Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives :: Books
Support This Site: Contributors :: Advertise


It is currently Sep Tue 25, 2018 2:25 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 105 posts ]  Go to page Previous  1, 2, 3, 4, 5, 6  Next
Author Message
 Post subject: Re: USSR 1969 RADIO PROJECT
PostPosted: Jun Mon 04, 2018 5:06 pm 
Member
User avatar

Joined: Jan Mon 22, 2018 5:59 pm
Posts: 280
This project still exists but I've just been a pretty busy working on my boats.
I have tested the transistors with my meter diode tester. I had to check the leads of course which differ for the RF and AF Soviet transistors. They all tested fine, except the voltage on the first Local Oscillator is not looking right. It should be much higher. The schematic gives reference points for the voltage tests.
Being able to pull transistors out of a socket is a good idea, I think.
I also suspect it may be a good idea to test the diodes. Actually there is a lot I can do but while the weather is good I'm forced to paint my boats and rig up a solar panel (while I'm at it).
Later at nights I still do what I can to swat up on transistor radios. When I look at the schematic to this radio (the VEF), I understand much better what I'm looking at. So, I will do my best to not lose this project and stay with it.


Top
 Profile  
 
 Post subject: Re: USSR 1969 RADIO PROJECT
PostPosted: Jun Sat 09, 2018 2:02 pm 
Member

Joined: Jul Wed 22, 2009 3:07 pm
Posts: 717
10.7Megahertz wrote:
...
I have tested the transistors with my meter diode tester. I had to check the leads of course which differ for the RF and AF Soviet transistors. They all tested fine, except the voltage on the first Local Oscillator is not looking right. It should be much higher. The schematic gives reference points for the voltage tests.
...
Glad to see you are still sticking with the project!

This is a schematics that shows the voltages on the transistors:
https://elektrotanya.com/orosz_ussr_vef-206_gelka_sm.pdf/download.html

There is a note (in Hungarian) on the schematics that states that the voltages on T1 (oscillator) and T4 (mixer) transistors are referenced to the collector of T2 (current generator) and not to the ground.
It is possible that this why your readings are different.

Regards, Peter


Top
 Profile  
 
 Post subject: Re: USSR 1969 RADIO PROJECT
PostPosted: Jun Sat 09, 2018 5:45 pm 
Member
User avatar

Joined: Jan Mon 22, 2018 5:59 pm
Posts: 280
Hello Peter. The schematic gives reference points for transistor voltages. Often these are busbar terminals. By the way feel free to enlighten me on T2 as I don't understand this stabiliser function
However, here is what I think I learned: It seems to me the radio uses a tickler oscillator. The drum switches in the various coil windings. Somewhere something is amiss as the RF section is doing nothing. I did have to change a broken capacitor adjacent to the diode and ought to test the diode too. Last time I worked on the set I think maybe I found a shorted resistor, in.that area. Do you think I should change the electrolytics and are these short lived?.I.always do.so with tube radios but these latter are fifties era.


Top
 Profile  
 
 Post subject: Re: USSR 1969 RADIO PROJECT
PostPosted: Jun Sat 09, 2018 6:10 pm 
Member
User avatar

Joined: Jan Mon 22, 2018 5:59 pm
Posts: 280
I noticed some points of interest - if I recall.one transistor collector was referenced to another transistor. Following the probe points for voltage tests T1 really should give around 3 volts if I recall. When I first physically removed transistors I was very wary of snapping one. However this turned out to be really easy and my diode meter tester indicated all components were sound. Of course I marked where the spots faced and the markings in fact differ between the RF and AF transistors. Guess I should test the diode, the resistors (that was hard going) and maybe new electrolytics. That is if the tree pollen doesn't kill me off. I've been sneezing 10 times in fits and starts.


Top
 Profile  
 
 Post subject: Re: USSR 1969 RADIO PROJECT
PostPosted: Jun Sun 10, 2018 2:29 pm 
Member

Joined: Jul Wed 22, 2009 3:07 pm
Posts: 717
Hi 10.7MHz,

As I mentioned earlier, you picked a hard to understand radio to get familiar with transistorized radio circuits!
None-the-less, these VEF radios are good performers, but they differ a bit from the usual circuits in other radios.

One peculiar circuit is the stabilizer stage built around T2.
This stage does not provide a stabilize voltage to the circuits, but a stabilized current.
The circuit of T2 is a current source, Wikipedia describes its operation pretty well:
https://en.wikipedia.org/wiki/Current_source#Zener_diode_current_source
The Zener diode in the radio is D1, according to my measurements its voltage is about 0.6V, so you can replace it with a generic small signal Si diode, like an 1N914 or similar, if it fails.
The current this circuits provides is about 1.5mA.
The other peculiarity is that the circuit is not "implemented to the ground", like the circuit on the Wikipedia page, but is sort of "hanging in the middle", providing the stabilized current at both ends.

