If you measure 66V across either R71 or R66, it means you must effectively be measuring across the full DC supply rails (+33, -33V) instead. You may have measured wrongly there but I would test the voltage across the C & E terminals of Tc or remove the power and test for continuity between the C & E leads in both directions, to see if that's true before attempting to continue as it could be that these transistors are toast in this channel. Fortunately, you have a good channel for all comparisons, so let's be extra careful with it.
Those resistors gives +33V supply for the current sources for each channel.
R32 for the Left channel, and R33 for the right channel.
The voltage should measured at the diodes D5 and D7
Reffered to the NEGATIVE 33V rail You should meassure about 1,5V at the cathode of D5 / D7
So about 66V measured over the resistors is probably all OK.
Please find the common piont for theese resistors, as the +33V supply is delivered there as the relay engages.'
Note: There is different versions of theese amplifiers.
In later ones the relay was skipped, and the diodes I mention is renamed:
D1, D2, D3, D4.
But common for them all: At the base of T4 there hould be about 1,5-2V refered to the negative supply rail.
This should in turn give about 0,7V across the emitter-resistor of T4
This is crutial for the function of the amp.
Last edited:
Ah, found R32/33 at last, sorry about confusing your results. These are voltage dropping resistors shown on a small diagram, and powered by a relay that switches a small current to bias up both amplifier input stages at the point marked "Input from relay circuit+". This looks to be a delay circuit to mute the amplifier whilst it powers up and thus block switch-on thumps. The resistors should have quite a high voltage difference across them as they apply the +ve rail 33V almost down to the -ve rail, so 65V across the 18k resistance is about right there. You can check the operation when the amplifier powers up - both channels should show a slight delay before the relay clicks after switch on and the marked 1.3V should then appear across either pair of diodes, D5,D6 or D8,D9 in series. As the muting circuit controls both channels, it shouldn't be the problem.
I see I'm in good company and we about agree there, thanks for your input Tandbergeren.
I see I'm in good company and we about agree there, thanks for your input Tandbergeren.
Last edited:
Base of T4 - neg rail is the good channel side and tests well. The bad channel is T12 and measured the same gives 0v, which in turn gives nothing across R71.
In my amp it's D4, D5 (good channel) and D7, D8 (bad channel), measuring across these in series gives 1.3v on the good channel and 0v on the bad channel.
In my amp it's D4, D5 (good channel) and D7, D8 (bad channel), measuring across these in series gives 1.3v on the good channel and 0v on the bad channel.
OK, it seems there are more than 3 known versions of the board and we only have one unspecific circuit diagram, though I doubt there is much real difference between them. We should now check the 18k resistors and the diodes individually. With the power off, you should be able to measure approx. 18k resistance across R32 or R33. Then I assume your meter's continuity check setting or perhaps a diode check setting as on a meter I have, can be used if you place the probes across either diode in the pair to check that they aren't open or short-circuit. Each diode of course, should read ~ 0.65V in one and the same direction only.
If you read nothing at all across one diode, you have your suspect - or perhaps it just has a poor solder joint.
If you read nothing at all across one diode, you have your suspect - or perhaps it just has a poor solder joint.
Last edited:
Good that you found it where you suspected too. Before ordering though, I would always go on to check the associated parts that are on the load side of the failed resistor. - i.e. the diodes, even the transistors, because downstream is what usually causes resistors to fail open or even burn. Maybe not here, but it's annoying and expensive to order twice or more for a minor problem.
QED 240 SA, problem with pre-amp
Hi. I have two of these amps, a 240 CD (upgraded with SA board) used as a pre-amp as one power channel is faulty, and a 230 which is used as a stereo power amp.
1. The 240 has gone faulty, the volume is now very low with dull sound, but equal on both channels. PSU rails are about right. It's the later version which does not have a buffer IC (next to the volume pot). Just wonder where to make a start?
2. For last 3 weeks I've been driving my Quad 57s with the 230 as it sounds clearer and sharper than many expensive amps (including valve) I have tried over the years; why I don't know, but it does! Same thing with the 63s I have. However, I have just discovered that the manual does not advise driving electrostatics.
