Another Son of Ampzilla rebuild.

[remilton]

I just bought a Son of Ampzilla on Ebay. I have examined it and done initial dis-assembly. It looks clean inside and all original with no obvious sines of any previous repairs. The right channel works fine with about 7mv dc on the output after tweaking. The left channel has a problem as R102 and R127 are burnt from excess current. However this is not a problem and I will be completely rebuilding the the boards.
The Son schematic shows the input transistors as 2n5551 and 2n5401 but my amplifier has MPS8099 and MPS8599. Can I assume either pair can be used? Or is it not that simple? If it is ok, I plan use the 2n5551 and 2n5401 as they are easy to obtain in quantity.

 

[east electronics]

According to the schematics yes it should be no problem ...Here is a couple of tips

--Lighting in the VU meters was not efficient or horrible .bulbs also fail very often I worked may alternatives by using led stripes ...Looks far better .... go for daylight color since the dark acrylic in the front will correct the tone .

--If you replace RCA plugs behind either you have to provide ground to them from the star ground or use non isolated types . First stages of the amp take ground from there and if you fail to provide one the amplifier will present many volts in the output since LTP will be out of balance .

--The VU meter circuit is very very horrible suffers from all the deceases that might occur when you use such a generic non logarithmic circuit to drive them

Here ..though is an interesting idea

Seen a kit @ ebay where the all thing is done with a 8Pin IC for both ch The IC is scratched so you cannot tell what is it I ve seen the circuit once but didnt manage to reverse engineer it ...

My assumption is that is a comparator or something similar with probably a voltage reference So when the signal is low the IC will amplify up to the reference and VU meter will play and then after that of volume goes up the VU meter then will be driven directly ...There is a trimmer that probably sets either 0db or what ever max is

It could be a comparator or some ALC circuit stands for Automatic level control .

The all point is not how accurate the VU meter is , but how joyful especially in low listening level ( ala Technics style )

PS
Anyone got the message and know how to do it please let me know I have seen how Technics does it but is way too complicated to incorporate to an average DIY amplifier ....

here is a link of the Ebay thing
Replacement TA7318P Amp Preamp Vu Meter Driver Circuit Board Stereo Module | eBay

 

[llwhtt]

A very common problem on most GAS units are the Fairchild TO-220 (GAS 116/117) transistors. Every piece I own had them already replaced or were bad. The latest failure of those was in my newly acquired Thaedra.

Craig

 

[remilton]

A very common problem on most GAS units are the Fairchild TO-220 (GAS 116/117) transistors. Every piece I own had them already replaced or were bad. The latest failure of those was in my newly acquired Thaedra.

Craig

I plan on replacing Q101 and Q105 with MJE15028 and Q102 and Q106 with MJE15029. The schematic shows different transistors for those two pairs, but it is my understanding you can use those two transistors for both pair.

The left channel which has high offset voltage very poorly matched input transistors, so I will be replacing those devices on both channels.

I may replace the outputs also, but I don't know what device numbers I need for those.

Other than that I will be replacing all electrolytic caps and anything else I find out of spec.

 

[llwhtt]

James B. would have told you to keep the Sankens if still good otherwise MJ21193/4s would be good modern replacements. The voltages in the Son are not very high so the 120V parts should be fine. I think I used 28/29s for one pair (VAS) and 30/31s for the other pair (drivers).

Craig

 

[remilton]

James B. would have told you to keep the Sankens if still good otherwise MJ21193/4s would be good modern replacements. The voltages in the Son are not very high so the 120V parts should be fine. I think I used 28/29s for one pair (VAS) and 30/31s for the other pair (drivers).

Craig

I'll keep the Sankens if the offsets are good after I complete the build.

I am not sure at this point since I can only check the devices on low voltage with an cheapo tester. Initially I get the following measurements on the Sankens.

Left channel: one pair has hfe 29 and 24, other pair hfe 59 and 45

Right channel: one pair has hfe 30 and 23, other pair 59 and 55

I don't have enough experience with transistor testing to know it that is ok. They are definitely working, but are they working well? That is why I say I will wait until I test the amp to decide if I will keep them.

 

[remilton]

According to the schematics yes it should be no problem ...Here is a couple of tips

--Lighting in the VU meters was not efficient or horrible .bulbs also fail very often I worked may alternatives by using led stripes ...Looks far better .... go for daylight color since the dark acrylic in the front will correct the tone .

--If you replace RCA plugs behind either you have to provide ground to them from the star ground or use non isolated types . First stages of the amp take ground from there and if you fail to provide one the amplifier will present many volts in the output since LTP will be out of balance .

--The VU meter circuit is very very horrible suffers from all the deceases that might occur when you use such a generic non logarithmic circuit to drive them

Here ..though is an interesting idea

Seen a kit @ ebay where the all thing is done with a 8Pin IC for both ch The IC is scratched so you cannot tell what is it I ve seen the circuit once but didnt manage to reverse engineer it ...

My assumption is that is a comparator or something similar with probably a voltage reference So when the signal is low the IC will amplify up to the reference and VU meter will play and then after that of volume goes up the VU meter then will be driven directly ...There is a trimmer that probably sets either 0db or what ever max is

It could be a comparator or some ALC circuit stands for Automatic level control .

The all point is not how accurate the VU meter is , but how joyful especially in low listening level ( ala Technics style )

PS
Anyone got the message and know how to do it please let me know I have seen how Technics does it but is way too complicated to incorporate to an average DIY amplifier ....

here is a link of the Ebay thing
Replacement TA7318P Amp Preamp Vu Meter Driver Circuit Board Stereo Module | eBay

Grounding the inputs to the chassis has been mentioned elsewhere in the forum and I have made a note to ensure the RCA plugs are firmly connected to the chassis. The rebuild just would no feel complete without gold connectors.

I thought about doing led's when I rebuilt my Gas Grandson's (I have 2 of them). But, I just never got around to engineering the circuit. I will have to give it some more thought since I really don't like the way Gas illuminated the meter on the Son.

 

[llwhtt]

If there are colored markings (felt pen?) on the transistor cases they are original and MATCHED. Output transistors are "usually" not the cause of DC offset. Offset problems in these amp are usually input diffs and/or the offset circuitry, bad zeners, caps, etc.

Craig

 

[remilton]

If there are colored markings (felt pen?) on the transistor cases they are original and MATCHED. Output transistors are "usually" not the cause of DC offset. Offset problems in these amp are usually input diffs and/or the offset circuitry, bad zeners, caps, etc.

Craig

Yes, I assumed the Sankens were original since I had noticed similar markings on the output transistors of the GAS Grandsons I have.

I agree the problem is not likely to be the output trans as I have found the NPN differential pair on the left input has a weak transistor. I will be sure to check all the components in that circuit, but I think I am on the right track.