UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Other Discussions > Homebrew Equipment

Notices

Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members.

Closed Thread
 
Thread Tools
Old 3rd Jan 2017, 5:29 pm   #21
Neil Purling
Octode
 
Join Date: Jul 2005
Location: Hull, East Yorkshire, UK.
Posts: 1,887
Default Re: Two or three IF stage Chinese radio kits.

How close a match do I need? I have a bunch of 2N3904 I could use for VT5,VT6.
Has anyone got any recommendations for the phase splitter? Eagle LT44 clone?
Neil Purling is offline  
Old 3rd Jan 2017, 5:54 pm   #22
G6Tanuki
Dekatron
 
G6Tanuki's Avatar
 
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 6,382
Default Re: Two or three IF stage Chinese radio kits.

For the phase-splitter you can't use a LT44-type driver transformer as that only has a single centre-tapped winding which won't work with a "totem pole" no-output-transformer stage.

Going to the "classic" push-pull ttransistor amp using LT44 and LT700 transformers would be better - at least that way each transistor has the full 3V of battery-voltage to work with.

[I've always had a weakness for the little 2-transformer push-pull transistor output stage; I built a lot of these as a kid - initially the little few-hundred-milliwatts versions using the likes of OC72 or OC81 transistors, then moving up to bigger stuff using OC35s in parallel to get 25 Watts or so...]
G6Tanuki is offline  
Old 3rd Jan 2017, 10:47 pm   #23
Neil Purling
Octode
 
Join Date: Jul 2005
Location: Hull, East Yorkshire, UK.
Posts: 1,887
Default Re: Two or three IF stage Chinese radio kits.

I can detect the audio with an ordinary diode, instead of that weird infinite impedance jobbie.
I obviously need an alternate audio stage.
Q. Does the circuit diagram provided back up the thread possess any sort of AGC?
Neil Purling is offline  
Old 3rd Jan 2017, 11:01 pm   #24
Herald1360
Dekatron
 
Herald1360's Avatar
 
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 12,331
Default Re: Two or three IF stage Chinese radio kits.

Yes. The falling dc level at the detector collector from strong signals biasses back the IF stage as well as the detector. It's a bit crude, but it's there.
__________________
....__________
....|____||__|__\_____
.=.| _---\__|__|_---_|.
.........O..Chris....O
Herald1360 is offline  
Old 4th Jan 2017, 1:23 pm   #25
G8HQP Dave
Dekatron
 
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,443
Default Re: Two or three IF stage Chinese radio kits.

A well-engineered infinite impedance detector should be better than a diode. This one is not well-engineered. However, you might not be able to simply replace it with a diode as the IFT secondary might have quite high impedance. You could perhaps try turning it into a emitter follower after a high impedance diode detector.
G8HQP Dave is offline  
Old 4th Jan 2017, 2:42 pm   #26
orbanp1
Hexode
 
Join Date: Nov 2009
Location: London, Ontario, Canada
Posts: 359
Default Re: Two or three IF stage Chinese radio kits.

Here is an interesting detector circuit that gives quite a bit of output:
http://www.edn.com/design/analog/444...n-simple-diode
Note the "difficulty" with the AGC control voltage for a transistorized IF-amp.
As a simpler solution you could keep a "regular" diode detector in parallel for the AGC.
DISCLAIMER: I have not built it, just simulated it!

Peter
orbanp1 is offline  
Old 4th Dec 2017, 2:30 pm   #27
Argus25
Octode
 
Argus25's Avatar
 
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 1,657
Default Re: Two or three IF stage Chinese radio kits.

I think the type of detector that shows a transistor as the detector as in post 5 is such that the transistor's base-emitter junction is being used as the detector diode. However, since current is sourced from the collector at about a 100:1 higher ratio to the B-E current (depending on the transistor's hfe) it simply raises the input impedance and lowers the output impedance of the detector. This might or might not be helpful though, depending on the design & bandwidth of the final IF transformer. At least the circuit in post 5 makes some use of the current in the collector circuit and that looks clever. Some of the transistors appear short on emitter stabilization.

