help with ground wiring

[mikelh]

hello
I have bought a beautiful L70 from the famous auction site and this one comes with a few diy features from a previous owner
the most interesting (to me) is the built-in phono stage
The combo sounds absolutely fantastic! The music is alive, the sultans of swings were playing into the room!







I have been searching a little and it seems to be a germanium transistors based phonostage with 3 transistors in each channel
forgive me but I don't know very much about electronics so I am asking for a little help:
the point is that the phonostage is coupled to the motor so when you start the motor from the turntable (on position) both devices switch on
And this produces a very heavy bump in the speakers/headphones even if this input of the amplifier is not selected ( I plug it in the tape input of the amp)
I want to dismantle the device from the top plate and put in a case with its own power cord and switch but then I have some doubts:
I have to desolder the inputs and outputs cables
from the picture below you can see all the ground wires : left and right negatives inputs and outputs, tonarm groung and phono stage ground
(the big grey cables are output and the thin black are inputs, yellow is tonarm ground and I guess the black and red cable coming from the right is the phono stage ground)
all of them are connected to a single point in the middle of the picture



 I guess this not very common,

I think that the best connection is:
left negative input with left negative output
right negative input with right negative output
the tonarm ground to phono stage ground to case ground
could you give me your opinion?


regards
Mikel

[Jessica_K]

Hi Mike,

Very interesting phono stage, germanium was the first material used to create transistors and were common in the 60’s, the OC71 being the most common small signal device, I cannot see what is being used in your amp so if they are marked it would be interesting to know.

To your question I am going to be generic rather than to say this wire or that. Maintain the turntable earthing as if the phono stage is there, obviously rewiring if the boards were used as part of the earthing. Take the cartridge wires from the tone arm and extend them with shielded cable to RCA’s to go to the external phono stage, grounding the shield, but do not connect to the RCA, run a separate ground wire. 

On the phono stage, treat it like any other phono, take the input return to each channel separately at the input end of the board and the output return to the output RCA from the output side of the pcb. Connect power and ground from PSU and ground the PSU to chassis and mains. Connect the TT ground wire to chassis (may not be required or could cause hum depending on how the TT is mains grounded so play that one by ear)

Have fun

Jessica xx

[fetteler]

+1 regarding curiosity about transistor markings.

Steve.

[mikelh]

dear Jessica, thank you very much for your help, I try to follow your advice today
the point is I haven't got a case, I have to order one (and more stuff) so I let it "on air"
I notice that one channel is louder than the other one but I guess it is a cable connection issue

regarding the transistors, they are marked 2n1305:



in any case the sound is really good (good  detail and dynamics) and it has overall more gain than the built-in phonostage in my amp
so I am considering to place an attenuator and try to connect it direct to the power section of the amp.

I didn't heard of that kind of transistors before and some say that they have a tube like sound, if it is a good comparison, I am tempted to try a tube phonostage in the future.

can someone tell me the purpose of those devices in the board?, they can be adjusted but I did not notice any change in the sound:

[fetteler]

It's a very interesting little phonostage and, if you like the sound then there's no reason at all why not to keep it. My first thought is, do you really need a separate enclosure for it, as there are distinct advantages to having it so neatly built int the turntable like that. For one thing it keeps the length of the small signal interconnects to the absolute minimum (well, with a little thought they could be shorter...) and there is a definite neatness to the arrangement.

Now, your switch on thump. Why not keep everything much as it is, take the 230V to the transformer and connect it before the Lenco's switch rather than (as it is) after it. In this way the little phono stage would be on all the time and you wouldn't get the thump. If you don't like the idea of it being on all the time perhaps you could find a nice place on the plinth for a separate mains switch just for the preamp?

As for one channel being louder than the other my first thought was that perhaps that was the purpose of the two 100k preset resistors you show in the photo but I see you say that adjusting them makes no difference to the sound. In addition to those two 100k preset resistors, I see above them in the pic what loos like two trimmer capacitors. It would be very interesting to try and draw the circuit if you can. It would help a little if you could at least identify which of the connections are the input and which are the output.

