Gibson G70 repair.

[gtrplayr1976]

Hi folks,

Trying to get an old G70 back in working order. I can't find much info. Would love to have some good high resolution gut shots. I think the schematic I have is correct. It belongs to a cousin. He said when he powered it up there was loud hum ,so I'm expecting to replace caps. I have noticed it looks to have been modified. I'm not sure what they were looking to do ,but it just looks like a couple of caps were added (badly) There is another thread I'm attaching that has some pictures I took. Any help or info appreciated. I've been striking out for the most part.

I've attached the schematic I'm using ,or will be using.

Gibson G70

Thanks folks !

[52 Bill]

The first test would be to find out if the hum is caused by bad filtering or dc voltage on the output.

If you have a voltmeter, read the dc voltage across the speaker terminals. If there is anything more than maybe 0.5 volt dc, disconnect the speaker before doing anything else. If you don't have a voltmeter look at the speaker cone when you turn on the amp. If there is dc on the output, you will see the cone move in or out and stay pushed in or out.

If there is little or no dc on the speaker output, then it's time to test the filter caps. The fastest test for me is to parallel a cap across the main filters and see if the hum goes away. The schematic shows the two main filter caps as being 2000uF at 35 volts. If you have spare a 1000uF to 4000uF cap rated at least 35 volts in your parts drawer try clipping it across one of the two main filters caps and turn on the amp. If the hum is reduced, then you know that the filters are going bad and need further testing.

I looked at the photos in the other thread and see the added caps. You need to figure out if they have been added or if they are just a really bad replacement job.

[gtrplayr1976]

The first test would be to find out if the hum is caused by bad filtering or dc voltage on the output.

If you have a voltmeter, read the dc voltage across the speaker terminals. If there is anything more than maybe 0.5 volt dc, disconnect the speaker before doing anything else. If you don't have a voltmeter look at the speaker cone when you turn on the amp. If there is dc on the output, you will see the cone move in or out and stay pushed in or out.

If there is little or no dc on the speaker output, then it's time to test the filter caps. The fastest test for me is to parallel a cap across the main filters and see if the hum goes away. The schematic shows the two main filter caps as being 2000uF at 35 volts. If you have spare a 1000uF to 4000uF cap rated at least 35 volts in your parts drawer try clipping it across one of the two main filters caps and turn on the amp. If the hum is reduced, then you know that the filters are going bad and need further testing.

I looked at the photos in the other thread and see the added caps. You need to figure out if they have been added or if they are just a really bad replacement job.

I do have a meter ,and can check that. I do have spare caps. I actually bought new electrolytics since these are most likely 35 years old ,and after looking closely again I think those added caps ARE and attempt at replacing some on the board. A bad attempt.

Thanks Bill !

[gtrplayr1976]

The first test would be to find out if the hum is caused by bad filtering or dc voltage on the output.

If you have a voltmeter, read the dc voltage across the speaker terminals. If there is anything more than maybe 0.5 volt dc, disconnect the speaker before doing anything else. If you don't have a voltmeter look at the speaker cone when you turn on the amp. If there is dc on the output, you will see the cone move in or out and stay pushed in or out.

If there is little or no dc on the speaker output, then it's time to test the filter caps. The fastest test for me is to parallel a cap across the main filters and see if the hum goes away. The schematic shows the two main filter caps as being 2000uF at 35 volts. If you have spare a 1000uF to 4000uF cap rated at least 35 volts in your parts drawer try clipping it across one of the two main filters caps and turn on the amp. If the hum is reduced, then you know that the filters are going bad and need further testing.

I looked at the photos in the other thread and see the added caps. You need to figure out if they have been added or if they are just a really bad replacement job.

Bill I just got back to troubleshooting this amp. I have replaced all of the electrolytics ,and replaced the power transistors as well. This schematic isn't 100% I have found. At least one of the transistors in the power section doesn't exist in the amp. I AM getting DC on the output. I get hum when I power up. I am using a limiter ,and variac as I do this. I'm only getting 10VDC to the power supply with the limiter ,and reading -5VDC at the speakers.
Early on after I replaced the transistors I was tripping the circuit breaker. I found that the new transistors collectors weren't isolated from the chassis , so they were going to ground. I replaced the metal screws ,and fixed that particular issue. Could this have damaged the transistors ? I was using the variac ,and was only getting a small amount of voltage before it tripped.
The attached is what I have for the power supply.
**UPDATE**I was getting erratic readings on the bias pot so I pulled it,and this is what I found.

[52 Bill]

Could this have damaged the transistors ? I was using the variac ,and was only getting a small amount of voltage before it tripped.
The attached is what I have for the power supply.
**UPDATE**I was getting erratic readings on the bias pot so I pulled it,and this is what I found.

The best way to tell if the transistors were damaged is to test them. Usually the limiter will protect them, but you never know until you test them.

The burned bias pot would be a major problem. Without it making contact the two sides of the power amp would be turning on at the same time, causing all sorts of problems. For testing, you can jumper over it or add a fixed resistor to see if it helps tame the output.

It may be difficult to find a replacement for the burned up bias pot. You may have to use one that mounts differently and run wires to the pc board.

[gtrplayr1976]

The best way to tell if the transistors were damaged is to test them. Usually the limiter will protect them, but you never know until you test them.

