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Old 29th Oct 2014, 5:00 pm   #1
paulsherwin
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Default Weird AM fault in Grundig Party Boy 700

The saga of my Party Boy continues.

Tony (Restoration73) kindly sent me a volume pot for this (requested here), and with a few minor mods it fitted a treat. I now have good FM operation.

However, a weird fault is affecting both MW and LW. Stations can be heard in the right places, but very, very faintly. I've unsoldered all the screening tinplate covering the IFTs and tested the detector diode - it's fine. All the switches have been thoroughly cleaned, and all the ferrite coil connections are good. I can't see any dry joints. The usual dodge of poking around components with a finger produces no obvious effects.

Before I fire up my ancient signal generator and do some signal injecting, can anybody think of some obvious gotcha that I've missed?

Probably unrelated, but near the last IFT is a component I don't recognise, labelled 'ST1' on the circuit diagram. It looks like some sort of diode and appears to be a voltage reference, but I don't understand how it works or what it's supposed to be doing. Can anybody shed light on this?
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Old 29th Oct 2014, 5:31 pm   #2
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Default Re: Weird AM fault in Grundig Party Boy 700

I would check to see if there is normal volume at the detector output with another audio amp. Could it be something as simple as an AM/FM switch problem?
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Old 29th Oct 2014, 6:07 pm   #3
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Default Re: Weird AM fault in Grundig Party Boy 700

Hello Paul,

St1 on my circuit diagram is identified as

BZ 102/2 V1 (hope this means something to you!)

Michael
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Old 29th Oct 2014, 6:56 pm   #4
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Default Re: Weird AM fault in Grundig Party Boy 700

Paul Stenning's service data DVD has an ERT sheet on this model and identifies ST1 as a 75C2/V1. I could not find any data on this device but when Michael came up with the BZ102/2/V1 I was able to find that that is a stabilizer diode which works in the forward biased mode as distinct from a zener. The 2 will be the nominal stabilized voltage.

I would speculate that the nominal bias for the IF transistors is developed across ST1 and the negative voltage from the detector diode (D1) is subtracted from that nominal voltage for AGC purposes.

This is pure speculation because there seems to be errors in the ERT sheet. The DC source for the IF collector circuits and base bias is drawn coming from chassis (the negative rail). I think perhaps one of the rails drawn as connected to chassis should in fact be a decoupled positive HT source.

The snippet below shows at least one more obvious error where C84 is shorted.
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Old 29th Oct 2014, 7:11 pm   #5
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Default Re: Weird AM fault in Grundig Party Boy 700

Many Grundig and NordMende radios of this era used similar stuff. Typically they have 0.4, 0.7, 1.7 v etc across them (Actual values off circuits). I think this one is a 0.7v type. Measure, and if they have any of the above voltages, stable, assume OK. You can just make up with Si and/or Ge diodes in series. I suspect I used to put a silicon diode in anti parallel, just like "making" a zener.
Les.
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Old 29th Oct 2014, 7:14 pm   #6
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Default Re: Weird AM fault in Grundig Party Boy 700

Mostly these are in the output section to stabilise. I never checked the IF section when looking just, but I think I have seen them there as well.
Les.
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Old 29th Oct 2014, 7:58 pm   #7
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Default Re: Weird AM fault in Grundig Party Boy 700

The circuit diagram in the service manual for the Grundig Party Boy 700 shows 1.9 V across ST1.

John.
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Old 29th Oct 2014, 8:06 pm   #8
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Default Re: Weird AM fault in Grundig Party Boy 700

Contacts g4-g5 on FM switch not making?

John
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Old 29th Oct 2014, 9:20 pm   #9
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Default Re: Weird AM fault in Grundig Party Boy 700

I will try shorting the contacts, or connecting the detector to the top of the volume control with a capacitor. I would be surprised if this is a simple switching fault though, because the symptom isn't in any way intermittent. I've tried vigorously switching between FM (the FM switch controls the AM/FM switching) and AM and nothing changes.

I appreciate the explanation of ST1 given by Colin and Les, but still don't understand how this component works or what it's supposed to be doing. Does it only operate on AM in this circuit?
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Old 29th Oct 2014, 9:39 pm   #10
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Default Re: Weird AM fault in Grundig Party Boy 700

It seems to be supplying the bias to the FM front end, so perhaps it's used as temperature compensation for the base-emitters?

John
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Old 29th Oct 2014, 9:52 pm   #11
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Default Re: Weird AM fault in Grundig Party Boy 700

Just expanding on what John has said, the FM front end (TR1 & TR2) is feed with bias directly from ST1, this line also provides TR3 with bias. TR3 is the AM mixer and an IF amplifier on FM.

TR4 and TR5 which are IF amplifiers on both AM and FM are fed with bias from ST1 modified by the output of the AM detector D1. This feed is direct to TR5 whereas TR4 is fed with DC from the emitter circuit of TR5. On AM only, the DC on this line will vary with the AM signal strength but on FM it will be determined by ST1 only.
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Old 29th Oct 2014, 10:19 pm   #12
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Default Re: Weird AM fault in Grundig Party Boy 700

OK, I see all that.

