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Old 11th Mar 2016, 4:13 pm   #1
longneck90
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Default Pam TB77

Hello to everyone. I am not sure if this is the correct place for this query. To Forum Moderators, please move if not OK. At Harpenden, I bought this Pam radio. It had suffered burning in the audio output area. R24 was destroyed by burning. R25, R26, R27, and C32 were blown to pieces. My problem is, what to replace TR5, TR6, and TR7 with. They are described as; TR5 - 'Black Circle Seven 5. TR6 - Black Circle Seven 6. TR7 - Black Circle Seven 6. The 'Trader' service sheet says that these transistors have been replaced on later radios by TR5 - NKT252, TR6 and TR7, by - NKT251. So far I can't seem to locate these transistors, or alternatives. I was hoping that some Forum member might be able to advise me about alternatives. All the best. John.
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Old 11th Mar 2016, 5:49 pm   #2
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Default Re: Pam tb77

NKT 252/ 251 = AC128 as far as I am aware.
John
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Old 11th Mar 2016, 7:25 pm   #3
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Default Re: Pam tb77

If you want to go the cheaper option of silicon, you could try SS8550.http://www.ebay.co.uk/itm/5x-transis...3D181789513840
John
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Old 12th Mar 2016, 12:50 am   #4
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Default Re: Pam tb77

Hello to all. Thanks 'jonnybear' (John) for information about transistor substitutes. I will try the well known electronics place in Cricklewood tomorrow. Thanks again. all the best. John.
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Old 12th Mar 2016, 9:46 am   #5
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Default Re: Pam TB77

That sounds like a lot of damage for a battery set to suffer.

Make me wonder if someone has directly connected mains to it

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Old 12th Mar 2016, 2:10 pm   #6
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Default Re: Pam TB77

Re reading your first thread, as Mike says, if it has had mains connected to it and C32 is blown to pieces, you could also be looking O/C transformers.
John
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Old 12th Mar 2016, 9:39 pm   #7
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Default Re: Pam TB77

Hello to all on Forum. To Cobaltblue (Mike T.) and jonnybear, (John). Thanks for all your information. As I said initially, Rs 25 / 26 / 27/ and C32 were blown to pieces. But also, R24, which is an adjustable resistor, was damaged. This resistor consists of fine wire wound onto a straight steel band, and that acts as a slider for an adjuster. The adjuster is slid forward or backward on the wire-bound band to regulate the output current. (I presume) More than half of the wire that is wound onto the steel band has gone, and more than a quarter of the steel band itself is gone, due either to burning, wear, or other means of deterioration. The wire that shredded from this resistor, may have come in contact with some part of the printed circuit, and caused the output transistors, resistors and capacitor to be destroyed. I almost forgot to mention. I got transistors and installed them today. The audio output seems to be working fine. There is however, a lot of noise. Stations can be tuned in, but not clearly, due to this noise. When I attach Signal Generator at C5, the output is clear. Ie. No noise. Anyway, I'll persevere, and see what happens. All the best. John.
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Old 13th Mar 2016, 12:18 am   #8
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Default Re: Pam TB77

Hi John
You will need to check quiescent current of the output transistors by breaking the connection between -9 volt rail and centre tap of the output transformer T5
then inserting a MA meter and adjusting R24 for a reading of 15ma, with no signal. You can use a 200 ohm skeleton preset in place of the slider.
The noise you have noted, is it there with the volume turned down, if so Tr4 could have gone noisey. I take it the noise is a loud hiss?.
John
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Old 13th Mar 2016, 1:21 am   #9
Edward Huggins
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Default Re: Pam TB77

Ahh, those Newmarket Germaniums. Yes, and I actually held the very first PAM portable radio prototype back in Radio Development, St. Andrews Road, Cambridge...
C. O. Stanley would not allow the Pye name on it in case Transistor Radios failed to take off and it might have damaged the Pye brand. He soon changed his mind! Edward
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Old 13th Mar 2016, 2:08 am   #10
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Default Re: Pam TB77

Signal generator to C5 takes in almost all of the radio. Assuming reasonable signal levels it sounds to be a problem with the aerial circuits or switching arrangements. Does connecting an external aerial directly to C5 help?
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Old 13th Mar 2016, 3:28 pm   #11
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Default Re: Pam TB77

Hello to all. Thanks to jonnybear (John), Edward Huggins (Edward), and AC/HL. I did monitor current yesterday, and could only reduce it to around 20mA with adjuster. I'll get an alternative adjuster/resistor. The 'noise' varies with Volume Control, right down to silence. It is not a 'hiss' type noise. More like a boat engine. It varies as a station is looked for; like as if the circuitry was unable to fully resolve ? the station. I have connected an aerial to C5, and there is a definite improvement, but still much noise. I will get some type of current adjuster tomorrow. Thanks again, and all the best. John.
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Old 13th Mar 2016, 5:18 pm   #12
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Default Re: Pam TB77

