Perdio PR73 - very poorly (dead)

[Tully Bascombe]

Hi all, got this Perdio PR73 for a modest amount from the usual place, sold as untested due to batteries not available. It's in quite good condition and appears to be totally original.

My plan was to re-cap it but unfortunately it doesn't work at all and on inspection I noticed that molten wax has dripped out of output transformer. I wonder what would cause it to get hot enough to melt wax. Seems an odd insulation material to use in a transformer. The chances of finding a suitable replacement are remote unless I can find a donor radio.
Any suggestions very welcome

[HamishBoxer]

First things first,why did the transformer fail?shorted turns or an o/put transistor etc.

Ed Dinning on the forum can rewind if needed.

[poppydog]

If it came from internet auction, get a refund/partial refund through paypal.

[HamishBoxer]

No refund when bought as untested I don't think.

[TrevorG3VLF]

Wax is often used in transformers to stop the windings or laminations moving but this is a transistor radio and there should be no problem. I do not see how a battery will have enough energy to significantly heat the transformer. More likely the radio was placed on top of a radiator.

Is the transformer all right? Measure the primary resistance with an analog meter. If you get a click then the transformer and speaker are probably OK.

[jonnybear]

I wouldn't mind betting that someone had tried wiring up a power supply, possibly causing the output transistors to fail (short circuit) and melted the transformer wax, the transformer may still be OK though I would say the transistors are not.
John

[Tully Bascombe]

Many thanks for the helpful replies. I'm going away for a couple of weeks and will do some more investigating when I return. All I can say at the moment is that it's drawing excessive current, more than my cheap Banggood variable power supply can handle.

[vinrads]

It looks like as someone suggested, it's probably been connected the wrong way round or excessive voltage applied. I would suspect the output transistors first, remove them then see what current it draws, check the transistors for short circuits. It's possible that the transformer has survived, check each winding now the transistors have been removed. Mick.

[Tully Bascombe]

I've found time to have another brief look at this 'baked' Perdio and have wired up 2 x 3 AA batteries to replace the AD28 type. The radio is working, only on MW though, I think it might be the switch.

Reception is rather weak and the output transformer is getting warm. From ambient it went up to 40C within a few minutes. At that point I switched off.

As I'm getting sound I assume the output transistors and transformer are working but drawing excessive current. Would a shorted transistor cause this but still allow the radio to work ?

[David G4EBT]

You don't mention if you have the data sheets on the PR73. As you're hearing MW signals, albeit weak, that means the RF, IF detector and output stage are all working, if only after a fashion. There are three sets of data - a Trader Sheet (1456), an ERT sheet and R&TV. You can download them all from the top RH of the page. They all consist of two pages, and without all three, you won't have a complete picture of the modifications, but if you only want to download one, I guess the Trader Sheet would be the one to go for.

The set uses several electrolytic capacitors, and given that it's approaching it's 60th birthday, it's an odds on cert that some, if not all of those caps, will have lost capacitance, developed a high ESR, and morphed into resistors. Some of them will simply degrade performance - others will make the transistors very unhappy indeed. Referring to Trader Sheet 1456, C23 for example (6uF) in series with R16 (3K9) from the volume control to the base of TR4, C25 (100uF) in series with R17 (22k) from base of TR4 to +VE, and C26 (100uF) in parallel with R19 - a 1K5 pre-set pot, which was factory adjusted to set the quiescent current of TR6 & 7 to 3mA. But if C26 is behaving like a resistor in parallel with R19 no doubt excess current will be drawn, and that could be why things are overheating.

If it were my set, at the very least, I'd replace those caps, refer to the datasheet on setting the quiescent current, then from the table of Voltages on the Trader Sheet, see how the actuals compare with the data. Some will argue that it's bad practice to replace electrolytic caps without first ascertaining whether they're faulty. On a set this old, with small electrolytics, I disagree. Testing them involves unsoldering one end, checking their capacitance and their ESR. Chances are, they'll fail on both counts, but if they don't they inevitably will at some point.

Caps are cheap and plentiful. Transistors on the other hand are long since obsolete and expensive, and the same goes for driver and output transformers. If I've counted correctly, there are seven electrolytics in all - the same number as transistors, so it's not a mammoth task, but needs care, on what is no doubt an SRBP PCB on which de-soldering can damage the tracks and pads.

Hope that gives a few pointers.

Good luck with the set.

[Tully Bascombe]

Thank you David for your very helpful and comprehensive reply. Yes I do have service data sheets and there's also one on the inside of the back cover which has remarkably survived intact.

My original plan was to re-cap but got sidetracked by the baked transformer. Now that I know the set is working I'll get the parts ordered up and crack on with it.
Regards,
Tully

[Tully Bascombe]

I've now finished the re-cap and disappointingly there's no improvement, the output transformer is still getting hot and performance is weak.

I'm not sure if the output transistors are also warming up as they are heatsinked. I guess my next move is to remove the transformer to properly test it for shorts as in circuit it's showing continuity between the secondary and primary windings.

[Boulevardier]

If I were you, the next thing I'd do is to remove the output transistors and test them - even the quick diode test will probably show whether they're faulty or not. Transformers are much more tolerant and robust than transistors - if the transformer is getting that warm it's likely that one or probably both o/p transistors are internally short circuited. As others have said, it could well be that someone has applied power in wrong polarity, which could wreck the o/p transistors (but probably not the transformer). If the o/p transistors are completely s/c, then they won't get warm anyway. When removing them, do take heed of David's advice in post 10 re damaging pcb traces.

Mike

[Tully Bascombe]

Right, I'll test the transistors next, but I would have thought that if one or both are blown there wouldn't be any sound?

[GeoffK]

Check the quiescent battery current (in one battery lead with minimum volume. If the current is over 10 to 15mA remove the output transistors and test it again, if the current drops then it is the output transistors that have failed, they can still carry on working but the sound gets less and the battery current goes up as the transistors get warmer and warmer. The black finish on the output transformer is just the tape it just finished with.

[Boulevardier]

No, I'm not sure about that either. But if the o/p transformer is overheating, then those transistors have to be prime suspects.

Mike

[Tully Bascombe]

Originally Posted by GeoffK

Check the quiescent battery current (in one battery lead with minimum volume. If the current is over 10 to 15mA remove the output transistors and test it again, if the current drops then it is the output transistors that have failed, they can still carry on working but the sound gets less and the battery current goes up as the transistors get warmer and warmer. The black finish on the output transformer is just the tape it just finished with.

I haven't dared measure quiescent current yet. It's enough to bake the transformer and melt the wax insulation. Any guesses?

['LIVEWIRE?']

I'd second what others have said-i.e., that the o/p transistors are almost certainly faulty.

[Tully Bascombe]

I've now managed to remove and test the OC 81 o/p transistors. The one with the red dot measures as a transistor but not great, hfe 15. The other one with the white dot is totally shorted, measures as two resistors which explains the cooked o/p transformer.
I'm going to replace them with AC128's.
Not sure of the significance of the white dot, would have thought both transistors to be the same.

['LIVEWIRE?']

Correct me if I'm wrong, but didn't the colour of the dot have something to do with the gain (hFE) of a particular device?