What's the significant advantage of using a voltage reference there though, as opposed to a couple of resistors? Why does the voltage need to be stabilised? We aren't dealing with lab equipment here, it's just a fairly basic transistor radio.

Also, what is the component? How does the technology work?
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Old 29th Oct 2014, 10:47 pm   #13
Leon Crampin
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Default Re: Weird AM fault in Grundig Party Boy 700

The voltage reference is fairly common in German radios of this period - Blaupunkt used it a lot. The idea is that a forward biased diode can be used as a low impedance bias source which thermally tracks (more or less) the b-e junctions in the transistors which are fed from it. This gives improved thermal stability to the stages fed from it, for fewer components.

Note that to provide AGC control, T5 is fed from the detector DC output ("sitting" on the bias line) with the preceding stage fed from its emitter. T5 is thus a dc emitter follower for AGC purposes as well as a common emitter signal amplifier. This is a clever scheme as it provides low impedance thermally compensated AGC for very few extra components.

The ERT sheet is just plain wrong. The inverted "T" symbol in these sets is a 6.9V positive supply derived from a zener in the power supply - the heavy line is negative.

These are good sets and well worth fixing. The bias diodes are notoriously unreliable and either fail completely or become noisy. As suggested in earlier posts, make a new one with forward biased Si and Ge devices to get the stated voltage. When, as is often the case, the diode fails o/c, you can connect your replacement "assembly" externally to the screened IF module on the back of the print - the connections are accessible.

Open the module at your peril - the print is fine and you need a big iron to remove the screen...

Leon.

Last edited by Leon Crampin; 29th Oct 2014 at 10:55 pm.
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Old 29th Oct 2014, 10:57 pm   #14
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Default Re: Weird AM fault in Grundig Party Boy 700

Actually, removing the screening metalwork wasn't that difficult - I did it with an 18W Antex, a solder sucker and a screwdriver. No print has lifted. The radio seems to work perfectly well with the screening removed, at least on FM. You need to remove it to access the detector diode, though 'ST1' is outside.
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Old 30th Oct 2014, 7:16 am   #15
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Default Re: Weird AM fault in Grundig Party Boy 700

If this diode indeed is faulty replace it with three 1N4148s in series. Sometimes these "zeners" are even drawn this way (three triangles pointing into each other, only the last with a rectangle for cathode). You could even try a green LED although the 1.6 V drop across it might be too low.

Tjerk, 9ZZ
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Old 30th Oct 2014, 9:53 pm   #16
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Default Re: Weird AM fault in Grundig Party Boy 700

A quick update for everyone who's been helpful about this.

The voltage stabiliser measures 1.9V across it, which is correct, so I've left it alone.

I tuned in a MW station with the volume at maximum, then connected a capacitor between the detector and the top of the volume control. The volume increased dramatically, so there is something wrong between those two points - bad switch contacts, bad capacitor, dry joint, whatever. I'll do some more digging tomorrow.
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Old 4th Nov 2014, 5:49 pm   #17
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Default Re: Weird AM fault in Grundig Party Boy 700

Further digging revealed the AM final IFT secondary was open circuit. I removed the IFT and dismantled it, but couldn't reattach the connection - it was hard to even see the connection never mind remake it, despite using a third hand plus a jeweller's loup and the smallest bit on my Antex.

Now I have to scavenge an IFT from a scrap set. I always find removing these miniature IFTs to be incredibly difficult - either one of the connections unsolders itself or a pin becomes detached and remains in the PCB.

Maybe I'll give up and just use it on FM - the radio is worth next to nothing and has no historical or sentimental significance. Sometimes I wonder why I bother, I must be mad
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Old 5th Nov 2014, 10:35 am   #18
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Default Re: Weird AM fault in Grundig Party Boy 700

Wouldn't a capacitor connected between the collector of the last I.F. amplifier and the anode of D1, or, as you've already tried, between the detector and volume control? leaving the faulty IFT in situ to complete the collector load circuit, restore AM operation to somewhere near it's proper level? Some experimentation might be needed to determine a suitable value for the capacitor, but IMHO, this should work
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Old 5th Nov 2014, 11:18 am   #19
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Default Re: Weird AM fault in Grundig Party Boy 700

That would certainly work after a fashion. I'll replace the IFT if I can scavenge one without wrecking it though - nothing to lose.
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Old 5th Nov 2014, 1:59 pm   #20
Leon Crampin
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Default Re: Weird AM fault in Grundig Party Boy 700

You need the dc isolation of the transformer to allow the rectified IF to float on the bais supply. You also really need the correct impedance to drive the detector load in order to preserve the bandwidth.

I would lash up a complete transformer from the same circuit position, taken from a scrap set. Sweep the IF amplifier with a generator and align your new transformer to the peak of the IF response. These sets have a superbly defined IF bandwidth through a clever combination of series and parallel resonant circuits nearer the frequency changer. The arrangement also avoids most of the AM/FM IF switching - just one shorting switch as I recall.

Grundig had some very clever circuit design engineers - pity their production engineers had no idea how to design a radio cassette player (a nightmare assembly), and a great pity they backed the wrong (over complex) VCR horse. Such is history, but I like the radios.

Leon.
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