"Above the T2 collector" is the oscillator stage. T1 is in series with T2 collector, the current goes through resistors R6 and R5 to the emitter of T1, the collector of T1 goes through R4, the oscillator coil, and through pin 11 of the band switch it goes to the filtered 9V supply.
(I am using the same schematics as I referenced in my previous post, other schematics might have different component designations.)

The mixer transistor is also "sitting on top" of the stabilizer circuit, the T4 emitter gets the supply current through resistor R20, again from the collector of T2, T4 collector goes to the filtered 9V supply through the first IF-coil, L31, and then through resistor R17.

On the bottom part of the stabilizer circuit is the bias to the RF-amplifier (T3) and for the IF-amplifier transistors (T5 and T6) are supplied.

The other peculiar circuit in that radio is the AGC regulation of the RF-amp stage. From DC point of view that transistor (T3) is in series with the AGC voltage of the IF-amp, the collector of T3 goes through R16 and R43 to the emitter of T5 (the first IF-amplifier) that changes its voltage according to the AGC regulation. The net effect of this is that at high input signals the collector voltage of the RF-amp is diminished. This sounds counter intuitive, you do not want small supply voltages at high input signals for distortion purposes, but it seems to work.

This is briefly how that stabilizer and the AGC regulation works. The rest of the circuits are pretty much standard.

I hope this is understandable!
Just keep reading the schematics repeatedly, till this makes sense ;-)

As for the oscillator not running, do pay attention to the pin-out of the various transistors, they are not he same, it is easy to mix them up!

In my experience only some of the small electrolytic caps were not up to standard from ESR-point of view, but I did check them with an ESR-meter.

Regards, Peter


Last edited by orbanp on Jun Sun 10, 2018 8:47 pm, edited 1 time in total.

Top
 Profile  
 
 Post subject: Re: USSR 1969 RADIO PROJECT
PostPosted: Jun Sun 10, 2018 5:06 pm 
Member
User avatar

Joined: Jan Mon 22, 2018 5:59 pm
Posts: 280
Cheers Peter, I will use your notes and feel I will get it working although I often stagger my applied work. You are correct this is in the deep end for me. Really it's wise to narrow down to "lines" such as AA5's or Japanese 7's but when I started I got all these radios randomly. Now I am stuck with two tricky challenges. The other is an East German pcb tube set with FM from 1964. Anyway on the transistor set I carefully traced the negative power line to the oscillator. One thing that puzzled me is often dc supply to transistors may be fed via a capacitor as in between chassis and lead. This I find in the RF part of the circuit. Anyway I think I should remove and test the detector diode as from experience I learned even that can stop a radio. Plus I suspect I found a suspect resistor reading zero
It is probably something simple but even so I learned new things and it's educational.


Top
 Profile  
 
 Post subject: Re: USSR 1969 RADIO PROJECT
PostPosted: Jun Tue 12, 2018 9:13 pm 
Member
User avatar

Joined: Jan Mon 22, 2018 5:59 pm
Posts: 280
R47 struck me as odd and it connects to the detector diode. It showed zero ohms so I desoldered it and tested again. All OK so I soldered it back. It was surface soldered so an easy check. Looking at its connection though I can't yet see why it should read zero across the track points. Plus there had been a detonated tiny capacitor right in that area. Really I need to spend a bit more time checking it all out. Lately I'm spread out very widely working on my boat and doing some HTML. So the radio work I will do at intervals. Of course it's better to focus more on less projects. Tonight I will study that diode part of the circuit. I ought to check as well for any shorts across tracks.


Top
 Profile  
 
 Post subject: Re: USSR 1969 RADIO PROJECT
PostPosted: Jun Wed 13, 2018 12:59 am 
Member
User avatar

Joined: Jan Mon 22, 2018 5:59 pm
Posts: 280
All checks out. The resistor seems to be bypassed via a winding which explains the resistance reading. Both diodes test OK. Transistors all tested fine. I was revising IF stages and will check the cans. Maybe someone had fiddled with the screws and ruined an IF can. I don't have a signal generator but am able to use another radio. Last time I did get a response but will experiment. For these more advanced sets I ought to have more diagnostic gear really. Had a break from radio work but feel a bit more in gear. Sometimes a break is good.


Top
 Profile  
 
 Post subject: Re: USSR 1969 RADIO PROJECT
PostPosted: Jun Wed 13, 2018 7:50 pm 
Member
User avatar

Joined: Jan Mon 22, 2018 5:59 pm
Posts: 280
Got my work cut out. I found a wire disconnected from the output transformer which had not been obvious to spot. It's polarity is positive. Another thing that is very obvious is the volume pot doesn't alter the volume. Yes, the switch works but turning the shaft makes no difference to volume. Wonder whether it's worth dismantling to clean.


