Tech TE-20 Sig Gen

[stitch1]

Hello,
I was going to do some radio alignment I got out my TE-20 (which I've used before) and unfortunately it's not working.

All the freq bands read 408Hz with the exception of range D which reads 230kHz to 1.7MHz but should be 3.2 to 11Mhz.

Reading other threads I suspect the oscillator V1 12BH7. I have checked all the resistors but not changed any caps, readings on V1 are:

1. 60V
2. -2.3
3. 0
4. 0
5. 0
6. 89V
7. -3
8. 4
9. 6VAC

This assumes I've got the pin numbers correct. The link below is for the valve data and I've attached the schematic.
http://www.r-type.org/exhib/aai0042.htm
Is it worth changing any capacitors or is it safe to assume V1 is faulty?

Any advice would be appreciated.

Thanks

John

[6AL5W-Martin]

TE20 is allready for service at old radios, it covers all frequencys.
Check please the band selektion switch if there is anything wrong or oxydated.

About your voltages: I have also a TE20 here, we can do a compare.

Martin

[stitch1]

Thanks Martin, it’s ideal for my occasion use and compact too. Yes I have cleaned the switch but no difference.

Unfortunately the valve is a bit pricey otherwise I’d order one so it would be uesful to compare the voltages, but only if it’s not too much trouble.

John

[Silicon]

Is the HT rectifier a Selenium diode?

These degrade over time and the HT voltage falls. I don't know what the correct HT should be.

If it is Selenium, try replacing it with a 1N4007 or similar.

I assume that you are getting the '400Hz' audio signal at the RF output socket on most ranges because the RF oscillator is not oscillating.

[stitch1]

No it is a silicon diode, I don't know what the HT should be either but I'm getting 129V on the cathode and 89V HT.

John

[Bazz4CQJ]

According to the link you gave in post#1, it looks very much as though an ECC81 (12AT7) might well work in that circuit; are they any cheaper than the 12BH7? They are common enough that you may be able to borrow one to try.

B

[tri-comp]

The ECC81 has a much higher amplification-factor.

ECC82 or 12AU7 is almost a spot-on substitute for a 12BH7 in a simple circuit like this.
The 12BH7 has a little lower Ra and a higher anode dissipation than the ECC81 but it's not too far off
The pin-out is the same and I would say go try it out.
As I too have a TE-20 I could possibly confirm this substitution will actually work as I have several ECC82 on hand.
Too late tonight, though ....

EDIT:
Just remembered I already did that.
Look here: https://www.vintage-radio.net/forum/...=121269&page=2

[stitch1]

Thanks Bazz and tri-comp for your suggestions, looking at the TE-20D thread it looks like the ECC82 it will do the job. It's is still a popular guitar amp valve and I don't have one in a radio or a spare here but I've seen a NOS tested one for £8 this is the best price I can see so I'll get one ordered and hopefully provide a positive update soon.

John

[David G4EBT]

Originally Posted by stitch1

All the freq bands read 408Hz with the exception of range D which reads 230kHz to 1.7MHz but should be 3.2 to 11Mhz.

408 Hz will be the AF oscillator also used for internal modulation of the RF signals. If you have the RF output set to 'Internal Modulation', in the absence of any RF, you'll see the AF at the RF output socket. If you switch to external modulation and see nothing at the RF output socket, that confirms no RF is getting there.

Changing the RF valve is the quickest and easiest thing to do, so good luck with that. Once sorted, the TE20D is perfectly adequate for hobbyist purposes, and given that they're several decades old now, we can expect a few problems. The circuit looks simple, but the internals are compact, and a bit of a rat's nest.

I've yet to hear from anyone who has confirmed that their TE20D works on Band 'F' but few of us want a genny that goes above 30 MHz,

Good luck with it - several thread on the forum about it, and on Internet in general.

[stitch1]

Hello David,
I've just tried the test you mention and switching to 'EXT' does result in no output.

My genny is just the TE-20 (not the D) https://www.pa3esy.nl/meetinstrument...TE-20_set.html as you say ideal for the hobbyist and importantly for me small, much as I'd like a Marconi I used in the RAF it would be over the top and bigger than my work bench!

I have an ECC82 on the way so hopefully back in action soon.

John

[tri-comp]

Originally Posted by David G4EBT

I've yet to hear from anyone who has confimed that their TE20D works on Band 'F' but few of us want a genny that goes above 30 MHz.

Did you look inside a TE-20 ?
You'll see different oscillator coils for the various bands except for the 'F'-setting.
Here the coils are replaced with just two jumper-wires prohibiting the oscillator to work.
What SHOULD have been there I guess no-one knows (yet) but it should be a simple matter of calculating values and do experiments to get the 'F'-setting up and running.

[tri-comp]

Originally Posted by tri-comp

The ECC81 has a much higher amplification-factor.

ECC82 or 12AU7 is almost a spot-on substitute for a 12BH7 in a simple circuit like this.
The 12BH7 has a little lower Ra and a higher anode dissipation than the ECC81 but it's not too far off
The pin-out is the same and I would say go try it out.
As I too have a TE-20 I could possibly confirm this substitution will actually work as I have several ECC82 on hand.
Too late tonight, though ....

EDIT:
Just remembered I already did that.
Look here: https://www.vintage-radio.net/forum/...=121269&page=2

I should of course have said ECC82 and NOT ECC81.
It's the '82 that's a near-equivalent to 12BH7 as stated in the link, but it looks like you've got it right.
Sorry about the confusion

[stitch1]

Not exactly snowed in but better to stay in today so I finally found some time to look at this again.
The good news is the new ECC82 arrived within 24hrs, fantastic service from Langrex (via eBay).
The bad news is it made no difference, so back to the drawing board.

