I've got a JVC JA-S41 integrated amp that will not engage the relay. It's been completely recapped and some of the transistors have been changed or replaced due to previous owner. Plugged into DBT and the relay is being feed 48 vdc. There are 2 transistors that seem to be controlling the relay X624 and X625. According to the schematic is is suppose to be 9.9 vdc from the collector of X625 and 6.9vdc from the emitter. The collector measures 48vdc and the emitter is .6 mv. X625 was originally 2sd438 and was replaced with Ksc2383 and X624 is KSC1845. I have attached the schematics and any help or ideas would be appreciated
I could use any advice. I'm definitely not a expert and still learning. There should be a negative 4.7 vdc coming out of the collector of X623 but I'm getting a positive .6 MV. Not sure how or why a transistor changes polarity of voltage. Is it because of the high voltage being applied to the base and emitter?
The first check to do are to confirm there is no DC offset on each channels power amp and that there is no over current situation in the output stage.
You should have less than 100mv DC (of either polarity) at the points marked TP54 and TP58 on the diagram.#
With the amp OFF measure the resistor pairs R643 and R645 and also R644 and R646. They should all be 0.22 ohm. These may be hard to measure accurately on a normal meter + leads but what you are looking for is any that are open or very high.
If the DC offset is OK then next measure the DC voltage between TP54 and TP55. It should be low, somewhere less than around 25mv. Do the same between TP57 and TP58. Polarity of the result is unimportant.
If all the above are OK then you really might have a relay drive issue but I suspect it will fall down on the above tests somewhere.
You should have less than 100mv DC (of either polarity) at the points marked TP54 and TP58 on the diagram.#
With the amp OFF measure the resistor pairs R643 and R645 and also R644 and R646. They should all be 0.22 ohm. These may be hard to measure accurately on a normal meter + leads but what you are looking for is any that are open or very high.
If the DC offset is OK then next measure the DC voltage between TP54 and TP55. It should be low, somewhere less than around 25mv. Do the same between TP57 and TP58. Polarity of the result is unimportant.
If all the above are OK then you really might have a relay drive issue but I suspect it will fall down on the above tests somewhere.
Thanks, I will do those tests in a bit. Helping my 17 year old son going out on his first date, sigh. This should be interesting
👍 that's good, I'm surprised it isn't a power amp fault, however......
I'll have a closer look later today but if we are looking at a relay drive fault then first suspect would be the cap C704. The only way to prove it easily is just to replace it. It 'smooths' the negative rail that is used to detect the presence of AC and to give a rapid relay disconnect when you turn the amp off. You would need a scope to look at the ripple on the cap to determine if it had failed, measuring the DC voltage isn't good enough here. Just change it.
If there are still problems then we need to work through it more closely but that cap would be a prime suspect.
Replace x624 and x625 . X625 should be a 600ma transistor. It has a resistive turn on with the 330 ohm emitter resistor , some heat is disapated here. 2sc1845 will die quickly.
Will replace C704.
Stocktrader what would you suggest for x624 and x625 as replacements. I do have access to some old 2SC1775 transistors for X624. What about X625?
Stocktrader what would you suggest for x624 and x625 as replacements. I do have access to some old 2SC1775 transistors for X624. What about X625?
Replaced C704, no change. I'm still getting 42vdc from the collector of X625. I have another S41 amp , pulled the cover and its putting out 10vdc from the collector. I just dont understand what the difference is. The voltage to the base and emitter are relatively the same.
Lets try and work forwards from the relay driver. We try and disturb as little as possible and gather evidence now from simple tests and voltage readings.
You have 42 volts on the collector of X625 because it is turned off and no current is flowing,
Work through these in order...
Edit... before doing these just stick a screwdriver 🙂 between collector and emitter of X625 and check the relay really does close OK. That just proves supply, relay and series resistors are OK.
1/ With the amp on measure the voltage on X627 base. It should be essentially zero.
2/ What voltage do you measure on the base of X625? (This is just for interest with it in the faulty state).
3/ If you apply a short from X624 base to emitter the relay should close. Does it? (this is a quick and dirty test to prove the driver transistor and relay is OK)
4/ If the relay does not close in step 3 above then remove X624. The relay should close.
Assuming you get this far and it behaves as outlined above:
5/ What voltage do you have on the newly replaced C704? (I would guess it should be around minus 7 volts based on the fact it is a 6 volt pilot bulb which might be under run. If we reckon on 5 volts AC then that would come in at around minus 7 volts DC when rectified)
Lets see where that gets you.
You have 42 volts on the collector of X625 because it is turned off and no current is flowing,
Work through these in order...
Edit... before doing these just stick a screwdriver 🙂 between collector and emitter of X625 and check the relay really does close OK. That just proves supply, relay and series resistors are OK.
1/ With the amp on measure the voltage on X627 base. It should be essentially zero.
2/ What voltage do you measure on the base of X625? (This is just for interest with it in the faulty state).
3/ If you apply a short from X624 base to emitter the relay should close. Does it? (this is a quick and dirty test to prove the driver transistor and relay is OK)
4/ If the relay does not close in step 3 above then remove X624. The relay should close.
Assuming you get this far and it behaves as outlined above:
5/ What voltage do you have on the newly replaced C704? (I would guess it should be around minus 7 volts based on the fact it is a 6 volt pilot bulb which might be under run. If we reckon on 5 volts AC then that would come in at around minus 7 volts DC when rectified)
Lets see where that gets you.
Good 🙂
Try and work through the other checks and report your findings. Doing it this way avoids unnecessary removal and replacement of parts hopefully.
Try and work through the other checks and report your findings. Doing it this way avoids unnecessary removal and replacement of parts hopefully.
Thanks for all the help, i'm learning everyday from people a lot more experienced than I. I would be very happy to get this amp going. I got it about 5 years ago, it was being junked out so I took it and have been slowly refurbishing it.
I have to work tonight so I'll be out of pocket until about 2230 my time. I know your in the UK so were what about 6 hours behind you folks.
Looking forward to the next steps
I have to work tonight so I'll be out of pocket until about 2230 my time. I know your in the UK so were what about 6 hours behind you folks.
Looking forward to the next steps
Could be. Unusual but there is not much else. Check for AC voltage on the cathode (stripey end). I would guess about 5 volts AC given the bulbs 6.3 volt marked value.
Just to explain the readings.
X627 base shows no DC that would turn on the transistor. If X627 were on it would pull the base of X625 to ground and stop the relay turning on.
X625 has no voltage to turn it on. It should be biased on via R668 provided nothing else gets in the way.
What may have happened...
The missing voltage on C704 is a definite issue. Depending what you find it is possible the original cap has caused failure of the diode. Small value caps like that hung straight off a rectifier are a common issue generally. The ripple current they see is relatively high for a small cap and they deteriorate and dry out. They don't normally take out the diode though 😀
The missing voltage is allowing X624 to turn on via R666. Normally its a balancing act and the negative voltage is pulling the base of X624 'down' via R675 and R667 which stops X624 turning on. If its on it pulls the relay driver base down and stops the relay operating.
The diode did it!!!. Relay now engages. Tonight I will adjust idle and dc offset. We get a solid 6.5 vdc at the diode and c704.
Again thanks so much. I buy amps that are in junk piles either from eBay or locally and refurbish them. I'll probably be asking for more help in the future. Thanks again for the help
Again thanks so much. I buy amps that are in junk piles either from eBay or locally and refurbish them. I'll probably be asking for more help in the future. Thanks again for the help