Top
 Profile  
 
 Post subject: Re: USSR 1969 RADIO PROJECT
PostPosted: Jun Thu 14, 2018 12:31 am 
Member
User avatar

Joined: Jan Mon 22, 2018 5:59 pm
Posts: 280
Maybe the problem is clear now. The schematic gives details of reference points for voltage as I said earlier. I had found the collector of T1 to be well below voltage. Of course if you're new to these more modern radios, locating transistor pins or even the correct transistor takes a bit of time. What I hadn't quite grasped is the manual also provides the resistance you would get (beteen collector and ref point). Thus my low volts is now explained as ohms between collector and ref is hjge. It should only be on the lower side. If my readings are correct there is no chance of oscillation
So I have to find out where that 21 k resistance is coming from. The schematic is very clear and I found today the polarities for the tracks have clear digit markings. So, testing between chassis earth and + feed wire the ohms should be low. And between the two track polarity points I get the expected 9 volts. I just have to be careful I accurately select the correct transistor. T1 checked out spot on for base and emitter resistances but way out on collector.


Top
 Profile  
 
 Post subject: Re: USSR 1969 RADIO PROJECT
PostPosted: Jun Thu 14, 2018 5:18 pm 
Member

Joined: Jul Wed 22, 2009 3:07 pm
Posts: 717
It must be my ignorance, but I have never seen a schematics of that radio with resistance measurements marked! (I have three such radios, the 204 - 206, and schematics/documentations collected from many sources.)
The voltage values should be perfectly enough for debugging purposes!
Not to mention, I would not trust a resistance measurement in a "cold" circuit, it could depend on too many things!
So as not to dig the hole any deeper for you than necessary, just measure the voltages!

Also, before you take apart that volume control pot, do measure its value and how does it change with a DMM!

Peter


Top
 Profile  
 
 Post subject: Re: USSR 1969 RADIO PROJECT
PostPosted: Jun Thu 14, 2018 11:45 pm 
Member
User avatar

Joined: Jan Mon 22, 2018 5:59 pm
Posts: 280
I have the original schematic in Russian. This is very detailed and includes a written account of the entire radio. It took me a while to get used to the layout. Around the board are numbers and these fortunately are included in a sheet. So if the diagram shows transistor pins it will give the numerical ref point for voltage measurement. Not only that but presumably resistance expected. If I understood correctly I should get just under 3 volts collector voltage between collector and board ref point. Resistance between those points should be 1.5 k. And that makes sense as it gives about 10 milliamps current. I got less than volt collector and some 20 k resistance. However base and emitter ohms measurements matched. As I said hopefully my interpretation is accurate going on the board and the diagrams. The written documents also tell you what resistors specifically to check. All in all I hope I have made sense of it - the info comes from Russian radio site with the diagrams. Some stuff never got translated.


Top
 Profile  
 
 Post subject: Re: USSR 1969 RADIO PROJECT
PostPosted: Jun Thu 14, 2018 11:54 pm 
Member
User avatar

Joined: Jan Mon 22, 2018 5:59 pm
Posts: 280
Also Russian may be a language worth considering. Electronics and software at the moment is pretty developed in Russia. I just joined Ukoz which is a website builder to suit most levels. I have to try and juggle the site building with any radio work I squeeze in. Not only that but the USSR produced some awesome tube radios in solid timber finish.


Top
 Profile  
 
 Post subject: Re: USSR 1969 RADIO PROJECT
PostPosted: Jun Fri 15, 2018 5:41 pm 
Member
User avatar

Joined: Jan Mon 22, 2018 5:59 pm
Posts: 280
orbanp wrote:
It must be my ignorance, but I have never seen a schematics of that radio with resistance measurements marked! (I have three such radios, the 204 - 206, and schematics/documentations collected from many sources.)
The voltage values should be perfectly enough for debugging purposes!
Not to mention, I would not trust a resistance measurement in a "cold" circuit, it could depend on too many things!
So as not to dig the hole any deeper for you than necessary, just measure the voltages!

Also, before you take apart that volume control pot, do measure its value and how does it change with a DMM!

Peter

Peter, here is a link for you:
https://kot-ivan76.livejournal.com/307493.html

If you scroll down you will see a table that shows the transistors and quite a few digits. You will also see there are some very useful images of how the pcb board actually looks. I must be honest and say I couldn't make at all figure out that transistor table. Really it was a matter of time for it to clear up. All in all, the design is very good as these tables match the numerical system on the board and this would have allowed a Soviet radio repairman (of the era) to hopefully trace faults.
You will see the digits 7 - 1 horizontal and also +8 - 13 vertical. These are the reference sources. To check my way around I tried simply measuring resistance between the + feed and the ground solder point and so on.
There are, of course, possibilities of getting things wrong. I have to try and be sure I am locating the correct transistor and that I get the pin layout correct. I am not really all that experienced with transistor circuits at all so I just hope to learn as I go along.
I don't mind translating this stuff is anyone feels a need for it although Google Translate works very well with Russian.
Very busy at the moment learning very basic HTML.


