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Old 21st Jul 2019, 2:29 pm   #1
Wendymott
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Default Levell TG200DM

Hi Peeps... I recently bought one of the above items, which turned out to be faulty...
As I had another project on.. I left it in the cupboard till "t" other day..

"T" other project has stalled, thus I decided to check out the TG200, I applied +/- 18V to the appropriate points and Zilch, "0" ,Nuffink out. After opening the item and rejigging the psu to allow access.............. what a NIGHTMARE. Who ever designed the pcb was not expecting some one to service it..still it was umpteen years ago.
However reading the previous post by Goodwoody in 2009... and subsequent comments... it was NOT going to be easy.. particularly when there are components shown on the schematic that are not on the PCB.
Basically all voltages are either at +18V or - 18V... meaning that the 0 line is suspect, but no... integrity is ok... I checked a few devices but seem ok..the items missing are two diodes a 4.7V zener and a Si probably a 4148 in series, that seem to offer bias to Q 10... these are shown on a pcb layout but have never been fitted...the +18V is drawing 50 m;/a approx, where as the -18V is drawing 10 m/A...... the service info.. infers that each rail should be 16 m/A.................... Disconnecting various points did not alter the situation. Unless some one has a magic bullet... it going to be a donor,
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Old 22nd Jul 2019, 12:35 pm   #2
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Default Re: Levell TG200DM

Hi!

I have started work on a new clearer circuit–diagram for this that will show the switching operations more clearly!

The oscillator is of the State–variable RC Feedback type (It's NOT a Wien Bridge because there is only one RC frequency–determining network!) where the output from the second amplifier section is fed back as positive feedback, and the third amplifier stage provides a negative feedback loop via the RA34 bead thermistor.

The 4V7 zener and the 1N4148 for the bias to J10 is shown on the diagram, it is connected from the collector of the NPN transistor J1 & emitter of the upside-down PNP transistor J2 to chassis, and my diagram in preparation will annotate these components more clearly! The 5.5V nominal bias at J1C/J2E is then fed to J10B via a close-tolerance 6k98 1% resistor.

The best way to deal with this instrument, because of the large number (20!) of direct-coupled transistors used, is to print off, on A3 paper, a large size copy of the PCB layout drawing from the last page of the manual, (in Acrobat Reader, click on "Take A Snapshot" from the "Edit" menu, draw a box round the PCB layout drawing, click "Print" - "Selected Graphic" should already be shosen for you, and under "Scale" on the Print Dialogue box, select "Fit"), then remove and test each transistor at a time, I recommend you obtain one of those little Chinese colour-screen LCR T1 testers for this, and as you check each transistor, refit it where it came from and highlight it with a highlighter pin to indicate it's been done!

If you find no obvious faults with the diodes and/or transistors, please let us know, and myself or another Member'll be glad to help you go thro' the feedback switching!

Chris Williams
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Old 22nd Jul 2019, 1:26 pm   #3
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Default Re: Levell TG200DM

Hi!

If you need to replace any of the transistors (the originals were an obsolete type of Motorola small–signal silicon device), use BC547C for the NPN and BC557C for the PNP ones – I recommend getting a cheap packet (about 20 each) from the cheap online sources if you can!

Finally a good bench supply with two channels is very useful for these generators, but if you don't have one, search for "LM317 LM337 Regulated PSU" – a small module providing positive and negative supplies that can be adjusted to provide the ±16.5V Levell suggests for testing this unit on the bench is only about a fiver!

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Old 22nd Jul 2019, 4:00 pm   #4
Wendymott
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Default Re: Levell TG200DM

Hi Chris. I have the schematic from the original posting in 2009...its a pity that Levell didnt use conventional component numbering at that time... I have a dual tracking psu and a lot of both BC types required..I am going to add the zener and Si diode later and report back.
Thanks for your time.
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File Type: pdf TG200DMP.pdf (600.7 KB, 162 views)
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Old 22nd Jul 2019, 6:56 pm   #5
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Default Re: Levell TG200DM

Hi!

Please keep us posted – I'm happy to go thro' the circuit in more detail with fault–finding hints if you need it!

Please note that alignment of these generators is a bit frustrating because it needs a counter to set the frequency points at the range–ends and many of the adjustments interact with each other!

Rather than try to do it to the (unnecessarily precise!) figures given in Levell's book, if you adjust the range upper and lower points so the output frequency is as close to the position given by the knob–cursor and the scale markings at each end of the ranges, that will be quite adequate for an instrument of this type!