For the circuit described here in post 26:

http://www.edn.com/design/analog/444...n-simple-diode

The diode is not really the detector and I think the diode could be removed (shorted out), the detection is happening at the B-E junction of the transistor, it is essentially the same circuit.

The benefit conferred by the series diode though is that if the drive voltage gets over about 7V to 8V peak, the diode prevents the transistor's B-E junction from zenering. The transistor has no base tie resistor which is never a good idea as the collector-base leakage is very variable.

Also, the method they used to help small signal recovery is not ideal; forward biasing the diode and the transistor's B-E junction from the power supply via a 1M resistor.

The better way to bias a detector diode is to establish a voltage reference, using another semiconductor junction with the same voltage drop and temperature coefficient as the detector diode.

This was already done by Sony in 1956. They also had perfected their IF transformers so that the detector loading was correct with respect to bandwidth considerations. I have attached the circuit (note NPN germaniums)

If you study the circuit carefully (the return point for the volume control and final IF output coil) you will see that Sony chose the base-emitter junction voltage of one of the IF transistors ( X2 ) to be the voltage reference to correctly set up the detector diode's DC conditions with a forward bias getting it to the threshold of conduction for low level signals. Not only that the AGC voltage developed with signal reduces the base current in X2. They also have additional agc diode for high level signal control.

X2's B-E voltage is also relatively independent of the battery voltage and this B-E junction is practically identical to the detector diode so the temperature tracking matches perfectly.

The circuit basically turns Sony's detector into a near zero voltage drop detector for low level signals out of their IF helping to eliminate square law distortion, but it doesn't diminish its large signal handling capacity either. In other words, Sony totally aced the detector/agc circuit in 1956.
Attached Thumbnails
Click image for larger version

Name:	sony.jpg
Views:	79
Size:	62.8 KB
ID:	153341  

Last edited by Argus25; 4th Dec 2017 at 2:36 pm. Reason: typo
Argus25 is offline  
Old 4th Dec 2017, 3:17 pm   #28
Neil Purling
Octode
 
Join Date: Jul 2005
Location: Hull, East Yorkshire, UK.
Posts: 1,887
Default Re: Two or three IF stage Chinese radio kits.

I did find that the signal at the detector was clean.
I have managed to test a bagful of 2N3904 and get two that were close.
I am building another of the 6 transistor radios. I will be using the trimmer in place of R4 & those matched transistors as VT5 & VT6.
Are there any other modifications I might incorporate to improve the audio stage?
I think that those speakers with the very low-profile plastic frame & disc-like cone are pretty poor.
Has anyone made one of these radios with a substitute 70 mm speaker?

For Winter construction I have two different 3-stage radios on the way: A HX108-2 and BS618. Both have push-pull outputs with two transformers. I will probably have questions when I get to the HX108-2 because the diagram seems to have at least one mistake.

Last edited by Neil Purling; 4th Dec 2017 at 3:26 pm.
Neil Purling is offline  
Old 4th Dec 2017, 5:34 pm   #29
radioman
Hexode
 
radioman's Avatar
 
Join Date: Apr 2003
Location: Ayr, Ayrshire, UK.
Posts: 398
Default Re: Two or three IF stage Chinese radio kits.

I built an HX108-2 a couple of years ago.
I scanned the instruction sheet and updated the overlay diagram with come coloured dots to make sure I put the IF transformers in the correct place as the supplied instructions were in Chinese and therefore difficult for me to read..
I've attached it below and hope it's of some help to Neil and other constructors.
I've had to replace the main tuning capacitor since I built it as tuning became 'noisy' presumably due to poor internal contacts.
I made a cradle out of cardboard plus added some foam to stop the batteries rattling about but once set up it works surprisingly well.