Top
 Profile  
 
 Post subject: Re: USSR 1969 RADIO PROJECT
PostPosted: Jun Fri 15, 2018 9:07 pm 
Member
User avatar

Joined: Jan Mon 22, 2018 5:59 pm
Posts: 280
The manual says for T1 check R7. To do this required some effort. First you need to locate the pins of T1 on the track side of the board. Then locate the various resistors which are tiny green components. To try and be sure I tested about 6 resistors around T1. In.the end I concluded R7 is way way below spec. I guess I can safely say I should change R7. It only read about 2 k and should be around 7k. The other resistors seemed to check out. I recall trying to find information on those tiny green USSR resistors but really can't say how good or bad they are. To do a neat replacement I could do with a similar sized component in dimension. Maybe I can find on a scrap board. Other than that there may well be other faults yet to be found.


Top
 Profile  
 
 Post subject: Re: USSR 1969 RADIO PROJECT
PostPosted: Jun Sat 16, 2018 12:55 am 
Member
User avatar

Joined: Jan Mon 22, 2018 5:59 pm
Posts: 280
Small correction. I think sometimes resistor readings can be suspect unless you remove at least one lead. As is known, other resistances may wind up in parallel. So R7 (although I located it would likely test as specified if I remove it). Never mind! At least it's good practice locating and testing parts of the circuit.


Top
 Profile  
 
 Post subject: Re: USSR 1969 RADIO PROJECT
PostPosted: Jun Sat 16, 2018 11:24 pm 
Member
User avatar

Joined: Jan Mon 22, 2018 5:59 pm
Posts: 280
Spent time today on basic stuff. Such as transistor layout. The transistors do take some effort to pinpoint I think. I wondered why they didn't inscribe T2, T2 and so on to save legwork. In the end I
was reasonably confident I am now testing the right transistors. I redid voltage tests. T1 the oscillator is definitely not right. I tried this time testing all leads of T1 against plain pcb positive ground. This way yields fairly high volts on all leads! Base was identical to one of the other leads. Same volts. Against ground you would expect some voltage on the collector and tinyvolts on emitter. Most of the other transistors voltage is more in line with the ones specified. I should add trying to deduce which socket solder blob is collector or emitter is a challenge.


Top
 Profile  
 
 Post subject: Re: USSR 1969 RADIO PROJECT
PostPosted: Jun Sun 17, 2018 6:49 am 
Member
User avatar

Joined: Mar Mon 17, 2008 5:05 am
Posts: 5363
Location: Ashhurst, New Zealand
Perhaps when you try to measure the voltages, your meter stops the circuit oscillating and so changes the bias conditions.

_________________
Cheers - Martin ZL2MC


Top
 Profile  
 
 Post subject: Re: USSR 1969 RADIO PROJECT
PostPosted: Jun Sun 17, 2018 3:04 pm 
Member
User avatar

Joined: Jan Mon 22, 2018 5:59 pm
Posts: 280
Doing the test with + probe on board ground which is positive indicates 4 odd positive volts between + chassis and collector / base. About 4 volts on emitter as well. Of course yesterday I figured I needed to make sure I am testing the right components so I went over it from the start. Now at this stage I am a bit clueless as to what's going on but here is what sprung to mind: My best guess could be leaky capacitors? Should one of these leak then DC voltage can creep around the circuit. From tube radios I know leaks of + volts on grids can be a known problem. I can perhaps study the schematic to see which caps might leak and swamp T1. I already checked for a short in the socket. Today also I may try again to inject an RF signal into the IF circuit. At this point too I can't say whether these sets fall short of western standards or not.


Top
 Profile  
 
 Post subject: Re: USSR 1969 RADIO PROJECT
PostPosted: Jun Sun 17, 2018 10:06 pm 
Member
User avatar

Joined: Mar Mon 17, 2008 5:05 am
Posts: 5363
Location: Ashhurst, New Zealand
Is the oscillator the same circuit as my VEF10?


Attachments:
Oscillator.jpg
Oscillator.jpg [ 34.67 KiB | Viewed 317 times ]

_________________
Cheers - Martin ZL2MC
Top
 Profile  
 
Post New Topic Post Reply  [ 105 posts ]  Moderator: Dave Doughty Go to page Previous  1, 2, 3, 4, 5, 6  Next

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 5 guests



Search for:
Jump to:  
























Privacy Policy :: Powered by phpBB