Adcom GTP-500ii FM Problem

Thank you to all the contributors to this thread. I was able to remove the tuner box on my GFT-555 II and all I have done so far is tap on CT-104 and the tuner works great now. I put the case back on and I can rap on the side of the tuner and shake it and it still works. I will seek out a replacement trimmer though.
there's one varacter diode in that circuit I replaced too as there was too much variation in signal intensity (little stacked LEDs in front flickered) after replacing the variable capacitor. Locks in fine, going on two years, and I did the same fix in several other model preamp/receiver units.

I've got a couple of the caps on hand just in case, insurance. I'm sure it'll all be in my estate sale someday.
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Hello all. This is an old thread but I hope someone is still monitoring it. I have a GTP 500 II for many years and two days ago the FM tuner crapped out.

I let it sit overnight and it worked again this morning but I'm sure it is not long for this world.

I tried many links within this post and so far none of them worked. I visited and they no longer have the recommended part for my tuner (it is marked obsolete), CT104 part # 659-GKG10015 so I ordered 2447-GKG10015-ND from DigiKey Electronics because it seems to have the same characteristics.

Can someone knowledgeable about this issue let me know if this looks like it might fix my problem?

At this point I am not even sure if it even looks like the correct part.

CT104 I hope.jpg
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Thank you NormB , I would post a "thumbs up" but I don't see how to do that in this forum. Some folks added a thumbs up to my post but I don't know how. Do you need to be a longer term member to have that ability?

Anyway, the first cap lasted since 1989, so I'm hoping I don't need to replace it again in my lifetime. :)

I only have a DVM. I was hoping to get it working using only steps one and two of the FM alignment procedure as others have described. Seems like it worked for others and I don't see any failure replies.

Someone suggested a non-metallic (plastic?) adjustment tool but the link to purchase it/them was non-functional.

I have an old plastic adjustment tool with a metal, screwdriver type tip (everything I have is old because I am). Will that work? The previous reply seemed to indicate that you need an allen wrench style adjuster.

Can you, or anyone else post a current link to the type of tool I will need or do I need to wait until I see what the cap I get has? TYIA
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Besides a couple of odd bits/tools I’d picked up over the years, like a Bournes trim pot adjustment tool (H90, although H91 is good to have on hand), I bought this set off Amazon a handful of years ago:

Aven 13016 9 Piece Anti-Static Alignment Tool Kit​

Many of the tools have a bit of metal at the end as some of the trimmers (like small rheostats) can be a bear to turn and have metal screws themselves with chew up the plastic part of the tool. This doesn’t affect the alignment procedure.

The point to using “non-metallic” alignment tools is you want something which doesn’t conduct RF or static such that you become part of the circuit you’re working on.

I’ve been through the “I’ll wait and see” routine enough to know as a hobbyist, more tools are better than fewer and I pay less shipping and handling this way.

The Bournes H90 is great for SMALL trimmers.

Potentiometer Tools & Hardware ADJUST. TOOL (1 piece)
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Thank you again NormB . Since I still cannot find a way to respond with a thumbs up, I hope you can settle for this :up:

Let me say that I'm not cheap but I am frugal. OK, I'm cheap. Now that you explained what the reason is for the plastic alignment tool it brought back a piece of my memory. I found the alignment tool I had from many years ago and also found one I made when I was in a bind at a customer's site.

My yellow alignment tool was too big so I took a smaller common head screwdriver and melted some heat shrink onto the shaft to insulate me. That tool has been in my tool-case for about 4 decades. I included a picture of my alignment tools.

The Trimmer Cap is supposed to be here tomorrow so I'll know whether they do the job or not. I can see where the screwdriver can still have an antenna effect so I hope the yellow one fits. If not, I will buy the tweaker you suggested. Thanks again.

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The speed adjustment coil on early REVOX reel to reels (A77 prior to an upgrade to an integrated circuit PLL circuit) was YUGE, about 1.5” across, 1” high, had this little ferrite core inside that could only be reached by a foot of… something.