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Old 23rd Jul 2019, 10:17 am   #6
Wendymott
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Default Re: Levell TG200DM

Hi Chris. Many thanks for your input...I feel guilty in wasting your time.... I originally bought it.. not as a requirement, as I have various generators of similar ranges, but because I had two Levell Micro/millivolt meters and I thought it would be good to continue the range. I spent most of yesterday.. chasing my tail. and decided that the end result was to scrap the item. I know this is not in the spirit of the forum, but at times you have to be pragmatic. The item is now a donor for other items that will be conceived in the future. Please close the thread MOD.. Thanks.

Incidentally.. if you want the main pcb or any of the other parts except for the meter or case, you are most welcome.
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Last edited by Wendymott; 23rd Jul 2019 at 10:19 am. Reason: Additional info
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Old 23rd Jul 2019, 10:53 am   #7
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Default Re: Levell TG200DM

Is this the one you got from me? If so it worked when it was here, so something silly must have happened. I'd check the bulb first, it is used as a negative resistance in the Wein bridge circuit.
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Old 23rd Jul 2019, 12:35 pm   #8
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Default Re: Levell TG200DM

Hi!

Quote:
Hi Chris. Many thanks for your input...I feel guilty in wasting your time.... I originally bought it.. not as a requirement, as I have various generators of similar ranges, but because I had two Levell Micro/millivolt meters and I thought it would be good to continue the range.
It's a pleasure to be able to assist where I can on this Forum for anyone, and there's nothing wrong with a desire to collect & restore a Manufacturer's range of products - I like Feedback Instrument's Emerald Green '600 Series and I'm waiting for another Member to give me the P & P price on a few I'd like to buy from him!

The lamp is only fitted to the basic instruments in place of a meter as a battery-indication device - negative-feedback amplitude stabilisation is carried out by the RA54 glass-bead thermistor between the output-stage mid-point (J12/J13) emitter circuit, and returned to a two-thirds tap in the emitter load resistor of J10 up from chassis-earth!

If there's a chance that rough-handling was a factor, then I would pay particular attention to all the connecting leads from the PCB - if any of the leads from the two RC frequency-determining networks to the multi-wafer frequency-range switch assembly break off, the circuitry will NOT oscillate, and because of the direct coupling employed, bias potentials of the transistors will alter resulting in whole sections of circuit turning off or hard-on, causing the large discrepancies in current-drain over the + and - 16.5V rails the OP found - the colours & lengths of these leads are given on the PCB layout diagram, altho' their destinations aren't very clearly indicated on Levell's diagram!

It might be worth obtaining a packet of Single-sided PCB Terminal Pins suitable for 1.5/1.6mm dia. holes to help protect the copper lands for the flying lead connecting points on the PCB - I think Levell's PCBs are only SRBP!

I'll try and draw a better wiring diagram as soon as I can!

Chris Williams
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Old 23rd Jul 2019, 8:41 pm   #9
Brian of Romsey
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Default Re: Levell TG200DM

Hi Wendy & Chris,

I have a TG200DMP too. Some while back I built it a little mains PSU to replace the batteries, figuring the cost of the PSU (mostly from the junk box) would be less than four batteries. That's OK but it is still a bit unstable. I had put it aside for later, but your thread has brought it back to a 'maybe now' status.

Ta for the manual and schematic. Having this is such a boon. Chris, hope you are able to improve the schematic, it is crowded IMHO and hence hard to follow in some places.

Cheers, Brian.
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Old 23rd Jul 2019, 10:04 pm   #10
Wendymott
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Default Re: Levell TG200DM

Hi Chris.. and Studio 263. Yes it was the one I bought from you, and as you can note.. I was not complaining.. things happen..however Chris....Its gone... finito.. this parrot is dead...yes I could have persevered...but as I said previously, I have other generators that cover ......and the recycled parts will be used...
What a DC Loop though, where all the parts, both active and passive have an effect on the operation.
Brian... I have kept the main pcb.. if you want it as a parts source to help stabilise yours. PM Me.
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Old 24th Jul 2019, 10:38 am   #11
Brian of Romsey
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Default Re: Levell TG200DM

Quote:
Originally Posted by Wendymott View Post
Brian... I have kept the main pcb.. if you want it as a parts source to help stabilise yours. PM Me.
I might take you up on that. Let me talk to my Huddersfield correspondent and see what gives. I might be able to arrange a local delivery or pickup.

Cheers, Brian.
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