Andy
Attached Thumbnails
Click image for larger version

Name:	Seven_radio.jpg
Views:	75
Size:	142.8 KB
ID:	153362  
radioman is offline  
Old 4th Dec 2017, 5:51 pm   #30
Neil Purling
Octode
 
Join Date: Jul 2005
Location: Hull, East Yorkshire, UK.
Posts: 1,887
Default Re: Two or three IF stage Chinese radio kits.

Apart from the tuning capacitor did you make any other modifications?
I am just wondering if you found the sound to be decent.
The big dislike with these radios is keeping the batteries in. They rock about on the edges of the speaker frame.
I use wadded t.p.
There is a version of the HX6 that has the batteries to the side of the speaker, which is the one I have yet to build & maybe modify.
Would it matter if you used better capacitors?

Last edited by Neil Purling; 4th Dec 2017 at 6:01 pm.
Neil Purling is offline  
Old 5th Dec 2017, 1:02 pm   #31
radioman
Hexode
 
radioman's Avatar
 
Join Date: Apr 2003
Location: Ayr, Ayrshire, UK.
Posts: 398
Default Re: Two or three IF stage Chinese radio kits.

I didn't make any major changes. I did make R1=100k (as per the circuit diagram) and R14=220k. R14 damps the primary of the first IFT.
These changes are shown on my updated overlay picture.
You'll never get a hi-fi sound quality from this radio !
The speaker is very small and so is the output transformer, there is no negative feedback in the audio amplifier and I doubt if it outputs any audio frequencies below 300Hz. I didn't change any of the fixed-value capacitors and I don't think that changing any of them would make a difference.
I thought I might have to modify it before I actually built it but it's performance exceeded my expectations (which weren't that high).
It is what it is.
If you accept these limitations I'm sure you'll be quite happy with it.
Good luck with the build and don't forget to bridge the four pads on the PCB which I think are there for test purposes.

Andy
radioman is offline  
Old 5th Dec 2017, 2:25 pm   #32
Neil Purling
Octode
 
Join Date: Jul 2005
Location: Hull, East Yorkshire, UK.
Posts: 1,887
Default Re: Two or three IF stage Chinese radio kits.

Having just looked at the sellers of the six transistor, one IF kits & one is definitely saying that: "R4 can change gain of the medium frequency amplifier.(V3)".
They also give V1 & V2 as 9018F & V3 as 9018G. I take it that the suffix denotes a particular band of hfe value.
Neil Purling is offline  
Old 5th Dec 2017, 8:11 pm   #33
Neil Purling
Octode
 
Join Date: Jul 2005
Location: Hull, East Yorkshire, UK.
Posts: 1,887
Default Re: Two or three IF stage Chinese radio kits.

My previous post was referring to the HX-6B, the S66D and the 9018-2. They are all the same circuit. The diagram you get with the HX-6B only gives 0.5 mA as the current one should see at measurement point 'B'. I was seeing more than that, but that break is related to V2. Nothing to do with V3 & my experiments altering R4.
The given 30K for R4 would give maximum signal? I don't quite understand what is going on there. I did have a 100 K trim pot as R4 on another board. I measured the resistance after extracting it & found it to be 89K. The output of the detector was not that strong.
I assume that one would adjust R4 on the basis of noting any distortion if the o/p to the speaker were 'scoped. The adjustment of R4 would change the strength of the AGC level and thus what signal is being amplified.
Neil Purling is offline  
Old 8th Dec 2017, 11:25 am   #34
Neil Purling
Octode
 
Join Date: Jul 2005
Location: Hull, East Yorkshire, UK.
Posts: 1,887
Default Re: Two or three IF stage Chinese radio kits.

I have finished a HX-6B & all I have is audio 'hiss'. Oh

I get no heterodyne whistle if I place the radio on top of another & tune up & down the band. You expect to get a whistle as the local osc of one beats with the other at a particular point. I must investigate the situation with a sig gen.
I have put the OSC & IFT cores at half way & will start from scratch.