Switch it over to PU is the noise still there?, you could even plug in a mp3 player to check. if the noise is not there with the radio switched to PU then the fault is in the front end RF/IF section, the cause can be any black or brown hunts capacitors as these along with waxies are nearly always the cause of instability,or it could be a dried up electrolytics.
John
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Old 13th Mar 2016, 9:48 pm   #13
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Default Re: Pam TB77

Hello to all. To John (jonnybear), I don't have anything to plug into PU/GRAM, so I decided to try Signal G. instead. Nothing. I then noticed that R27 was heating badly. I disconnected power, and have now discovered the radio is totally dead. I haven't had time to investigate yet. Tomorrow I will look further. Thanks for all the help and information. All the best. John.
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Old 14th Mar 2016, 1:02 am   #14
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Default Re: Pam TB77

If R27 is heating up you must be drawing a lot more than 15ma, what sort of battery are you using?. hope you are not using a smps, if you are that is where your problems lie.
John
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Old 14th Mar 2016, 1:32 am   #15
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Default Re: Pam TB77

Hello to all. To John (jonnybear), What I am using to power radio, is, a regulated, variable, power supply, that produces 4.5, - 6, - 9, - and 12 volts. I have used it to power various devices, so I thought it was OK. Thanks John for your time and information. All the information from you and other Forum members will be very helpful. All the best. John.
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Old 14th Mar 2016, 9:59 am   #16
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Default Re: Pam TB77

That PSU should be OK, provided it is set to the correct voltage and polarity, the latter being down to ensuring that the o/p leads from the supply being connected with correct polarity to the battery leads of a vintage radiio such as your PAM TB77, which presumably doesn't have a DC power socket.
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Old 14th Mar 2016, 11:00 am   #17
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Default Re: Pam TB77

Hi John
With reference to the power supply, if it is a switch mode power supply, that could be where the noise was coming from, if it is a transformer version then that is fine. If you need a small skeleton pot I have a 300 ohm ceramic that will do the job for R24 that is if you can not source locally. A little tip put a 12 volt 1 watt bulb in series with the power supply this will limit current while you are working on it and provide a warning if you start drawing current. Another item I would add to the radio is electrolytic between - and + rails, as the radio was originally designed for battery use only,so one was not required, not having this could cause ripple which may damage transistors.
John
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Old 15th Mar 2016, 12:16 am   #18
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Default Re: Pam TB77

Hello to all. Thanks to ' LIVEWIRE', and (jonnybear) John, for all the information. Thanks John for offer of 300 Ohm Adjuster/resistor. I found one (250 Ohm) today in a electronics store. The 12 V, ! W bulb is a great idea. (when I get a bulb). I can only power up for 3 or 4 seconds, or R27 will disintegrate. I don't believe I have made a mistake with the legs of the AC142s. However, where there is supposed to be 22 Ohms between the bases of the AC142s, there is over 100 Ohms. (around 120 Ohms) So there must be something wrong there. There is something else that is exercising my mind. The original power output transistors (I think they were NKT252) were not insulated from the heatsink. Accordingly, with the new ones (AC142s), I didn't insulate them from the heatsink. Now I am wondering, should I have insulated them? Thanks again for all the information and offer of potentionmeter. All the best. John.
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Old 15th Mar 2016, 3:16 pm   #19
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Default Re: Pam TB77

Hi John
I do not think the AC142 transistors need insulating from the heat sink.
It looks like you could have cooked the AC142s or at least one of them if R27 heats up in a couple of seconds.
Going back to your original thread (burn up)and not being able to get the quiescent current down to 15ma.
I would check the driver transformer and the output transformer, and compare the resistance of the windings, as there was a burn up originally there could be a shorted turn or o/c winding. other than that the output stage is fairly straight forward. The AC142 leadout is E B C the collector being the lead nearest the little tag.
John

Last edited by jonnybear; 15th Mar 2016 at 3:21 pm.
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Old 15th Mar 2016, 10:12 pm   #20
longneck90
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Default Re: Pam TB77

Hello to all. To John (jonnybear). I installed 12V - 1 W, bulb in series with power line. It is a great help, as one can monitor, as it were, current usage. At the moment, there is no current being registered, either by the bulb, or the DVM. I have removed Base and Collector legs of AC142s one at a time, to see how that affected R27. When I removed one Base leg, R27 stopped heating. I then replaced all the legs, and R27 is not heating now. But then, of course, there is no current being registered. I have checked voltages on all legs of AC142s, and they seem very close to what is in 'Trader' service sheet. I have also disconnected T4 and T5, (taken wires off) and checked resistance, and they all seem OK. There is no sound from speaker when I attach Signal G to legs of AC142s. Well... when I attach Signal G to AC142s, you can hear the output from SG in the speaker, but it isn't being powered by the radio. Thanks to John, and everyone , for your help. See what happens tomorrow. All the best. John.
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