Signal Generators

Simply, A demodulation probe usually consists of a Diode that rectifies a modulated RF signal, it strips off the RF portion and leaves only the audio portion intact. This type of probe can be used with VTVMs and O'scopes to measure audio voltage.

Yes the signal looks like the signal you would see in a radio in the IF stages.
What is a demodulation probe?

So what your saying is, your not getting a clean signal?

If the generator is working fine, I'd suggest leaving it alone.
They did make errors in parts lists and schematics. More often than one would like.( And mismarked parts too. Cathode bands on diodes at wrong end, resistor color codes marked wrong on resistors, to name a few problems I have run across.)
often they would include an update or errata page with the manuals too. That may have gotten lost over the years too.

I bought this off of Ebay. It turns on and the scope sees a signal. I watched the Utube video on a generator like mine and he has a clean waveform. Whoever built this did a nice job. I found a resistor R6 that is wrong according to the schematic. He had a 4.7k and it calls out for a 10K. This resistor goes to the plate of the 6AN8. I tested all the resistors with my meter and they are good. Any suggestions?
One of the plates of the variable capacitor was shorted and I fixed that.

frank1956 wrote:

Project completed. Signal generator is back up and working.

what did you find to be the problem?

Project completed. Signal generator is back up and working.

Cliff, here a copy of the schematic. I did replace the cap's across the AC line. When I first finished all replacement ie. cap's, resistor and the solder bridge I turned it on and took my voltage reading per the schematic the voltage coming from the diode to the 2.4 k resistor R1 should be 125 V I was getting 150 V. That when the 2.4 K resistor started smoking. Since I have removed the solder bridge I get about 108-109 V. I literally removed everything and started over, ensuring the diode was installed correctly, electrolytic were correct pos,neg, neg, pos , and replaced and checked the 2.4 K resistor to ensure it was good. Everything was ok. Prior to doing the re-capping the generator was working 100%. The only reason for the work was a couple of nasty looking cap's.


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From the video there looked like a solder bridge also. I can't find the schematic right away, but that looks like the B+ as its a red wire. Either your polarity with the caps is an issue which I doubt, if you were careful. Or a short in a tube. Also did you replace the caps across the ac line?
If that was taken are of, remove the tubes and check the power transformer voltages. Disconnect the diode first.
If they are correct voltages then you will need to check for miss-wiring. Remember this was a kit and builders do not always know what they are doing. Let us know what you find.

Since I finished my RCA X551 the other day I decided to re-cap my eico 324 signal generator. I changed all the resistor and the two (2) electrolytic. I found a you tube video on how the electrolytic were done and followed that process since it was straightforward and seemed fairly simple. The processed called for a 5 prong terminal strip,  two (2) 20 mf @ 150 v electrolytic, one (1) silicon rectifier, 1000 PRV 2.5A, and to replace the original 2.2 K resistor with a 2.4 K resistor.

I doubled check the video after I was done and notice that between the 2.8 K resistor and the positive side of the second electrolytic there was what looked like as I call a solder bridge between the pos side of the electrolytic and the hot lead going to the 12AU7 tube. After all was said and done and I turned it on the 2.4 k resistor started smoking . Needless to say I turn it of and removed the solder bridge, problem solved wrong. Since I have removed the solder bridge between the two the signal generator does not work. Do not want to proceed any further without some advice, any idea!!  Not sure if this make a lot of sense, so I have attached so pic's of what I have done so far. If you would like to see the video [You must be registered and logged in to see this link.]

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I also obtained about a Month ago a Navy Signal Generator TS/413/U
It has two Meters which is extra nice, one is R.F. Modulation -Audio Volts, and the other Meter is R.F. Volts rms.
OH Boy more knobs to twiddle with Muh Ha Ha!!!!!



Image is on Radiomuseum.org
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HI Cliff, nice catches on the signal generators! If I'm not mistaken that URM-25 is the source of the "Holy Grail" of crystal radio air variable capacitors. That one air variable cap could bring more than the $60 you paid. Seems like the URM-25D had the single gang silver with ceramic insulators and the URM-25E had the dual gang silver with ceramic insulators. Have you checked inside that one yet? Can you confirm the air variable cap in the B model please?

