Unit-8

Re: [BUILD] NU VARIMU 436 build & help thread
« Reply #60 on: June 20, 2019, 09:09:22 AM »
Hi Bernd,

Thanks for the useful tips. This morning I started calibrating the compressor and I encountered the first problem. On test points B, C, D, E, I and J I have the same voltage on both channels between 297 and 300VDC which does not change using the VR103 and VR104 A-B trimpots. On the test points F, G, H the voltages measured are 0VDC and even here the trimpots have no effect. All the tubes do not glow up on both channels. No problem on relays and heater rails where I get 12 and 6.3VDC respectively. The tubes I used in the circuit are: RCA 6AL5, Mullard CV4024 (12AT7) and Brimar 6BQ7A (6BZ7 equivalent). Also, I checked all the components on the PCB to verify the values and their exact place on it and I viewed all the solders. No problems found. Any suggestions?

Ta

Andrea


bernbrue

Re: [BUILD] NU VARIMU 436 build & help thread
« Reply #61 on: June 20, 2019, 10:04:50 AM »
Maybe a stupid question, but did you do the heater wiring on the solder side of the pcb?
Bernd

« Last Edit: June 20, 2019, 10:12:41 AM by bernbrue »

Unit-8

Re: [BUILD] NU VARIMU 436 build & help thread
« Reply #62 on: June 20, 2019, 05:50:41 PM »
Yours is not a stupid question. I completely forgot to connect the heaters together :) now solved.
Progress to follow...

bernbrue

Re: [BUILD] NU VARIMU 436 build & help thread
« Reply #63 on: June 21, 2019, 06:09:06 AM »
 ;D

Unit-8

Re: [BUILD] NU VARIMU 436 build & help thread
« Reply #64 on: June 24, 2019, 11:43:32 AM »
Hi Bernd,

I started testing again today and unfortunately I haven't made much progress since last time.
First of all, the needles of the two VU meters do not move.
These are values ​​measured by the test points of the two channels:

CH-A:                 CH-B:

B: 295                 B: 293
C: 289                 C: 282
D: 289                 D: 286
E: 289                  E: 286
F: 14.46              F: 13.4
G: 0.90               G: 1.08
H: 0.81               H: 1.04
I: 285                    I: 283
J: 285                   J: 282

Furthermore, turning the trimmers these have only a slight effect on the voltages probably because of the above-mentioned VU meters problem.

Andrea

bernbrue

Re: [BUILD] NU VARIMU 436 build & help thread
« Reply #65 on: June 24, 2019, 12:06:52 PM »
1) Can you tell me more about your VU meter? In fact the unit needs an amperemeter (1mA or lower)
2) Does the meter react to the meter trimmer on the control pcb?
3) Is there a ground connection going from mainboard to control pcb?
4) Did you test, if the unit passes audio?
Pictures would be helpful. After powering up the unit needs about 5-6 seconds before the meter moves up.
Bernd
« Last Edit: June 24, 2019, 12:14:46 PM by bernbrue »

Unit-8

Re: [BUILD] NU VARIMU 436 build & help thread
« Reply #66 on: June 25, 2019, 06:18:23 AM »
Hi Bernd,

Thanks again for your valuable tips.
Even here I had forgotten to connect the grounding from the main board to the control boards ... damn it!
Now the measured voltages are close to those of your prototype.
However, the needles of the VU meters go to full scale and the trimpots have no effect on them.
It seems that the VU meters get more current than the one needed although they are 1ma fsd as required by the circuit.
What do you recommend: change the resistor from 270R to a higher value, or put another in series at the positive terminal of the VU meters, or what?
I have yet to test if the sound passes through the compressor.

Best

Andrea

bernbrue

Re: [BUILD] NU VARIMU 436 build & help thread
« Reply #67 on: June 25, 2019, 06:33:18 AM »
You could increase the value of the 220R resistor (470) or use a 1K trimpot as meter trimmer.
Bernd

Unit-8

Re: [BUILD] NU VARIMU 436 build & help thread
« Reply #68 on: June 26, 2019, 10:17:44 AM »
Hi Bernd,

I replaced R106 A and B (220R) with a 6K49 resistor, but it is a temporary solution because the VU meters need one with a slightly lower value than I currently don't have.
However, now they work even if they do not reach full scale.
The primary problem now is that the audio does not go through both channels and I still haven't managed to find the cause.
Any suggestions?
Meanwhile, I continue to investigate..

Andrea

Holger

Re: [BUILD] NU VARIMU 436 build & help thread
« Reply #69 on: August 03, 2019, 02:21:38 PM »
Some notes on this project:

- the PCB as well as the schematic have an error, the heater wiring for the 12AT7 is wrong.
Pin 4+5 of the two 12AT7 must be wired to F+, Pin 9 must be wired to F-.

- the 436 is built around attenuators. If a 1:1 input transformer ist used, these attenuators don't make so much sense because the internal level is relatively low.
The input TX for the 436 suggested by Sowter is the 1231 which is a 1:2.3 (15k:80k). Such a transformer rises the input level to a point where the attenuators are usable.
I'm currently playing with an Edcor 600:10k which gives a very high internal level, maybe a bit too much.

- I had to solder a 5k trimmer in series with the 220R meter resitor to get the value down to 1mA for the meter.

- The suggested OP TX for this project is the Sowter 8650 which is a 9CT:1 TX. Sowter has also the 1232 12.4CT:1+1 which could work here. Both are expensive (100 pound sterling).
I'm currently toying with an Edcor 15K:600 which I doubt has the right gap but it works.

The compressor is interesting. My version distorts the low end a bit but I don't have the right transformers right now. Can be improved.




bernbrue

Re: [BUILD] NU VARIMU 436 build & help thread
« Reply #70 on: August 03, 2019, 03:00:50 PM »
Thanks for pointing out your observation, Holger. When designing the pcb Tony and I were concentrated  on getting the details right and didnt see the more obvious things. Sorry for that.
I'm going to alter my prototype accordingly and report back. Please keep us informed about your progress.
Cheers
Bernd


 

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