For record keeping - got another A500 with identical symptoms: protection triggering with no reason.
It was done some maintenance - full recap, relay contact cleaning, volume pot replacement. Had to replace zenners all 3 (orange ones) as they being rated 16v but actually across zenner voltage was much bigger 17.5v - 18.6v. New zenners fixed issue but they heats up a bit so need to install at least 1W rated. Oh and had to re-solder some dry joints on PCB as usual for that age amplifier.
Measurements:
DC offset is normal 20mV and 40mV.
Idling current as per manual 15mV.
No AC in speaker terminals from PSU.
No speakers connected so no short in speaker terminals.
With current UK mains voltage voltage out of PSU is +-50.5V where suppose to be 49.5V.
Symptoms:
After warmup period approx 5-15 mins clicks relay and disconnects speakers.
Then after couple of seconds clicks back.
Intervals between cycles - random but in range from 15sec to 2 mins.
One more interesting observation - if to tap with finger area near output transistors or especially where protection small transistors sits, always triggers protection on tap.
Based on djk advice I have replaced all small and big protection el.caps in protection but as it happens, all measured with ESR Meter as aged by a little but still working.
Protection sensing transistors (TR125, TR126, TR129) (those were sensitive to touch on legs from solder side of PCB) measured as working (measured voltage across legs and with diode mode on tester) while on PCB.
Now main part of this report: I have de-soldered each of them from the board and put into multimeter - again all were in specs ~400hFE and ~600mV. And when all 3 were soldered back, protection stopped to trigger not only by itself, and stopped triggering by finger tapping around the same areas.
So root cause might be:
- close to fault one of protection transistors, which were magically (might be temporary) fixed by giving heat stress during removal and putting back.
- less likely but still might be - it was conducting dust or flux around those protection transistors terminals.
Time will show if it was fixed for good, or for how long, but for now amplifier works fine.
And to final note - even for the same symptoms cure might be totally different...
It was done some maintenance - full recap, relay contact cleaning, volume pot replacement. Had to replace zenners all 3 (orange ones) as they being rated 16v but actually across zenner voltage was much bigger 17.5v - 18.6v. New zenners fixed issue but they heats up a bit so need to install at least 1W rated. Oh and had to re-solder some dry joints on PCB as usual for that age amplifier.
Measurements:
DC offset is normal 20mV and 40mV.
Idling current as per manual 15mV.
No AC in speaker terminals from PSU.
No speakers connected so no short in speaker terminals.
With current UK mains voltage voltage out of PSU is +-50.5V where suppose to be 49.5V.
Symptoms:
After warmup period approx 5-15 mins clicks relay and disconnects speakers.
Then after couple of seconds clicks back.
Intervals between cycles - random but in range from 15sec to 2 mins.
One more interesting observation - if to tap with finger area near output transistors or especially where protection small transistors sits, always triggers protection on tap.
Based on djk advice I have replaced all small and big protection el.caps in protection but as it happens, all measured with ESR Meter as aged by a little but still working.
Protection sensing transistors (TR125, TR126, TR129) (those were sensitive to touch on legs from solder side of PCB) measured as working (measured voltage across legs and with diode mode on tester) while on PCB.
Now main part of this report: I have de-soldered each of them from the board and put into multimeter - again all were in specs ~400hFE and ~600mV. And when all 3 were soldered back, protection stopped to trigger not only by itself, and stopped triggering by finger tapping around the same areas.
So root cause might be:
- close to fault one of protection transistors, which were magically (might be temporary) fixed by giving heat stress during removal and putting back.
- less likely but still might be - it was conducting dust or flux around those protection transistors terminals.
Time will show if it was fixed for good, or for how long, but for now amplifier works fine.
And to final note - even for the same symptoms cure might be totally different...
Well... After 12 hours of working amplifier went into protection triggering on-off cycle again. I must admit it was expected in case if fault was caused by transistor which started to work after heat-stress...
This time I have invested a bit more time for proper diagnostic.
Findings: measurement shows that TR125 on Base had fluctuating voltage in range 200-375mV. When the same TR126 for the other channel had constant voltage approx 40mV.
So after replacement of TR125 with some random transistor (just checked max voltages as per spec), on Base it got constant 5mV voltage and amplifier is no longer triggering protection.
Interesting case when protection circuit is triggered because of itself fault, not the main amplifier circuit fault...
This time I have invested a bit more time for proper diagnostic.
Findings: measurement shows that TR125 on Base had fluctuating voltage in range 200-375mV. When the same TR126 for the other channel had constant voltage approx 40mV.
So after replacement of TR125 with some random transistor (just checked max voltages as per spec), on Base it got constant 5mV voltage and amplifier is no longer triggering protection.
Interesting case when protection circuit is triggered because of itself fault, not the main amplifier circuit fault...
If you have a decent sound card and ARTA or REW, looking at distortion spectrum you can set the bias to minimize distortion. For some reason, many amps seem to be under-biased, including some of the cheaper Yamaha models I checked. From my experience the distortion would probably improve if you increase bias voltage to 25-30mV (just guessing, measurement is needed).
Maybe the measured THD number would not change so much, but the harmonics spectrum usually looks much cleaner with a little more bias than they specify. '
But I have also seen distortion decrease of more than 10dB on some amps compared to specified bias (think it was a Denon AVR). Be aware of higher temps though.
Maybe the measured THD number would not change so much, but the harmonics spectrum usually looks much cleaner with a little more bias than they specify. '
But I have also seen distortion decrease of more than 10dB on some amps compared to specified bias (think it was a Denon AVR). Be aware of higher temps though.
