12-02-2013, 01:21 AM
I have a number of vintage items where there is no manual or service sheet available.
Top of the list being the amplifier a have as my avatar.
The first thing I have done is to power it up having had trouble with the tone control network and bass response.
Most of my troubles look like they are to do with earthing and this has been on the watch list for a long time and joins phasing of the mains inputs to the two mono block amplifiers to keep hum at bay (do you fancy a bid on some hum?).
It turns out that I had been testing two out of four mono block amplifiers one having a wrong value resistor in it. This lead to one or two odd effects that would not be noticed in a mono system and looks like it has been there from new.
It was a 68K resistor identical to the ones used in the phase splitter in place of a 6K8 wire wound type.
This runs the preamp stage at low HT and we have a problem with big signals inputs form modern kit.
It can be seen that the correct green 6K8 resistor next to the 1K 5% has been replaced by a 68K 1% high stability resistor of the type used in the phase splitter.
The other three mono block units all have the 6K8 green wire wound type.
[attachment=7169]
[attachment=7170]
The next issue is the phase splitter itself.
It is not built like the Mullard circuit and I am pretty sure I had seen one many years ago and was lost as to where I had spotted it.
The Mullards are cathode coupled with the capacitors connected to the anodes.
These sets have the double triode connected as a voltage amplifier followed by a single triode with the phase splitter caps wired to the anode and cathode.
The designer of this circuit does difficult to follow circuit diagrams for sure
[attachment=7171]
[attachment=7172]
I thought this was going to be an easy project but with the challenge of putting the audiophool capacitors up head to head against the usual parts we use this resistor problem bounced the job making me have to start over.
I had to be very careful where I put the scope probe when I was doing a full power test. The signal generator is also earthed and The amplifier oscillated.
They also pick up the ballast in the magnifier lamp so that all the traces go all wiggly.
I must get my fragmented circuit diagram drawn onto a single sheet so that we can share it. I am not going to inflict four ropy hand drawn scrawls on anybody.
Top of the list being the amplifier a have as my avatar.
The first thing I have done is to power it up having had trouble with the tone control network and bass response.
Most of my troubles look like they are to do with earthing and this has been on the watch list for a long time and joins phasing of the mains inputs to the two mono block amplifiers to keep hum at bay (do you fancy a bid on some hum?).
It turns out that I had been testing two out of four mono block amplifiers one having a wrong value resistor in it. This lead to one or two odd effects that would not be noticed in a mono system and looks like it has been there from new.
It was a 68K resistor identical to the ones used in the phase splitter in place of a 6K8 wire wound type.
This runs the preamp stage at low HT and we have a problem with big signals inputs form modern kit.
It can be seen that the correct green 6K8 resistor next to the 1K 5% has been replaced by a 68K 1% high stability resistor of the type used in the phase splitter.
The other three mono block units all have the 6K8 green wire wound type.
[attachment=7169]
[attachment=7170]
The next issue is the phase splitter itself.
It is not built like the Mullard circuit and I am pretty sure I had seen one many years ago and was lost as to where I had spotted it.
The Mullards are cathode coupled with the capacitors connected to the anodes.
These sets have the double triode connected as a voltage amplifier followed by a single triode with the phase splitter caps wired to the anode and cathode.
The designer of this circuit does difficult to follow circuit diagrams for sure
[attachment=7171]
[attachment=7172]
I thought this was going to be an easy project but with the challenge of putting the audiophool capacitors up head to head against the usual parts we use this resistor problem bounced the job making me have to start over.
I had to be very careful where I put the scope probe when I was doing a full power test. The signal generator is also earthed and The amplifier oscillated.
They also pick up the ballast in the magnifier lamp so that all the traces go all wiggly.
I must get my fragmented circuit diagram drawn onto a single sheet so that we can share it. I am not going to inflict four ropy hand drawn scrawls on anybody.