G-7700 kicking into protection after repair

moodydan

I love the punk rock.
I'm working on a G-7700 for a friend; in transit, it was dropped, kicked, smashed, who knows? The result was the F-3058 circuit board for the left channel was shattered. When he plugged it in to see if it worked upon receiving it, transistors shorted all along the left channel.

So, I got a new (old) F-3058 board (from a 6700), replaced the main outputs with pairs of NJW3281G/NJW1302G, and replaced all of the following in the power amp section: VR01, VR03, TR01, TR03, TR05, TR07, TR09, TR15, TR17, TR19, R03, R37, R53, R51, R57, R59, R65, R81, R85, R87.

My DC Bias and Offset have been set and both are staying set (not drifting).

I power up, get the relay click, and get great sound out of the right channel (which didn't have any damage). The left channel sounds good, but if I turn past the "10" mark on the volume dial (approx. 7 o'clock position), the receiver kicks into protection every time. If I disconnect the left speaker, this does not happen.

Some other measurements I've taken:
Voltages at both sides of
R601: 41.5v/23.99v
R604: 43.5v/23.99v

Votage across ZD601: 22.99v

I feel like I'm close, but I'm looking for a lead!
 
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That series doesn't like driving low Z loads.....If you flip speakers does the left still cut out??
Try a series "R" like 4 ohms 25 watt... just for test
 
That series doesn't like driving low Z loads.....If you flip speakers does the left still cut out??
Try a series "R" like 4 ohms 25 watt... just for test

To be honest, I don't understand what you mean by Z loads or a series "R," but I did try swapping speakers, and using both A and B outs, as well as tested with a different set of speakers, just in case. The symptoms remained the same.
 
To be honest, I don't understand what you mean by Z loads or a series "R," but I did try swapping speakers, and using both A and B outs, as well as tested with a different set of speakers, just in case. The symptoms remained the same.

Z means impedance......

If you are running low impedance loads such as 4Ω speakers, these amps are not particularly good at driving them.....
The over current protection will kick in and shut it down.

They do however like driving higher impedance loads such as 8Ω speakers.....

So what are your speakers, and what are the output meters reading when it cuts out......?
 
Z means impedance......

If you are running low impedance loads such as 4Ω speakers, these amps are not particularly good at driving them.....
The over current protection will kick in and shut it down.

They do however like driving higher impedance loads such as 8Ω speakers.....

So what are your speakers, and what are the output meters reading when it cuts out......?

Thank you! All of my speakers are 8Ω; the output meters don't get a chance to read anything if I have speakers hooked up to the left channel. If only the right channel has speakers hooked up, I'm still good. If I don't hook any speakers up, with the Speakers A or Speakers B selected, or Speakers A & B selected, the meters read up to 120 watts with no problem.
 
Yes your speakers might SAY 8 ohms but for some reason the Sansui doesn't like em on the left. So try this, hook both speakers in series and try the left ( only ) ....
See if that changes anything
 
Well the series test did tell you something...
Plus Glen ( I think ) has just given you what you need to know ( or check )
 
Then you haven't replaced everything necessary to use the board in a 7700 receiver.
For example, some of the zener diodes are different when going to a 7700.
You need to get into the service manual and carefully note "all" of the differences, then make sure they have been applied.

I only needed to replace the F-3058 board, not the power amp board (but components on the power amp board were replaced due to them blowing when power was supplied before knowing there was damage). I'll dig in and double check, but all that I recollect being on that board are 2 resistors, 4 output transistors, and a varistor.
 
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Some other measurements I've taken:
Voltages at both sides of
R601: 41.5v/23.99v
R604: 43.5v/23.99v

I am assuming that you replaced the single load emitter resistors from the G-6700 F-3058 bd with the double load emitter resistors from the G-7700's 3058 bd.

Are the above measurements from the same channel?

My suggestion is to measure and report the voltage readings for
B+ & B-
R59&60 - - - TR601/B
R85&86 - - - TR603/B
R65&66 - - - TR602/B
R87&88 - - - TR604/B
R69&70 - - - TR601/E
R41&42 - - - Junction of all 4 emitter resistors
 
I am assuming that you replaced the single load emitter resistors from the G-6700 F-3058 bd with the double load emitter resistors from the G-7700's 3058 bd.

Are the above measurements from the same channel?

My suggestion is to measure and report the voltage readings for
B+ & B-
R59&60 - - - TR601/B
R85&86 - - - TR603/B
R65&66 - - - TR602/B
R87&88 - - - TR604/B
R69&70 - - - TR601/E
R41&42 - - - Junction of all 4 emitter resistors

I did replace the emitter resistors, but I had to put two single load resistors in series to replace one of the originals because it was also broken in shipping.

Please explain to me what you mean by B+ & B-, and I will get the voltages reported. Also, I don't know where the junction of all 4 emitter resistors is...I will look at it harder after I've rested a while, long day working in the sun...
 
B+ & B- is a common reference to the principal rail voltages, coming off the big filter caps. On your schematic the circled #17 is B+, and the circled #18 is B-. There are many places to measure this but I prefer to clip onto the leads coming right off the filter caps. To be on the safe side I like to clip a mini grabber to a component lead with the power off, turn power on for the measurement, then turn it off to move the mini grabber. This prevents the probe from slipping and bridging two points that should not be touching. That can (and has) take out a string of components in a flash.

I also make it a habit to measure B+ & B- every time I start taking a series of voltage readings. The LINE voltage feeding your equipment varies during the day, and the P/S voltages in your gear will also vary proportionally. If you are comparing voltage levels to measurements you took at some other time, this could be the reason they do not agree.
 
B+ & B- is a common reference to the principal rail voltages, coming off the big filter caps. On your schematic the circled #17 is B+, and the circled #18 is B-. There are many places to measure this but I prefer to clip onto the leads coming right off the filter caps. To be on the safe side I like to clip a mini grabber to a component lead with the power off, turn power on for the measurement, then turn it off to move the mini grabber. This prevents the probe from slipping and bridging two points that should not be touching. That can (and has) take out a string of components in a flash.

I also make it a habit to measure B+ & B- every time I start taking a series of voltage readings. The LINE voltage feeding your equipment varies during the day, and the P/S voltages in your gear will also vary proportionally. If you are comparing voltage levels to measurements you took at some other time, this could be the reason they do not agree.

Thank you for the break down; it's humbling to say it when something is over my head, but I certainly appreciate the patience and information. I will get back into this as soon as I can and report back; I'm going to check back over the F-3058 board first thing to make sure my soldering was good and that everything is squared away, then I'll get into the voltages.
 
My voltage readings for:

B+ --> +70.6V, B- --> -70.7V
R59 +.353, R60 +.540 - - - TR601/B
R85 +.359, R86 +.546 - - - TR603/B
R65 - .365, R66 - .557 - - - TR602/B
R87 - .367, R88 - .557 - - - TR604/B
R69 - .006, R70 +.002 - - - TR601/E
R41 - .001/-.011, R42 .000/+.002 - - - Junction of all 4 emitter resistors

Clearly, the left channel has lower voltage than the right, but I don't know exactly what that means, or what that leads to...

EDIT: ghazzer cleared some things up for me, so I cleaned up the post!
 
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Could it be something as simple as a bad resistor further up the circuit? If something is choking the voltage down on the left channel, what else could it be?
 
I would be inclined to hook up your meter and measure that left channel bias as the volume is increased to see if the amp is oscillating and driving the bias way up.
 
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