By the way, I did find the schematic for my old RCA signal generator kit. Should anyone else need a copy, I have posted it on my website!
Thank you! Bill NT9K..

I now have an HP 606A signal Generator (I picked it up after the Ham Fair MIKE and Key Club in Puyallup) that I am starting to tear into. There is a problem with the Modulation meter setting at 40%. So I started to test the tubes first. ARRRRGH First three tubes I tested were all reading LOW so I suspected My B&K 747 tube tester of being in need of alignment. Started the alignment and ran into Murphy's Law again. So now I have two repair projects.

Getting back to the signal Generator, I am trying to remove the chassis Box where most of the tubes are hidden. I have run into a problem with the hardware. Around the box are a number of screws, easy to remove, they are all regular slot heads. But on the side between two of the boxes are Socket Head screws. It takes a special wrench that has a couple of Fahnestock clips to store it inside the Generator. Of course the wrench is gone.

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I found the description of the wrench in the manual :
According to the HP-606A manual I have, the wrench in question was part number 1470-0010. The manual defines this wrench as Hex socket 0.1087 inches across the flats.
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The closest SAE size is 7/64 inch (0.1094 inch). I was actually able to insert that size in the socket screw and had a nice tight fit.
The shaft length should be 12" for the handle to clear the end of the chassis.

The chassis from the screw to the end of the chassis is 10" (+) so I would consider the extra (2") = (12"total) for clearance for the "T" handle.
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So my friend is going to braise the tip to a long extension to get at those pesky socket head screws.

I see about 200-300mv ripple and the voltages are +13.4 & -14.8vdc. The positive rail does have greater ripple.

Even with the scope ac coupled, I see the signal jump around similar to the AM ouput. I little better with a 470pF cap blocking DC. Probably need to drop 4 new 1000uf caps and 1 2200uF in it.

Intersting side note is the RF section is very similar in components and layout to the Sine/Square section. That gave me some comparable voltages and signals expected since the sine wave side was working.

FrankB mentioned caps so as old as they are you might check the ripple across them and cleaning of all connectors and switches. Possibly look at the regulation and stability of the power supply. I will be doing that on my units, that's for sure.

Found some bad transistors in the front end of this Generator. Finally have a Modulated output, although not quite as stable as I would have expected. It's not like this is a lab grade generator. Probably good enough for educational purposes as intended.

Seems to jump around on the scope +/- 5-6Khz with it centered at roughly 455Khz. About half that much deviation without modulation. Scope could be triggering on some harmonics? Scope is a Tektronix TDS2022C. Not that the measuring capability of these digital scopes is accurate on an analog signal either.

Can anyone comment if this is stable enough for troubleshooting and tuning AM radio circuits?

I think I paid $189.00 for mine and $25.00 for the protection kit.
There are cheaper ones out there, but this one came with full schematics and troubleshooting info too. the others I looked at did not.

I paid for this one in the first 2 TV repairs I did after I got it.
It saved mass time, especially with vertical circuit problems.

Frank, That Capacitor Wizard looks like a nice tool. A bit out of my league for very part time hobby stuff, but I'll keep an eye out for one. I did see a vid on YouTube using a scope to check caps and such. Needing them out of circuit is not very convenient though.

Well, with much perseverance I finally have the overload circuit working in a manner I suspected it should. This required some slight modifications to the circuit a bit. I redrew it in cad to better reflect the intent of the design.

The principle of this circuit is primarily to flash the LED only. This is enabled when the transistor turns on hard in saturation pulling the ground side low. The LED was always on giving a false indication of an overload condition. As connected the transistor never did go into cutoff and the LED was tied to emitter not the chips common reference ground at the collector. Both issues were causing current flow through the LED although the 555 timer was not fully on. Now it works like I thought it should. I may add in green power on LED just cause I can....

With that part out of the way I can concentrate on the lack of RF output. I'll post back with those details as I progress.

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I would suggest the Capacitor Wizard as the best out there for an ESR meter. Also be sure to buy the overload protection kit.

IIRC I got mine from Howard Electronics. GREAT folks to deal with.
The digital ones are not as handy, IMHO, and they don't come with a full schematic or repair info like the Cap Wizard does.
It was originally made by Independence Electronics, whom IIRC, the owner of died. Quality equipment too.
The only down side to it was no "on" indicator. You just have to remember to turn it off.

