Hi everyone.
Since last time I posted here I have learned a lot and I am getting into some more advanced repairs but I need some advice...
So basically i have an amplifier from 1980s that seems to be pretty solid for what it is and sounds pretty good to me when it is working but the problem is that it had a hissing/crackling noise on left channel so I traced it to the bc237 transistor in pre-amp stage so I took a bc248 that I had on my workbench and put it in there temporarily and it worked so I got pair of bc547 transistors and replaced them in both channels so they are matched but after this repair left channel was making some weird noise and speaker excursion again but a little bit less than the first time. So I suspected that another transistor had gone now and I started cooling them down with alcohol to see if they act up and as soon as I touched bc414 on the right channel which seemed to be working before it started making crackling noise from the right side too and when I flick bc1414 with my finger it acts like a pickup. It seems to react to physical movements. I re-solderes all the pins off the transistor and no change.
So since it looks like the transistors in the pre-amp are dying one by one should I replace them all ? And what about the caps ? It has electrolytes for coupling between the stages.
Maybe the pre amp transistors are bad because they look like they are one of the ones that were made by radio industry Zagreb in Yugoslavian times so maybe bad batch ?
At least output stages seem to have STM transistors for some reason...
Since last time I posted here I have learned a lot and I am getting into some more advanced repairs but I need some advice...
So basically i have an amplifier from 1980s that seems to be pretty solid for what it is and sounds pretty good to me when it is working but the problem is that it had a hissing/crackling noise on left channel so I traced it to the bc237 transistor in pre-amp stage so I took a bc248 that I had on my workbench and put it in there temporarily and it worked so I got pair of bc547 transistors and replaced them in both channels so they are matched but after this repair left channel was making some weird noise and speaker excursion again but a little bit less than the first time. So I suspected that another transistor had gone now and I started cooling them down with alcohol to see if they act up and as soon as I touched bc414 on the right channel which seemed to be working before it started making crackling noise from the right side too and when I flick bc1414 with my finger it acts like a pickup. It seems to react to physical movements. I re-solderes all the pins off the transistor and no change.
So since it looks like the transistors in the pre-amp are dying one by one should I replace them all ? And what about the caps ? It has electrolytes for coupling between the stages.
Maybe the pre amp transistors are bad because they look like they are one of the ones that were made by radio industry Zagreb in Yugoslavian times so maybe bad batch ?
At least output stages seem to have STM transistors for some reason...
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It is more likely that passives like capacitors degrade over time or that dust/dirt buildup will cause leakage currents from and to high impedance nodes. First job is to wash both sides with an alcohol (and dry before power up) to eliminate those possibilities.
I am going to try it tomorrow but I don't think it will help because pcb doesn't seem dirty. Actually whole amp is in almost mint condition cosmetically but it just has these problems which I solved by replacing the transistor temporarily first time but after putting matched transistors in both channels problem came back I even switched the transistors around and problem was following the transistor but now it looks like the bc414 on the right channel died after thermal shock from alcohol cooling test.. .
I will update tomorrow if I get some time because currently it is 1 AM in Croatia and I should be going to bed lol.
If anyone has any more suggestions please let me know. Maybe old coupling capacitors messing up the bias and killing stuff ?
I will update tomorrow if I get some time because currently it is 1 AM in Croatia and I should be going to bed lol.
If anyone has any more suggestions please let me know. Maybe old coupling capacitors messing up the bias and killing stuff ?
BC2xx transistors are very low voltage types by comparison with most small signal transistors used in audio amplifiers since the 1980s. The low voltage itself, is not a problem but the transistors could be more prone to transient voltage surges and spikes which, since the failures are quite recent and grouped together, suggests an external, probably power supply fault, not necessarily an original transistor manufacturing problem.
So, lacking a schematic, I would replace all electrolytic power supply and decoupling capacitors for the low voltage power rail(s) but you may as well replace the others too, unless they have already been replaced in the last 30 years. You might also consider using good quality film types for values below 1uF where they should be small enough to fit and more more reliable.
So, lacking a schematic, I would replace all electrolytic power supply and decoupling capacitors for the low voltage power rail(s) but you may as well replace the others too, unless they have already been replaced in the last 30 years. You might also consider using good quality film types for values below 1uF where they should be small enough to fit and more more reliable.
