Ampro 20 rescue

[Brandon Brown]

I joined this forum in the hopes that I can get some assistance....
So I have an Ampro 20 Premiere that my Dad bought for me in 2011 to replace our Ampro that was damaged from a flood. He bought it on ebay and was told that it was fully functional. We never had the time to try it out and a few months later my Dad became ill and passed in Jan 2012. Anyways, all this time later, I finally got it out. Mind you that it sat in the garage for 10 yrs. I have ordered a service manual and currently awaiting for its arrival. I cleaned it up, got all the dust off of it and gave it some lubrication. Opened it up and it has obviously been worked on before. There is what appears to be lamp cord now running to the exciter bulb. Also, someone folded electrical tape over the insulation on the bulb over the control panel. It does power on. Seems to run very smooth and the lamp lights. That is as far as I dare go. Scared to turn on amp. Pretty sure it is not going to work. I had the tubes out and since no way to test them, will probably get all new ones. One tube, a 6v6 I believe, was pretty black at the top and another one the glass moved slightly up and down. It is probably shot. My main concern is in the picture. On what I believe is called the small board, there is a blackened area under / besides a resistor. Any idea what could have got hot enough to do that? Is the resistor bad? No other apparent damage anywhere. Suggestions greatly appreciated on plan of action.

[Maurice Leakey]

It appears that the resistor just above the blackened area is a replacement, probably from a burnt out resistor some time previously.
I say this as the coloured code rings are in a reversed position as opposed to the other two resistors.
I am sure that the original wiring would have all resistors placed in the same way so that quick identification of their value could be determined.

[Brandon Brown]

Thank you Maurice, my electronics knowledge is limited and would never have seen that. I have just enough knowledge to make me dangerous. Assuming, the resistor was replaced then. Is there a way to test the resistor with a multimeter without removing it?

[Joerg Polzfusz]

When your multimeter is battery powered and can be switched to Ohm (uppercase Omega), then you can check the resistor. Simply select an Ohm-value on the multimeter that is higher than the resistor’s. (There are several webpages that will tell you the resistor’s value according to the colors of the rings, e.g. https://resistorcolorcodecalc.com/ )

[Maurice Leakey]

You can't check a resistor whilst in circuit. You would need to unsolder one end before checking. See reply number 2.
How to Test Resistors: 9 Steps (with Pictures) - wikiHow

[Brandon Brown]

Thank you both. I'll see what we come up with.

[Brandon Brown]

So, I threw caution to the wind last night and decided to run some film to see what it would do. I had a silent film of Yosemite and it did very well. Great picture. The picture had a little jitter to it so that will have to be adjusted later. Also upon rewinding, the bottom take up seemed to be struggling near the end, so that will have to be addressed in the future. The top of the lamp housing had a small amount of smoke from time to time, however, it was not of the electrical sense and just smelled hot. If I remember correctly, these will do that from time to time d/t the heat. Now for the amp. I turned it on and of course, nothing. Not to my surprise. Still waiting on my service manual, however, it is still in England d/t the "ransomeware" attack. My question now is the convert plug on the back. Should it be throwing 120v when the unit is on to feed the amplifier? I put a multimeter on it and it said 1-2 volts. Now also keep in mind that this unit had been worked on before and this plug had been replaced. I actually believe that I have found someone that solders worse than myself. The lead to the exciter bulb that they replaced has a bad solder on the board. Seems like I need to trace the convert plug to its attach points and see if it is making contact. Any other suggestions? I feel that I am making slow but steady progress. Thanks again to all.

[Brandon Brown]

ok, after opening up the unit... scratch that statement about the plug. I get it now.

[Brandon Brown]

So I brought the Ampro amp back from the dead. Got new tubes and replaced the fuse. Now I have panel and threading lamps and tubes glowing. No I have never run this as I was too little when we first had it. My question is that in order to get the exciter lamp to light, I not only have to turn on the amp vol, but also the microphone. Also, the exciter lamp increases in brightness when I turn the vol up. Is this correct? I thought that the exciter lamp was always lit. Any way upon doing so, I noticed that the resistor shown in the first post is getting very hot. Something is not right. Will replace the resistor when I finally get the service manual. Something just does not look right inside there. Also turning on the mic sometimes blows a fuse. I said before that it has been worked on before and after further evaluation, it has mostly been re-capped. I am also running it through a veriac. Any insight would be greatly appreciated. Just getting power back and tubes lit made me very happy.

