Question about Solid Audio F2500D 1-Channel Amplifier Car Audio Amplifier

3 Answers

Power connected but no lights

I'm an electronics engineer whos been given a broken F2500D to 'look at' by a mate. It seems to be the voltage-inverter section that had a few dead components, i.e. 10uF cap C357 was blown to bits, 2 FR104 diodes were short-circuit, D356 and the one it's common-anode connected to (number covered by silicon-rubber). Plus the bit of track on the back of the PCB, between the anode of D355 / cathode of D356 and the T301 connection (feedback winding?) was burnt off. I've replaced these bits, put a 5 Ohm 25W resistor at the output and applied 12V. I can see the 12V at various places, but the LEDs are out and there's no sign of any high voltage or attempts for the inverter to start-up. Any suggestions, or a schematic would be much appreciated!

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  • 13 more comments 
  • Alan Clarke Jun 14, 2007

    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.

  • Alan Clarke Jun 14, 2007

    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?

  • Alan Clarke Jun 16, 2007

    Take a break!

  • Alan Clarke Jun 18, 2007

    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.

  • Alan Clarke Jun 18, 2007

    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.

  • Alan Clarke Jun 18, 2007

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

  • Alan Clarke Jun 18, 2007

    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.

  • Alan Clarke Jun 19, 2007

    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!

  • Alan Clarke Jun 20, 2007

    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.

  • Alan Clarke Jun 20, 2007

    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.

  • Alan Clarke Jun 20, 2007

    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?

  • Alan Clarke Jun 20, 2007

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

  • Alan Clarke Jun 21, 2007

    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!!!

  • Alan Clarke Sep 07, 2007

    Hi Dyllan, I didn't get any diagrams hence had to give up, having spent way too much time already. It doesn't look like the owner wants it back (in a basket), since it's still in my workshop. Any offers considered! regards, Alan.

  • Alan Clarke Sep 07, 2007

    Hi techman, I hated having to give up, but as you say I learned something. Must have fixed a dozen things since, including a couple of Hi-Fi amps, so not lost it yet!
    Cheers, Alan
    PS My stereo has 13 tubes.

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Mostly out of curiosity, did anyone ever go back and find those diagrams, or were you able to figure anything out on this one through other sources (since obviously we weren't able to fix it)?

Posted on Sep 07, 2007

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Try following the "trigger on" wire into the amp and see where it goes. You may find a transistor, or diode, or thermistor that it must go thru to get to the 494. Do your main power supply MOSFET's have the +12v going to them correctly? It is fairly common for the 494 to get power from that source, but sometimes it is switched via a transistor from the trigger input wire. Don't forget to look for breaks in the traces or bad solder connections. It sounds like since you can force the amp to switch on that it is a very minor part that has failed or a bad trace, etc. Good luck. Hope this helps.

Posted on Jun 18, 2007

  • Mike Duncan
    Mike Duncan Jun 19, 2007

    You said "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." You might want to start with NO speaker load and watch the output on a scope BEFORE you connect an actual load. If this is a stereo amp DO NOT!!! connect the two "+" together and the two "-" because that can possibly fry the output sections. When you bridge the outputs you are indeed tying the two channels together but it is across a load and not a dead short.

  • Mike Duncan
    Mike Duncan Jun 20, 2007

    In response to "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 would say that you can but watch your load. Better still don't use a speaker load at all.

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Yes there is a remote wire that has to be switched on from the head unit. If you want the amp top be on all the time, simply connect up the remote power wire to the B+ wire, but I would not recomend that. Good Luck

Posted on Jun 18, 2007

  • 4 more comments 
  • Larry  Dillon
    Larry Dillon Jun 18, 2007

    If you have voltage on the fets, the amp power supply is working, have you tried to connect up an audio source and speakers? Is there any voltage on the speaker outputs? If there is, you still have a short or a leaky componet on the side that has the DC on the outputs, or speaker leads.

  • Larry  Dillon
    Larry Dillon Jun 18, 2007

    I would try the diff. circuit. You should cirtainly see the 40CPS but you really should go to 1000CPS and at a low voltage. if you use 0 DB you will pop that resistor, as I am sure you know.

  • Larry  Dillon
    Larry Dillon Jun 19, 2007

    what you should do is install a light bulb in place of the fuse. This is an old trick to help prevent other componets from blowing out as if there is a short, the bulb will light up bright and absorb the excessive current and will not blow out your other components. Use around a 75 to a hundred watt bulb.

  • Larry  Dillon
    Larry Dillon Jun 19, 2007

    No Duncan, this is a single channel amp

  • Larry  Dillon
    Larry Dillon Sep 07, 2007

    thats a bloody shame after all the time you spent on it. At least you have to say you learned a few things in the meantime, like the old timers light bulb trick. I have used that from way back when they had tubes in TV sets as well as stereo systems.

  • Larry  Dillon
    Larry Dillon Sep 07, 2007

    Thats a cool thing, you still have a good Stereo unit. I happen to of just overhauled a 200 watt Mackintosh Amp and receiver that I found at a flea market for 50 bucks. The tubes and parts cost me about 200 bucks, but the labor was free as I did it. Nice rich true sound from these amps!

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