I have had this great sounding QED CD240 amp for decades. Over the years it's had several sets of BD911/912 replacement output devices and a few other small driver components.
About 3 years ago it suddenly stopped working after a period of running quite hot to the touch.
I then changed the bridge rectifier and it worked fine until recently when it started playing up by taking progressively longer to start working
after switching it back on. Even now, once it starts after 15 minutes and if I don't switch it off it works fine until I switch it off. Any ideas anyone? Thanks
About 3 years ago it suddenly stopped working after a period of running quite hot to the touch.
I then changed the bridge rectifier and it worked fine until recently when it started playing up by taking progressively longer to start working
after switching it back on. Even now, once it starts after 15 minutes and if I don't switch it off it works fine until I switch it off. Any ideas anyone? Thanks
Is yours this version of the A240CD?
Or the one below with the separate listen and record selection switches?
I have owned the former version since 1986 and it is still performing faultlessly - there's never a component failure!
My A240CD has 4 discrete rectifier diodes, so I presume yours is the latter version since you mention a bridge rectifier.
Anyhoo, that's got your thread up and running - I hope.
Or the one below with the separate listen and record selection switches?
I have owned the former version since 1986 and it is still performing faultlessly - there's never a component failure!
My A240CD has 4 discrete rectifier diodes, so I presume yours is the latter version since you mention a bridge rectifier.
Anyhoo, that's got your thread up and running - I hope.
You can download the QED A240 Series 2 service sheet from here: https://elektrotanya.com/qed_a240_series_2_service_sheet_sch.pdf/download.html
Attachments
Try to troubleshoot while the amp is misbehaving. From the symptoms you describe, I suspect the delayed-on relay circuit. Do you hear the relay click when audio begins?
The schematic clarity is very marginal, but I interpret the controlling time constant as R31 and C31, i.e. 100K and 47uF, a nominal 4.7 second time constant. Try monitoring the rise of C31 voltage at power on. If it's slow, the resistor may have drifted high in value; the cap may have become leaky, thus limiting voltage developed; the emitter follower transistor's Beta may have deteriorated.
Good luck!
The schematic clarity is very marginal, but I interpret the controlling time constant as R31 and C31, i.e. 100K and 47uF, a nominal 4.7 second time constant. Try monitoring the rise of C31 voltage at power on. If it's slow, the resistor may have drifted high in value; the cap may have become leaky, thus limiting voltage developed; the emitter follower transistor's Beta may have deteriorated.
Good luck!
I have a QED 240 that also gave me problems for some time, and in the end, I found a power supply filter capacitor that was really degraded compared to the other one (the amp had a very big DC Offset at the output at cold start with a very big difference between the positive and negative voltage) but if I gradually increased the AC supply voltage with a variac, it worked for hours in an acceptable way but with a slight distortion audible on the Forté.
I'm not at home, I don't remember if there is a delay circuit, I can tell you when I get home, I kept this amp.
That sounds like the place to look. I don't think my Series 1 model has the relay. However The Series 2 does and that may cause some confusion.
For Rol G's information: After operating the power switch there should be a short delay before the relay clicks and brings the amp into operation. During this delay the amplifier is muted in order to prevent switch-on thumps.
The relay circuit is not shown in the schematic attached in post #5. I believe it is on a separate board that feeds into the point marked "Input From Relay Circuit". This input is required to bias up the input stage of each channel.
BSST, Am I correct in presuming that it is on a separate board where the R31 and C31 time delay components you mentioned are to be found?
Hi Galu,
Yes, I found ithe circuit in the SM as you describe, but in a different page than you show in your helpful post.
Thanks.
Yes, I found ithe circuit in the SM as you describe, but in a different page than you show in your helpful post.
Thanks.
Thanks BSST.
I dug out an old Hi-Fi World article on QED amplifiers and found an explanation of the marginal clarity of the schematic to which you referred.
The A240CD was introduced in 1986 and at this point in time QED's circuit diagrams were "still being hand-drawn" and its amplifier circuit boards "laid out using masking tape and a craft knife".
I dug out an old Hi-Fi World article on QED amplifiers and found an explanation of the marginal clarity of the schematic to which you referred.
The A240CD was introduced in 1986 and at this point in time QED's circuit diagrams were "still being hand-drawn" and its amplifier circuit boards "laid out using masking tape and a craft knife".
No relay click ever at the point when the amp decides to start working.
No click ever after pressing the on switch.
No click immediately before the amp starts working again after
The longer it's been switched off via the power switch the longer it takes to start producing audio after switching on.
Once it's been working for a few minutes, if you then switch it off and back on immediately, it won't start instantly but it only takes a few seconds to produce audio.
No click ever after pressing the on switch.
No click immediately before the amp starts working again after
The longer it's been switched off via the power switch the longer it takes to start producing audio after switching on.
Once it's been working for a few minutes, if you then switch it off and back on immediately, it won't start instantly but it only takes a few seconds to produce audio.
This may be progress.
Take off the cover so you can hear the relay click. If the relay driver is working properly, you should hear the relay click in only a few seconds.
Am I correct that neither channel works, then they both begin to play simultaneously? Measure both PA output voltages, before and after the relay contacts. You should probably use expendable speakers until the problem is resolved.
Take off the cover so you can hear the relay click. If the relay driver is working properly, you should hear the relay click in only a few seconds.
Am I correct that neither channel works, then they both begin to play simultaneously? Measure both PA output voltages, before and after the relay contacts. You should probably use expendable speakers until the problem is resolved.
Perhaps you should post a large, clear photograph of the interior of your Series 2 amplifier so we can locate the relay.
Have you located the relay control circuit diagram in the hand-drawn service manual?
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"PA output voltage" is Power Amp output voltage at the junction of the emitter resistors and labeled "output" in the drawing.
The caution about the speakers was motivated by uncertainty about the relay. If the relay is closing at power on, why isn't audio present? Is there really a relay present? Faulty amplifiers can be dangerous to speakers--- hence caution until there's better understanding.
Are both channels both quiet, then both come to life?
The caution about the speakers was motivated by uncertainty about the relay. If the relay is closing at power on, why isn't audio present? Is there really a relay present? Faulty amplifiers can be dangerous to speakers--- hence caution until there's better understanding.
Are both channels both quiet, then both come to life?
I left it on non stop until today. It worked fine. After switching it off it now stays dead and cold. Pics attached.
