Why Oh Why do I need help with a Pam 713?

[ms660]

Originally Posted by Boater Sam

I'm using the square wave off my DMM, between 50c/s and 4kc/s.

Not being a DMM person that's a new one on me, is that variable frequency? and how square is it?

Lawrence.

[Boater Sam]

Its an AN8008, very versatile and useful.
Also dirt cheap.
Its proving to be reliable too, seems to tolerate having 250v DC poked up it when on square wave. Waveform is acceptable and variable from 50 c/s up to 4kc/s.
There is a test and review by Mark Hennessy on another forum.

[ms660]

Cheers, just seen the square wave output on the EEV blog video.

Lawrence.

[Boater Sam]

Spent an inordinate length of time today getting this Pam to receive on FM.
I think that the FM packed up first and then the twiddler had a go at all the slugs and lost the AM too.

With painstaking effort with signal generator and 'scope I have finally got it to make a much distorted sound interspersed with whistles and motorboating. Receiving the BBC programs as well as some regional stations.
Sensitivity seems to be acceptable.

There is instability, made worse when the second IF is peaked. I will explore the decoupling capacitors C17, C28, C5 and see what happens.

The heater supply to V1 was intermittent due to a strained pin3 in the holder.

So the trial goes on....................

Sam.

[Sideband]

Also the discriminator cap....usually between 4 and 10 uF with positive to chassis. In fact I'd go for that first....

[Boater Sam]

Discriminator cap C43 already changed but its only 2uf. I may increase that one once the instability is sorted.

[Chris55000]

Hi!

For anyone who has difficulties getting a signal from a generator thro' an unresponsive valve or transistorised I.F. strip, this is what you need a "dummy aerial" for – this isolates the tuned circuits from the generator's low O/P impedance!

Make it from a 22uH RF choke, in parallel with a series network of a 390 ohm resistor and a 390p capacitor, and interpose a 10n 275VAC safety isolation cap (needs to be Class X2!) in both live and "earthy" output leads for safety with a.c./d.c. or a.c. "live–chassis" sets – many service signal–generators of the period had one supplied with them!

If you can't find a dummy aerial ready made, search "dummy aerial" in American Radio History's P.W. pages – you should get a few ideas back for physically making one!

Chris Williams

[Boater Sam]

Played about with the alignment, one of the IF coils doesn't peak very well, its in the area were there is most corrosion so I suspect its been damp.
Recovered the FM reception to a fair standard though, with some distortion on the sides of the correct tuning point.
Rechecked the ratio detector capacitor. Increasing the value makes no difference. There is some looseness in the combined tuning capacitor which affects the tuning somewhat, the tuning spindle and drum are poorly engineered in my opinion, putting too much sideways stress on the bearing.

The FM front end is a thing of horror, to get at any components inside involves removing the tuning drive completely, I think I'll pass on that.
The ECC85 valve holder has a skirt for a can but there is not one in the set.
Fitting a spare one had a bad effect on the sensitivity so perhaps it was never intended to have one. Slight tweaking the RF amp tuning made no difference.

Spent some time repairing the card back. There is a damaged area in the middle into which I have glued a patch made from an old Bush AC1 back board.

Will see how it performs tomorrow with the back on, the FM aerial is strung on to it. If its reasonable I think I'll move on to the Goblin Timespot that is lurking on the end of the bench, its a much more interesting set than this poor Pam and its earlier so worth a restoration rather than just a repair. It also looks to be unmolested, in fact I can find no repairs in it at all. There is something slightly disheartening about working on a set that someone has messed about and made a shambles of.

[ms660]

Originally Posted by Chris55000

Hi!

For anyone who has difficulties getting a signal from a generator thro' an unresponsive valve or transistorised I.F. strip, this is what you need a "dummy aerial" for – this isolates the tuned circuits from the generator's low O/P impedance!

Make it from a 22uH RF choke, in parallel with a series network of a 390 ohm resistor and a 390p capacitor, and interpose a 10n 275VAC safety isolation cap (needs to be Class X2!) in both live and "earthy" output leads for safety with a.c./d.c. or a.c. "live–chassis" sets – many service signal–generators of the period had one supplied with them!

If you can't find a dummy aerial ready made, search "dummy aerial" in American Radio History's P.W. pages – you should get a few ideas back for physically making one!

Chris Williams

You can feed straight to the grid via a blocking capacitor no problem as the grid is high impedance, the dummy antenna was for feeding in at the antenna terminals so as not to damp the RF tuned circuits, for IF you can feed direct to the mixer grid via a blocking capacitor, usually with the tuning set to the low end of the MW band.

Lawrence.

[Techman]

Originally Posted by Boater Sam

The mains transformer has shed a considerable amount of black pitch under the chassis but not straight down onto the cabinet floor but sideways onto the mains lead and components as though the set has been operated stood on end, most odd.

Sam.

I've had one like that, except it was melted wax from the transformer that had seemed to have at first sight, defied gravity!

A good sign that someone has had the set on its side for a few hours while head scratching over why it's not working properly, while being unaware of a fault that's causing the set to draw too much HT current, probably caused by a leaky coupling capacitor to the output stage, which by now you'll have changed anyway. From their 'core twiddling' it shows that something like a grid coupling capacitor was the last thing on their mind - or even if they had the knowledge to know what its purpose would have been anyway.

[val33vo]

Originally Posted by Boater Sam

I don't get stuck often but this Pam 713 (Trader sheet 1297) is giving me the run around. Stone deaf all bands but crackles with wave change from AM to FM and to Gram.

Audio working from the detectors onwards on all bands.
Signal tracer from secondaries of the 2nd IF transformer comes over fine.
But absolutely nothing from the primaries.

Local oscillator is running (g1 of V2 is -5V to -7V) and the frequency changes with tuning.
Injecting 470kc/s into V3 anode and grid gives me nothing at all.
All valve voltages are close enough to the sheet.
Have replaced capacitors C33, C34, C43, C44 and C45. Resistors changed R10 and R11.
Continuity through 2nd IF windings is OK.
Anode current of V3 is 12mA.
Please Help!

Though continuity is ok through 2nd IF primary it might be too good ,ie short cct ( windings or associated tuning capacitor

[Boater Sam]

It isn't the parallel capacitor unfortunately, I'm pretty sure its the winding itself.
With a bit more time I would try to measure its resonant frequency but for now its back on the pile of working but could try harder.

Its the old story, something more interesting always comes along or pressing things to do.