I'm sure mot of the components are OK but if the board is the same vintage as the turntable (1963) then it is a very good idea to replace any electrolytic capacitors as they will be well past their sell by date now! Who knows, replacing them may sort out your unequal gain issue.

Finally (!) the transistors. They are indeed germanium types:  PNP TO5 Germanium 25V 300mA 10MHz.

Steve.

PS I think the addition of an hours meter is a really good idea - I never saw it on a turntable before but I like it.

[mikelh]

hello Steve,

I have found the problem in the channel balance, there are two resistors a both extremes of the board marked 1K (shown with the point of the screwdriver) and they are very sensitive so I guess I slightly move one during the extraction/rebuilt operation, now both channels are almost equal because they are tricky to get the very exact point of balance (maybe because they're old and do not slide/turn fluently):





I am listening to it right now and the music is very engaging even at low volume which is great!

I want to place the phonostage in a case for different reasons:
I would like to place some plywood boards under the top plate of the turntable to reinforce the plinth, this one is better than the stock l75 plinth but I guess a little more stiffness won't hurt so I need to make room for it.
I also would like to try the device with other turntables.

I agree with you in the design which is neat, however I don't know very much about electronics it is easier for me to separate all the devices in order to "see" the different functions and possible issues,
for instance, the bump comes from the turntable switch when disconnected(not from the phonostage as I thought) so I guess I should replace the big capacitor and resistors coupled to it.
There is also a very slight hum and even if I am pretty sure it comes from the tonearm ground cable (because it makes some "scratches" from time to time when playing and when I return it to the rest position) I want to be sure all wiring is as perfect as I can do with my skills.

regards
Mikel

[Jessica_K]

I am making a big guess here because without a schematic it is impossible to know the actual design, however.

I think the design is likely to be an active 2 transistor RIAA as you are not hearing highish noise levels (or you have not mentioned it) with an output or input gain stage or even an emiter follower output stage as you say level is good and these transistors could be only hfe of 40 though they could be selected higher.

Unlike valves that have a pretty fixed gain (mu) depending on construction, all transistors have a min to max range per type that can be large in this case min is 40 but could be as high as 200 which in valve terms the difference between fitting a 12AY7 and a 12AX7 it would be easy to select the high gain samples for use in this stage.

Ge transistors behave the same as modern Si bipolars (not FET) but have a few characteristics that are different, biggest is Vbe (forward bass to emiter voltage) is only 0.2V instead of 0.7V, generally much lower power and current, typically 10mA instead of 100mA small signal types and very susceptible to temp specially durning soldering, they cannot take the 3-400C for short periods during soldering that a Si can. That said once fitted they do not need any protection.

The transistors used could be bettered? But all Ge devices are obsolete and if happy I would leave alone. Changing the electrolyletics as mentioned is a very good idea those old ones are probably pretty dry now and will not be working well, also changing the pots. While at it replace the resistors too as the carbon can crack and increase in resistance, I would still use carbons as they will still give that vintage sound you are liking

Good luck with the project, do make sure you use the exact same power voltage that was used before, the devices are only 10V Vce and therefore the circuit would be designed to keep them within range with the selected power voltage, any higher and they, and bearing in mind they are very old, may get stressed

Jessica xx

[mikelh]

hi,

I made the diagram this afternoon, I did my best but please remember I am not an expert and some values or connections may be wrong so feel free to ask me to look better





right now the device has a very very little hum and unnoticeable at my listening level, I guess this comes in part (if not all) because of the bad ground of of the tonearm
so I am very satisfied with it and I do not intend to change anything
but if I change my mind I will ask you, of course!
well the only thing I want to try is to add an attenuator to try it with a power amp, there is enough gain for it.
which value in this case?
and thank you for your help

regards
Mikel