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  • #8513
    Vince Cappuccitti
    CVRS Member

    I have recently picked up a Sentinel radio and I’m having problems with a nasty hum. This is a portable with option of plug or battery. I have replaced all the caps and a couple resistors. The radio had 2 Electrolytic caps of 40 uf that I replaced with 47 uf caps and I’m wondering if this value is too high and may be causing the problem. The hum is constant / stable throughout the volume range.

    Fairly new at this and would appreciate your advice. I did some voltage checks on the tube pins and found them all to be higher than listed readings from the schematic/manual. The tube compliment is 117Z6GT, 3Q5GT, 1H5GT, 1N5GT x (2) 1A7GT.

    Look forward to your insights.

    Forum Participant

    I think you are just fine with the cap size. others might help more with the source of the hum. if you have any fluorescent lights in the house try to turn them off as they emit horrible interference. If you suspect the voltages are out , try checking the resistors, in particular any that are physically large. If you re stuffed the caps you might be ok but if you put them elsewhere I guess there could be a chance of interference from the wire routing but I dont’ know if that is the first things others with more experience might suggest. I think you have some 1 volt tubes there and I think sometimes people try to check the filaments with a digital meter and blow them because they are only one volt and the meter puts out 9V from it’s battery. of course if you replaced the electrolytics you paid attention to which way around they were wired right?

    it could be that the alignment is off and the radio volume is turned up to compensate but I dont know that you should start trying to align it without more instruction as it will almost always mess things up if you dont’ have some direction to do it properly. Often it’s best for those who are inexperienced not to turn any screws. some radios have good alignment instructions and some assume you know things. one way to ease into it is to start with radios that explain the procedure thoroughly.

    Its probably more likely that it is not an alignment issue anyway so I’d suggest not turning screws for now but if you have already , then that’s different as that could definitely throw it way off. sometimes people fiddle with them and make it not work right or not at all. . most radios will play pretty well without touching them.
    Were any of the caps you changed inside the IF cans? maybe that would affect alignment if you ran leads under the chassis?

    a very simple thing that anyone can try is to pull the plug and flip it over and plug it back in the other way if the plug is non polarized it can make a difference which way. often people will put a polarized plug and a fuse on it and of course hook it up the right way around and I believe one could say the hot wire or smaller prong goes to the switch usually. sometimes there is a cap near the line cord to chassis and you could look at this:

    there is a note on Justradios site about hese safety caps here and that might relate.
    if you read where it says:
    Y2 Safety and Interference Suppression Capacitors

    your post was quite old so I responded but others here might chime in with better advice. so in that month or so did you get any further? some others here could talk circles around me and give better advice but your post wasnt’ quickly responded to.

    Vince Cappuccitti
    CVRS Member

    Phil, thank you very much for responding to my post. I have not worked on the radio since posting due to not being sure how to proceed. What I didn’t mention is that the radio works very well with excellent reception on many stations and very good volume. Only the loudness of the hum is the problem.
    The caps I replaced were not stuffed into the old cans but instead are in the underside of the chassis with the other components now. I left the cans in place and cut off the connections at their base. I did pay attention to the negative for the caps. I shifted the caps around within the chassis but it doesn’t seem to have any effect on the hum. Should I remove the cans and put the new caps on the topside of the chassis? could that help?
    I didn’t check the tube filaments with a meter but did check them on a tube tester and even tried a couple new tubes with naive hope that would be the problem. Thanks for pointing out the possibility of burning out the filament with a 9v meter. would never have thought of that.
    Are there caps in the IF cans? If there are I didn’t change those. Should they be? I will take a look at the schematic again.

    I am using an isolation transformer and a dim bulb tester (not sure of the terminology)when I work on these old radios so reversing the plug didn’t help. I have tried turning off the flourescents as well with no change.

    The justradios link is interesting with a lot of information to absorb and understand. I will take some time to dig into it.
    I’ve checked some of the resistors but I’m not sure how far out of tolerance is acceptable. I did use a digital meter to do this. Could that have done something if the tubes were not removed.

    The radio didn’t work at all when I got it, has taken a lot of effort to get it to this point and I would hate to give up on it when I’m so close.

    Thanks again for responding Phil.

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