Last night I decided to lube the 230's pots and sockets and check the bias. I found that it was around 63mv, so I corrected it to (I presume) 16mv. The heat-sink is now cool as opposed to "very warm" before, but the sound is
now duller and has lost its sharpness.
Any comments welcome
Hi. I have two of these amps, a 240 CD (upgraded with SA board) used as a pre-amp as one power channel is faulty, and a 230 which is used as a stereo power amp.
1. The 240 has gone faulty, the volume is now very low with dull sound, but equal on both channels. PSU rails are about right. It's the later version which does not have a buffer IC (next to the volume pot). Just wonder where to make a start?
2. For last 3 weeks I've been driving my Quad 57s with the 230 as it sounds clearer and sharper than many expensive amps (including valve) I have tried over the years; why I don't know, but it does! Same thing with the 63s I have. However, I have just discovered that the manual does not advise driving electrostatics.
Last night I decided to lube the 230's pots and sockets and check the bias. I found that it was around 63mv, so I corrected it to (I presume) 16mv. The heat-sink is now cool as opposed to "very warm" before, but the sound is
now duller and has lost its sharpness.
Any comments welcome
When we overbias an amplifier, as the 63mV measurement across the output transistor emitter resistors suggests, we force it to remain in class A operation up to a higher output level than required to minimise distortion. At low volume levels, it might even appear to perform like a class A amplifier. As you find though, QED amplifiers weren't designed for class A operation or the nasty load that Quad electrostatic speakers are.
There may be something else at play here with the way you have used 2 amplifiers to keep your system working but that would need a closer look and probably bench testing with appropriate instruments to see what goes on. As there are already unresolved problems there, you should fix the A240 before trying to change anything with the A230 and a professional repair may be best. If that seems expensive, just consider what it costs to buy new and what problems you'll be facing all over again with untested fleabay gear from liars looking to offload their own problems.
Meantime, I suggest you beg, borrow but not steal
a much larger integrated amplifier in good condition and listen to your speakers with that for a few hours and see what you think. It may even be that its the sweet class A sound you really seek.
There may be something else at play here with the way you have used 2 amplifiers to keep your system working but that would need a closer look and probably bench testing with appropriate instruments to see what goes on. As there are already unresolved problems there, you should fix the A240 before trying to change anything with the A230 and a professional repair may be best. If that seems expensive, just consider what it costs to buy new and what problems you'll be facing all over again with untested fleabay gear from liars looking to offload their own problems.
Meantime, I suggest you beg, borrow but not steal
a much larger integrated amplifier in good condition and listen to your speakers with that for a few hours and see what you think. It may even be that its the sweet class A sound you really seek.Thanks Ian for the comprehensive reply. You've confirmed my suspicion about the biasing, especially as the heat-sinks begin to get hot! I am fortunate in that a friend regularly trades on ebay so I get the opportunity to try various amplifiers etc and hear the the effects on frequency responses that the Quads impose on them (bearing in mind that older amps can have deteriorated from their original specs).
I use moderate volume levels in a small room 16ft long, so yes the power being drawn will be small. Prior to using the 230 my Sony 1130 (although not class A) drives the Quads very well. I also hear that amps that are specified to larger currents into low impedances can also be suitable.
I use moderate volume levels in a small room 16ft long, so yes the power being drawn will be small. Prior to using the 230 my Sony 1130 (although not class A) drives the Quads very well. I also hear that amps that are specified to larger currents into low impedances can also be suitable.
The TA1130 amp probably produces the current necessary to drive the Quads comfortably at your sound levels and if you still have it, that could do for a reference amplifier, if has had at least one full service including electrolytic capacitor replacements, since it was built. If it's still battling along with >40 year old main power supply caps, it's high time you did something about that amplifier too.
Compare again, now that you have the A230 bias sorted and see what you think. If you find the sound is still unacceptable in your QED setup, I don't think it will be just a case of the wrong type of amplifier but there is going to be a problem there - perhaps it needs capacitor replacement or there may other components such as semis that have died too.
First things first.
Compare again, now that you have the A230 bias sorted and see what you think. If you find the sound is still unacceptable in your QED setup, I don't think it will be just a case of the wrong type of amplifier but there is going to be a problem there - perhaps it needs capacitor replacement or there may other components such as semis that have died too.
First things first.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.