$30 to the forum when amp is fixed. It's a GAS Grandson amp. It has 6 volts offset on each channel. I replaced ALL the electro's including the 10,000 power supply caps. 1 channel has Toshiba outputs 2sd426 & 2sb556. The other has MJ2955 & 10261. It had incorrect fuses... Nothing is burnt, all the resistors are within specs. Each channel has a 14 pin dip socket with 4 transistors plugged in for matching?? Everything looks original except for the 1 set of outputs. I'm not sure where to measure the bias current also? .. Your expertise is absolutely appreciated!
Thx, The schematic I found is at .. ABELTRONICS.CO.UK under GAS. It doesn't have much for voltage test points. If you have a more detailed repair manual, I surely could use it....
That's the schematic that I have and there were no service manuals that I'm aware of. Anyway IC301 (and associated parts) is the servo that is responsible for keeping the offset at 0VDC or close to it. Check for the proper DC voltages, pins 4 and 8 if they are OK swap out the IC. If they're wrong check the zeners, and there are two more caps(C303/4) that could use replacing. Since both channels are bad the servo is very suspect. Also I would consider replacing all of the small electrolyic capacitors, there are several in the audio path and in the servo itself. I'm doing the same on a Son. Digikey has Panasonics that are popular on this forum and Mouser has Nichicon Muse capacitors.
Craig
Craig
Craig seems to have covered most of the possibilities. From the schematic, it looks like the 14 pin sockets with the transistors in them are part of the Biasing IC detail. (SAE used CA3086 or LM3086 transistor arrays, and they are no longer available. I'm guessing that GAS used similar chips. Are they mounted so the transistors are on the heat sinks near the outputs or drivers? Good luck.
A few potshots:
That servo is limited to a max offset range of +/- 700mV because of the 1N4148 diodes clamping the output of the servo to ground. At 6V, it is not able to affect the offset at all. Removing the clamping diodes may help with the offset, but may introduce other issues.
You can measure voltage across R14/15 0.2 ohm resistors to figure out the bias at idle. Maybe a more experienced member will comment on what it should be at, about 60mA should be a fair starting point.
Are the power meters (if that's what they are) at rest position when the amp is turned on and at 6V output offset, or are they about halfway?
That servo is limited to a max offset range of +/- 700mV because of the 1N4148 diodes clamping the output of the servo to ground. At 6V, it is not able to affect the offset at all. Removing the clamping diodes may help with the offset, but may introduce other issues.
You can measure voltage across R14/15 0.2 ohm resistors to figure out the bias at idle. Maybe a more experienced member will comment on what it should be at, about 60mA should be a fair starting point.
Are the power meters (if that's what they are) at rest position when the amp is turned on and at 6V output offset, or are they about halfway?
Actually, 600 mv at the output of the Servo IC would produce several volts out at the output. The servo goes through two 150k resistors, as a voltage divider, to the + input of the amp, which gives about 300 mv (minus the input current difference between the PNP and NPN input transistors), which is amplified by the amp (gain is 25) to 7.5 volts on the output which is pretty close to what is occuring. But, as it is on both channels, I would still go for the 15 volt zeners or the resistors (r302 and r303) supplying them power.
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The 14 pin IC socket IS for the input diff. transistors, all GAS products used that technique, I think my Thalia and Thaedra preamps have it also. The bias IC SHOULD be on the heatsink somewhere with thermal goop on it's top, though I haven't seen a Grandson yet.
Craig
Craig
Yes, I'll check the 15v zeners and resistors. I see they could affect the servo IC and the input transistors. The bias IC is between the output transistors with goop to tranfer the Heat Sink temperature. It's a bit of a pain to work on because the output transistor/bias circuit board is permenantly attached/soldered to the main circuit board at a right angle, so it can be attached to the amps back & heatsinks. So the whole thing has to be pulled out carefully at one time. I have no doubt we can revive this oddball....
I'd leave the bias ICs alone unless having distortion or blowing fuses (high current draw). Check the voltages out of the servo IC and change it if necessary.
Well after many hrs of testing, here's a couple things that everyone should know. There are at least 2 version (different circuit boards). The schematic on the web is close to the other version BUT with different part numbers in the layout. In my version, it's critical the input jacks are grounded to the chassis. This is the grd for the +/- 15v supplies. Someone decided to upgrade to gold input jacks with insulators and started this whole mess. Then they replaced many parts! So make sure the supply voltages are good. Then I found the 1458 IC for the servo sensitive. Once you get the DC on the ouput down to 50mv, try swapping out several 1458's. This is not a fun amp to disassemble. So test carefully, make notes, replace suspect parts, & hopefully you only have to do it once. There's not a lot of info on these amps. If anybody has both schematics, I'd like them....
