• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

SA7700 protection relay dropping out

Stereoboy70

Stereoboy70

Active Member
Hello again. I’m having trouble with this amp. When I’m running the volume about half way the ice cube relay will open up until you turn down the volume and then about 5 seconds later it closes again and I have sound. At low volume it’s fine and will play all day. The relay has been replaced by another owner. I replaced Q26 transistor with fresh heat paste and still have the issue. I’ve heard maybe the PA 3004 chip might be failing or a zenor diode may be bad
Wondering if anybody has a solution or has had this issue before. Thanks in advance.
 
Register to hide this ad
I am thinking you may have an issue with the dc offset, since it works at low volume but not high. If you disconnect your speakers and turn on the amp with no audio signal going into, you can measure the dc voltage at the speaker removals - that might give some indication to the problem. You can also measure the voltages going into the PA3004 chip when the relay is open - that will tell you which part of the circuit is failing.
 
alexhastings said:
I am thinking you may have an issue with the dc offset, since it works at low volume but not high. If you disconnect your speakers and turn on the amp with no audio signal going into, you can measure the dc voltage at the speaker removals - that might give some indication to the problem. You can also measure the voltages going into the PA3004 chip when the relay is open - that will tell you which part of the circuit is failing.
Click to expand...
Measured .3 mV in R channel and 6.1mV on left channel. I don’t see any adjustment pots for the dc offset unless it’s controlled by the idle current pots.
 
I don't think it's adjustable with a pot in the 7700... When the volume is up and the relay is open, is the dc offset the same? If so, you may have to measure the voltages at the PA3004 when the relay is open to see what's causing it to happen
 
try balance all the way left and all the way right ..
i have a feeling it might be q19 q20 protect circuit at fault .
 
petehall347 said:
try balance all the way left and all the way right ..
i have a feeling it might be q19 q20 protect circuit at fault .
Click to expand...

If the DC offset is the culprit, Q19 and Q20 is a great place to start! I had a lot of trouble with one of those leaking current and tripping the relay
 
alexhastings said:
If the DC offset is the culprit, Q19 and Q20 is a great place to start! I had a lot of trouble with one of those leaking current and tripping the relay
Click to expand...
Somebody had replaced Q 22 with a transistor that should not be there. I’m going to replace it with a suitable one. I’ll also try swapping those two transistors and see what happens. Thank you for your help
 
alexhastings said:
I used KSC1845FTA for Q19 and Q20.

For Q22, I used KSC2690AYSTU.

Good luck!
Click to expand...
Hello. I replaced the Q19,20 and Q22. Also the PA 3004 chip. Also Q26. Relay still drops out at about 50% volume or a little less. Checked bias and it’s hovering around 40mV per channel with 8ohm resistors on the speaker terminals. This is correct according to the service manual. Any caps related to this circuit? All caps look good just using a visual.
I’ve read that there are a couple of zenor diodes that might be an issue also.
 
Q19 & Q20, what about C319 & C320 ?

Checking the service manual they are the dreaded type CSZA, the sky blue Sanyo solid aluminum capacitors (Sanyo's try for imitation Tantalum caps) that leak electrically until they become solid short circuits.

Replace C319 & C320 and your protection problems as currently described should clear up.
 
markthefixer said:
Q19 & Q20, what about C319 & C320 ?

Checking the service manual they are the dreaded type CSZA, the sky blue Sanyo solid aluminum capacitors (Sanyo's try for imitation Tantalum caps) that leak electrically until they become solid short circuits.

Replace C319 & C320 and your protection problems as currently described should clear up.
Click to expand...
Will do. Can I use
markthefixer said:
Q19 & Q20, what about C319 & C320 ?

Checking the service manual they are the dreaded type CSZA, the sky blue Sanyo solid aluminum capacitors (Sanyo's try for imitation Tantalum caps) that leak electrically until they become solid short circuits.

Replace C319 & C320 and your protection problems as currently described should clear up.
Click to expand...
Can I use film caps or electrolytic??
 
EastPoint said:
Use film. Wima is usually what’s used to replace those.
Click to expand...
Hello again. Replaced the blue caps with Wima films and it is still doing the same. About 3.5 on volume the relay drops out. I’ve replaced all the transistors that are in question and set the bias to spec. Also replaced some questionable caps Could it be related to the output transistors??? All the questionable components have been swapped. The relay looks like it has been replaced.
 
dc offsets are good, not the problem.

the amp sounds ok until it trips?
q26, q22, q19, q20, pa3004, C319, C320 all new?
WHICH questionable caps?

We have hit almost all of the usual suspects. Cept this:

try a new C405 (47uf / 16v) . No fudging... this circuit position is deceptive.

IF it STILL trips after a new C405:
pull q21 and see if it still trips.
 
markthefixer said:
dc offsets are good, not the problem.

the amp sounds ok until it trips?
q26, q22, q19, q20, pa3004, C319, C320 all new?
WHICH questionable caps?

We have hit almost all of the usual suspects. Cept this:

try a new C405 (47uf / 16v) . No fudging... this circuit position is deceptive.

IF it STILL trips after a new C405:
pull q21 and see if it still trips.
Click to expand...
I replaced cap 405,406,407. I used a 33/50 at 407 instead of a 33/16. The others are original values.
 
You must log in or register to reply here.
Back
Top Bottom