I had a look around the capacitors and found one that was giving different readings from the others of the same type (circled in red on the attached diagram) so with some difficulty I changed it, the value on the diagram is 1500pf but its actually 2000pf so I replaced with a 2.2nf, other capacitor values are also different to the diagram, good news is I now have oscillation, but the frequency is quite a bit out at one end of the dial but the error decreases with the frequency.

I decided to dig out the rarely used scope I have and look at the waveform, and to me it looked un-modulated, am I correct?
I removed V2 and there was no difference in the waveform, but the frequency readings are spot on (I'm using a DMM to read the freq').

Maybe that faulty capacitor was a red herring or I've got 2 faults.

So I looked for a new 6AR5 but these seem very hard to get, some overseas and not expensive but nothing at Langrex or elsewhere on eBay and there don't appear to be a sensible equivalent, but I thought that about the 12BH7, does anyone know if there is an equivalent?

I could change some more caps but I'm reluctant to do it without a good reason not least because it's just like new ... I may make some temporary replacements, any thoughts to likely culprits?

John

[newlite4]

I have worked on one of these recently. The 6AR5 is the oscillator that provides the 400Hz internal modulation signal. Since the 400Hz is present then this valve is OK. Possibly replacing the capacitor has just knocked out the tracking slightly, there may be some padders around the band switch to compensate for this replacement. The waveform looks right for one of these - a bit top heavy, but does the job.
Hope this helps, Neil

[David G4EBT]

Given that test gear gets so little use, I think it's rarely going to be a defective valve, but it's the easiest option to try if one is to hand. If not, it becomes inconvenient, and not a little expensive.

In your first post John, you were getting 408Hz on all bands except band 'D', which suggests that the 6AR5 audio oscillator used for internal modulation and as an AF output source was working, but not the RF stage, so all you were getting at the RF output socket was the AF output with no RF component. At that stage, the indications were that the AF oscillator was working, so I'd be inclined to check the waveform at the AF output socket to check that, where you should get a nice clean sine wave if all is well with the AF oscillator section.

Re the waveform in your pic, yes, it's unmodulated, but it should be a nice sine wave, which it isn't. If the switch is set for the waveform to be modulated, then either the AF oscillator has stopped working, or the RF stage is malfunctioning.

Below, I've attached some pics of a working TE20(D) I'd repaired back in 2015.

The first pic shows the unmodulated RF output.
The second pic shows the AF output (which happened to be 506 Hz - the actual frequency is unimportant, so yours at 408 Hz is fine).
The third pic shows the modulated RF output.

I think as a first step, I'd be inclined to check some voltages, as follows:

Is there 120V or so AC into the rectifier?
If yes, it there 150 - 160V DC output from the rectifier. If not, the rectifier is suspect.
If there's 150 - 160V output from the rectifier, is there approx 50V at the anode (pin 5) of the 6AR5?
Is there approx. 115 - 120V at pin 5 of V1?

How do the actual values of the resistors compare to the circuit (ie, within +/- 10%)?

A couple of caps that I'd be inclined to check are the 50pF one that goes to pin 2 of V1, and the 0.5µF that's connected to the 100k/30k potential divider on pin 7 of V1.

(I think the 5pF cap is a 'gimmick cap - a couple of bits of PVC coated wire twisted together).

I hope these thoughts might help a bit.

Good luck with it.

[stitch1]

Thanks Neil and David for your responses, reasuring and ideas for more checks.

I will do some more testing tomorrow (Sunday)

Thanks

John

[tri-comp]

Originally Posted by stitch1

So I looked for a new 6AR5 but these seem very hard to get, some overseas and not expensive but nothing at Langrex or elsewhere on eBay and there don't appear to be a sensible equivalent, but I thought that about the 12BH7, does anyone know if there is an equivalent?
John

A sensible equivalent would be the 6AQ5 / EL90 / 6005.
Same pin-out, slightly more filament current 450mA vs. 400mA, a little lower Ra and somewhat beefier with 12W Pa vs. 8.5W.
I realize it's not the 6AR5 that's causing the trouble with your TE-20 as it seems to actually be the only thing that works, so this is just fyi.

/Torben

[6AL5W-Martin]

have a nice sunday.
I will open now my TE20 and give you assistance to repair yours.
First results in some minutes.
greetings from germany
Martin

[6AL5W-Martin]

self test of the TE20, using Systron Donner Counter 6053, Range A

Settings: the Modulator must be set to EXTERN ! If there is modulation the frequency counter will display wrong results
HF set full clockwise. Output plug red upside and ground (black)

Range A, lo =120kHz and hi 300kHz = OK

no counter? use a radio, set 200kHz and search this on long wave.

[6AL5W-Martin]

Testing Band.

B: 350 = 3570 // 1000kHz = 1048
c: 1000 = 1018 // 3000 = 3069
D: 3500 = 3596 // 11000 = 1112
E: 11000 = 11600 // 35MHz = 34,93
F: 40MHz = 35,6 // 60MHz = 52,4 // 100MHz = 86,9

Result of the self check: Band A, B, C, D, E may be ok, Band F needs urgend adjust.

I found always a low modulation ripple, on scope. (TE20 is no HP or Racal)

To be continued with Voltages inside.