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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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8th May 2009, 2:44 pm | #121 |
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Re: Roberts RT7 - problem
The AC128's have a TO-5 layout of 'pin out' as shown in http://www.vintageradio.me.uk/info/germanium.htm#cases. The OC81M's I had suspected, as was conformed by another poster, as having a grey line on the jacket in place of the paint mark against the collector pin. I still think this may have been correct, and believe I did the install in the correct order.
However, I have contacted Cricklewood for confirmation of the 'pin out' on the AC128, and as both Tim and Graham have stated, the metal tag protrudes nearest the emitter. Prior to testing, battery Voltage at 9.6V Results of checks: Bias: Followed instructions exactly, with TR6 & TR7 NOT installed, meter set to 0 - 20V DC Red lead to chassis and black to junction of R26 & R27 = 0.2V Voltage check on TR5; Actual/Service sheet Base: 1.87V/1.69V Collector: 9.15V/8.7V Emitter: 1.75V/1.6V Current: Meter set at 20mA range (3 o'clock on my meter) = 0.00 Meter set at 10ADC (with plug moved to correct socket) = 0.01 (The latter being about the same as current passed through my body between fingers held on probes). After testing, battery Voltage = 9.48V. How are we doing? |
8th May 2009, 3:14 pm | #122 |
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Re: Roberts RT7 - problem
That all looks OK apart from the last test. I'm surprised that you didn't get a reading on the 20mA range. 0.01 amps is 10mA, so I would have expected to see a reading on the 20mA range. I assume the set was turned on? Are the normal sockets the ones to be used on the 10mA range?
I guess the next thing to do is fit the output transistors. Then switch on whilst monitoring the current drawn from the battery. If OK check the voltages on the output transistors.
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8th May 2009, 5:57 pm | #123 | |
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Re: Roberts RT7 - problem
Quote:
I switched off and then tried the bottom and top sockets with the dial at 10ADC range = 0.01 I'll run another check, and if I find anything changed I'll post again. If not, I'll put the AC128's in and see what happens. Derek PS Picture of meter in Post 93 Page 5. |
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8th May 2009, 7:36 pm | #124 |
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Re: Roberts RT7 - problem
OK The TR6 &TR7 have been installed.
Battery Voltage 9.6V Switched on with meter in 20mA range and set up between battery + and radio, same result as before = 0.01 Switched off, removed meter, and connected radio to battery. Switched on, no sound from speaker, on either Long or Medium, went up and down the bands with no result - no sounds. No sound from speaker when removing battery clip and replacing. Checked Voltage at TR6 & TR7 between chassis; TR6 Emitter 0.18V Base 0.00V Collector 8.95V TR7 Emitter .19V Base 0.2V Collector 9.48V Switched off, battery reading 9.57V. None of the components seemed to get warmed up, the new TR's were just about body temp, no more. I must be missing something. Derek PS I'm thinking if the base from TR6 & 7 connect to the T1 transformer, and one shows 0.2V and the other 0.00, might there be a problem with the T1? Last edited by stonehopper; 8th May 2009 at 7:44 pm. |
8th May 2009, 7:47 pm | #125 |
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Re: Roberts RT7 - problem
The bases of TR6 and TR7 are connected together through the seondary winding of T1, so they should both have the same voltage (0.2V) on them. Check your soldering of T1 and TR6 base carefully.
The readings on the collectors and emitters of TR6 and TR7 indicate that the primary winding and one of the secondary windings of T2 is OK.
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8th May 2009, 7:55 pm | #126 | |
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Re: Roberts RT7 - problem
To quote from post #66:-
Quote:
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8th May 2009, 8:40 pm | #127 |
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Re: Roberts RT7 - problem
I have gone back and rechecked the soldering. It seemed OK. So with power on I began to gently tap the TR's and C's on top with a plastic tool. Then went under the board to do the same on the joints. Still no squeaks or pops, so went to check the Voltage readings on the TR6 & TR7 again. Felt around and all cool to touch.
