Murphy B40D Audio Output
 

Just wondering if any B40D owners out there have measured the AF output power available from their set(s)? The manual says 2.5W maximum from the 600 Ohm line. I have two of these radios, and the best clean output I've seen with an AF generator input and a directly-connected 600 Ohm resistor load is about 0.5W. With an elderly RS Hygrade transformer coupling a 4 Ohm speaker to the output line both sets do sound weaker than I would expect. The valves are good, and I've been through the AF amplifier stages of one radio: found a few carbon resistors that had drifted high by 10-20%, and replaced them, but without significant difference. The coupling caps were still good.

Thanks for looking: any comments welcome.

Re: Murphy B40D Audio Output
 

Hello Noel

I have three of the variants, B, C & D.
All seem to have similar output (to my ear), but I only use the tiny built-in speaker when in use.

All seem to provide enough volume for general listening, all-be-it with 'communications' quality.

Regards
Pete

Re: Murphy B40D Audio Output
 

Thanks Pete,
Not having heard any other B40s apart from these two, another user's experience is useful. These radios are not built to have their AF amps scoped easily, so building a couple of extension leads to permit separation of the PSU/amplifier module from the parent frame may be a future move. I reckon that the speaker matching is ok, but will double check just in case.

Noel

Re: Murphy B40D Audio Output
 

A lot of the B40 receivers I have owned (many) have had problems with the anode and screen feed resistors of the AF driver stage - this will also give a very low output.

To be honest, once they have been sorted, there is normally plenty of audio - Ive never actually bothered to measure it though.

Re: Murphy B40D Audio Output
 

Thanks Sean,
I found the driver's 82k anode load resistor had drifted up to just over 100k: the screen feed had fared better, only up to about 230k from the nominal 220k. Both replaced anyhow.

Today I substituted an old 8" speaker for the (very modern) 4" unit I had been using, and no doubt the audio does sound better. Tuning to our local BBC Radio on MW, I still need the AF gain at close to 100% to get what I consider a good listening level in my workshop. Maybe this is normal, and I can live with it.

After 60+ years I'm surprised these two radios run so well. I don't know what the difference is in their exact build dates, but one has much tidier wiring than the other, with laced looms and 90 degree bends that can only be described as artistic. This unit also has component-reference ID painted on individual capacitors, etc., with nice blobs of protective lacquer. Someone took a lot of pride in his/her work!

Noel

Re: Murphy B40D Audio Output
 

The manual states that 2.5W at 600 ohms should available at pins 'B' and 'E' of the 6 pin output socket.

To get the impedance matched correctly to a 8 Ohm loudspeaker you would need a step down transformer with a ratio of 8:1. A 240V to 30V mains transformer would be suitable for testing.

If a 4 Ohm speaker is used the ratio needs to be 12:1. A 240V to 20V mains transformer would do.

Re: Murphy B40D Audio Output
 

The Hygrade audio transformer mentioned in post #1 takes care of the matching: lots of ratios available, from 46:1 down to 11:1 used with the B40. For reference, a 1 kHz test signal can be fed into the AF section at the junction of R223 & R224. This is easy to do in R/T or CAL mode at switch wafer SW202f which lies about halfway along the IF deck. In my case, monitoring the output level across the 620 Ohm internal dummy load, limiting started at +/- 15V peak with an input of 250 mV peak. If anyone makes a similar measurement I'd be interested to hear the result.

Re: Murphy B40D Audio Output
 

If I get time tomorrow (and a healthy shot of enthusiasm) I will drag a B40 D I have here onto the bench and see if I can persuade it to work.

If it goes to plan, I will post results for you

Re: Murphy B40D Audio Output
 

Ok Sean, I appreciate that: these radios are not exactly portable, so are a bit of a job to move around. Thanks also to the others who came back to this thread.

Noel

Re: Murphy B40D Audio Output
 

Hi Noel,

I dragged the beast from the heap of "To Do" projects this afternoon.

I must at this point say that this receiver is almost exactly as found.

I had to perform a couple of repairs - as expected the Anode load, and SG feed resistor for the AF driver stage were well out of spec, robbing the anode of nearly all of it's volts.

I also found the AF coupling condenser to be leaking, and whacking around 8v onto the grid of the output valve.

With these three components replaced the audio amplifier seems very lively.

I have tried it briefly in the receiver, and have no problem getting room filling audio on my extension speaker - sadly, it does not have the internal speaker (an all to familiar problem on these sets :( )

I will couple the receiver up to some measuring gear tomorrow, and take some true measurements, but I doubt I will find any issues with low output.

Something that might be worth checking on your RX - the AF driver stage has it's cathode grounded via the multiway plugs that feed the IF and RF chassis - I have no idea why Murphy chose to do this, but it might be worth grounding the chassis end of the 220R cathode resistor, and just see if that improves matters.

More to do tomorrow, but I doubt there will be much wrong with your B40.

