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
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
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
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
Ha
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
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
rail balance and caps
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.
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.
VMat said:
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.
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,
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,
I agree with you to a certain point, Andrew, but I guess the speakers are the ones I should bin - as a matter of fact, I've been only using them for tests for a couple of years.
After some 4 or 5 CDs I got the courage to hook my main speakers to the amp - they are no state-of-the-art at all, but they do sound much better than the other ones - so that now I don't really need the EQ anymore (but I'm still using it to add a bit of bass).
Sakis, I hope the "bad road" refers to the offset only...
Thanks again,
After some 4 or 5 CDs I got the courage to hook my main speakers to the amp - they are no state-of-the-art at all, but they do sound much better than the other ones - so that now I don't really need the EQ anymore (but I'm still using it to add a bit of bass).
Sakis, I hope the "bad road" refers to the offset only...
Thanks again,
no.....
it refers that you cannot follow procedure and that you cannot look to this amplifier as a complete device .....you only try to work with fragments of it
you need to check everything .....forget the big capacitors in the power supply the small ones here and there can create a lot more complications
the big caps will create problems ( if aged ) in some ammount of power ....in casual listening they still might be ok ....the small ones need to be checked ....
VMat said:
it refers that you cannot follow procedure and that you cannot look to this amplifier as a complete device .....you only try to work with fragments of it
you need to check everything .....forget the big capacitors in the power supply the small ones here and there can create a lot more complications
the big caps will create problems ( if aged ) in some ammount of power ....in casual listening they still might be ok ....the small ones need to be checked ....
Oh, OK, I got it.
I guess I have not expressed myself well. Reading back my posts, when I said "until I fix everything else", I should have said "until I believe I have a good chance of fixing everything else" instead. Meaning, if I decide to go and restore it, I'll go all the way. But until then I want to have an idea of how much I'm going to spend before I start buying 47000uF capacitors, which, as I said before, are quite expensive here (the small ones shouldn't be a problem, I even have some new ones around). Furthermore, if I can't see any possible fix for something (e.g., the radio interference), there's no point in doing a full restoration. I'll just keep it as a piece of furniture, and save my money to buy a better amp.
I also have a Technics amp (no radio) that used to sound beautifully, and seems to have similar bias problems. I may try to restore that one instead. I started with the Telefunken because I have the whole set (turntable and tape deck).
Worst case, I'll just keep using the HT amp. It's "consumer-level", you know how those amps sound. But it's OK for casual listening.
I'm probably waiting until Saturday before I can play with it again. I'll keep you posted.
Thanks again!
I guess I have not expressed myself well. Reading back my posts, when I said "until I fix everything else", I should have said "until I believe I have a good chance of fixing everything else" instead. Meaning, if I decide to go and restore it, I'll go all the way. But until then I want to have an idea of how much I'm going to spend before I start buying 47000uF capacitors, which, as I said before, are quite expensive here (the small ones shouldn't be a problem, I even have some new ones around). Furthermore, if I can't see any possible fix for something (e.g., the radio interference), there's no point in doing a full restoration. I'll just keep it as a piece of furniture, and save my money to buy a better amp.
I also have a Technics amp (no radio) that used to sound beautifully, and seems to have similar bias problems. I may try to restore that one instead. I started with the Telefunken because I have the whole set (turntable and tape deck).
Worst case, I'll just keep using the HT amp. It's "consumer-level", you know how those amps sound. But it's OK for casual listening.
I'm probably waiting until Saturday before I can play with it again. I'll keep you posted.
Thanks again!
Hey, sorry Rafael, I almost missed your post.
Fuses are fast-blowing. I'm glad they are. Probably saved the output transistors. As I said, they protect the amplifier board only - differential/drivers/power. One for each channel. Why did you ask?
Yeah, I know. The best we can get. But still, I don't like buying them at the local stores, I always have that feeling of being cheated with QC-rejected batches or something. I might try Farnell, but their selection is very narrow in Brasil.
Anyway, the thump is gone, so I changed focus for a while (it does need a follow up, though).
No. It's from the radio inside the receiver (I can even change stations). It's very low, but still unacceptable.
Thank you too,
Rafael.luc said:
Fuses are fast-blowing. I'm glad they are. Probably saved the output transistors. As I said, they protect the amplifier board only - differential/drivers/power. One for each channel. Why did you ask?
Epcos
Yeah, I know. The best we can get. But still, I don't like buying them at the local stores, I always have that feeling of being cheated with QC-rejected batches or something. I might try Farnell, but their selection is very narrow in Brasil.
Anyway, the thump is gone, so I changed focus for a while (it does need a follow up, though).
No. It's from the radio inside the receiver (I can even change stations). It's very low, but still unacceptable.
Thank you too,
Hey, has it been 2 years already since I came here? Geee...
Well, this weekend I decided to give that receiver another try. After some tests (with a light bulb inserted between the +Vcc rail and the power amp), I noticed there was something wrong with the cabling between the power amp and the output transistors - just touching the cables made the bulb light up. I heated up the soldering at the cables to melt it and added a bit of new solder, in the board end.
I also noticed one of those cables was very close to the heatsink at the transistor end, although I couldn't really see if they were touching each other. So I also re-soldered it far from the heatsink, just in case.
The receiver played for 4h at moderate to high volume, no issues. And the radio interference is gone. The sound is nothing to write home about, but considering its age and the speakers it's driving, I'm satisfied. The only thing I tried but couldn't fix was the ligths behind the VU meters. But they are not essential, maybe I'll try again - maybe not.
I'm gonna test it for a few days and send it to my dad, he needs an amp in the computer room.
Thank you again,
VMat
Well, this weekend I decided to give that receiver another try. After some tests (with a light bulb inserted between the +Vcc rail and the power amp), I noticed there was something wrong with the cabling between the power amp and the output transistors - just touching the cables made the bulb light up. I heated up the soldering at the cables to melt it and added a bit of new solder, in the board end.
I also noticed one of those cables was very close to the heatsink at the transistor end, although I couldn't really see if they were touching each other. So I also re-soldered it far from the heatsink, just in case.
The receiver played for 4h at moderate to high volume, no issues. And the radio interference is gone. The sound is nothing to write home about, but considering its age and the speakers it's driving, I'm satisfied. The only thing I tried but couldn't fix was the ligths behind the VU meters. But they are not essential, maybe I'll try again - maybe not.
I'm gonna test it for a few days and send it to my dad, he needs an amp in the computer room.
Thank you again,
VMat
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