Power connected but no lights

Thanks Duncan, you're certainly in the ball-park! I now have a datasheet on the 494. It has no +Vcc feed to pin 12, (or 8 and 11, the output emitter-followers). This track is quite high impedance to 0V, so it's not the IC dragging the rail down. I'm feeling a bit in the dark without a schematic, but either the 12V feed to the 494 comes via a transistor which has +12V on it but no base drive?... or it comes from a different direction, which I haven't spotted yet. Any ideas why the +Vcc isn't there? Many thanks, Alan. Thanks Duncan, you're certainly in the ball-park! I now have a datasheet on the 494. It has no +Vcc feed to pin 12, (or 8 and 11, the output emitter-followers). This track is quite high impedance to 0V, so it's not the IC dragging the rail down. I'm feeling a bit in the dark without a schematic, but either the 12V feed to the 494 comes via a transistor which has +12V on it but no base drive?... or it comes from a different direction, which I haven't spotted yet. Any ideas why the +Vcc isn't there? Many thanks, Alan.

I've cheated and put a 10k resistor between the collector and base of Q341. This puts volts on the 494 and it starts-up at 26kHz, turning 13.2Vdc into about 167Vdc. The green LED is ON and the red LED is OFF. I've never been involved with an amplifier like this before. Have I missed something like a 'remote enable' signal? I note that the 2 relays, (possibly speaker protection?) don't operate. Any ideas on where to go from here? I've cheated and put a 10k resistor between the collector and base of Q341. This puts volts on the 494 and it starts-up at 26kHz, turning 13.2Vdc into about 167Vdc. The green LED is ON and the red LED is OFF. I've never been involved with an amplifier like this before. Have I missed something like a 'remote enable' signal? I note that the 2 relays, (possibly speaker protection?) don't operate. Any ideas on where to go from here?

Take a break! Take a break!

Hi techman, I don't want it to be on all the time... but I would like to see it working!
Assume a reasonable background in electronics, but no product-specific knowledge. I didn't even know about the head unit!!!
I've put +12V on the 'Rem' connector at the front. (I tried the non-grounded pin of the 'Remote' control port at the rear first... this is the calibre of person you're dealing with!). The inverter now starts up without the 'cheat' enable, and the 2 relays click. Looking around with a scope, there are plenty of volts around the amplifier O/P FETs but no evidence of PWM. Hi techman, I don't want it to be on all the time... but I would like to see it working! Assume a reasonable background in electronics, but no product-specific knowledge. I didn't even know about the head unit!!! I've put +12V on the 'Rem' connector at the front. (I tried the non-grounded pin of the 'Remote' control port at the rear first... this is the calibre of person you're dealing with!). The inverter now starts up without the 'cheat' enable, and the 2 relays click. Looking around with a scope, there are plenty of volts around the amplifier O/P FETs but no evidence of PWM.

Hi Duncan / techman,

Operator error may be playing a big part here!
The inverter section powers-up fine with the 'Rem' enable applied (Doh!) and the whole amplifier may be working... if I just wire it up correctly. At the moment I have a 5R 25W resistor across the output in parellel (I think) i.e. between the '+' connections together and '-' connections together. There is a signal generator supplying a low level 40Hz sinewave at the single 'Parallel' audio (what I assume to be) input. There is no output and no evidence of PWM in the amplifier section. Hi Duncan / techman, Operator error may be playing a big part here! The inverter section powers-up fine with the 'Rem' enable applied (Doh!) and the whole amplifier may be working... if I just wire it up correctly. At the moment I have a 5R 25W resistor across the output in parellel (I think) i.e. between the '+' connections together and '-' connections together. There is a signal generator supplying a low level 40Hz sinewave at the single 'Parallel' audio (what I assume to be) input. There is no output and no evidence of PWM in the amplifier section.

Sorry, by PWM in the amplifier section, I assume there should be a Class D carrier frequency present? Sorry, by PWM in the amplifier section, I assume there should be a Class D carrier frequency present?