Has anyone found that the cores of all the inductances were so far out they would not pass any signal & the set was otherwise silent apart from audio stage hiss.
Neil Purling is offline  
Old 8th Dec 2017, 3:15 pm   #35
ukcol
Dekatron
 
ukcol's Avatar
 
Join Date: May 2003
Location: Harlaxton, Lincolnshire, UK.
Posts: 3,851
Default Re: Two or three IF stage Chinese radio kits.

I wonder how long it will be before the volume control goes noisy? D.C. on a volume pot is always a bad idea.
Attached Thumbnails
Click image for larger version

Name:	VC.jpg
Views:	35
Size:	34.8 KB
ID:	153622  
__________________
Best Regards,
Colin McLaughlin
BVWS Member
ukcol is offline  
Old 8th Dec 2017, 3:51 pm   #36
Neil Purling
Octode
 
Join Date: Jul 2005
Location: Hull, East Yorkshire, UK.
Posts: 1,887
Default Re: Two or three IF stage Chinese radio kits.

I think that we can agree that the one IF stage/six transistor radio is a very crude creature. But what do we expect for six pounds or less, shipped to your door?

I think that the fact that people have found the White & black IFT's are not often set near to the proper frequency would immediately discourage the inexperience constructor.
You switch on & you don't get radio, it seems silent. If the cores were 'way out'. Can you do anything 'by ear'?

I saw Shango066 doing one on the 'Tube. He has a tasty HP signal generator, it has a built-in digital frequency readout. The point is the IF seems to posses two peaks, or else the stage can still tune out to over 500Khz. He had it all reasonably aligned by ear only to discover the IF was over 500Khz.
So he had to use the generator and do the tracking all over again.
Where the gentleman is (Los Angeles) there are stations all over the band, wheras for my area the only station at the low frequency end is R. Caroline. Alignment by ear is not an option for best results.

I don't even think that the Chinese printed 'instructions' has the actual IF frequency printed on it. I think that Shango066 was guessing it was about 460Khz. IF in a AM radio is usually 455-470 Khz.
Neil Purling is offline  
Old 8th Dec 2017, 3:57 pm   #37
ParcGwyn
Hexode
 
ParcGwyn's Avatar
 
Join Date: Jun 2005
Location: Aberystwyth, Wales, UK.
Posts: 274
Default Re: Two or three IF stage Chinese radio kits.

Quote:
I wonder how long it will be before the volume control goes noisy? D.C. on a volume pot is always a bad idea.
Shouldn't be too bad as no DC flows through the wiper.

Dave

GW7ONS
ParcGwyn is offline  
Old 8th Dec 2017, 5:45 pm   #38
Neil Purling
Octode
 
Join Date: Jul 2005
Location: Hull, East Yorkshire, UK.
Posts: 1,887
Default Re: Two or three IF stage Chinese radio kits.

How many volts ought there to be on the collector of V3?
There is electrical continuity through T4's secondary?
It is doing my head in and I want my tea.
Advice anyone?
Neil Purling is offline  
Old 10th Dec 2017, 9:40 pm   #39
Neil Purling
Octode
 
Join Date: Jul 2005
Location: Hull, East Yorkshire, UK.
Posts: 1,887
Default Re: Two or three IF stage Chinese radio kits.

I got the little sod working after finding the print had lifted on V2 collector & I had the wires on the ant coil mixed. It was gratifying to discover that setting the cores mid-way produced audio, even though T3 & T4 were off the peak. The IF bandwidth seems quite broad.

Then I found that the damn thing would go off again at the slightest touch. I don't know whether to waste any more time on it or build the other one. I don't know whether there is any more lifted print around V2, or elsewhere. These printed circuit boards have thin tracks or they lift easily.
Neil Purling is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 2:36 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2018, vBulletin Solutions, Inc.
Copyright ©2002 - 2018, Paul Stenning.