pioneer SA 7800 protection relay, fault or function?

hugo22

New Member
Dear all, please advise. I have gone through discussions but no simillar issue mentioned.
My SA7800 works perfectly but in case of AUX or Phono active input, play CD or vinyl, and volume set about -25db and more power the relay starts cutoff speakers, click on/of in peaks.
It is true that blue pwr indicators, especially in drums, touch maximum but it seem to be early? In case of Loudness switch on, this happen from -40db.
using Tuner input is OK, no ploblem. I can volume till -20db and no problem.
I do not understand what is wrong? Speakers BW 703, 8ohms,
My other amp SA 708 works with no problem, the same setting

Really appreciate your help.
 
Sounds like the over current or overload detector is cutting the speakers off to save the amplifier from blowing. Either that, or the amplifier itself has a problem and it is cutting out to save your speakers.

Might be a good thing to measure your DC offset. Connect a voltmeter on a very low DC scale, 2V or lower, and measure the voltage at the speaker terminals. It should be 0mV.
 
And that measurement is taken with the volume turned ALL THE WAY DOWN, Tone controls centered, Loudness OFF, Selector in AUX, with NO SOURCE either On or Connected to the Aux PORTS.

Agree with QSilver that the protection circuit is working to keep your unit alive, and you've got a problem in the amplifier board(s) itself. Get the Service manual and check the IDLE CURRENT. No Offset adjustments on the SA7800. ( NSA or Non-Switching Amp). If you don't understand the instructions in the manual, (they are somewhat vague at times) ASK QUESTIONS 1st!!!! The NSA's can be a TOTAL PITA to fix if you screw it up. On your probes INSULATE ALL BUT THE EXTREME TIP!!!! This will help prevent "Aw Sh**'s" when you DO slip(and we've ALL BTDT so welcome to the club). Take the measurements as listed in the manual and report back.
 
IF it correlates with volume events in the audio material, (use the balance control to localize which channel)
then GWH-118 C21(L), C22(R) CEANL caps could be at fault. Unusual and unlikely, but not impossible.
Then GWH-118 Q27(L), Q28(R) 2sc1914a can be replaced with ksc1845 transistors if it continues.


And... probe tips slip when we take our eyes off the probe to read the meter. It happens to us all..
 
Two more possibilities.
One: (most likely if problem occurs when either channel is driven) C25 (330 uF 6V) in the protect circuit is open or off value. This is part of a low pass filter which keeps the signal out of the DC protection circuits. If so, either channel will trigger protect. It would also be better if this cap were a NP (non polar) or bipolar (same thing, different name) capacitor. If you have something close, you could tack it on to see if it affects the problem.
Two: (presuming the problem is due to one channel), the overload circuit monitors the current going through the emitter resistors, so if one of those is higher than it should be, that would cause problems.
 
Hi All, speakers terminals measured, output A, R= 13mV, L=0-2mv, output B, R=0-2mV, L=0-2mV so , does it target to more specific problem?
thanks for help
M.
 
Hi All, caps C21, 22, 25 changed,fault still there, Q27,28 dismantled from PCB and measured, seems to be ok. Replacement KSC 1845 FTA comes on Friday but do not expect any change.
Please advise any other way to go???
 
Have you tried turning balance control to see if only one channel triggers - i.e. turn it to left only, then see if problem occurs, then turn the balance to right only, then see if it occurs. Try to narrow down where problem comes from.
 
Q27,28 replaced and all work well now !!!!! I do not belive that Qs were wrong( i measured them out of the board, OK) but replacement solved the problem.
Thanks for valuable advise, really appreciate.
I will be back soon with my SA 708, output A Left is weak or completely off but " nocking" the case solve it. Likely need to be resoldered….
 
Hi Again, back with SA7800
For testing I used different set of speakers ( 8ohms), works well no cutoff issue but whet i returned " repaired" SA back to the rack and connect BW703 the problem is back.
Now tested:
output B or output A - problem is there when volume more than -40db, balance control either max L or R still the same problem, looks problem both L and R??
BUT only with BW703 speakers, other set Radiotehnika, Supersound S150 no problem.

I would say, thare is problem with BW703 but vith SA708, SA8800 work well

Could you advise?? any idea?? Why? Does my SA7800 has allergy to BW?
 
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