HMV radiogram no AM reception

[lesparapluies]

Hello, any advice on this appreciated: I've been doing some work on this HMV radiogram. First problem was the seized tuning capacitor, which I was able to solve thanks to advice gathered here. Radio still totally dead, so investigated and replaced the two AF115 / AF116 with AF125 and AF126 (VT4 and VT5). VT6 was already an AF126. The radio roared into life on MW, but within about half a minute the station faded and now all it will play is "between station" noise, with no stations obtainable. The noise responds to the tuning cap being turned, and to the ferrite rod being touched, but no stations whatsoever. Curiously, the FM band is working perfectly, so the IF would seem to be fine. There is also a bit of continuous random crackling from the left hand audio channel which I think is probably a bad cap or two. (Callins plastic body caps, which, depending on what you read, are either wonderful or total garbage..) I will probably replace these but I don't think that's the problem with the AM bands. I'm going to do some voltage checks tonight on the AM front-end but any ideas would be welcome, thanks.

[G6Tanuki]

The AM frequency changer is VT3- has the original whisker prone transistor here been replaced as was done for the transistors in the IF strip??

[Sideband]

Originally Posted by G6Tanuki

The AM frequency changer is VT3- has the original whisker prone transistor here been replaced as was done for the transistors in the IF strip??

Typical of the local oscillator not running so I would agree that it's time to change VT3.

Unusual to have a separate oscillator for AM. Normally only found in good quality Comms receivers.

[lesparapluies]

Thank you both for the replies. VT3 is actually an AF126 rather than an AF116, so I assumed no whiskers, but yes I think I will order a replacement AF126 and fit it. Off on holiday this week so it may be a couple of weeks before I reply again but I do appreciate the advice

[lesparapluies]

Have ordered a new AF126 but also noticed after a bit of probing that the 3 resistors around VT3 are all about 30-40% higher in value than they should be, so will replace these too

[lesparapluies]

Update - have replaced the 3 resistors associated with VT3 (R9, R10, R11) as they were very high in value. Fault remains. I tested the voltages on the VT3, which is an AF126, not an AF116, and the general feeling is that these tend to be reliable. The figures were: Base 0.75V (should be 1.1V according to the schematic). Emitter - 0.47V (1.0V) and Collector 8.6V (12V) - so these figures are also way out of tolerance. Waiting for arrival of "new" AF126, so would welcome any comments on these voltage figures. I can't think of anything else in the circuit which would cause the fault, just a little concerned that at least one contributor says he has never known an AF126 to fail..

[smiler411]

Is there a resistor feeding the collector that could be high resistance? The collector voltage and emitter voltage both being low doesn't seem right (collector voltages reduces with current through transistor while emitter voltage should increase) so seems whatever is feeding the collector is causing the voltage to be too low.
The voltage between base and emitter should be around 0.2 to 0.3 volts ish so looks OK suggesting transistor may be OK

[lesparapluies]

Just got back from holiday and re-visited the set. Have replaced VT3 but no luck - I didn't really think this one was faulty as it was an AF126 to begin with. I also renewed all the resistors around VT3. In general all the resistors are high-ish in value. Virtually all the voltages in the RF / IF stages are low, which I assume to be a result of the resistors combined. But something which may be significant is that switching between FM and AM doesn't alter the voltage readings very much, if at all - for example the collector of VT4 should be 9.5 (FM) and 12.0 (AM) whereas it's actually 8.5-ish on all bands. I find I'm renewing components randomly which I know isn't the right approach, and to be honest my knowledge of IF stages is limited. I'm wondering if any of the AM cans have silver mica disease in the caps, as the FM band is working perfectly. I don't quite get the circuitry of the 2nd and 3rd IF amps, as both the FM and AM transformers seem to be active at the same time, wheres in the first stage they are switched in or out depending on the wave switches. I have a small audio amp which I use as a signal detector when repairing audio stages, and I believe it's possible to trace an RF signal by connecting a diode to the input of the amp.. if anyone can point me to anywhere else on these forums which would boost my understanding of this I'd be grateful.. I'm sure the fault is staring me in the face!

