Re: Drip U73b
« Reply #20 on: April 27, 2020, 12:22:17 PM »
Update:
Finally I got a proper working unit, really sounds great! It was a hell of a ride but finally this beast is alive.

Thanks to Walter who helped me out with most of the issues.

Bugs:
Psu
Main pcb-relay  comp/lim, plate choke (v72 is the wrong one, take 1377 ...sowter)
Some resistors to replace or take out the circuit (R60, R47).

I have followed the thread with interest. So I'm going to ask some questions.

What problems has the relay part caused you?

As for the problem of the comp / lim part of the circuit, I understand that it refers to the change of the plate choke. In the original schematic Telefunken shot it is a CTV 72/2 with a 10: 1 ratio; Could you be a little more explicit in your arguments about why the Sowter 1377 is better?


Re: Drip U73b
« Reply #21 on: April 27, 2020, 07:07:21 PM »
I have followed the thread with interest. So I'm going to ask some questions.

What problems has the relay part caused you?

As for the problem of the comp / lim part of the circuit, I understand that it refers to the change of the plate choke. In the original schematic Telefunken shot it is a CTV 72/2 with a 10: 1 ratio; Could you be a little more explicit in your arguments about why the Sowter 1377 is better?

Hi,
sure.

As far as I know the original choke was the BV514 , the right equivalent is the Sowter 1377. It got only half as many henries as the 9375.
value at the cathode resistor R47 was about 2.4V (should be 1.8V), what causes distortion on the output.
Replaced the choke with a 15K resistor and replaced R47 with a 390 resistor. Still not at 1.8V (now it´s around 2.0V) but distortion is gone. And I took of the R60 resistor (a tip from Walter) for more headroom.

Alex (also posted here) left the 9375 in the circuit and soldered a 10k resistor in line the the choke, but I don´t know if this worked out. But i think so.

I don´t know if current revision of the main pcb still have this bug, but on the first revision (bought mine almost 2 Years ago) the contact of lim and cmp on Relay SW78B was fliped...so you need to cross them. Otherwise you will not be able to calibrate the unit im Lim mode.



So far..hope this will help.

So far
« Last Edit: April 28, 2020, 06:00:56 PM by Impulsstudio »

Sc

Re: Drip U73b New
« Reply #22 on: May 01, 2020, 03:37:11 AM »
Hi - just got into the forum …

The relay issue - only on the first Edition 2018 board - the switch lim/comp acts wrong on the R79 cathode bypass – R79 is open in comp mode instead of bypassed to ground. You can fix it by crossing two pins on the relay.
The issue with the anode-choke is technically quite difficult - the original BV514 (Haufe) choke is Sowters 1377. It has a lower DCR and about half inductance  compared to 9375E (the V72 choke).
I was not able to get the prescripted 1.8V at the cathode E88CC (actually I had around 2.5). But the 1.8V are absolutely essential, because at 2.5 one of the halfwaves goes into cutoff at lower signals already. You can see a flattened halvwave on the Scope. This gives a nasty distortion. Switched to 1377 and lowered the cathode resistor to 390R - fine. I dont think you will have to change the output transformer, unless you want to have 10:1 instead of 12:1. The parallel resistor over the primary  of the OT (R60 - 10k) was used to correct frequency response  and was aligned in the factory. Since the original OT probably has slightly different properties, the value of 10k might not be correct. In our case it works fine without parallel resistor - frequency response is perfect.
Finally the issue with the PSU. I can't verify this exactly, because we used the stronger 273EX from the beginning. But whenever B+ drops under load, the load is probably higher than it should be (or the transformer to weak to deliver).
 
Good luck and carry on - it's a fantastic compressor.

Regards Walter   

« Last Edit: May 01, 2020, 01:06:00 PM by Sc »


 

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