There was a company out there whose owners name was Lance. This company mainly deals in refurb Sencore equip and was/is in S. Dakota. He was selling the ESR meters too.
Avoid him like the plague. Non- responsive to my phone calls and emails.
I bought a refurb. Sencore LC53 from him and it failed on the
2nd component I tested, right out of the box. He ignored all my calls after teh intial ones and then finally replaced it, but I had to pay the return shipping on the defective one. What a slime.
It took over 3 months to get me a replacement, and I lost hundreds of bux, as I had many pounds of "Bumble bee" caps I had intended to list on the net for the audio fools, that I had needed to test. This was in the Xmas sales season too!.

I keep looking at the ESR meters. There was a kit I had my eye on, but never pulled the trigger on it. At the time I was working on some micro-controller projects and using 1-2F supercaps and luck of the draw with high ESR in a few. 30ohms would eat up its own voltage and starve the circuit it was intended to hold up at power loss.

Progress update: The overload circuit seems to work as intended. I did find the 2N5810 transistor bad. It would not switch on to pull the ground low enough to start the astable oscillator circuit blinking. However it does still keep the LED on stable. I am starting to wonder if this is a power on/off/overload on flash dual purpose indicator.

I can simulate an overload by shorting across the A1-A2 terminals of the triacs. The relay drops out and the light flashes as long as it is in simulated overload. Remove the short and the LED comes back on steady. This suggests the 555 timer chip is fine and the caps are functional. However the circuit never really shuts down and can be monitored on the O-Scope oscillating just under making the LED blink. Still trying to figure out if the transistor is not quite in cut-off or if other components keep the 555 supply pins with enough differential to run. I was thinking about increasing the 330 resistor on the transistor collector pin or the 390 between +15 and pin 8. This in an effort to get the LED and timer chip to shut down unless a real overload occurs.

I will see about that change FrankB, Thanks!

My power supply section is an all in one faceplate, it has 2 separate adjustable DC outputs.
They both have 2 ranges 0-10vdc and 10-25vdc 1-amp.

There is a 3rd bigger knob voltage adjust 0-300vdc 100-ma. push button selected.
It also adjusts an AC output 0-130 vac. 1.75amps
One power switch for the entire console rack with power led indicator.
It also has a power strip for 115V ac (2 plugs) + power switch and indicator with circuit breaker and Speaker jacks 8 ohms with rear mounted internal speaker.

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Note there are 5 of them on Ebay for 5 days. $38.40 + $35.00 shipping looks like a good deal IMHO.
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Don't forget that bad electrolytics can cause this too, especially in older equipment.
Check your filters for ripple and any electro in it for ESR. and value

It's been so long I can't even remember how I picked these up for nothing around the early-mid 198X's. I do have the FET Meter with the RF/SINE/SQUARE wave generator. I also have a seperate unit with a 0-25vdc power supply I have been using often over the 20 years for misc. projects. Not sure what was in along with that supply originally. Very handy empty space  for my leads and catch all though! A supply with both AC/DC would be great too. One of these days....a decent lab bench supply will find its way home.

I have an old tube type Knight RF generator My Dad or Uncle built from the early 50's. Not sure if it works either. I almost fear turning it on if the spiders decide to run.  

I haven't used my set yet, However I did hook it up to a scope and got some flakey signals. So I have determined it has to do with most of the switches needing cleaning. AH another project. Mine has the RF-Audio section, FET multimeter and Variac with AC-DC adjustable Power source and speaker for testing radios. lots of Power plugs in the front bottom and master switch with breaker.

Thanks Cliff. Any info at this point would be helpful.

Will send you an Email on problem.
If the trip point is set to sensitive then this may be a consideration, and it also mentions Q22 as a fault for lack of RF.

I have had this Generator for about 20 years. It is an ECI 40500. Never used it, but knew I had it for some day projects.

Went to check it out finally only to find it did not put out RF, although it does put out sine and square wave. Clean inside with no nube fiddlings. 2 boards in it. One for the generators and one for the overload protection circuit. Looks to be capable of internal modulation and external modulation.