[smiler411]

If the voltages around VT4 are not changing when you switch bands have you checked switch S2 maybe it is faulty. May be why am oscillator is not running too

[lesparapluies]

Thank you, yes will investigate the switch. Last night I replaced the 2 dropper resistors R18 and R32 which were both about 40% off value, and the voltages are now more like what they should be. Strangely, when I switched the set on yesterday there was briefly the faint glimmer of a broadcast station on MW as I swept the band but this gradually faded over about 10 seconds. The set picks up very well the thyristor control of my soldering iron, and to the ferrite rod being touched, just no stations... more fun tonight

[lesparapluies]

No luck yet, but today I switched the set on while it was set on MW, and for about 10 seconds it received stations, but quickly faded to the usual continuous background noise.. I'm suspecting perhaps a faulty capacitor somewhere in the signal chain which is quickly failing when current is applied, would anyone else agree with this, or am I overlooking something else? Any thoughts appreciated, thanks.

[frsimen]

Check the following voltages:
1) The voltage at the junction of S1A and R32 (should be 23.5V)
2) The voltage at the junction of R32 and R18(should be 14.5V AM, 13.5V FM)

If the 23.5V measures lower by more than a volt or so, there is a problem in one of the AF amplifiers.

If the 23.5V is correct but the 13.5/14.5V is much lower than it should be, try replacing C50. If you have already changed C50, check that you have the polarity right.

If the problem remains, check the voltage drop across R20 (expect about 1V) and R25 (expect about 1.5-2V.) If either of these voltages is much higher, the associated transistor is probably faulty.

Paula

[lesparapluies]

Hi Paula, thanks for your reply. I will double-check those voltages tomorrow, but I don't think the fault lies with the power amps as the FM band works perfectly. And all the AF115/6 transistors have been replaced. C50 has been replaced. All the resistors around VT3 and VT4 have been replaced, and the values of the caps checked out of circuit. This is really driving me nuts and I'm thinking the only way forward is going to be to get (or build) an RF signal detector and try to trace it through from the front end... but will keep on trying!

[60 oldjohn]

I take it that you have cleaned the wave change switch? Have you checked the diodes? Some may be in the IF cans. Signal Diodes usually fail OC so connecting a good one in parallel should get the radio working to prove the point.

John.

[lesparapluies]

Hello John, all advice gratefully received. After watching one of David Tipton's videos last night I'm convinced that the oscillator is not running (or is failing after about 10 seconds running). This is the most frustrating aspect, as taking readings during the few seconds when reception is coming through is a very quick window of opportunity. And the set needs to be switched off for about half an hour before it will repeat this performance (see my postings from 3rd and 7th Oct)

[smiler411]

The oscillator not running could still be because the wavechange switch, the symptoms you describe are exactly what you get with a poor wavechange switch. If it works some of the time it is unlikely to be transistors or even capacitors.

[smiler411]

Earlier you measured the collector voltage on the oscillator transistor and it was lower than expected even though the current through it was low as shown by the low emitter voltage, this suggests a high resistance in the feed to the oscillator transistor which could easily be the wave change switch as the supply for the oscillator passes through the switch, so think this switch definitely needs more looking at as a likely cause of your symptoms

[lesparapluies]

Originally Posted by smiler411

Earlier you measured the collector voltage on the oscillator transistor and it was lower than expected even though the current through it was low as shown by the low emitter voltage, this suggests a high resistance in the feed to the oscillator transistor which could easily be the wave change switch as the supply for the oscillator passes through the switch, so think this switch definitely needs more looking at as a likely cause of your symptoms

I'm going to check all the wave change switches tonight. However, another development is I think I've been obsessing too much about the oscillator stage, whereas I noticed yesterday that the voltage on the collector of VT5 is only 7V on both FM and AM, so also looking into that part of the circuit (although FM performs well).. it's strange but passes the time while my wife watches Strictly

[smiler411]

Suppose IFT2 could be faulty to cause low volts here possibly enough capacitive coupling to work at FM is FM coils OK, what is the emitter voltage is it high suggesting biasing wrong or low suggesting problem with IFT2

[frsimen]

Could you tell us the voltages that I mentioned before, please?

1) The voltage at the junction of S1A and R32 (should be 23.5V)
2) The voltage at the junction of R32 and R18(should be 14.5V AM, 13.5V FM)

Include the voltage at the other end of R18 and the voltage drop across R20 and R25.

It should then be possible to point you in the direction of the fault.

Paula