The burned bias pot would be a major problem. Without it making contact the two sides of the power amp would be turning on at the same time, causing all sorts of problems. For testing, you can jumper over it or add a fixed resistor to see if it helps tame the output.

It may be difficult to find a replacement for the burned up bias pot. You may have to use one that mounts differently and run wires to the pc board.

I found a 100 pot with three contact legs from Mouser, so I guess the wiper needs to be connected to one side or the other ? According to the schematic it is suppose to have CW rotation.

[J M Fahey]

The schematic shows that it is a 3 leg pot with wiper soldered to one end so it becomes a 2 leg variable resistor.
Wiper to which end?
The one necessary so when turned clockwise total resistance becomes zero, in the schematic wiper is soldered to bottom leg and when fully turned CW it reaches the other end, so effectively shorting thepot.

For bias setup, set pot to zero ohms, and increase resistance bit by bit (by turning CCW) until you read suggested 10/20mV across R26 or R27 .

[gtrplayr1976]

The schematic shows that it is a 3 leg pot with wiper soldered to one end so it becomes a 2 leg variable resistor.
Wiper to which end?
The one necessary so when turned clockwise total resistance becomes zero, in the schematic wiper is soldered to bottom leg and when fully turned CW it reaches the other end, so effectively shorting thepot.

For bias setup, set pot to zero ohms, and increase resistance bit by bit (by turning CCW) until you read suggested 10/20mV across R26 or R27 .

Thank you sir. The one schematic piece is one you supplied me with before. The full schematic doesn't seem to match all the way through. Do you have any idea what the rating on the pot should be ? I have a small trim pot on the way.

[gtrplayr1976]

The schematic shows that it is a 3 leg pot with wiper soldered to one end so it becomes a 2 leg variable resistor.
Wiper to which end?
The one necessary so when turned clockwise total resistance becomes zero, in the schematic wiper is soldered to bottom leg and when fully turned CW it reaches the other end, so effectively shorting thepot.

For bias setup, set pot to zero ohms, and increase resistance bit by bit (by turning CCW) until you read suggested 10/20mV across R26 or R27 .

Would it really matter which leg you wired with the wiper as long as you can start at zero ohms one way or the other ? As I have it wired zero is full CCW. When I bring the voltage up with the variac I get increasing hum ,and the bias voltage doesn't move until I get to about 70volts. The hum sound changes ,and then the meter goes out of range. I have the meter set on 200mV. It goes silent when it hits that point. I thought I tripped the breaker , but when I reduced the voltage the hum came back ,so it was still pulling power.

[52 Bill]

Would it really matter which leg you wired with the wiper as long as you can start at zero ohms one way or the other ? As I have it wired zero is full CCW.

No it really doesn't matter, but when the pot is at zero ohms the idle current will be at the lowest level. Having it wired the opposite way may seem more intuitive.

When I bring the voltage up with the variac I get increasing hum ,and the bias voltage doesn't move until I get to about 70volts. The hum sound changes ,and then the meter goes out of range. I have the meter set on 200mV.

What is this meter reading? If you are reading more than 200mV across one of the emitter resistors at idle, them you have a problem.

[gtrplayr1976]

No it really doesn't matter, but when the pot is at zero ohms the idle current will be at the lowest level. Having it wired the opposite way may seem more intuitive.

What is this meter reading? If you are reading more than 200mV across one of the emitter resistors at idle, them you have a problem.

Bill I changed it just to be sure. Once I got to that point in the voltage again I started adjusting the bias ,and got it set. No hum with all controls at zero ,but when I turned the volume up I got hum. It seems to be a grounding type of hum because when I touch the metal control pot it changes. I didn't have all the pots ,or jacks tight to the chassis either ,and I didn't have a grounded plug. I just replaced the power cable, and I'm working on getting everything tightened down. Hopefully that will help.
I have also found the lamp (which I think is an LED) isn't working. I have some lamps ,but they're rated 250v, 2 amp. I know the higher voltage would probably make it burn dim ,but I was concerned about the 2 amp rating. That could cause problems with the current draw ? Attached is the PS part of the schematic. I think the original lamp is 120v.

[52 Bill]

I think the original lamp is 120v.

My guess would be that the original pilot light was a neon lamp assembly, with a neon lamp and a dropping resistor. If yours doesn't work you can replace it or rebuild it.

[gtrplayr1976]

My guess would be that the original pilot light was a neon lamp assembly, with a neon lamp and a dropping resistor. If yours doesn't work you can replace it or rebuild it.

You are correct Bill. It is a neon lamp. The dropping resistor drifted to 128k also. I will investigate further, and replace the neon lamp if needed.

[gtrplayr1976]

The lamp is in the amp and working. I have it to full power ,and adjusted the bias initially to 12mV. It drifted up to about 16mV then suddenly dropped to 10mV. It has been hovering between 9,and 11 mV since. What would cause this ?

[52 Bill]

The lamp is in the amp and working. I have it to full power ,and adjusted the bias initially to 12mV. It drifted up to about 16mV then suddenly dropped to 10mV. It has been hovering between 9,and 11 mV since. What would cause this ?

Warm up of the power amp parts? How long was the amp on? I usually allow for a 15-20 minute warm up before setting final bias.

Remember no signal and on some amps no load either when setting the idle bias.