R1155 bias voltages

[cmjones01]

Right, I've got the BFO box fitted back in to my R1155 with new capacitors fitted, and so it works without the 'meter amplitude' knob creating an HT short!

I've been measuring the bias voltages, since I know these are critical to getting the receiver working properly. However, in 'omni' (manual gain control) mode, it seems to behave oddly. There is silence for the first three-quarters of the volume knob's rotation, then, in the last quarter of its rotation, signals start to appear but distorted at first, as if something's mis-biased somewhere. The useful range of the volume control is all compressed into the last few degrees of rotation.

I've been round and measured the bias voltages and this is what I've found. All voltages are relative to chassis and yes, my HT- is isolated. Master switch set to omni.

Bottom end of R8 (RF volume control): -31V
Top end of R8: -3.6V
Wiper of R8: varies from -31V to -3.6V, but at mid-rotation it's at -28V
Grid of V4 and V5 (mixer and first IF): same as wiper of R8, -31 to -3.6V
Grid of V3 (RF): just over half the wiper of R8, -18 to about -3V
Grid of V6 (second IF): about -5 to -3V

This all seems to be about right, given the way the bias is distributed by R10/R11/R12. Running in AVC mode, the bias never goes much lower than about -5V, even on a fairly strong signal, though I haven't tested this properly.

Is what I'm seeing normal? It would be really useful to compare with someone else's experience. I find it particularly odd that the volume control behaves in the way it does. It's very non-linear.

Thank you
Chris

[G4XWDJim]

Here's a voltage table taken from Instructions on T1154/R1155 from the Marconi school of Wireless Communication.

I'm not sure how much help it might be. If it's any use and you'd like it bigger I can send it as an email.

My working R1155 is heavily modified so I can't help much I'm sorry.

Jim

[cmjones01]

Thanks, Jim - that's really helpful. I hadn't seen that document. The manual I've got for the R1155 doesn't show much in the way of expected voltages, so now I can check things more clearly.

Chris

[G4XWDJim]

Glad it's useful. The book also shows resistance readings between various points which I can provide if you' d like it although I think the AP 2548 has the same info.

Jim

[GW3OQK Andrew]

Chris, I had about -30v as the bias voltage on the last R1155 I made a psu for. Originally I seem to recall it was more negative at about -40 and the set was dead, the fault being the one resistor between HT- and chassis.
Best of luck
Andrew

[G4XWDJim]

Chris,

I've had a look at my modified R1155. The RF gain circuit is fairly similar to the standard set with a swing of -32 volts to -2.7. With the pot centralised it has -19.5 volts on the wiper. I think it's a linear 50k pot but I can't read the value and measuring it is no good due to the various parallel circuits. Readings taken with an AVO 8. I don't think it's worth giving other readings as this set is quite modified. They're not a million miles away from yours though.

The Marconi voltages I gave above in the earlier post were measured with a 1000 ohms/volt meter so will be slightly different again

I do have an 1155 that I completely restored to standard but don't right now have a suitable PSU I can fire it up with. If you are still stuck at some later stage I'll endeavour to power it up and take some comparative measurements.

Jim

[cmjones01]

Jim,

That's very interesting. Your RF gain pot behaves differently to mine - I get the same voltages at the ends, but mid-rotation I get -28V and you get -19V. Yours is much more like I'd expect. Perhaps my pot is poorly. I'll try and do some more tests to find out.

Chris

[G4XWDJim]

Looking at the circuit Chris the voltage across R8 the RF gain pot is limited by the fact that R3 1.2K is in parallel with it. If your pot was O/C at some point it could then give very funny readings depending which side of the break the wiper was.

Jim

[cmjones01]

I've just investigated the pot more closely. I'm fairly sure it's the original part, because it has the right part number marked on it. The tracks measure the right resistance: 500k for the audio volume control, and 50k (well, 53.9k) for the RF gain control. I disconnected the wiper and one end and did a rough-and-ready plot of the law of the pot:

rot  Ra    Rb
0    53.9   0.0
1/4  52.4   1.5
1/2  50.5   3.4
3/4  32.2  19.8 
1     0.1  53.9

The resistances of both 'halves' vary smoothly, but it looks for all the world like it's a log pot and it was made that way. It's an odd choice for adjusting the bias on vari-mu valves, which have a more-or-less log law of their own. Maybe they realised that it wasn't working very well and corrected it in later production - a linear pot in this position would be much better.

I'm in two minds about this one. I'm trying to restore this receiver to pretty much original condition, but I might allow myself a reversible modification (a resistor from the wiper to the top end of the RF gain control) to make it work usefully.

Chris