Soft start for class AB

[VMat]

Hello!

I have this Telefunken receiver - HR240 - that I'd like to put to work. It has an apparently standard class AB output (differential + driver + power) powered by +39V/-39V rails. What seems odd to me is that it has a fuse (3.15A) for each channel, at the +39V rail - only for the amp board.

One of the channels is working fine, the other is blowing the fuse very often - sometimes after working for some minutes, most often when powering up. Also, when I switch on the receiver, I hear a thump from the woofers.

So, for the fuses, I'm gonna check and readjust the quiescent current in the driver stage - I've just found out that it should be 30mA. But if that doesn't fix the issue, I'd like to make some mod to add a soft start circuit for the amp board, which would also prevent the thump - what do you think?

In case I decide to make it, do you think it's a good idea to add a timed relay to break the power rails from the amp board at switch on, for a couple of seconds? I was thinking I could insert the relay right "below" the fuse.

Sorry if there's not enough info. I have a rough idea of what I'm talking about, but I don't have the experience to tell you what is really important. Please ask me specific details, and I'll get them for you.

Thanks in advance

[h_a]

Adding a softstart is not a cure, but just a little makeup. The problem will still be there and that it's a problem (and not normal) shows that you have this only for one channel.

The thumping you could probably reduce by replacing all electrolytic caps, as turn-on thumps are caused by differently fast rising rails. Could also be caused by differently aged caps.

Have fun, Hannes

[sakis]

THE ABOVE ADVICE IS VERY CORRECT .....

many times we said that audio problems should never cured with kortizon ....meaning that the point is to solve the problem not to supress it ....

take a look at this you might find it handy vintage amplifier repair/restore manual

[VMat]

That's an invaluable tutorial. Thank you VERY much.

I'll start by looking into the capacitors. They haven't been used a lot, but are probably worn out by age.

[djoffe]

you can check the theory that caps are not balanced, so the rails come up at different rates...

Match two resistors, about 47K Ohms each. Connect them in series.
Call one end A, the middle B, and the other end C. Connect the A end to the positive rail, the C end to the negative rail, and a meter (or preferably a scope) between B and the ground connection. Turn on the power and watch the voltage from B to ground. To the extent that the supplies rise time differs...the B node will divert from ground.

If you find that the node stays pretty well controlled (e.g. within a volt or two of ground), you may have other issues.

[nigelwright7557]

Originally posted by VMat
Hello!
In case I decide to make it, do you think it's a good idea to add a timed relay to break the power rails from the amp board at switch on, for a couple of seconds? I was thinking I could insert the relay right "below" the fuse.
Thanks in advance

For an amp with that power I wouldnt bother with a soft start, it simply doesnt need it.

I dont even use one with a 1000VA transformer on one of my amps. The only down side is I cant get away with a 5 amp mains fuse as the surge at poeer up blows a 5amp fuse. So I use a 13 amp fuse which isnt ideal.

[VMat]

Thanks for all the input (djoffe's suggestion seems very simple and effective, I'll try it later).

However, as I adjusted the idle current - which was WAY off the recommended value in the right channel - the thump is gone. So I'll leave the capacitors alone until I fix everything else (mainly because good capacitors are very expensive and hard to find around here). Besides, I have the impression that one pair of them was already replaced, based on the dust and brand (still didn't check the soldering). There's another pair which looks much older.

Problems now are:

1) Hiss and FM interference. It is evident that the amp has been serviced before, and the guy might not know exactly what he was doing - he may have changed the internal cabling layout. I'll have to experiment with that. If you have other suggestions, they are always welcome.

2) Output DC offset is around -40mV, is that too bad? I don't see a trimpot in the schematics to adjust that. At least both channels are very close to each other (2mV difference).

Sakis, in step 10 of your tutorial you say "absolutely no load to the amplifier", are you referring to the speakers? I always thought it wasn't a good idea to power up a class AB without a load.

The good news is that the amp is stable now. I've listened to a whole record (yes, vinyl) yesterday, and today I'm listening to some CDs. It's been on for about two hours now. Sounded awfully "mid-ranged" in my test speakers, but I hooked up a 7-band equalizer to it, and it's much better now.

Thanks,

[sakis]

this is exactly what i mean ...

class AB solid state amplifiers have no problems to operate without load for testing procedure


still you are in a bad road i think .....

[AndrewT]

Originally posted by VMat
. Sounded awfully "mid-ranged" in my test speakers, but I hooked up a 7-band equalizer to it, and it's much better now.

if a 7-band equaliser improves anything, then it's time to bin it.

[Rafael L]

Originally posted by VMat
Hello!
One of the channels is working fine, the other is blowing the fuse very often - sometimes after working for some minutes, most often when powering up. Also, when I switch on the receiver, I hear a thump from the woofers.

The fuses are slow (retardado) or quick, are before or after the source of capacitors?

Originally posted by VMat

So I'll leave the capacitors alone until I fix everything else (mainly because good capacitors are very expensive and hard to find around here).

Epcos

Originally posted by VMat

1) Hiss and FM interference. It is evident that the amp has been serviced before, and the guy might not know exactly what he was doing - he may have changed the internal cabling layout. I'll have to experiment with that. If you have other suggestions, they are always welcome.

Can be external interference, Cd player power-on, computer, etc..