Of course, there was a “special tool” but I’d looked at various kinds of plastic/nylon rods, fishing rod even, and settled on a 3/32 length of brass rod I’d used for a 2M antenna project, soldered a flat bit of brass on the end, trimmed to fit the coil, then covered the whole length with heat-shrink tubing and put a L-shaped bend in the end. worked great. Sold decks.

Good luck.
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OK, now I'm into it. I received the part and wow the leads are short. In the old days the components would come with long leads and you would put them through the holes, solder them, and then clip off the excess. No "excess" here!

Anyway, on to more important stuff. My unit failed intermittently so it is actually still working now. I wanted to see what the readings were between TP3 and TP4 before taking out the original CT104 expecting the reading to be approximately 3V when tuned to 87.5MHz and approximately 23V when tuned to 108MHz but it was not the case.

My unit measured 5V when tuned to 87.5MHz and 21.4V when tuned to 108MHz with the original components.

So, this question is for an expert if there are any out there. Do I insert the Trim Cap and follow the directions in the service manual or do I adjust it to the readings I got from the working unit before removing the original? I realize that the intermittent component MAY be affecting the readings, and the new component may put different characteristics into the circuit. Then again, maybe they are better for my unit IF that is what it came out to be when it was set up at the factory with all their equipment. I only have a Digital DVM.

Anyone else able to measure this before replacing CT104?
Things are looking really good.

The part arrived and it physically looked like a good replacement for my component.

I opened the pre-amp this morning and (since my problem is intermittent) I took readings from the test point under the range of frequencies they recommend in the service manual. They were off but fairly close. They may have been off because the component was failing and out of spec. Anyway it is supposed to read ~3V @87.5MHz an 23V @ 108MHz. They read 5V and 21.4V respectively.

After replacing the Trim Cap (CT104) I measured the 108MHz voltage first since that is the one that calls for the Cap to be adjusted and it measured 33V. I tweaked the Cap until the reading came down to 23V and then checked the voltage at 87.5MHz. It was right on at 3V without touching it. So, as soon as the new Cap adjustment brought the high end down to 23V, the low end came into alignment automatically for me.

I put it all back together and tuned all the FM presets and it is working like an amp! (play on words for “working like a champ” pun). Rock on!

Thank you all for your help and input.
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Amazon has a set of 5 each variable trimmer capacitors covering several ranges 5, 10, 20, 30, 40 and 60 pF. Same 6 mm pin spacing as the Digikey 2447-GKG10015-ND. Less than $10 US.
Amazon ASIN B06Y4MP4JM

Their white 10 pF trimmer worked for me for CT104 to set the TP3/TP4 to 23 volts at 108 MHz. The voltage at 87.5 MHz was then at 3.00 volts without needing coil adjustment.
I was able to use a small metal screwdriver that while producing a different voltage when inserted into the trimmer, could be offset from the desired value and then checked after removing the screwdriver.
I did notice the voltage being about 0.15 or 0.2 volts lower when the metal cover was in place. Since it was more difficult to adjust the trimmer through the hole in the top of the metal cover when in place, I simply offset the value with cover off and checked when put back in place.
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In spite of replacing CT104 and adjusting voltages, my initial GTP-500ii issue remains. Several high signal strength stations that are received on other tuners in stereo are not being picked up in an FM scan. When they are manually tuned to, the Mute/Hi Blend switch must be depressed to hear them but the stereo indicator does not illuminate. The signal strength indicator is at 4 or 5 bars. The Muting Adj trimpot inside the unit has no effect on this problem.

FM scan stops at many weak stations that are heard with static. The stereo indicator illuminates on many and signal strength is 3-5 bars. Mono button turns off the stereo indicator and does reduce high frequency noise a bit. Mute/Hi Blend switch has no effect.
Thanks for posting this link. I’m sure many who are attempting to fix their FM tuners will find it helpful.

Mine’s still working well after several years.

I recently acquired a Technics ST-9038/SH-9038 Tuner/timer set which pulls in stations the ADCOM only presents as static, but for a garage stereo set tuner, the GFT-555 is still good enough.