The holes for the input jacks have had the anodizing scratched thru to bare aluminum on my SON, is yours like that also? If so the person who replaced the jacks should have taken note. I haven't really looked at the grounding scheme other than that, I'll have to check it out on the schematic to see if that's the only ground. I think Mike Bettinger of GASWORKS/GASAUDIO(?) mentioned something about the grounding in a SON on a previous post. I'll have to go back and find it. The main thing is you figured it out and are back up and running. Thanx for sharing that BIG little tidbit.
Craig
Craig
Yes the input jack holes were scored to expose bare metal. This amp has Hitachi output transistors and Elna caps (which were replaced). I was lucky to have a friend drop off a working amp and I could compare voltages and parts. The repaired amp is a lot brighter sounding than the old stock amp. I hope it settles down after burn in. The old amp sounds really good! I guess I need to find a supplier for Elna caps for future repairs. Any suggestions??
Elna standard and low-ESR caps (RC2, RC3, RJ4, RJJ, RJH, etc.) are available at lots of places. If you want premium audio-grade Elnas, Parts Connexion (Canada, with web-based ordering also) has most of the Elna Silmic II and Cerafine series. You may find them a bit bright in comparison to the older and standard Elna series, even when broken in.
More Grandson info for Insiders...
Hi 2 all !
Let me refresh this once more as after finally receiving my new Son Of Ampzilla 2000 I decided it is about time to give my Grandson some love now and not missing it badly during downtime.
Back story is that I got the Thalia/Grandson combo in 1985 and I am obviously totally addicted to the sheer musicality of it.
I tried loads of other amps, even insanely expensive high end types and while finding some preamps that I could live with (and finally went for a Technics SU 9070) power amp-wise I always came back to the Grandson.
So I got used to the idea that James Bongiorno seems to be able to do something in his designs that has become vital for my life as a musician and producer/engineer: Musicality.
But I needed a bigger amp.
Living in Europe however makes it pretty hard to find a GAS or SUMO as there are veeeery few 220V units on the market anyways and to find one in mint condition is nearly impossible.
The best I could find was a Crown K2, it is awesome - but still it doesn't really have the "Groove" that Grandson adds to the musical performance.
Yeh and then one day I found out that Guru James introduced Ampzilla 2000.
Decision was clear within milliseconds - but to get one over here is not really the easiest of tasks.
Now it arrived and it does exactly what I expected. This is clearly the best amp I ever heard. 'nuff said.
Now for the Grandson.
After almost 30 years of non-stop delivery of performance it needs some refreshing. Bass precision has gotten quite poor and also there is a strong hum component which I can only get rid of by running the left input cable in a special loop across the transformer to cancel it out...
Of course I encountered the same problems with the schematic and board mismatching.
The boards in most of the units seem to be 721, whereas the schematic on the web is for 721 B which I now think maybe never made it into production...
As I am in contact with James Bongiorno himself about the Ampzilla 2000 anyways I decided to just ask him about rebuilding Grandson and in return of (hopefully) getting some expertise here I'd like to share with you what he suggested:
"[...]If you want to tackle this then merely replace the ceramic compensation caps with silvered micas.
Then replace ALL the bypass caps with .1ufd/100V green drop polys (Panasonic).
If the output devices are Sankens, then leave them alone. If they are something else, then replace them with ON Semis MJ21193 and MJ21194.
The rest of the semis should be OK.
Replace the 1558 type servo dual opamp with a FET input type like a LF353, TLO72 etc. This should give you some small improvements."
Then I found Toshiba output transistors in my unit and did a web search for "green drop polys" which I couldn't find anywhere. They seem to be grey now...
And instead of the 1558 there is a 1458 in my unit.
This is what he says about it:
"The output devices should definitely be changed to the ON (formerly Motorola) devices. The grey Panasonics are OK. The 1458 is a bipolar and definitely should be changed to a FET input type opamp."
So I ordered a bunch of spare parts, except for the Micas which don't seem to be available over here in Europe so easily.
I have to say that I don't really have a history in servicing vintage amps, so any help on how to adjust bias after replacing the OP and output transistors is greatly appreciated.
Also I am not 100% sure if I identified the bypass and compensation caps right.
I believe that (on the schematic) C114 and 214 (.05uF) and the ones on the output daugther boards 722 and 723 are the compensation caps that he wants to be replaced with Micas but honestly this goes beyond my knowledge of reading schematics.
Another question is, does he really mean that ALL the ceramic caps which on my PCB have values of 10pF, 100pF and 220pF are to be replaced by .1uF polys ? Does this make sense for you guys ?
