Pignose to Chuck

[Dragonmac]

Newly joined. Am not an electronics whiz. Last thing I ever built from scratch was a Heathkit car radio.

Need some insight to a problem with an old 7-100 made in the early 70's.

When I plug into the jack, get this loud low frequency motorboating. No guitar comes through.

Prior to experiencing the motorboating:

After years of not using the amp, I decided to clean it up. Took off the metal circuit board cover. Noticed the blue wire with the resistor attached to the pre-out jack had come detached.

Using this photo I resoldered it to the "+" terminal on the board. Is this right? AFIK the board below is of the germanium trasnsistor '70's model. T'was all I could find.



I should've tried the amp before I took it apart. Nothing seems detached now.

I also tried to pull off the metal cover that houses the input jack and volume/switch knob. Couldn't get the cover off after taking off the four screws but did loosen the nuts thinking the cover would come off. No go. So I just put back the nuts on the jack and pot. Tried not to make the jack and the pot spin from its position as I tightened the nuts.

Lastly I reflowed some of the solder joints on the board. Nothing is touching.

Thick board and with enough space between the traces. Made in Japan.

Any ideas what's wrong or what I did to screw things up?

 

[zgrav]

motorboating can be caused by an old electrolytic capacitor., likely the largest one on the other side of that board.

 

[Dragonmac]

Thanks zgrav!


One of these? Arrows are not mine. Pre-existing photo.

 

[zgrav]

not a transistor. the capacitors are the barrel-shaped objects.

 

[Dragonmac]

Yes, I realize that. The photo is someone's who was pointing to the transistors.

So the largest cap there would be suspect for the motorboating?

What does one do on a 7-100 circuit, change all the caps as SOP for something this old?

 

[zgrav]

Do a visual inspection of the caps and look for something "off" -- swollen, blistered, etc. If you find any, change those with the same values and voltage ratings.

And if none of the caps look bad, I would start replacing them largest to smallest and test it after each one to see if the problem is fixed.

Obviously an easier thing to to if you already have an inventory of capacitors on hand. If you don't, I would suggest ordering enough to do a full replacement even if it turns out you do not need all of them. Or if you want to have an inventory, order some spares as well since the shipping charge will not be much different if you order 10 caps or 20.

 

[Dragonmac]

I understand zgrav. Thank you!

Yeah these days have to get while the getting is possible.

 

[Chuck D. Bones]

It could be something else entirely, so before you start soldering and unsoldering parts, we should do a little investigation. These boards are easily damaged by soldering/unsoldering.

I'll think on this and get back to you.

 

[Dragonmac]

Thanks Chuck!

I used one of those inexpensive temperature controlled 60 watt pencils with the digital read out. Had it set to like 300 +.

Been sourcing caps in the meantime as zgrav suggests. Don't see any rebuild kits for the 7-100 circa 1973 or any year for that matter. I see there are parts lists made on other forums but 1980's is mentioned.

If it matters the serial number on the unit I have is 0408.

Also the original 3x2 AA battery holders cracked long ago. I put in a RShack 9 volt batt holder and is connected to what wires come out of the housing with the logo/label. That was done long ago as well and the amp worked then.

It's been years since I've used the amp. Saw an auction for the 7-100 that Clapton used that went for USD$9k. Sparked my interest in the Pignose to use it again.

The motorboating happens no matter if a 9 volt batt is in the holder or I use a wall-wart 9 vdc 300 mA. Had to use alligator clips to attach the adapter to the cap-batt terminals. The 2.5mm plug is uncommon unlike barrel plugs. Same noise, same loudness. The volume knob does nothing to the loudness of the motorboating, It's all the same sound and volume no matter the position of the pot. Turning off the amp the motorboating stops. Only happens when the jack has a plug in it. Unplug the guitar cable, motorboating stops. Switched jacks?

Could something behind the jack and pot inside the housing I couldn't get off been pushed up against something?

 

[music6000]

Have you tried the Amp with that ''Blue'' wire unsoldered?

 

[Chuck D. Bones]

Mine are made in Taiwan, similar, but not identical layout. Wire colors are different. That blue wire looks like it goes to the ground lug on the 1/4" jack, is that right? The 1/4" rear jack on mine is Preamp Out. I don't use it. I noticed that one of the aluminum electrolytic caps on mine is leaking electrolyte, so it's time to change 'em all. Can you show us a picture of the component side of your board? The expected life of an aluminum electrolytic is 10 years, 20 at most. At this point, I'm inclined to agree with Zgrav, replacing the caps is a good idea. Let's see if that solves your motorboating. Personally, I like motorboating, but that's another story entirely.

 

[Dragonmac]

Have you tried the Amp with that ''Blue'' wire unsoldered?

No. I assumed that since the wire is there it belonged somewhere. I could try it and see.

 

[Chuck D. Bones]

What's the rationale for disconnecting the blue wire?

 

[Dragonmac]

Mine are made in Taiwan, similar, but not identical layout. Wire colors are different. That blue wire looks like it goes to the ground lug on the 1/4" jack, is that right? The 1/4" rear jack on mine is Preamp Out. I don't use it. I noticed that one of the aluminum electrolytic caps on mine is leaking electrolyte, so it's time to change 'em all. Can you show us a picture of the component side of your board? The expected life of an aluminum electrolytic is 10 years, 20 at most. At this point, I'm inclined to agree with Zgrav, replacing the caps is a good idea. Let's see if that solves your motorboating. Personally, I like motorboating, but that's another story entirely.

Yes it goes to ground lug on the preamp out jack. Hence I got confused as the trace on the board says "+" but viewing the photo of the board posted sort of shows the blue wire goes at that spot.

Isn't ground "positive" on this amp?

Will take photos as requested.

 

[Chuck D. Bones]

AFAIK, they are all positive ground. Hence the + sign on the ground trace.

Am I right in thinking that all of the pix you have posted so far are not of your unit?

<(-.-)>
"Confused me you have."

 

[Dragonmac]

AFAIK, they are all positive ground. Hence the + sign on the ground trace.

Am I right in thinking that all of the pix you have posted so far are not of your unit?

<(-.-)>
"Confused me you have."

Sorry Yoda! Yes that's all I could find on short notice. And when registering for this forum it didn't take right away. Had to get in touch with admin to fix the problem so I wasn't sure I was going to be able to get to post anything here. And I'm assuming the ones I chose are like the amp in my cabinet. But am taking photos as we speak. Have to charge the batts for the camera. So waiting for that. Don't have a smart phone.

 

[knucklehead]

Don't have a smart phone.

No one phone-shames here . . . . . .

 

[Chuck D. Bones]

The fact that you are NOT using you phone to take pix increases my respect.

Let's not make any assumptions about this or any other circuit.

And about that Heathkit car radio, did it have tubes? I cut my teeth on Heathkits and my first build (not a car radio) did have tubes.

 

[fig]

Don't have a smart phone.

you are my hero

 

[Dragonmac]

The fact that you are NOT using you phone to take pix increases my respect.

Let's not make any assumptions about this or any other circuit.

And about that Heathkit car radio, did it have tubes? I cut my teeth on Heathkits and my first build (not a car radio) did have tubes.

No it was solid state. An FM stereo car radio.

I do have an EICO HF-85 that has tubes. That needs a rebuild.

Yes I figured someone would say not to assume on the circuit boards.

My camera isn't that great either so here goes.














I appreciate the assist!