The O/P was doing 'something' at the 40Hz input frequency. It looked like the negative peak section of a sinewave but only going one side of 0V. Then there was some odd squeaking from the amplifier and my 13.2V @ 8A supply started to current-limit. It won't enable anymore without the current-limit operating and it cycles, with the relays clicking on and off.
I've measured that O/P devices Q220, Q221 & Q222 are now much lower resistance than all the other devices... Ooops! Game-over for a day or two, I'll see if I can find some new parts.

Get back to you later, thanks and regards, Alan. The O/P was doing 'something' at the 40Hz input frequency. It looked like the negative peak section of a sinewave but only going one side of 0V. Then there was some odd squeaking from the amplifier and my 13.2V @ 8A supply started to current-limit. It won't enable anymore without the current-limit operating and it cycles, with the relays clicking on and off. I've measured that O/P devices Q220, Q221 & Q222 are now much lower resistance than all the other devices... Ooops! Game-over for a day or two, I'll see if I can find some new parts. Get back to you later, thanks and regards, Alan.

Hi Duncan, Fried it is!!!
I only intended to warm-up the 25W resistor slightly with some 40Hz sinewave before giving it back to the owner, to try back in his installation. I hoped to be reasonably safe with a PSU that limits at 8A, i.e. less than the continuous current rating of any individual power device... not so! There seems to be enough energy stored in various capacitors? Or perhaps it was a voltage transient on the 200V output FETs, due to the 'squeak' applied to the inductors in the output filter - before it started current limiting? In any event, I have some IRF640N replacements on order now. It clearly wasn't amplifying properly before it died for good, since I could only get output going one side of 0V.
I'll stick to separate resistor loads between the + and - outputs next time, even though they do seem to be hard-wired in parallel inside the unit, across an internal 1k 5W resistor (for open-circuit stability I suppose). How I wish I had a circuit diagram! Hi Duncan, Fried it is!!! I only intended to warm-up the 25W resistor slightly with some 40Hz sinewave before giving it back to the owner, to try back in his installation. I hoped to be reasonably safe with a PSU that limits at 8A, i.e. less than the continuous current rating of any individual power device... not so! There seems to be enough energy stored in various capacitors? Or perhaps it was a voltage transient on the 200V output FETs, due to the 'squeak' applied to the inductors in the output filter - before it started current limiting? In any event, I have some IRF640N replacements on order now. It clearly wasn't amplifying properly before it died for good, since I could only get output going one side of 0V. I'll stick to separate resistor loads between the + and - outputs next time, even though they do seem to be hard-wired in parallel inside the unit, across an internal 1k 5W resistor (for open-circuit stability I suppose). How I wish I had a circuit diagram!

Hi techman,
I like the bulb trick, it's gone straight into my repertoire! When I removed O/P FETs Q220, 221 & 222, it was only Q222 which continues to read low-resistance gate to source. I changed all three however.
On enable, the high-voltage DC rail came up, and then the relays clicked. (With no input), my scope showed a small, unstable ?ve DC offset across the load resistor, plus the voltage would occasionally dive off the scope screen for a short ?ve transient? which corresponded to a ?ticking? from around the area of the O/P filter. Then the (4x 12V, 21W) bulbs flared up and the thing started to cycle again with the relays clicking. It did the whole thing again a couple of times, but now the bulbs flare as soon as the relays click, and it starts to cycle. I can?t see any dead O/P FETs this time. What were you saying earlier about leaky or S/C components? Where do I start looking? Regards. Hi techman, I like the bulb trick, it's gone straight into my repertoire! When I removed O/P FETs Q220, 221 & 222, it was only Q222 which continues to read low-resistance gate to source. I changed all three however. On enable, the high-voltage DC rail came up, and then the relays clicked. (With no input), my scope showed a small, unstable ?ve DC offset across the load resistor, plus the voltage would occasionally dive off the scope screen for a short ?ve transient? which corresponded to a ?ticking? from around the area of the O/P filter. Then the (4x 12V, 21W) bulbs flared up and the thing started to cycle again with the relays clicking. It did the whole thing again a couple of times, but now the bulbs flare as soon as the relays click, and it starts to cycle. I can?t see any dead O/P FETs this time. What were you saying earlier about leaky or S/C components? Where do I start looking? Regards.

