Telefunken V76

[ubxf]

Hello,
I have a V76 that is starting to fail. After an hour of use a low frequency rhythmic thump (like a heartbeat ) creeps up. The preamp
still functions ok after this happens (the audio still goes thru and seems ok ). Does this symptom point at anything in particular ( a cap , a tube ). Does anyone have a schematic with voltages. Any help would be greatly appreciated.

 

[CJ]

probably the caps,

try running some alligator clips to a good cap and see if it stops,

 

[ubxf]

do you think it could be one of the 33uF power supply one

 

[ubxf]

i just found this schematic and i don't know if it is accurate

 

[PRR]

C29

 

[ubxf]

Thank you CJ and PRR i will try and report back, i found another schematic probably closer since it's a V76m

 

[ubxf]

I changed all 4 capacitors in the power supply and now the problem changed. For the first 20 min all is good then a low level 60hz  appears as well as hiss and random crackle. I didn't wait an hour to see if the low rythmic was still there.So far i've only used the preamp going through a mixer, if i do bench test do i need to put a load on the output like i would for a tube amp ?

 

[CJ]

the B+ voltage probably went up when you changed the caps,

so now a tube that was borderline might have gone noisy,

try tube substitution with V1 V2 and V3,

tap on the ef804s to hear microphony,

there is possibly an old selenium rectifier stack that does not like the new caps,

you can upgrade to 1n4004 but voltage will go up even more, check the condition of the cloth leads near the transformers, if they vaporize when touched, then the transformers could be in danger with the higher voltage,

also make sure that you did not reverse polarity on one of the caps,

symptoms are 120 hz hum due to excessive ripple current  followed by a small chemical explosion, but non caustic so no need for hasmat squad,

 

[ubxf]

Thanks CJ, Do i need a load on the output (like i would for a tube amp) while  i have the preamp on the bench and check voltages and look at the ripple.

 

[tubestation]

Hello!!
i also have some V76 restored and fixed and all i can say what wasn t said before  is be very carefull with the switch.after i recapped all electrolytics- the Unit was fully working ,but i thought i should clean the switch with isopropyl.iwas very carefull but i toucht the little metalllip(there are two of them on each deck(3) of the switch) and he went out of bench but i didn t reallised that.when the unit was repowered again it sounded like in your dicription  after a long search i found the problem.I tried to push the metalllip back in place but than it felt into the unit. it was very difficult and timesucking to fix  .in worst case you have to reasable the whole switch.one deck is conected with wireleads to the resistorboard.i had to desolder them to get access to the switch.
Good luck!!

 

[ubxf]

Thank you for all the suggestions, i recapped the power supply once more with new caps just to make sure. I changed the tubes except for
E83 F (i'm still waiting for that one to arrive ) . No microphony on the tubes but gentle taps on V1 and V2 did trigger a few crackles.
Transformer leads seem good. No more hum but random crackles. I didn't touch the switch so i hope nothing is broken in there.

 

[mjrippe]

If tapping on the tubes generates crackles, be sure to clean the tube pins and sockets.  It is amazing how many problems this will solve.

 

[gemini86]

Just out of curiosity, what do y'all use to clean up tarnished tube pins? I've just been cleaning with alcohol and scrubbing with a toothbrush to remove most of the oxidization, but I feel like I could do better. Maybe some of that silver cleaner? DeoxIT?

 

[emrr]

Deoxit works.  I apply with a Q-tip to tube pins, you don't want to soak sockets. 

Really just excercising tubes in sockets takes care of 90% of contact issues.  Many old broadcast manuals recommend doing this every month.  I find I need to do so seasonally when there's a large temperature or humidity change outside. 

 

[mjrippe]

+1 for DeOxit.  I have some tiny FlossTech interdental brushes that are perfect for miniature pin sockets.  One drop of DeOxit is plenty.

 

[gemini86]

Sweet, thanks guys.

 

[ubxf]

Thank you guys, all good now