ah pro 2

[dladams76]

hello i recently purchased an ahpro 2.it drifted horribly so i sent it to intex systems (formerly ah electronics)they were very easy to deal with however the detector still drifts horribly.in other words after turning the detector on i adjust the sens control to obtain threshold.as soon as i reach a level suitable for hunting and let the control go it immediately starts drifting and within 5 seconds it is either at full audio with the needle pegged or it has went silent..it can go either way but usually gets louder..this model has the thumbwheel instead of push button retune.i would appreciate any input if any of you have ever had this problem..as it is now it is totally worthless and i would really like to get it going thanks daniel

[technos]

hello i recently purchased an ahpro 2.it drifted horribly so i sent it to intex systems (formerly ah electronics)they were very easy to deal with however the detector still drifts horribly.in other words after turning the detector on i adjust the sens control to obtain threshold.as soon as i reach a level suitable for hunting and let the control go it immediately starts drifting and within 5 seconds it is either at full audio with the needle pegged or it has went silent..it can go either way but usually gets louder..this model has the thumbwheel instead of push button retune.i would appreciate any input if any of you have ever had this problem..as it is now it is totally worthless and i would really like to get it going thanks daniel

This is a seventies era detector. Interesting that this company still takes care of these early machines;
http://www.intexsystemscorp.com/ahpro.htm

Could you post a picture of the machine so we could match it up with a schematic from a patent?

It needs to be remembered that the technology and construction on these machines is fairly primitive. Your coil may not be shielded which would cause some of the instability right off.

[mikebg]

There are two patens ussued to mr. Hametta (president of A.H. Electronics):
US 4,678,992 Electronic metal detector
and US 4,263,553 Discriminating metal detector with compensation for ground minerals

[mikebg]

Daniel, to which of these models is similar your machine?

[dladams76]

the one on the left is similar.mine has the fine tuner on the end of the handle and a coarse tune on the box..these are labeled sensitivity but basically just control the threshold.also mine has a three position discriminate switch labeled nails,foil,tabs,all metal....thanks for the replies daniel

[dladams76]

ill try to post some pics after work..

[cyclops]

maybe it´s GE diodes

Hm, 70ies MD... Have a look at the PCB and try to find out if they still used GE diodes. If yes, they may be a part of the problem, as they are much more sensible to rising temperarures than SI diodes. If you live in a sunny country, this may be a partial explanation.
but you cannot easily replace GE by SI diodes.

[cyclops]

older resisters could be a reason too:
coal layerd resistors are sensible to heat to, heat modifies the coal grains working as resistance: warmth may change the grain structure (intergrain connections generally increase, thus the resistance drops)
anyway, with age the grain structure suffers > resistance changes > MD behaviour changes.
Try to change resistors by same values weather metal coat (good), metal oxide (better) or metal film (best) resistors.

Did you replace the eletrolytic caps (if there are)? At that age they presumly aren´t good any more.

Basically, out of some 70ies detectors, from around 8 bad working or not working units (BH, Compass), that I restored, I replaced most components (electrolytic caps & resistors) with new ones. In 3 cases I had really better results, 1 or two were a bit better, 1 no results, 2 losses (where one transistor heated before and after, so it may have been one). Did not find a good substitute at that time. Kept it for spareparts.

I tend to keep the original ennamelled or "ceramic" caps, as I think, they are best, after having been replaced and not replaced in two identical units. Just better feeling.

I think in your case it won´t be the transistors or, if there already are, first IC´s.

Hope this helps.

[dladams76]

well first off im sorry for not posting pics..the guy that worked on it sent all the old components back with the detector.he replaced the 14 pin ic and all the transistors and about 5 small caps.not sure if he replaced the electrolytics but i think so.also replaced the metal tuning can.ive got a schematic that he sent me if i can figure out how to post it..ok i think ive got it..thanks for the replies so far

[dladams76]

and i wanted to clarify.i know that older detectors were subject to drift...in this case the threshold is never stable not even for 3 seconds..it is either rising or falling mostly rising.it rises until the needle is pegged and at full audio.this might take 2 seconds or it might take 5 seconds..the funny thing is that ive seen it work three different times for 5 minutes or more...it just doesnt make sense

[technos]

.the funny thing is that ive seen it work three different times for 5 minutes or more...it just doesnt make sense

Actually it would kinda make sense if this was an intermittent failure. From the schematic that you posted above this detector is represented by the 3,896,371 patent shown above.

The question I have are which transistors were replaced, what the numbers were on them, and what are the numbers for transistors Q1, Q2?

But I do not think those are the problems, My bet is that you are dealing with a dirty pot R19 which is 30 years old and could have loose crumbs of carbon on the wiper causing loss of tuning. I bet that the company did not replace that, as it's not always an obvious problem. According to the patent the value of this pot is 1K, but I would measure it to make sure. In fact, if you take it out of circuit and put a digital meter on it you might be able to see the value wander on it over time. A couple of ohms is not bad, but tens of ohms would be.

If it was me I would switch out the resistors in the part of the Q1 Q2 circuit (R1, R2.R3) with metal film resistors which will add some stability. Change C3 to a polystyrene capacitor.

If the transitors Q1 and Q2 are germanium types (not unheard of mid 70s) then that would be another source of instability. Ge transistor's gain can skate all over the place with a change in temperature. I would use a couple of matched 2N3906 transistors there instead.

But I would change R19 first and see how the performance improves and if this fixes the problem or not. Then chase after improvements.

This is an interesting circuit operating at about 53 KHz that doesn't fit in any specific detector category, but probably has the same sensitivity as a BFO. Apparently it does have some discrimination ability that at the time would have been pretty rare.

[dladams76]

well i changed r19..same thing happens.i may try to find replacement resistors and get them ordered..thanks for the help

[technos]

well i changed r19..same thing happens.i may try to find replacement resistors and get them ordered..thanks for the help

Two other things. I would also change out all of the electrolytic capacitors at this point. I really did not read carefully your post about how the needle would get pegged. This really could be an old cap problem - C14, C16 are primary suspects. I would also get some contact cleaner and clean the various rotary switches in the machine. After that I would get an O'scope and trace the signal (after the caps were replaced) if the problem persists. Welcome to the world of troubleshooting!

[dladams76]

well as far as temperatures go,it is inside.it remains a constant 75 degrees.i dont have access to a scope but will try other things..i really appreciate the input here!thanks daniel