TR7 E 0.08V; B 0.23V; C 9.24V TR6 E 0.18V; B 2.78V - at which point I stopped, because that read zero before. I then went back to check TR7's Base again, as I thought I might have got the decimal point in the wrong place. It read 4.6V and rising. I then put my finger on the TR6 & TR7, and now have a circular burn for it! Immediately unclipped the battery and left it alone. battery now at 8.73V. T2 was definitely warm to touch now. (Finger OK, not stinging, just a shiny circle!) Will leave it for tonight. Derek |
8th May 2009, 10:10 pm | #128 |
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Re: Roberts RT7 - problem
Sounds like we're back to square one. Best to check whether T1 secondary is faulty.
Remove TR6 and TR7. Meter on 200 ohm range. One lead of meter connected to junction of R26 and R27. Other meter lead connected to point to which TR6 base was connected. Note reading. Other meter lead connected to point to which TR7 base was connected. Note reading. If you get an over range indication switch to the 2000 ohm range. Report readings here. Both halves of the winding should show continuity. Meter on 2000mV (2 volt) range. Red lead of meter connected to chassis Turn on the set. Black meter lead connected to point to which TR6 base was connected. Note reading. Black meter lead connected to point to which TR7 base was connected. Note reading. If necessary switch up or down a range for a more accurate reading. Report readings here. You should see about 0.2V (200mV) on each base track.
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9th May 2009, 11:27 am | #129 |
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Re: Roberts RT7 - problem
OK, chores done, back on the set.
Exactly as instructed; 200Ohm range: R26 & R27 - where; TR6 base was connected = 34.5 TR7 base was connected = 37.5 2000mV; TR6 base was connected =194 TR7 base was connetced =195 Did not try any other range - are these accurate enough? Derek |
9th May 2009, 1:21 pm | #130 | ||
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Re: Roberts RT7 - problem
Quote:
Quote:
I am at a loss to explain why you saw first 0V and later 4.6V on TR6 base. Visually check for breaks in the track going to TR6 base and contacts with other tracks. ----------------------------------------------------------------- While the output transistors are out check the windings on T2:- Resistance readings are always measured with the set turned off. T2 primary. Measure resistance between chassis and the points where TR6 and TR7 emitters were connected. T2 upper secondary. Battery removed. Measure resistance between the negative battery lead going into the set and the points where TR6 and TR7 collectors were connected. Measure the resistance of R30 and tell us what its colour code is. T2 lower secondary. Disconnect one lead of the speaker. Disconnect one end of R28. Measure the resistance between the speaker wires going into the set. While R28 is disconnected you might as well check its value. Reconnect speaker and R28. ------------------------------------------------ Now for some voltage readings. Voltage readings are always taken with the set turned on. T2 primary. Measure the voltage between chassis and the point where the TR6/7 emitters were connected. Expect a low or zero reading. T2 upper secondary. Measure the voltage between chassis and the point where the TR6/7 collectors were connected. Expect to see the full battery voltage. --------------------------------------------- It would be a good idea to check the operation of your meter's 20mA range. Battery out of set. 2.2K resistor (a spare one, not the one in the set) to positive of battery. Positive lead of meter to other end of resistor. Negative lead of meter to negative side of battery. Expect a reading of about 4 to 5mA.
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9th May 2009, 3:00 pm | #131 |
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Re: Roberts RT7 - problem
The AC128 connections are as the picture viewed from underneath.