Hi Rallyfinnen,
Thanks for suggestion. Initially drifted addling current was approx 25mV so amp was running a bit too hot. Especially when there is other device put on top of it.
In general I'm not questioning Yamaha's idling current recommendations, especially for this model which is equipped with ZDR (zero distortion rule) circuit which cancels an distortion and it works on recommended idling current in mind.
Usually Yamaha does not saves on idling current and calculates it based on requirements and size of heatsink reasonably. Not as Rotel with funny 4mV which makes amplifier run totally cold and not warmed by a bit while working.
Anyway when I have tried to hear difference, when adjusted idling for some other amp when at the same time I could listen to see how it affects or if affects sound, so I must say, maybe it is visible as a result for distortion numbers or spectrum picture, but it is not hearable. I mean if to add e.g. 2x more than recommended. So not playing with idling current a lot.
This is for Yamahas, but for Rotels I do first thing couple of tricks a) disconnecting driver emitter resistors from output stage, b) increasing idling current up until usual 15-20mV/0.22ohm to make it warm upon working.
Thanks for suggestion. Initially drifted addling current was approx 25mV so amp was running a bit too hot. Especially when there is other device put on top of it.
In general I'm not questioning Yamaha's idling current recommendations, especially for this model which is equipped with ZDR (zero distortion rule) circuit which cancels an distortion and it works on recommended idling current in mind.
Usually Yamaha does not saves on idling current and calculates it based on requirements and size of heatsink reasonably. Not as Rotel with funny 4mV which makes amplifier run totally cold and not warmed by a bit while working.
Anyway when I have tried to hear difference, when adjusted idling for some other amp when at the same time I could listen to see how it affects or if affects sound, so I must say, maybe it is visible as a result for distortion numbers or spectrum picture, but it is not hearable. I mean if to add e.g. 2x more than recommended. So not playing with idling current a lot.
This is for Yamahas, but for Rotels I do first thing couple of tricks a) disconnecting driver emitter resistors from output stage, b) increasing idling current up until usual 15-20mV/0.22ohm to make it warm upon working.
A500 - what is behind R and C in in driver's bases?
Hi,
as we have Yamaha A500 repair thread it might be right place to discuss what is the reason for R229 (2.2k), C185 (1200p) and R231 (2.2k), C187 (3300p) in bases of driver transistors?
If all this is for oscillation suppression, why not single 100 ohm resistors? And perhaps 220p across BC?
And why caps values different for NPN and PNP drivers?
Strange that A500 must be simpler circuit than A700, but in A700 there is no such RC in driver's bases.
Another resemblance I saw in NAIM NAP140, but there was slightly different configuration, but again similarity to this is the different values for R in NPN and PNP driver's bases.
And again, what for all this stuff?
Thanks.
Kes
Hi,
as we have Yamaha A500 repair thread it might be right place to discuss what is the reason for R229 (2.2k), C185 (1200p) and R231 (2.2k), C187 (3300p) in bases of driver transistors?
If all this is for oscillation suppression, why not single 100 ohm resistors? And perhaps 220p across BC?
And why caps values different for NPN and PNP drivers?
Strange that A500 must be simpler circuit than A700, but in A700 there is no such RC in driver's bases.
Another resemblance I saw in NAIM NAP140, but there was slightly different configuration, but again similarity to this is the different values for R in NPN and PNP driver's bases.
And again, what for all this stuff?
Thanks.
Kes
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greetings to the group.
I turni it back on this old post to ask for a remote diagnosis too.
for sale is an A-500 that works in mono only.
the seller, correctly, wrote that the same problem had already been solved about ten years ago but he doesn't remember what had been replaced.
before taking it home and finding myself with yet another irreparable wreck, I ask you experts (I'm less than a hobbyist) if you have an idea of what could create this inconvenience.
Thanks in advance for any answers/suggestions.
I turni it back on this old post to ask for a remote diagnosis too.
for sale is an A-500 that works in mono only.
the seller, correctly, wrote that the same problem had already been solved about ten years ago but he doesn't remember what had been replaced.
before taking it home and finding myself with yet another irreparable wreck, I ask you experts (I'm less than a hobbyist) if you have an idea of what could create this inconvenience.
Thanks in advance for any answers/suggestions.
Hi Guys - I too have a similar problem...
My a500 goes into protection mode - I replaced the caps around the relay etc.
However i spent some time monitoring offset on Both channels and these seem ok when first switched on , but after a while BOTH channels go a bit wonky with offset rising and falling slowly up to about 150mV...not staying constant at all...relay seems to trigger at 80mV from memory.
Any ideas
THanks in Advance
George
My a500 goes into protection mode - I replaced the caps around the relay etc.
However i spent some time monitoring offset on Both channels and these seem ok when first switched on , but after a while BOTH channels go a bit wonky with offset rising and falling slowly up to about 150mV...not staying constant at all...relay seems to trigger at 80mV from memory.
Any ideas
THanks in Advance
George
Hi, seems I had repaired same amp model with the same issue. When left turned on, even without signal, protection relay clicks after 5-30mins without reason. So I had to replace one of small protection sensing transistors (in red). Assume you replaced small electrolytic capacitors around protection transistors (in yellow) before.
Attachments
Hi Kesrmrk
From memory the caps were replaced -- i will check - however the transistor you refer to (on both channels ) may need a look- again from memory there seemed to be a couple of "freefloating" transistors were sort of "clamped "to the heatsink- i added some thermal compound- to no avail...
maybe the one you refer to....??
From memory the caps were replaced -- i will check - however the transistor you refer to (on both channels ) may need a look- again from memory there seemed to be a couple of "freefloating" transistors were sort of "clamped "to the heatsink- i added some thermal compound- to no avail...
maybe the one you refer to....??