Re: Murphy B40D Audio Output
 

Hi Noel,

Okay, I have had the B40D coupled up on the bench - Took me a while to make up all the Mk 4 connectors :)

Some details of the test equipment used:

Marconi 2022D RF Signal Generator set to 1.07MHz, 5uv carrier, 30% modulation at 1000Hz

Marconi 893 C Output Meter - Set to match 600R Impedance, and running on 3W FSD


With all of these settings applied, the AF gain on the receiver set to maximum, the Bandwidth set to 8KC, and the AGC off, RF gain to maximum.

I measured a peak (distorted) power of 2.95 Watts

At undistorted (to my ears) output, I measured 2.5 Watts

I had to back the AF gain down to clear the distortion.

So, in a nutshell, the amplifier in mine will certainly do 2.5 Watts.

I guess the question now is, why isn't yours doing the same?

Re: Murphy B40D Audio Output
 

Ok Sean,
That's most helpful, many thanks for doing this test. I'll go back and make a few more measurements later this week. I was aware that the driver's ground path was a bit convoluted: will check this and the other plugs/sockets involved in the audio path for good continuity. At least I now know that I'm looking for a result that's achievable. I'll report back in due course.

Noel

Re: Murphy B40D Audio Output
 

I did an RF test similar to Sean’s: I don’t have an AF power meter, so again monitored the voltage across the set’s internal 620 Ohm load. A similar result to that found previously, with early AF clipping and quite severe distortion with the AF gain at maximum. At this stage I pulled the ‘original’ PSU/AF chassis and set it up on the bench alongside the one from my ‘spare’ B40D (this hadn’t seen AC mains in 15 years, so I wound it up slowly). I had 620 Ohm loads connected directly across the output transformers, plus the preamp’s ground lines and 2.2M input bias resistors (R301) taken to earth: just needed to switch the sig-gen and ‘scope connections to compare the amplifiers. The HT rails do run a bit high (320V) with only the 6BA6 & 6BW6 taking current, but so far everything has survived. The good news is that I got a nice 110V p-p sine wave on the spare’s output (near enough 2.5W into the load), while the original amplifier started to clip at about 45V p-p (0.4W). The corresponding p-p anode swings were 400V and 180V before non-linearity set in. Quiescent DC levels across both amplifiers were the same, within 5% or so. The only suspect item left was the output transformer TR301 and it turned out that’s the problem. I connected my Hygrade output tranny with a 30:1 ratio feeding a resistive load, and was rewarded with a 400V anode swing and full output on the secondary. Transplanting TR301 from the ‘spare’ receiver also worked well. There may be shorted turns on the poor transformer, though a winding-resistance comparison didn’t reveal any vast difference between it and the good one. The transformers are built slightly differently, so some mystery remains. I’ll either fit the Hygrade permanently or look out for a genuine replacement from a scrap receiver, if only to preserve originality. Thanks to all for your interest and comments.

Re: Murphy B40D Audio Output
 

Ah, that's a pity, but not uncommon - previous owners may well have just run the audio stage into an 8 ohm speaker - the 2.5 watt output gives plenty of volume even into a horrendous mismatch, well at least until the optx dies.

Hopefully you will be able to resolve the matter with some spares from a donor set - sadly I no longer keep a spares donor.

Re: Murphy B40D Audio Output
 

Why would an 8ohm load cause any grief to a 600ohm valve output transformer?

The output valve is pretty much a voltage controlled current source, so the current into the speaker will be much the same at 8ohms as 600ohms- much less audio but no drama.

Shorting the speaker output on a normal valve amp is similarly undramatic.

Re: Murphy B40D Audio Output
 

Doner receivers do crop up - I've bought two in recent years.

Another option is to fit a 'standard' valve output, which would be more suitable for the extension speaker you plan to use.
I had a BR2169 (CAT receiver) at one time, which had started to leak oil. I replaced this with a CR100 output transformer which worked very well.

Regards
Pete

Re: Murphy B40D Audio Output
 

Chris, Just speaking from some experience - Particularly on the TCS - an otherwise completely restored receiver developed a similar fault, the only possible cause was due to incorrect loading

Re: Murphy B40D Audio Output
 

Wrong load impedance and the load line rotates so far as I can gather.

Lawrence.

Re: Murphy B40D Audio Output
 

I did an impedance test on the faulty transformer's primary with the secondaries open-circuit: the result was 1.9k (mainly loss resistance?), and only a couple of hundred Ohms less with the 600 Ohm load connected properly. The RS Hygrade unit returned >20k under o/c conditions and 9.6k under load. Little wonder that the amplifier's AC load line was a bit depressed. Oddly I can't find any DC leakage between windings or windings-to-core. I don't know the history of this radio, but it has probably had a hard life and Sean's diagnosis is not far off the mark.

I'll post under the 'Wanted' section just in case anyone has an original transformer for disposal, though interested parties have probably read the story here anyhow. Again, thanks to all for your interest and comments.

Noel