Westrex RA-1479C Power supply questions

[Kid Squid]

Hi Chaps,

Just wondering if any of you could shed some light on some questions I have regarding a Westrex RA-1479C power supply.

One of my close friends inherited a Westrex RA-1593A Compressor, and the power supply for this is a Westrex 1479C PSU ( sharing same 'modular' chassis)
Since he's had it, its never worked, as previously the power transformer suffered a meltdown. ( quite possibly tried on 240V instead of 110V..)
Transformer markings are - TE 4851 DP
https://photos.app.goo.gl/ADgT9s82qeoGcYu57
and this sharpie'd on
https://photos.app.goo.gl/9pk2CJALdvyYaEbn6
Actual unit
https://photos.app.goo.gl/7FA65eAaQLY2giJRA

(I've just swapped over to google pic, but cant Share actual images ??)

He asked me to go over the units, re-cap etc, and see if I could either get the transformer rewound, or get a replacement unit.
I've got around to having a look at the PSU, found the correct schematics, and it is a variable regulated power supply.
However,
I'm a little stuck with trying to work out whats going on, specifically what would be the secondary AC volts pre-rectified / regulated value ( the variable supply has thrown me a little !).

As far as I can see (please correct me if I am wrong), that it is a 'degenerative' regulated supply, with the 6AQ5 acting as the pass element, then sort of identifying and comparing the other circuit blocks against a discrete SS regulator -  error amp, reference Volts (0A2), comparison amp etc.
From the RA-1593A manual, it states that B+ is 275VDC @ 44mA, and the valves require DC - 1.2A @ 12.6V.

Also, there is another winding supplying raw AC for the power supply module heaters, but the centre tap of this winding is brought out to the terminal strip and is marked 'filament' bias. ( I will check later where exactly this is going).
Granted, that the variable output was such, that this power supply module could be used with other Westrex kit, such as the 1474 amplifiers (EDIT- just checked the 1474 schematic and it shares the same power rails..)
Maybe the variable element is just a rough trim ??


6X4 rectifier - tube drop , Ib =70mA (22V each plate) feeding a CLC filter ( 4uF caps are around top limit of max capacitance 'seen')
0A2 regulator - mean stabilised volts - 150V, Imin - 5mA, Imax - 30mA, Vreg 6V ?, striking volts - 185V

Choke input Full wave - AC volts x 1.414 = DC
B+ DC =275V ( minus 6X4 voltdrops)

AC= (275 + 44) / 1.414 = 225.6V, not taking into account the variable supply, or even if these calcs are correct !!! :-\ ;D
Anyway, heres the schematic, very interested to see what you chaps make of it.


https://photos.app.goo.gl/Q3CYsJsyWLf4zTqi9


Best,
Steve

 

[ruffrecords]

The topology is  a pretty standard regulated power supply - the implementation is a little unusual.

The 6X4 provides raw dc across C2 which is smoothed by L1 and C3. V2 (6AQ5) acts as a series regulator and the regulated dc ouptu appears at its cathode.

The 0A2 is fed fromt the regulated output and produces a reference voltage of about 150V. The OA2 output feeds pin 6 of V4 which is configured as a low frequency long tailed pair (note the cathode decoupling). The other grid of V4 is fed by the output of the set point pot P1. The error voltage appears on V4 pin 2 and is fed to the grid pin 5 of V5. V5 is set up as another long tailed pair. I think this is acting simply as a level shifter so that its output on V5 pin 2 is right for driving the cathode follower V3. The output of the cathode follower feeds the grid of the 6AQ5 and so closes the loop.

I am not sure what the transformer HT secondary voltage will be but the voltage rating of C2 should give you a clue. I would guess you probably want at least 50V across the 6AQ5 and allowing for some voltage drop in L1 and 20V or so in the 6Sx you probably need  a raw dc voltage around 375V to 400V which implies a secondary voltage of 265VAC to 280VAC.

Cheers

Ian

 

[gridcurrent]

Save yourself and your friend grief and money,  and get improved performance.

Use an off the shelf linear supply for the high voltage:
https://www.mouser.co.uk/ProductDetail/International-Power/IHB250-01?qs=0xCm9DOQnC5apaiylMDitA%3D%3D
and likewise for the low voltage:
https://www.mouser.co.uk/Power/Power-Supplies/Linear-Power-Supplies/_/N-brwqiZ1yzvvqx?P=1yx5k7vZ1yxt792Z1y9f5fk

Restored 2 of those Westrex RA-1479c compressors recently.
Made client happy by keeping the psu's as original as possible,
but would have much preferred linear supplies.

 

[Heikki]

Attached is schematic of Western Electric power supply which also has 275V output. Looking at this schematic and couple other tube regulated power supplies it seems raw DC voltage from the rectifier is usually about double the regulated voltage.  Like ruffrecords suggested, C2 voltage rating will give you a clue. If the voltage rating is over 500V, I would guess they would not have put expensive high voltage cap there if it isn't needed.  Or perhaps gridcurrent remembers what the voltages are supposed to be?

 

[gridcurrent]

RA-1599a  indicates 440 VDC (no load) and 400 VDC (loaded) at  filter input.

 

[Kid Squid]

The topology is  a pretty standard regulated power supply - the implementation is a little unusual.

The 6X4 provides raw dc across C2 which is smoothed by L1 and C3. V2 (6AQ5) acts as a series regulator and the regulated dc ouptu appears at its cathode.

The 0A2 is fed fromt the regulated output and produces a reference voltage of about 150V. The OA2 output feeds pin 6 of V4 which is configured as a low frequency long tailed pair (note the cathode decoupling). The other grid of V4 is fed by the output of the set point pot P1. The error voltage appears on V4 pin 2 and is fed to the grid pin 5 of V5. V5 is set up as another long tailed pair. I think this is acting simply as a level shifter so that its output on V5 pin 2 is right for driving the cathode follower V3. The output of the cathode follower feeds the grid of the 6AQ5 and so closes the loop.

I am not sure what the transformer HT secondary voltage will be but the voltage rating of C2 should give you a clue. I would guess you probably want at least 50V across the 6AQ5 and allowing for some voltage drop in L1 and 20V or so in the 6Sx you probably need  a raw dc voltage around 375V to 400V which implies a secondary voltage of 265VAC to 280VAC.

Cheers

Ian

Ian, thank you very much for the explanation. I really appreciate it.
Fantastic stuff 

Save yourself and your friend grief and money,  and get improved performance.

Use an off the shelf linear supply for the high voltage:
https://www.mouser.co.uk/ProductDetail/International-Power/IHB250-01?qs=0xCm9DOQnC5apaiylMDitA%3D%3D
and likewise for the low voltage:
https://www.mouser.co.uk/Power/Power-Supplies/Linear-Power-Supplies/_/N-brwqiZ1yzvvqx?P=1yx5k7vZ1yxt792Z1y9f5fk

Restored 2 of those Westrex RA-1479c compressors recently.
Made client happy by keeping the psu's as original as possible,
but would have much preferred linear supplies.

Grid, I hear exactly what you are saying, and if it was mine, I would certainly look nto that option. Thanks for the links !
However, after a conversation, he would like to keep it as original as is possible.
How did the two restorations go ? any problems ?

Best,
Steve

 

[rmburrow]

Look around on this site or the diyaudio site for CJ in California somewhere...he can probably rewind that transformer....nice to keep that old vintage stuff original if possible....

 

[emrr]

Yeah, the overall value of the piece warrants a rewind.