the MPSU07's.
burnedfingers said:
Hi Joe,
Sorry for the delay, I lost my internet. Yes the GAS 107 is the same as the MPSU07, I double checked on a couple real devices.
Mike.
I'm a little late for this but found this topic in a search.
In the late 70's, I worked for the Great American Sound Company. It was the period between James Bongiorno and the closing of the firm. Starting employment as a production tech, I was promoted to tech supervisor and lastly served as the service manager. I still have information, schematics, manuals and part cross reference data for most of the product line.
It was an honor to work with Andrew Hefley. There were just so many great people that worked there. I was fortunate to absorb some of the knowledge working at the little company down the road from Fisher.
Ah, the memories...
Gary
In the late 70's, I worked for the Great American Sound Company. It was the period between James Bongiorno and the closing of the firm. Starting employment as a production tech, I was promoted to tech supervisor and lastly served as the service manager. I still have information, schematics, manuals and part cross reference data for most of the product line.
It was an honor to work with Andrew Hefley. There were just so many great people that worked there. I was fortunate to absorb some of the knowledge working at the little company down the road from Fisher.
Ah, the memories...
Gary
- Status
- Not open for further replies.