Car amplifier Restoration - Need help identifying cirucit type Helix HXA400MKii

[cman]

Hello Everyone,

I am a member of a few other online forums but I know this one seems to have the most tech savvy people who actually know what they are talking about when it comes to circuits so I have signed up for an account in hopes of getting some clarity on my amplifier I am restoring. This is a car audio amplifier (I know I know) This is an old German amplifier made by Audiotech Fisher. They still make a similar model today but this one in particular is at least a decade if not older.. The pictures are ones i got off a website because my amplifier is still coming in the mail from abroad (I bought two really old ones online from europe to restore and i am waiting for them to get to the USA)

I am going to be recapping it, but most solid state items like transistors will be left original if they function. Switches and potentiometers may be replaced, or hard wired. Crossovers will be bypassed for active use.


My question is this.. Please help me understand what is going on with the Darlington Transistors inbetween the TO-3's. Normally in Class A/B amplifiers you will see one Film or wire wound resistor for each output transistor. This is some kind of Darlington and TO-3 (TO-3 are BJT's, right?) combination. I am pretty familiar with standard Class A/B circuits but not when it comes to Darlington Pairs. Can someone help me understand what is going on in the output section of this amplifier with the TO-3's and Darlington transistors? Is the TO-3 and darington working together to create one big NPN and one big PNP transistor?

It is 4 channel so as you can see there is two TO-3 output devices and two darlington transistors per channel. It puts out 88watts per channel into 4 ohms @ 13.5V power supply voltage before clipping.

BD681 (NPN) & BD682 (PNP) - Darlington Transistors (Little black ones inbetween the TO-3's)

There are 3 film resistors underneath on the back side of the board per channel (a regular class A/B would only have one film resistor per TO-3 but there are an odd number here?)

TO-3's are proprietary or not labeled.. If anyone can help me determine a similar replacement in the case i might need to replace the TO-3's I would also appreciate it, although i know that might not be possible without trial and error. I mostly would like to understand the circuit so I can have a grasp on how my equipment is working when i am restoring it.


Thank you for your help

 

[AnalogSteph]

It's probably an EF3 output stage topology, which makes sense given that car audio amps routinely have to drive very low-impedance loads. Either that or a CFP beefed up with an extra input driver, but that would require more resistors. 3 resistor = 1 between driver emitters and another 1 emitter resistor per output.

The TO-3s are probably some sort of custom-branded Motorola / ON Semi parts, presumably rated 15-20 amps.

What I don't quite get yet is how the DC/DC section works, and there has to be one since 88W / 4 ohm = 53 Vpp (+/-30-ish V supply?). I don't see any big inductors, unlike the round cans aren't actually big electrolytics but hiding toroids (but then where's the secondary-side electrolytics?). Presumably it involves some of the other heatsinked transistors.

The package design is screaming mid-'90s to me, ca. 1994-97. The 219 datecode on the TO-3s may be indicating week 19 of 2002, the construction is too modern for 1992 for sure. (These are common Chinese film caps and Yageo metal film resistors, so early/mid-2000s seems about right.)

Depending on how much there is to recap and whether you do this on a regular basis I would contemplate getting an ESR meter. This device is not yet at an age where bulk recapping makes that much sense, unless you know it had a hard life or capacitors used were of dubious quality to begin with (check which series / manufacturer they used).

 

[cman]

It's probably an EF3 output stage topology, which makes sense given that car audio amps routinely have to drive very low-impedance loads. Either that or a CFP beefed up with an extra input driver, but that would require more resistors. 3 resistor = 1 between driver emitters and another 1 emitter resistor per output.

The TO-3s are probably some sort of custom-branded Motorola / ON Semi parts, presumably rated 15-20 amps.

What I don't quite get yet is how the DC/DC section works, and there has to be one since 88W / 4 ohm = 53 Vpp (+/-30-ish V supply?). I don't see any big inductors, unlike the round cans aren't actually big electrolytics but hiding toroids (but then where's the secondary-side electrolytics?). Presumably it involves some of the other heatsinked transistors.

The package design is screaming mid-'90s to me, ca. 1994-97. The 219 datecode on the TO-3s may be indicating week 19 of 2002, the construction is too modern for 1992 for sure. (These are common Chinese film caps and Yageo metal film resistors, so early/mid-2000s seems about right.)

Depending on how much there is to recap and whether you do this on a regular basis I would contemplate getting an ESR meter. This device is not yet at an age where bulk recapping makes that much sense, unless you know it had a hard life or capacitors used were of dubious quality to begin with (check which series / manufacturer they used).

Interesting, I did notice a fourth film resistor by the speaker outputs.. i dont know why it would be all the way over there but it looks almost the same as the 3 on the back of the board by the output devices so maybe it is the 4th missing resistor..

I read on a German audio forum that there was a filter/choke type inductor under one cover and the main transformer under the other cover. It was using Google Translator but basically what I got out of it was one of them was different so that is what I am guessing.

I do believe it is from around the 2000's - I was also reading about sending an AC signal through the caps for testing.. i do not have a function generator but that makes alot of sense so i think i will try to find some ways to test them, check them against their original spec granted they are not proprietary unlabeled parts..

I will probably just take it apart, clean it, test caps with an ESR meter to verify they have not drifted out of spec too far. And clean switches and potentiometers. I suppose there is no sense in going deeper into it if most of the parts are still good.


Thank you for the info

 

[Doodski]

I suppose there is no sense in going deeper into it if most of the parts are still good.

Good choice. If the caps are bad you'll know it when you power it up and torture test it for thermal tracking. Use a digital amp meter with 20A rating if you can get one or a 10A and then load the amp and wind it up to a good amount and watch the current. If the current drifts to far upward without turning the volume up you may have a thermal runaway issue or something causing the amp to use more power than required for the volume level as it gets hot. A slight increase of current as the amp heats up will be normal but if it keeps going up shut the test down and find the issue. The power supply will usually screech/squeel or shut the unit down. Most of these car amp won't cascade if they blow and so usually just the outputs will require replacement or the power supply switching FETs will go if they are overheated and the power supply does not shut it down. Other than that it appears to be a pretty kOOL piece. I'm guessing those emitter resisters are in multiples to keep the package size smaller for compactness.