[John Burgess]

Hi Brandon
From what you are saying there is definitely a problem with the wiring. The exciter lamp should light as soon as power is turned on to amplifier, it also should stay at the same brightness and not be adjustable. If I was you I’d wait for the schematic to arrive and check the wiring as per factory.
Good luck John

[Maurice Leakey]

...The exciter lamp increases in brightness when I turn the vol up. Is this correct? I thought that the exciter lamp was always lit....

Brandon
Many years ago I had a pre-war Bell & Howell 138. Its amplifier didn't have a normal volume control. The increase/decrease in volume was achieved by the "Volume Control" adjusting the voltage of the exciter lamp.
Perhaps your quite early Ampro does the same.

[Brandon Brown]

So the plot thickens.. After doing a lot of reading on various forums, I am just as confused as before. It appears that both John and Maurice are correct. I have read in other forums that the exciter lamp does work like a rectifier and will increase and decrease in brightness with vol control. However, I should not have to also turn on the mic vol in order to get it to light. I do have the orig owners manual and it says to turn on both the vol and the mic and after the tubes warm up, turn the mic off. Well, in my case, that will also turn off the exciter lamp. Still do not have my service manual, but have been studying the wiring diagram that is online. I don't know very much but I am learning. The resistor picture in my first post got extremely hot with the mic and vol controls on for a few mins. Then the fuse blew. According to the diagram, this resistor was replaced as the orig one had two resistors in its place. Will be looking to get it replaced, however, why is it getting so hot? Bad resistor? Obviously now it is. Or is something else causing it.

[Maurice Leakey]

I do have the original owners manual and it says to turn on both the vol and the mic and after the tubes warm up, turn the mic off. Well, in my case, that will also turn off the exciter lamp.

It does seem that there is something wrong with the wiring. I would have thought that the exciter lamp would go off when the microphone circuit is switched in. This would obviate any unwanted noise whilst doing a commentary over a silent film.

You say that the resistor you installed replaced two resistors. I suggest that the two were installed for a reason.

Have you a radio repair shop near you who could check out the amp? Some years ago I bought a modern 16mm projector which had a transistor amp. The exciter lamp would not light up. I took it to a local radio repair man who at first said he didn't repair film projectors but relented when I said it had amp trouble. He fixed it within a few days with new electronic parts. It's still going strong.

[Brandon Brown]

Good day Maurice, I did not replace the resistor. It came that way and while comparing it to the wiring diagram, noticed that the diagram shows two resistors instead of the one currently there. This is the on that gets hot. So in response to the exciter lamp going off with the mic on, this would not be necessary for a silent film d/t the projector would be switched to silent speed and the bulb may not even come on. I totally agree with the wiring. I can see that the threading lamp is wired into the wrong tube but again still waiting for the manual to come. I'm kicking myself as 11 yrs ago I through out our orig premier 20 d/t it was ruined by a flood. I could have used it as a road map. It also had a brand new transformer in it. That is how we ended up with this one. It is a passion project. As far as repair shops around here. Not for 30 years. This whole area has died out. I did however, may have found an amp repair business about 45 mins away in Pennsylvania that has been around since the 60's. Have not called and inquired. They will probably say that they only work on guitar amps. It says that they are Fender and Marshall certified, so they must know what they are doing. Will possibly look into that later. Still want to try and figure it out on my own first, without doing any harm.
Thanks for the reply and hope all is well.

[Patrick Ryan]

I have an Ampro Premier 20 and have spent a lot of time recapping, replacing resistors, wiring, Jones plug, et al. The main motor runs great but whenever I turn the amp on it blows. I can't test anything in circuit because it never holds the power long enough! I need to take it all apart and check the transformer and switches now, but being a learner, its been demoralising to keep plodding on with no result. Going to try one more time, hey ho.