Transformer output in PYE P53 radio

[ms660]

The voltages so far aren't that far out except for the AGC voltage, if the RF/IF/Det stages are working ok then the AGC voltage will depend on the signal strength at the antenna.

So far it's still chicken or egg, if you're using Bluetooth for the gram input signal then I would re-connect the -ve feedback circuit, connect the Bluetooth output to the top end of the volume control (as viewed in the schematic) via a 0.01 or 0.047uF capacitor, set the tone selector switch to the B position, switch to gram input and see what the volume is like from the Bluetooth with that set up.

Lawrence.

[Nuvistor]

Hi Nicola,
You will soon be looking for a signal generator and a scope.

Joking apart its a times like these that they are so useful, perhaps something to think about for the future.

Agree with Lawrence, you need to confirm audio or IF/RF fault.

Frank

[Aerodyne]

I've assumed here that you do not have a signal injector of any kind such as an RF/AF generator. You might like to try this simple test: take a capacitor of, say, 0.01-0.1uF and, holding one lead wire in your fingers, touch the free lead on the output valve control grid. Do the same with the AF amplifier grid. Needless to say - have volume up at max and do NOT allow your other hand to contact the chassis or any part of the receiver. You should get a medium to loud 50Hz hum at the output valve grid and a considerably louder 50Hz hum at the AF amp grid. If you do, this rules out an AF fault. If you do not get a loud hum, or if the hum level is much the same whichever grid is contacted, you definitely DO have an AF problem.
At least this should help narrow the field a little.
The fact that V1 triode section acts as an audio pre-amplifier means that when switched to 'gram' the same capacitor to grid test can be carried out and should provide even louder 50Hz hum. Again, if there's no improvement in the volume level with the hum test between the AF amp grid and the triode preamp grid, the triode circuitry should be investigated.
Hope this makes sense...
Tony

[indigo.girl]

Thanks Tony - I've been away and just getting back to this now. I've done the test (hopefully correctly!)

On GRAM:
V1 pre-amp control grid (triode) = pin 6 = LOUD HUM
V2 IF amp control grid = pin 6 = QUIET HUM (higher pitched)

On RADIO:
V4 output amp control grid = pin 6 = LOUD HUM
V2 IF amp control grid = pin 6 = silence!

Hopefully I've selected the right valves and pins based on your description.
I have absolutely no idea what the concept is behind this test either - could you give me a quick laymans guide to what I've just tested?

Looking forward to seeing what people make of these results....

Nicola

[ms660]

You won't get much hum from the control grid of V2 because it's an IF amplifier, not an audio amplifier.

The idea of that hum test is to get an idea how each of the audio stages is working amplification wise, normally with that receiver circuit you would start at the output valves control grid by injecting the "hum" signal there and try to memorise the volume of hum coming out of the loudspeaker, the next place to inject the hum would be at the input circuit of the second audio amplifier (V3) the best place to inject would be at the top end of the volume control (as viewed in the schematic) with the volume control turned up, if the hum is then too loud (risking damage to the loudspeaker) then reduce the volume to a comfortable level and memorise that level, if all's well so far then the next place to inject the "hum signal" would be at the gram input and compare the hum level in the loudspeaker with the hum level from the previous test. it should be more but possibly by not by a great amount because the anode load resistor for V1's triode is a relatively low value (10k) compared to V3's anode load (220k), don't forget that it's the AC voltage (the signal) that's developed across the anode loads that count because it's that voltage that feeds the next stage.

Using the human body via a capacitor as a 50Hz signal source is ok when you know what to expect eg: knowing what's what etc because the amplitude of that source can vary due to the influences of impedance, here's a trick if you want a 50Hz test signal at a fixed amplitude in an AC only receiver, there's already 50Hz at 6.3 volts across the valve heaters so you could connect a couple of resistors across that supply to form a potential divider, the resistors mustn't be a stupidly low value otherwise the mains transformer might cook, I would suggest something like a 6.8k in series with a 1k, connect the free end of the 6.8k to the heater supply and the free end of the 1k to chassis, take the 50Hz test signal output from the junction of the two resistors via a 0.01 or 0.047uF capacitor (value not that critical) that should give test signal voltage of approx 0.8 volt which wouldn't be that far removed from the voltage output of a crystal pickup cartridge and it's amplitude will be more or less constant.

Failing that then use the output from a Bluetooth job for a test signal.

All tests in gram mode of course.

Lawrence.

[ms660]

Ref my previous post regarding obtaining a fixed amplitude 50 Hz test signal from the heater supply via a potential divider and capacitor, forgot to say that if you are holding the capacitor and using the free end as the test probe then ideally the capacitors outer foil connection should be connected to the potential divider, sometimes the outer foil end of the capacitor will be marked with a band, some times not, to determine the outer foil connection connect the capacitor you want to use between the top end of the volume control (as viewed in the schematic) and chassis, switch to gram and turn up the volume, grasp the capacitor's plastic body between thumb and forefinger and note the level of hum from the loudspeaker, then reverse the capacitors connections and follow the same procedure, the test that gives the lowest amount of hum means that the capacitor lead that's connected to chassis is the outer foil lead, that lead should be connected to the potential divider, that will ensure that any 50Hz induced by your fingers while holding the capacitor and probing around will be reduced to a minimum. For the capacitor use a tubular type with axial leads.

Ideally for this type of work you need a proper audio signal generator, the cheapest option for that would be a simple signal injector, these are simple enough to build and are also available as a DIY kit, the other option if you are getting into radio's is a service signal generator, these provide a tunable RF frequency that will normally cover all the RF and IF frequencies encountered in a receiver, such as those required for receiver alignment, they normally also contain an audio frequency generator, the frequency of that is usually fixed, normally 400Hz or 1kHz or thereabouts, the audio generator can be used as a stand alone test signal for testing audio stages or it can be used to modulate the RF section of the generator thus simulating a test transmission.

Hope this is of some use.

Lawrence.

[Aerodyne]

Sorry for late reply, Nicola. I've been away on holiday.
I think Lawrence has more than adequately described the methodology behind the 'hum injection' test. 50 Hz hum won't be amplified by RF stages, which is where your fault appears to lie. In other words the receiver (RF) part of your set is not functioning correctly, but the audio (AF) stages are OK.

What you do now is dependent on whether you have a signal injector of some type: a simple square wave oscillator can be made quite easily and this will generate harmonic waves through audio and radio frequencies.

Access to a 'proper' RF signal generator would allow you to inject through the IF stages, checking again where the gain drops. Otherwise, with no generating tools at your disposal, I believe you should check the IFTs for high resistance or open windings and (forgive me, I don't have the circuit at hand) the parallel capacitors that tune the windings for open-circuit, short-circuit (the latter is unlikely) - assuming voltage checks are, as you indicated, normal. Then it is a matter of working from the final IF through to the first IF, then the RF stages for tuning coil and switch faults - possible dampness in the coil formers or 'green spot' copper corrosion of the winding terminations.
Tony