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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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27th Aug 2015, 12:53 am | #1 |
Pentode
Join Date: Jul 2015
Location: London & East Sussex, UK.
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A frustrated Motorola
Hi there,
Does anyone have any experience of restoring these, and perhaps know of somewhere (ideally in SE London) where I can get a couple of them fixed? I am patiently trying to restore them myself, having got hooked into this hobby involuntarily via a) buying a classic car (Peugeot 304 convertible) and b) being stubborn and stupid enough to think that if I worked hard enough at it I would eventually figure things out. Alas, despite an decent enough education and having been reasonably successful in most things in my life so far, I have failed at electronics. I hate the fact you can't SEE any of it. ****** electrons, eh?! And I've bought more and more kit as I have dug myself deeper into this wonderful obsession ... All I crave is a BIT of success, a little sign that these inanimate machines are responding to my loving care and best intentions of bringing them back to life! Anyway I am prepared to put in the work. Ive loved watching countless YouTubers and followed their insights with geeky thirst; but I would like to talk some actual specifics to someone about the Motorolas, which after all turn up on eBay like every day. They are common as muck; but you can't even get the chassis off without desoldering a few things. So here's the findings so far: Having followed religiously the couple of threads on this (Livewire, you are an absolute radio god, I worship thee!) I have replaced the C19 and C20 caps; taken out a couple of last stage transistors TX5-8, and they all seem fine. I can play my phone through the last big capacitor and the small circuit around the loudspeakers (well, hey, what a Lazarus moment that was!), but everything is dead from the left side of R28, ie. not much going ooown at all, baby! I would welcome ANY thoughts, helps, tips and if you are a retired kind-hearted soul who lives in the Plumstead/Woolwich area, or "Blackheath" as the posho's try to call it, I would love to make you tea and listen to your whole life story. Thanks in advance |
27th Aug 2015, 6:47 am | #2 |
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Join Date: Mar 2011
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Re: A frustrated Motorola
It would help us to help you if you told us the model number of the Motorola radio.
If you dont know then post some photos and somebody may be able to identify it, they don't all use the same circuit. Mike |
27th Aug 2015, 8:57 am | #3 |
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Re: A frustrated Motorola
Would I be correct in thinking that the radio is one of the Motorola 112/114/122/124 series, which are all similar electrically. Two are Manually Tuned radios, the other two having a 5-push button tuner. Some models are Neg. Earth only, whilst others are Dual Polarity. The service manuals(all 4 covered in one manual) are available for £1.99 from VRSD. It seems as if the fault is in the RF/IF stages somewhere.
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27th Aug 2015, 12:07 pm | #4 |
Pentode
Join Date: Jul 2015
Location: London & East Sussex, UK.
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Re: A frustrated Motorola
Thanks both for the quick responses. This is the Motorola 114 Ive been working on.
Nothing working from the neck down. Please see attached photos, including a schematic with my annotations. The red lines on the right edge represent the only places where feeding in an audio from my mobile phone works. The last places you can play anything that registers in the speaker are the emitters of the two power amps, TX7 and TX8. Ive run a multimeter over them while in situ and also the TX6 P2U driver - plus the TX5 pre-driver (took this out to test) and they seem fine (I think) by those tests. I've taken out C19 (400 uF, 16v) and C20 (1,000 uF, 16v) but they seem ok. I replaced them anyway with modern electrolytics: 470, 25v and 1000, 25v respectively. Presume that is ok to do? I know there have been threads on this 112/114/122/124 series, but the most interesting one came to an unresolved end mainly because the op did not report back the ending ... like reading a novel and finding someone has torn the pages out. If you look at the schematic, the last place I can play audio through is the right-hand end of the R28 resistor. Why does it not play if I simply put the audio lead to the other end of this resistor To test the RF and IF stages, do I need yet further kit? eg. a function generator or something? A scope? (I've been looking for a Tektronix 465 retro-style; they are cheap as chips in the US, but few good ones for sale here on eBay!) Thanks in advance again. I promise to report back for the benefit of future readers and not leave the end hanging! You WILL find out whodunnit! |
27th Aug 2015, 3:02 pm | #5 |
Dekatron
Join Date: Jul 2008
Location: Charmouth, Dorset, UK.
Posts: 3,601
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Re: A frustrated Motorola
Would the Peugeot 304 not have used a valve radio?
