Help with 4 valve mantle superhet

[retailer]

I've had an early 1950's 4 valve AM mantle for long time, the bakelite cabinet is beyond saving, the half that is left is badly cracked - the plan is make a Covid 19 lockdown timber cabinet - if I can get the chassis working well enough to make it worth the effort. I found a copy of the service manual which says that there were 2 versions a KT61 output and a N78 output version, I have the KT61 version, the KT61 tested a bit weak at around 65% so I made an IO8 to B7A adaptor and installed a new N78.

The chassis was totally dead apart from some bad hum - with no experience on mantle radios I started some basic testing, electro caps both bad so I replaced with salvaged and reformed newer versions, C21 failed an insulation test so it was replaced, all of the other caps passed a 500V insulation test and also within tolerance for capacitance values on my universal component tester. I haven't removed the IF cans to check the internal 70p capacitors but all seems good here with around 8-9 ohms across the IF coils, service manual quotes 10 ohms.

R4 had drifted high to 3.5M so I replaced with a 2M2 1watt. The voltages check out close enough to the values quoted in the service manual with the exception of the 6BE6 and 6AR7 anodes (circuit voltages are in black my voltages are in red). There seems to be an error in the circuit voltages as both of the anodes are connected to same HT supply line as the N78 screen which is very close to the correct 160V value so I can't see how they could be at 200V.

I now have reception - but the high frequency end of the AM band is very weak the volume control needs to be at full volume to hear anything at all with quite a bit of background noise, the low frequency end of the band has good performance and is way too loud with the volume control full. The antenna is a good length at around 2.5m long - leaving it coiled up or extending out to 2.5m makes very little difference. The 6BE6 and 6AR7 both test good and I've swapped them out with known good ones - results are the same.

I really don't know where to go from here and can't find anything that stands out. just wondering if any one with more experience than me has any ideas, not sure if it has any significance but when I check the anode voltages on the 6BE6 or 6AR7 touching the connection with my DVM probe kills the sound output.

[cathoderay57]

Sounds like the alignment is out. Worth following the alignment procedure in the service data. Cheers, Jerry

[paulsherwin]

2.5M isn't very long for a basic superhet radio if you want good reception. It would normally work a lot better at the HF end of the BC band than the LF end though, so the symptoms are odd.

[Sideband]

Originally Posted by cathoderay57

Sounds like the alignment is out. Worth following the alignment procedure in the service data. Cheers, Jerry

I'd agree with that. As you have tried new valves it sounds like the RF (aerial) circuits are in need of a trim. Good sensitivity at one end of the band and poor at the other is a classic sign of the aerial circuits not optimised.

[Techman]

Originally Posted by paulsherwin

2.5M isn't very long for a basic superhet radio if you want good reception. It would normally work a lot better at the HF end of the BC band than the LF end though, so the symptoms are odd.

I think he may be confusing wavelength and frequency.

[retailer]

Thanks for the response.
It is the high frequency end of the band that has lower volume, a station at 1323Khz is at the high frequency end and Gov funded ABC station is at 891Khz. I tweaked the aerial trimmer that sits on top of the tuning gang and this bought up the volume enough so that it could be listened to - but the volume control is on full - perhaps I'm expecting too much, it is after all only a 3 valver in reality, can't count the rectifier.

I'll do some more internet searching and find out how to do an alignment - I have a sig gen that goes to 2Mhz but it is for audio ie it is not modulated so I'll have to find a method that does not use a RF sig gen.

[petervk2mlg]

I'd first align the IFs by applying 455 KHz as per the instructions in the service data. Then I'd open the gang right out and apply a 1600 KHz signal to the aerial input. Then using the oscillator coil trimmer set it to receive that modulated signal. Then peak the aerial coil trimmer for maximum output. Have you fitted a three core power cord so the chassis is earthed?

[usradcoll1]

Originally Posted by paulsherwin

2.5M isn't very long for a basic superhet radio if you want good reception. It would normally work a lot better at the HF end of the BC band than the LF end though, so the symptoms are odd.

