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Old 10th Sep 2019, 1:14 pm   #1
coopzone
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Default Perdio Super 7 advice

Perdio Super 7

Hello everyone,

I have a few questions about this radio and would appreciate any comments people care to make. Just to be clear I will refer to the trader service sheet part numbers (as provided on the wonderful CDROM up top)

The main problem is that the OC81D (TR5) and the two output transistors TR6 and TR7 (both OC81) are missing! Admittedly this does make testing somewhat tricky.

So while I await some (hopefully not to dodgy PNP germanium replacements) I decided to test the RF/IF stages, according to the trader sheet the bias for TR1-TR3 is provided mainly by the current flowing through R17 (470ohm) and TR5 when present. In suggests a voltage of around 1volt should be across R17 to provide the bias. So as a temporary test i added a resistor where TR5 is (collector-emitter) and set the voltage across R17 to 1.05v. Connected my tried and tested headphones via a capacitor to the output of X1 and gave it a go. Nothing!

Next i checked the voltages on TR1-3, in all cases they where low. Trader sheet says, for example, the collector should all be 7.3v. They came in at around 5.7v. I double checked R17 and R12 both supposed to be 470ohm, both where high (near 800ohm) So they got replaced. Result Still nothing, still low voltages.

Then I checked if any of the transistors TR1-3 was causing the voltage to be low by disconnecting the collector(s) this did not appeared to make a significant difference, next disconnected C13 again no real difference.

Just by way of experimenting i changed my substitute resistor so that the voltage across R17 was round 1.8v and bingo! Life! Radio 4 (not sure a should call that life really). But still the voltages are low.

My worry is that even when i put the new transistors in place it’s not going to work due to the bias voltage and supply voltage being low.

For now if anyone can suggest why the difference 7.7v down to 5.7v ?. Or perhaps point me at something else to check or suggest any reason why the increase in bias voltage appears to make it work i would be grateful.

When the transistors arrive i’ll update this post.


Thanks

Derek
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Old 11th Sep 2019, 7:28 pm   #2
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Default Re: Perdio Super 7 advice

You seem to have checked all the components I would have aimed for first.
Couple of maybe silly thoughts -
has someone replaced the germaniums with silicon?
you were measuring the volts from battery + and not the "ground" line?
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Old 11th Sep 2019, 7:53 pm   #3
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Default Re: Perdio Super 7 advice

Thanks for the reply (nice cat by the way).

I was taking the reading from the battery + (as per the instructions on the trader sheet).

And no silicon, all the transistors that are present are germanium.

I think i'm just going to have to wait for the transistors to arrive and take it from there.

I did wonder if maybe the OC44 was leaking, but i have no real way to test it easily and i have no idea what is within spec anyway i would assume less than 1ma ? would be ok. - so back to wait and see.
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Old 11th Sep 2019, 10:10 pm   #4
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Default Re: Perdio Super 7 advice

Ah well - it was just a thought.
If you are dropping from 9v to 5.3v across R12 then I make that 7mA.
Going by the emitter voltages in the table for the OC44 and OC45s, I make it 0.3mA and 2*1mA respectively. Similarly using the voltages for TR4 it looks like it should be taking about 0.9mA. That gives a total of 3.2mA for the transistors to the left of R12 so you are losing an extra 3.8mA somewhere.
I would check voltages across R1, R2, R5 and R11 to see if they make sense.
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Old 13th Sep 2019, 9:52 am   #5
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Default Re: Perdio Super 7 advice

As suggested i took some readings and did some basic maths:

ohms voltage current
r12 470 3.09 6.5744680851
r1 3300 0.906 0.2745454545
r2 1000 5.39 5.39
r5 1000 0.895 0.895

It's fairly obvious where the current was going (but strange that when i disconnected the collector of tr2 you did not see such a big difference). So a quick look around R2 and tr2 shows a possible culprit, namely 0.04uf C15. It was quickly dispatched/replaced and the voltage readings are now about right.
Still a little high on TR5 emitter.

So thanks to buggies for helping out. I am now really going have to wait for the replacement transistors to arrive from either china or the Ukraine (it's a race!)
to go further.

I'll update the thread later.

Derek
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Old 13th Sep 2019, 12:57 pm   #6
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Default Re: Perdio Super 7 advice

Well caught sir!
I was going to post a PS the other day as I dug out my old PR7 and measured the emitter voltages etc (powered from an 8.6v rechargeable PP3) but found all of them to be within a whisker of the service sheet figures.
I took it apart today and located C15 - did not attempt to remove it. It seems to be a brown axial thing but I can't read the manufacturer. Would be interested to know if your's was the same type and if it shows any leakage on a DVM. There are others on the PCB as well...
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Old 13th Sep 2019, 1:05 pm   #7
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Default Re: Perdio Super 7 advice

Yes it's a small brown job, about 10mm x 6mm, it's marked DCC 0.04 150v. I tried to get a picture for you but no good.

I just measured it on a DVM it came in at 39 ohm, yes thats right almost a dead short!. I'm sure when i took it out the set last night it was around 20k - talk about rapid decline!

Derek
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Old 15th Sep 2019, 9:18 am   #8
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Default Re: Perdio Super 7 advice

Some ac128 transistors have arrived, the radio chassis is now working. Not got as much volume as i would have hoped for but good enough. I still have to have a go at a new volume on/off switch and the entire radio case need re-stiching (one side completely fell off!) So a way to go yet.

Attached are some pictures showing the replacement transistors in situ and my temporary volume control! I also added (not to clear ) picture of the 0.04 replacement cap.


Derek
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