Peter |
27th Aug 2015, 3:07 pm | #6 |
Pentode
Join Date: Jul 2015
Location: London & East Sussex, UK.
Posts: 118
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Re: A frustrated Motorola
No, the radio was originally a 112. I bought the car off the daughter of a deceased old lady in Dumfries. Everything on the car was orginal, but fragile. It had been garaged and hardly used (which for the engine was a problem at first! It had quite a few cobwebs to disperse).
Righto, I've tested most of the trannies now, including the IF - all fine as far as I can see. Any further thoughts? |
27th Aug 2015, 4:27 pm | #7 |
Dekatron
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Re: A frustrated Motorola
As always – the ‘half split rule’.
Feed an audio signal in at the volume control – that’ll tell you if the AF amplification stages are working. If nothing can be heard then take a signal out from the volume control and feed it into a separate amplifier – this will tell you whether the RF stages (left hand side of the circuit which I can’t really read as it’s a little out of focus) are working or not. This divides the radio in half and will tell you which half the fault is in – hopefully not multiple faults in both halves! |
27th Aug 2015, 4:37 pm | #8 |
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Re: A frustrated Motorola
On the circuit diagram are voltages at various points around the transistors.
Identify each of the wires from each transistor using the diagrams on the service sheet and comparing with the symbol for a NPN transistor. see attached image. the connections are also identified on the layout drawing in the service information. note that Tx6 is different it is a PNP type Then note down what voltages you measure for each transistor connection. Like this Tx8 e 6.45v b 5.8v c 0v Tx7 e 6.55v b 7.2v c 13.8v Do this for all the transistors starting from Tr8 and working back to Tr1. Then let us know the results. Mike Last edited by crackle; 27th Aug 2015 at 4:49 pm. Reason: correction |
27th Aug 2015, 7:37 pm | #9 |
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Re: A frustrated Motorola
You really need an RF signal Generator to test the RF/IF stages, which is all circuitry to the left of the detector diode in the diagram. Alternatively, do as Techman suggests, and take a signal from either the 'hot' -i.e top, or wiper (middle tag) of the volume control and feed it into a known good amplifier. Signals from an audio sources uch as a CD or MP3 Player can only be used to test the audio & output stages, i.e. from the volume control onwards.
A device called a Signal Tracer (basically a diode probe) can be used to check the RF/IF stages in the absense of an RF Generator, but to do this, an aerial must be connected to the radio, which is then tuned to a known station frequency. |
27th Aug 2015, 8:49 pm | #10 |
Pentode
Join Date: Jul 2015
Location: London & East Sussex, UK.
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Re: A frustrated Motorola
All great and fertile advice chaps, many many thanks ... I may look up a signal generator on eBay and see if I can nab one.
Re the transisters, today I took out all five from the left-hand side, ie. TX1-TX5 and tested them with a MM with an HfE tester. They all seem to be fine, so I popped them back in. If nothing else it has improved the speed and daintiness of my soldering and desoldering. Am quite nimble and quick now. You can't remove TX6-TX8 without enormous trouble, but yesterday I managed to put leads to all of them, testing for tranny type and for shorts. (I named TX6 "the Corbyn" as I did indeed notice it was a bit way out there and a Channel 4 News piece on the Labour leadership campaign came on just at that moment!) I will try all these wonderful ideas this weekend and report back as promised on all the points. Cheers again all I love this : |
28th Aug 2015, 9:13 am | #11 |
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Re: A frustrated Motorola
For future reference, to access/remove the driver & output transistors, it is neccesary to first remove the RH sise (as viewed from the rear) of the outer case. This involves removing the cable clamp(s) securing the power & speaker cables, and also removing two screws at the front, & one at the rear. The side panel can then be carefully lifted clear of the PCB (This may be soldered to the side panel), then the driver & O/P transistors can be removed. It goes without saying that this should only be undertaken if really neccesary. By desoldering transistor leads from the PCB, and making sure they are insulated from the board, they can be tested in situ. Voltage readings can also give a clue as to where the fault lies, since, e'g', the reading at the junction of the o/p transistor emitters should be approx. 6.5vDC (Half-supply)