Looking at the schematic, I don't see a first audio amplifier stage. The output of the detector and second IF stage go right to the audio output grid.
The 6ar7 is similar to our 6sf7, but our designs use a first amp stage, before the output stage.
Strange to me.
Dave USradcoll1

[paulsherwin]

It's a short superhet. These were very popular in Australia in the valve era because of the geography - if you lived in a city there was no point in paying extra for more sensitivity, because a short superhet was good enough for your local transmitters and the ones in the next big city were too far away to listen to on anything.

Aussie radio design incorporated elements of US, European and British practice, with local idiosyncratic stuff thrown into the mix too. They do seem a bit odd if you're not an Aussie.

Some British manufacturers made short superhets, notably Pye who were very keen on the concept in the postwar years and even used it in upmarket models such as radiograms. Again, if you lived within 50 miles of a big BBC transmitter complex as millions did, you didn't need anything better.

[Synchrodyne]

The Australian valvemakers also specifically catered for the four-valve receiver. There were early miniature-era releases to this end. AWV had the 6BV7 double-diode output pentode, whose pentode could be driven directly from the demodulator diode, whilst Philips, in its “Innoval” series, had the 6AD8 double-diode pentode, with the pentode serving as both an IF amplifier and a (reflexed) AF amplifier.

In the US, I think there were some “AA4” type circuits, an example being the GE model 419, although in this case it was the IF amplifier rather than the AF amplifier that was “missing”.


Cheers,

[usradcoll1]

Originally Posted by Synchrodyne

The Australian valvemakers also specifically catered for the four-valve receiver. There were early miniature-era releases to this end. AWV had the 6BV7 double-diode output pentode, whose pentode could be driven directly from the demodulator diode, whilst Philips, in its “Innoval” series, had the 6AD8 double-diode pentode, with the pentode serving as both an IF amplifier and a (reflexed) AF amplifier.

In the US, I think there were some “AA4” type circuits, an example being the GE model 419, although in this case it was the IF amplifier rather than the AF amplifier that was “missing”.


Cheers,

That is what I'm used to seeing! No if amplifier, the F/C transformer coupled to the detector and A/F amplifier stage.
RCA, Motorola and Zenith never seemed to build a short superhet.
Every other US manufacturer did.
Dave, USradcoll1

[Synchrodyne]

To avoid hijacking this thread I have taken the "AA4" aspect to a new thread, "AA4", "AA6", at https://www.vintage-radio.net/forum/...d.php?t=170951.

[retailer]

I had some more time to look at the superhet, the interference/noise was tracked down to my LED lighting system - I have 9W fluro type LED lights, 10 in total spaced through the workshop, turned these off and all interference/noise disappeared, from comments received I thought that signal strength might be an issue, my workshop is all metal - frame, walls, roof and a steel reinforced concrete floor, I have issues with my mobile phone losing signal so I carried the chassis out and onto the outdoor table, a noticeable difference in volume and probably more like I would have expected.
The alignment is probably out - I though about playing around with an AM modulator and my 2 signal generators to see if I could come with a workable alignment oscillator, but by chance I leafed through an old copy of Silicon Chip and there was an article on the Minispot a modulated oscillator, specifically for aligning superhets, it is fixed at 455Khz but it is all I need, I've ordered a ceramic resonator which is the only thing I don't have in my spares box.

I recall reading that that high slope valves like the KT61 were used so one audio stage could be eliminated, mine is a bit on the weak side so I'm using a new N78 via an adapter I made.

[red-duck]

Don't forget there is also a big difference in power between the old 5DN 1323Khz transmitter (2Kw) and ABC 5AN 891 Khz (50 Kw) transmitter.
The set is probably not deaf at higher frequencies just that the lower frequency ABC transmitters are more powerful.
I guess your location could play a part too with the ABC transmitters being south of the city and 5DN just to the north.

[retailer]

While we were building a few years back we temporarily moved down south to Reynella around 1Km from the ABC transmitting tower, I could see it from my back porch, I had some trouble getting ADSL internet working, traced it to filters on the incoming phone line, once removed all was good - however you could listen to the ABC by picking up the land line handset.