Hi
The title is essentially the issue.
A more bass than treble "thump".
No crackling when the volume is altered up or down.
Plays music well otherwise.
Now I will confess to being a novice here: basic electronics theory and kits behind me (1980/90s), soldering iron and multi meter available, and a renewed interest in electronics (nothing too sophisticated mind you) in recent years.
This amplifier is from the late 1980s. I like it because of the slim height - like the A&R Cambridge - and it fits my cabinet shelf slot whereas most amplifiers don't.
Now I'm thinking something to do with the power capacitors ... but like I said, I'm a novice.
Any advice is appreciated.
Regards
Fud
The title is essentially the issue.
A more bass than treble "thump".
No crackling when the volume is altered up or down.
Plays music well otherwise.
Now I will confess to being a novice here: basic electronics theory and kits behind me (1980/90s), soldering iron and multi meter available, and a renewed interest in electronics (nothing too sophisticated mind you) in recent years.
This amplifier is from the late 1980s. I like it because of the slim height - like the A&R Cambridge - and it fits my cabinet shelf slot whereas most amplifiers don't.
Now I'm thinking something to do with the power capacitors ... but like I said, I'm a novice.
Any advice is appreciated.
Regards
Fud
The "turn on" delay relay isn't working,check out the relay circuit .
Circuit diagram available on hi-fi engine website
Circuit diagram available on hi-fi engine website
Appreciate the speedy reply. How would I check out the relay circuit?
Now I'm happy to learn here and if there are references/books /web sites to look at please direct me to them. I have another amp sitting in storage (one channel gone) so "one fine day when its raining" - as my father used to say, I'd like to get to that also.
I've got the circuit diagram.
Cheers
Fud (NZ)
Now I'm happy to learn here and if there are references/books /web sites to look at please direct me to them. I have another amp sitting in storage (one channel gone) so "one fine day when its raining" - as my father used to say, I'd like to get to that also.
I've got the circuit diagram.
Cheers
Fud (NZ)
Relays click when they switch. Shouldn't click immediately, but after a delay to hide the transient (thump) from the speakers.
Suspect either the relay is welded shut, or perhaps the electrolytics in the protect circuit are failing.
Suspect either the relay is welded shut, or perhaps the electrolytics in the protect circuit are failing.
Same probleme with the same amp in CD version (without MC/MM card)
the relay works well even if I find it a little fast, however the thump is huge and on both channels, I have to look more closely, it has just arrived on the table.
the relay works well even if I find it a little fast, however the thump is huge and on both channels, I have to look more closely, it has just arrived on the table.
I actually have some of those relays that I bought when QED went under. I also acquired the the owner, but that's another story...
Can you tell me more about that ?I actually have some of those relays that I bought when QED went under. I also acquired the the owner, but that's another story...
Most conventional AB amps will "thump" if the switch-on delay is too short or the switch-off not fast enough. The delay circuit is generally controlled by small electrolytic timing caps and these predictably fail because the circuit is either located near the output stage and gets hot or the caps are just cheap parts. If you ever see obsolete speaker protection SIL ICs type NEC μPC1237, Toshiba TA7317 or Chinese copies of them, you'll know what I refer to. When the timing cap or caps begin to fail, the discharge/timing effects disappear and because the power supply rails of the amplifier rise or fall at different rates, it "thumps" according to what then looks like a DC spike to the input stage. It still "thumps" whether you have speaker protection or not but without a speaker connected, it can't do any damage.
QED's relay drive design is simple and uses discrete parts but still requires timing caps so locate them from the speaker relay control circuit diagram in the hand drawn "service manual" copy and replace with decent modern types or preferably, just replace all small electrolytics in such old amps because few will be 100% now anyway and small electrolytics tend to be least reliable.
QED's relay drive design is simple and uses discrete parts but still requires timing caps so locate them from the speaker relay control circuit diagram in the hand drawn "service manual" copy and replace with decent modern types or preferably, just replace all small electrolytics in such old amps because few will be 100% now anyway and small electrolytics tend to be least reliable.
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thanks guys for the help 🤗
I stopped my professional activity of troubleshooting at the beginning of the year because I did not earn enough money and I resumed my job as a full-time refrigeration mechanic, so I have less time to devote to electronics.
I will return to this amp as soon as possible and I will provide information on this thread.
I stopped my professional activity of troubleshooting at the beginning of the year because I did not earn enough money and I resumed my job as a full-time refrigeration mechanic, so I have less time to devote to electronics.
I will return to this amp as soon as possible and I will provide information on this thread.
Originally, I got this amp for free because it burned its fuse directly when it was turned on.
once opened, no traces of burnt components or anything else.
after a few basic tests it turns out that it is the diode bridge that is dead (rare but nothing surprises me anymore).
Then comes the chapter bang at the switching of the relay, after investigation, nothing special or strange, just a DC and AC peak when the relay switches, in doubt I test some capacitors but all are perfect and I have not at all from offset to output.
Well, I ended up controlling the biases, and there I found almost zero values (+/- 1.5mV) so I set it to the original values (+/-15mv) and there, surprise, almost no more bang when the relay switches...
no idea why or how but it works perfectly... 🤷♂️
once opened, no traces of burnt components or anything else.
after a few basic tests it turns out that it is the diode bridge that is dead (rare but nothing surprises me anymore).
Then comes the chapter bang at the switching of the relay, after investigation, nothing special or strange, just a DC and AC peak when the relay switches, in doubt I test some capacitors but all are perfect and I have not at all from offset to output.
Well, I ended up controlling the biases, and there I found almost zero values (+/- 1.5mV) so I set it to the original values (+/-15mv) and there, surprise, almost no more bang when the relay switches...
no idea why or how but it works perfectly... 🤷♂️
As the "thump" is due to the different rate of power supply rails rising/falling, the increased bias current may be holding off the relay long enough to allow the thump to dissipate before it closes. You could probably see this on a CRT oscilloscope connected before and after the speaker relay contacts.
I admit that I didn't have the courage to disassemble the main pcb to check the rails when powered up, it's painful these architectures with a single pcb but I think I'm going to do it anyway insofar as I don't do not know who did what before me especially when you know that the diode bridge was really dead.
That's understandable. If you need to get serious and perform surgery to isolate the problems on the board, it sure takes courage but a dead bridge is rare unless a power output stage has shorted or "someone" has been trying to fix a problem with the wrong type of tools, wrong diagnosis etc.
I think that as many technicians now leave the consumer electronics repair business for reasons like yours, users are now helpless and try a lot of stupid things of necessity. Practical choices are tough - try to find a generous retired tech. who still has the time, tools and components, throw it away-buy another or simply go without.
I think that as many technicians now leave the consumer electronics repair business for reasons like yours, users are now helpless and try a lot of stupid things of necessity. Practical choices are tough - try to find a generous retired tech. who still has the time, tools and components, throw it away-buy another or simply go without.
In fact, I keep a lot "for my retirement", but it must be recognized that after 25 years spent in there, I think I lost the flame a little, so I store while waiting for me to get back to enjoying myself to do it .
Many things have changed over the past few years and when I see that my partner's son (22 years old) does not understand the concept of "HIFI" and does not know what an amp, tuner or turntable is, I tell myself that a great era may have passed and that we are part of an old world.
Many things have changed over the past few years and when I see that my partner's son (22 years old) does not understand the concept of "HIFI" and does not know what an amp, tuner or turntable is, I tell myself that a great era may have passed and that we are part of an old world.
voltage dissymmetry at start-up = a supply capacitor at the end of its life.
well seen 😉