A temporary way to find out if it is the transformer would be to disconnect the two emitters from a, b, windings and use just one of the AC128’s as it will need an NPN such as an AC176 as well as the AC128 to make a push-pull output. It does work as I have modified a radio with a duff transformer and it still gave acceptable volume. Fit two resistors (quarter Watt will do, low value 2.2 Ohm minimum as shown or even up to 5 Ohm each (this is to limit the current through the transistors) Disconnect the speaker from the transformer secondary winding , but leave the feedback resistors in circuit Connect the emitter resistor common junction to the negative end of an electrolytic capacitor (value not critical 100uf to 1000uF 12Volt) The capacitor is to block the DC. Connect the positive end to the speaker, This will produce a mismatched output (that is why the output-matching transformer is there) but if there is a signal, it will show up. It is shown with the negative at the top of the diagram The approximate circuit is in the diagram. Geof |
9th May 2009, 3:19 pm | #132 |
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Re: Roberts RT7 - problem
Firstly, the explanation to why my reading of 0.00V on the base initially might well have been down to not making sufficient contact to the chassis with the probe. On the second attempt, I had to jiggle the tip into the metal and voila! Down to me I'm afraid.
Results:T2 primary measured on 20k Ohm range TR7 - 0.01 TR6 - 1 .00 i.e. open circuit T2 upper secondary TR7 - 0.01 TR6 - 0.01 R30 code: Red Violet Brown Silver = .37 (20k Ohm range) Voltage: T2 primary; chassis to emitters - TR7 = 0V TR6 = 0V Upper secondary; TR7 = 9.23V TR6 = 9.23V Meter check revealed a zero reading using the resistor connected as instructed. Duff meter range? Attached AC128's with tab by emitter. |
9th May 2009, 3:37 pm | #133 | |
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Re: Roberts RT7 - problem
Quote:
Recheck the meter current range, these do sometimes have a fuse inside which can blow. R30 = 270 Ohms Red Violet Brown Silver 10% tolerance Geof |
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9th May 2009, 3:39 pm | #134 | ||||||
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Re: Roberts RT7 - problem
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9th May 2009, 3:49 pm | #135 | |
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Re: Roberts RT7 - problem
Quote:
Why not check out my circuit, it will prove one way or another if the transformer is faulty. And if it gets the radio working at reasonable volume you may even leave it that way, which is the equivalent of amore modern circuit. Geof |
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9th May 2009, 3:50 pm | #136 |
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Re: Roberts RT7 - problem
Geof and I seem to be in agreement.
Look at the picture of the meter in post #93. It's set to 20K with the leads not touching. Reading is "1 .", not 1 .00.
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9th May 2009, 3:57 pm | #137 | |
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Re: Roberts RT7 - problem
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But to measure 270 and such it needs to be on the lowest Ohms range of course Had a job finding that post, so far back Geof |
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9th May 2009, 4:04 pm | #138 |
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Re: Roberts RT7 - problem
If the reading was 1.00 rather than 1 .00 this would indicate a resistance of 1K which would indicate a faulty transformer. 1K is far too round a figure though. I'd expect something like 1.03K or similar.
Once we get confirmation of the readings from stonehopper we should know more about the state of the transformer. ---------------------------------------------------- EDIT. Just noticed that the service sheet gives resistance readings for the windings. T1 secondary 35 ohms each half. (Measures OK) T2 primary 2.5 ohms each half. T2 upper secondary 0.5 ohms each half. T2 lower secondary 0.2 ohms. We need readings taken on the 200 ohm range.
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9th May 2009, 4:53 pm | #139 |
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Re: Roberts RT7 - problem
Sorry chaps, getting lots of exercise back and forth to the shed.
Dammit! Fuse gone in the tester. No spare. For some inexplicable reason I have been putting two zeros after the decimal point when showing open circuit. As you can see from that picture of the meter on Page5 post 93, open circuit with probes touching nothing = 1 . - not 1. or 1 .00 which is what I have been putting. Doh. Switching to 2000 screen reads 1 Switching to 200 reads 1 . two spaces after the 1 My error, now I need a fuse. |
9th May 2009, 5:14 pm | #140 |
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Re: Roberts RT7 - problem
That would appear to confirm that one half of T2 primary is open circuit. As a final check what are the readings for each half of the winding with the meter on the 200 ohm range?
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