Sounds to me more like dry joints. Before you change anything else resolder all components on the board.
If its going to be a transistor (rather than dries etc) then the VAS stage has to be the first suspect as that one will run hot. And its the most likely to have dries.
OK, I will look at some of the suggestions later today...
I actually have a circuit diagram for one of the other models that is pretty much identical to this one.
But the circuit diagram is really blurry and hard to read...
It has a phono pre-amp for the phono input and all the other inputs go directly to pre-amp with tone controls and then the signal goes to the pre out for eq and from the pre out it comes back to the power amplifier if I recall correctly
Here is the schematic
P4060 — ImgBB
I actually have a circuit diagram for one of the other models that is pretty much identical to this one.
But the circuit diagram is really blurry and hard to read...
It has a phono pre-amp for the phono input and all the other inputs go directly to pre-amp with tone controls and then the signal goes to the pre out for eq and from the pre out it comes back to the power amplifier if I recall correctly
Here is the schematic
P4060 — ImgBB
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Here is a little update...
So I ruled out the bad solder joints and dirty pcb.
I swapped the two bc414 transistors between the channels and the problem followed the transistor and it seem like there is a bad connection or intermittent short inside the transistor itself. It acts almost microphonic and interesting thing is that it started happening after I tried cooling it down with alcohol to see if it is faulty and it failed completely.
Although it was making some weird pop noises on loud volumes from time to time which I thought was my test speaker bottoming out... Look like it was that transistor the whole time.
The other channel which was originally giving me problems seems to have calmed down now but I don't thrust it because it was doing some serious excursion to my test speaker when it was acting up...
So I ruled out the bad solder joints and dirty pcb.
I swapped the two bc414 transistors between the channels and the problem followed the transistor and it seem like there is a bad connection or intermittent short inside the transistor itself. It acts almost microphonic and interesting thing is that it started happening after I tried cooling it down with alcohol to see if it is faulty and it failed completely.
Although it was making some weird pop noises on loud volumes from time to time which I thought was my test speaker bottoming out... Look like it was that transistor the whole time.
The other channel which was originally giving me problems seems to have calmed down now but I don't thrust it because it was doing some serious excursion to my test speaker when it was acting up...
My hunch is the circuit is oscillating in the ultrasonic region - perhaps replace the rail capacitors in case they'd dried out.
The symptoms of oscillation can be many and various (odd distortion, noise, gain changes) - the best diagnosis (without a 'scope) is probably monitoring the supply current and probing various parts of the circuit with a voltmeter. Sudden jumps or dips in current from the supply indicate onset or ceasation of oscillation (due to the capacitance changes from probing).
Its very common for electrolytic capacitors to dry up over time, become less and less effective for decoupling the supply rails. 40 years is long enough for this to be probable.
The symptoms of oscillation can be many and various (odd distortion, noise, gain changes) - the best diagnosis (without a 'scope) is probably monitoring the supply current and probing various parts of the circuit with a voltmeter. Sudden jumps or dips in current from the supply indicate onset or ceasation of oscillation (due to the capacitance changes from probing).
Its very common for electrolytic capacitors to dry up over time, become less and less effective for decoupling the supply rails. 40 years is long enough for this to be probable.
Well maybe there is some more problems but this is definitely second transistor that failed in the pre amp section... I always try swapping transistors between the channels to confirm that that transistor is indeed faulty and in both cases fault followed transistors.
So what would be best replacement for old bc237 and bc 414?
So what would be best replacement for old bc237 and bc 414?
You can still buy BC237 at least, from Element 14 or their associated distributors. Almost any low noise, small signal types will be OK at those low circuit voltages. BC549,550 for example, should be fine. Check that their grade (A,B,C or no suffix) or their hFE measurements are comparable to original semis, as some may have been selected on test and others are specified BC237B grade. Just type the part number followed by "datasheet" to find the specific details and "buy BCXXX" etc. into your browser to find sources, if there are any.
thanks for the tip. I already fixed that amp. Replaced some capacitors and few noisy transistors. I replaced them with modern equivalents pretty much same as you suggest.