Checking into it further I have discovered it is showing an overload LED constant on although it is for certain not in full overload. The partial overload board schematic is attached (anyone have an electronic copy?). I've reverse engineered this much into cad and hope it is accurate. This appears to be an astable flasher for the LED and the transistor appears to turn it on. The relay is on which it should be without a true overload. The Triacs(one shown, others in parallel) basically short out the power to the relay coil in a true overload. I find nothing going to the gates of the triacs. Any one of the 4 of them react to overloads on any of the sine, square, RF or external modulation signals.

One question is there is a slight imbalance between the 15v sources. One at -19.08 and the other at +17.18. I suspect this could contribute to the LED being on although not flashing via the timer circuit. The IC only has 3.5v between pins 1 and 8. I find 4.5 is typically minimum. The slight imbalance may be due to something loading the + supply side on the main board RF Generator circuitry.

I don't have a bench lab supply I could use to test it further in balance. Since it is not really in overload....I could carry on with the main board troubles and see if this LED issue resolves itself presuming the balance returns as if that has anything to do with the LED.

Any thoughts, guidance or advice would be appreciated. It's more about the challenge than its worth.

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I now have a SG 85/URM-25B.
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Paid $60 for it.
Has a few connectors that are needed.
Minor inconvenience.

Subject: Audio Generator questions
Paid $56.00 which included shipping

[You must be registered and logged in to see this image.]Subject: Found on EBay   [You must be registered and logged in to see this image.]Fri 4 May 2012 - 23:27	[You must be registered and logged in to see this link.]
A Philips/Fluke PM5135 Function Generator. I am in need of an English Manual for it, does anyone have a copy? Generator sold for $34.99 plus Shipping and Handling $21.62 = Total $56.61 Nice look from what I can tell, will know more when it arrives. I tried fluke but no luck on manual. It looks to be a 1993 production. Function generator is easy to use The PM 5135 is a low-cost, easy-to-use function generator. Frequency is selected by setting front-panel pushbuttons and a large logarithmic dial and vernier control. Also easily selected by pushbuttons are the standard sine, triangle, and square waveforms. Duty cycle is a standard 50%. Dc offset can be added, if needed, with a <+/->10-V range. Offset dc voltage can also be produced without any ac component. The PM 5135's sweeps can be single or continuous, with a logarithmic characteristic and a continuously variable sweep ratio from 1 to 2,000. The sweep range is 3.5 decades, and sweep period is continuously adjustable from 10 to 150 s. Triggering can be set internally manually, via or from an external source. The PM 5135's output is short-circuit-proof.

The picture attached isn't the best quality but gives you an Idea anyway.
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This is my first modern Signal generator, so I am unfamiliar with its different uses and functions, so any advice and suggestions would be appreciated as to uses. I cannot find a users manual, and I have really looked. I found one in Spanish,( Click Here for PDF ) and I found a previous model which is very close. but nothing that describes each button and functional use.

The front panel from left to right :
frequency/sweep start   dial
determines output freq. or start freq. if you want to use it as a sweep generator

frequency Hz  push buttons multiplies the dial by xfactor ,lo end 100th of a second ,hi end 10khz times main dail marking

x0.01_x1_x100_10khz

sweep buttons with one indicator light:
On - allows sweep  mode
Cont internal osc- runs all the time
Trig internal osc- started by external trigger source,

Waveform  buttons:  sine_triangle_square_DC voltage that follows the freq. of the main osc

Attenuation   buttons -all out no attenuation of the osc. output
10dB_20dB_30dB

Power Light

Bottom row consists of knobs and one bnc jack
Frequency Offset-pos or neg limits of sweep  -20%   to  +20%
Sweep Stop   (I think this interacts with sweep time. )   01 to 200
Sweep Time       1s to 150s
DC Offset          - 10v to +10v
Amplitude     3v pp to 30v pp

Output BNC Jack  Zo50 Ohms symbol on the side of the jack is an upside down capital letter T

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Back panel has 4 BNC Jacks
What are the purpose of these inputs?  
Inputs:
External input
Sweep  usually a triangle waveform that sets the start freq and stop freq of the sweep
Trigger input pulse that locks the internal osc to an outside source
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Outputs :
Sweep internally generated ramp wave form that follows the sweep set up
TTL 0v to 5v logic level of internal osc waveform.
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Each jack also has the upside down letter T. It means that all ground connections are tied internally together