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29th Aug 2015, 10:04 pm | #12 |
Pentode
Join Date: Jul 2015
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Re: A frustrated Motorola
Mike,
I have now done some work on the transistors on the 114. With 12v being pumped into the +ve lead and the -ve on the case. The results, with -ve probe of Ohm Meter on the radio case and the +ve on either base, emitter, collector etc is as follows: TX8: b: 0.1v which then goes to 0.0 e: 0.59v c: 0.1v which then goes to 0.0 TX7: b: 0.03v e: 0.59v C: 11.86v (I guess as close to the 12v being pumped in?) TX6: b: 9.21v e: 11.8v c: -0.07v TX5: b: 2.2v e: 1.54v c: 9.22v TX4: b: 1.37v e: 0.76v c: 2.19v TX3: b: 0.68v e: 0.0 c: 11.29v TX2: b: 1.75v e: 1.14v c: 11.09v TX1: b: 10.87v e: 0.21v c: 9.95v Bear in mind that I have taken out and put back in TX1, TX2, TX 3, TX 4, TX 5 as they are accessible. They all seemed to show a decent reading from the hFE setting on cheap MM. eg. TX1 = 80 hFE; TX2 = 62hFRE; TX3 = 128 hFE. Happy to take out all the caps as well and test them. Question to all: how do I test a choke/filter, or any transformer/coil either in or out of circuit with basic tools? Thanks in advance all DC |
29th Aug 2015, 11:32 pm | #13 |
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Re: A frustrated Motorola
There are some very funny voltages.
What are the transistors, pnp or npn, Ge or Si. The transitors may be AF117 et al with tin whiskers in them, they may test OK if the screen is not connected. |
30th Aug 2015, 9:41 am | #14 |
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Re: A frustrated Motorola
This is the point at which I sometimes ask for help but The voltages around TX6, 7, 8 all seem wrong so they could all be faulty.
The voltage on the base of TX1 is wrong, can you check this again or that you have typed the value 10.7 correctly, it should be 0.9 but 0.7 would be acceptable as the circuit states the measurements were measured with a 13.8v supply. If there was a problem with the figure you wrote down for Tx1 and it is in the region of 0.7 to 0.9 then the RF and IF section of the radio looks as if it may be OK. If you connect from the collector of Tx4 via a 0.5uf capacitor to the LINE INPUT of another working amplifier with the shield connected to the ground of the radio, then I would expect to hear the radio picking up stations with a few feet of wire connected to the aerial. I think you need to be looking at buying 3 new transistors for TX6, 7, & 8. Tx6 and TX8 are both the same P2U types which are PNP transistors. Tx7 is a A6D type which is a NPN transistor. These transistor type numbers are specific to Motorola and someone else may be able to advise the best equivalents to buy. But in the mean time you could try the test with another amplifier to see if the RF section of the radio is working. Mike edit; I have just seen that post 4 on this thread https://www.vintage-radio.net/forum/...ad.php?t=85156 it is suggested that BD433 can be used for Tx7 (the NPN type) and BD434 can be used for Tx6 & Tx8 (the PNP type) Can anyone else please confirm. Last edited by crackle; 30th Aug 2015 at 9:53 am. Reason: added info on other post |
30th Aug 2015, 10:38 am | #15 |
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Re: A frustrated Motorola
There seems to be a voltage difference of about 0.6V between emitter and base so the transistors will be npn and there should be many alternatives.
I think you have labelled TX1 wrongly |
30th Aug 2015, 2:09 pm | #16 |
Dekatron
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Re: A frustrated Motorola
Silicon rather than germanium but not necessarily npn unless that 0.6V is specifically base positive to emitter.
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30th Aug 2015, 2:37 pm | #17 |
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Re: A frustrated Motorola
To help others advise I attach a part of the circuit diagram.
Mike |
30th Aug 2015, 5:28 pm | #18 |
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Re: A frustrated Motorola
Thats better, now the dog can see the rabbit.
Harald of course is quite correct. TX6 must be dead or there is a bad connection with 2V e/b. |
31st Aug 2015, 12:56 am | #19 | |
Pentode
Join Date: Jul 2015
Location: London & East Sussex, UK.
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Re: A frustrated Motorola
Quote:
I hadn't realised any signal when through the pot; I thought it went straight to the speakers. Thanks in advance |
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31st Aug 2015, 2:03 am | #20 |
Pentode
Join Date: Jul 2015
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Re: A frustrated Motorola
They are NPN and PNP,
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