How strange! Where did all those extra ??? come from?
I had - signs and ' marks in what I thought I'd posted. Hope it's still intelligible. Let's see if this is. How strange! Where did all those extra ??? come from? I had - signs and ' marks in what I thought I'd posted. Hope it's still intelligible. Let's see if this is.

I'll not be doing much diagnosing tonight, on account of a local tradition involving beer.
I have however discovered that the O/P FETs are either in 4 banks of 6, or 2 banks of 12. In any event, I now think that 12 FETs are broken, on the opposite side to the newly replaced short circuit FET.
Having acquired a datasheet for IRF640Ns, I should be able to measure the source-drain diode? which just isn?t there on 12 devices.
I also discovered a really bad dry-joint on one of the brass wire links, joining the common source connection between the outer 3 FETs on one side. Looks like I may need a big bag of FETs.
Do you know if it?s possible to get the thing running at a low level with just 4 or 8 FETs (representing the parallel blocks), in order to see if the drivers are OK? I'll not be doing much diagnosing tonight, on account of a local tradition involving beer. I have however discovered that the O/P FETs are either in 4 banks of 6, or 2 banks of 12. In any event, I now think that 12 FETs are broken, on the opposite side to the newly replaced short circuit FET. Having acquired a datasheet for IRF640Ns, I should be able to measure the source-drain diode? which just isn?t there on 12 devices. I also discovered a really bad dry-joint on one of the brass wire links, joining the common source connection between the outer 3 FETs on one side. Looks like I may need a big bag of FETs. Do you know if it?s possible to get the thing running at a low level with just 4 or 8 FETs (representing the parallel blocks), in order to see if the drivers are OK?

It looks like ' then I post and it turns into ? It looks like ' then I post and it turns into ?

I had high hopes, but it's time to give up I think.
True to my nom-de-plume, I'd only seen a load of FETs with IRF640 in the name... once I'd decided to try it with no O/P FETs the volts told me I was missing the fact that 12 devices had to be P-channel i.e. IRF *9* 640
I built it back with 8 good O/P FETs, (4x N channel and 4x P channel), representing the 8 separate blocks driven from the SM PCB with the class D switching drivers. No luck! 1 device gets hot, in the group of 3 IRF640N devices which I previously identified a S/C device in.
Oddly, all the SOT-89 emitter-follower driver transistors and associated resistors measure OK. But... (and it's a big but)... there are other SOT-23 transistors further back with serious problems, and some 0805 4.7k resistors which have blown O/C. Alas there are also ICs there that have no visible markings.
Unless anyone has a schematic, parts-list and general assembly diagram for the SM board, it's probably time to admit defeat... although it rankles somewhat!!! I had high hopes, but it's time to give up I think. True to my nom-de-plume, I'd only seen a load of FETs with IRF640 in the name... once I'd decided to try it with no O/P FETs the volts told me I was missing the fact that 12 devices had to be P-channel i.e. IRF *9* 640 I built it back with 8 good O/P FETs, (4x N channel and 4x P channel), representing the 8 separate blocks driven from the SM PCB with the class D switching drivers. No luck! 1 device gets hot, in the group of 3 IRF640N devices which I previously identified a S/C device in. Oddly, all the SOT-89 emitter-follower driver transistors and associated resistors measure OK. But... (and it's a big but)... there are other SOT-23 transistors further back with serious problems, and some 0805 4.7k resistors which have blown O/C. Alas there are also ICs there that have no visible markings. Unless anyone has a schematic, parts-list and general assembly diagram for the SM board, it's probably time to admit defeat... although it rankles somewhat!!!