Instead of the TL072 I'd like to go for an Analog Devices OP249GPZ which I believe to remember sonically did a great job in my Harrison console.
Any confessions ?
Thanks,
spocintosh
Hi 2 all !
Let me refresh this once more as after finally receiving my new Son Of Ampzilla 2000 I decided it is about time to give my Grandson some love now and not missing it badly during downtime.
Back story is that I got the Thalia/Grandson combo in 1985 and I am obviously totally addicted to the sheer musicality of it.
I tried loads of other amps, even insanely expensive high end types and while finding some preamps that I could live with (and finally went for a Technics SU 9070) power amp-wise I always came back to the Grandson.
So I got used to the idea that James Bongiorno seems to be able to do something in his designs that has become vital for my life as a musician and producer/engineer: Musicality.
But I needed a bigger amp.
Living in Europe however makes it pretty hard to find a GAS or SUMO as there are veeeery few 220V units on the market anyways and to find one in mint condition is nearly impossible.
The best I could find was a Crown K2, it is awesome - but still it doesn't really have the "Groove" that Grandson adds to the musical performance.
Yeh and then one day I found out that Guru James introduced Ampzilla 2000.
Decision was clear within milliseconds - but to get one over here is not really the easiest of tasks.
Now it arrived and it does exactly what I expected. This is clearly the best amp I ever heard. 'nuff said.
Now for the Grandson.
After almost 30 years of non-stop delivery of performance it needs some refreshing. Bass precision has gotten quite poor and also there is a strong hum component which I can only get rid of by running the left input cable in a special loop across the transformer to cancel it out...
Of course I encountered the same problems with the schematic and board mismatching.
The boards in most of the units seem to be 721, whereas the schematic on the web is for 721 B which I now think maybe never made it into production...
As I am in contact with James Bongiorno himself about the Ampzilla 2000 anyways I decided to just ask him about rebuilding Grandson and in return of (hopefully) getting some expertise here I'd like to share with you what he suggested:
"[...]If you want to tackle this then merely replace the ceramic compensation caps with silvered micas.
Then replace ALL the bypass caps with .1ufd/100V green drop polys (Panasonic).
If the output devices are Sankens, then leave them alone. If they are something else, then replace them with ON Semis MJ21193 and MJ21194.
The rest of the semis should be OK.
Replace the 1558 type servo dual opamp with a FET input type like a LF353, TLO72 etc. This should give you some small improvements."
Then I found Toshiba output transistors in my unit and did a web search for "green drop polys" which I couldn't find anywhere. They seem to be grey now...
And instead of the 1558 there is a 1458 in my unit.
This is what he says about it:
"The output devices should definitely be changed to the ON (formerly Motorola) devices. The grey Panasonics are OK. The 1458 is a bipolar and definitely should be changed to a FET input type opamp."
So I ordered a bunch of spare parts, except for the Micas which don't seem to be available over here in Europe so easily.
I have to say that I don't really have a history in servicing vintage amps, so any help on how to adjust bias after replacing the OP and output transistors is greatly appreciated.
Also I am not 100% sure if I identified the bypass and compensation caps right.
I believe that (on the schematic) C114 and 214 (.05uF) and the ones on the output daugther boards 722 and 723 are the compensation caps that he wants to be replaced with Micas but honestly this goes beyond my knowledge of reading schematics.
Another question is, does he really mean that ALL the ceramic caps which on my PCB have values of 10pF, 100pF and 220pF are to be replaced by .1uF polys ? Does this make sense for you guys ?
Instead of the TL072 I'd like to go for an Analog Devices OP249GPZ which I believe to remember sonically did a great job in my Harrison console.
Any confessions ?
Thanks,
spocintosh
hallo.
mica caps are perhaps available at reichelt.de...............glimmer kondensator.......cy..........(for use as compensation/miller cap ).
grüsse
mica caps are perhaps available at reichelt.de...............glimmer kondensator.......cy..........(for use as compensation/miller cap ).
grüsse
Should be "Silber-Glimmer". Can find these only up to 47nF here in Europe, although CDE is producing higher values also.
Prices are $ 50,- and up also.
I went for some NOS micas from a russian supplier now, partly out of curiosity, partly for the cool price (€ 2.50). They come with a mil specs tolerance of .5% and the size of building bricks !!
Cool.
Prices are $ 50,- and up also.
I went for some NOS micas from a russian supplier now, partly out of curiosity, partly for the cool price (€ 2.50). They come with a mil specs tolerance of .5% and the size of building bricks !!
Cool.
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