Philips 650A Low Volume

[Voxophone]

When I mentioned replacing all tar capacitors, I forgot to mention that this did not include the flat types in the r.f. circuits (i.e. C6, C14, C15 and C16). These are still the originals.

Regarding the trimming capacitors, I'm afraid I'm still not sure how to go about adjusting these. I've attached a picture of the type I have below. From your description, am I right in thinking that the whole top of the assembly including the nut and rivets should rotate?

Edit - I've also been through V1-4 with the DMM. All readings appear normal, except that the h.t. line appears to be about 20V low. I think this may be due to a higher than nominal value of the smoothing choke.

Aside from this I think V2 may be passing a slightly excessive anode current (4.94mA vs 2.8mA). The final remaining round tar cap is in parallel with the cathode resistor of this valve. Then again, since the volume is very high on LW and SW, this would not seem to point to components common to all bands.

Thanks

Liam

[GeoffK]

If the capacitors in the oscillator stage are all original, apart from C13 then it is best to leave this stage alone, unless the station positions are way off. The top trimming capacitors appear to screw up or down on the central screw by fixing a suitable adjuster on the hexagon nut, but if there is any risk of breakage then don't try adjusting, especially if the radio is working reasonably well. You could try replacing V2, the cathode capacitor C11 has two resistors which add up to 660 Ohms on medium wave so some leakage in the capacitor would make little difference to DC.

[Voxophone]

Another quick update on the 650A restoration. I’ve been through all three wavechange switch assemblies with the DMM and confirmed that all are operating correctly, with coil resistances being more or less correct. There were a couple of high resistance switch connections in the SW circuits which have now been cleaned.

The set has now been reinstalled in its cabinet (minus the dial glass, as the surrounds still need replacing). See picture below (excuse the state of the speaker cloth!)

MW remains far quieter than SW, but is still very listenable. I recently built a 2 foot tunable loop aerial for the MW range; however this does not improve the performance of the 650A much beyond that of the improvised outdoor setup.

Station positions seem roughly ok to me, although without the dial glass I can’t be certain. I’ve noticed that BBC Newcastle (205m) seems to be missing however. I did take a video of the set in operation over the wavebands but I’m having problems uploading it at the moment.

I think there may be hope yet for the original tone pot, which I’ve managed to open up and clean. This will later be reinstalled.

Overall this has been a very satisfying project, and the set works great even as it is. The MW issue still bugs me however, considering how it can make the room shake on SW!

Cheers

Liam

[PJL]

SW is working so there is no gain or IF problem and MW/LW keeps the RF stage so it should be very sensitive. You could try touching a short length of wire on the grid of v1 then v2 when tuned into a MW station. If reception improves dramatically then there is something wrong with the RF tuned circuits. It's also worth checking the cathode voltage of V2 when in MW as the bias conditions are changed in SW.

The Philips wire wrapped trimmers are one time use as you can't increase the value so fiddling with these would be a last resort.

[Voxophone]

Ok, now we're getting somewhere. I tried tuning to a station on each band and then transferring the aerial wire to V1 control grid as suggested. The results were as follows:

SW - Volume decreased.

MW - Volume increased significantly (although still not as high as I've heard it on some SW stations).

LW - Volume of background noise increases but station volume decreases.

So there could be some detuning in the MW aerial circuit. I'm fairly certain however that the wavechange switches operate correctly in this area and that the coil resistances are ok (although L4 is probably too low to be measured anyway). Is this looking like a job for the trimmer C35 then?

Since I also have a similar problem with MW on my Marconi 911 (also pre-war), could it be that the factory set trimmers on these older sets need readjusting for my area?

I've confirmed that C35, the MW trimmer, is still in its wax seal. Unfortunately I still don't have a suitable trimming tool.

[Voxophone]

I've now repeated the above test on V2. Transferring the aerial wire to the grid results in a lot of loud interference but the station beneath it remains at more or less the same volume on MW. It looks like the issue remains with the tuned circuits preceding V1 then.

Even a couple of feet of wire touched to the V1 grid increases the volume significantly beyond that of the outdoor setup and moves the EM1 correspondingly.

V2 cathode voltage reads as follows across the wavebands:

SW - 1.63V

MW - 3.299V

LW - 3.287V

The MW/LW readings seem slightly higher than I would have expected from the Trader values, while the SW valve seems slightly lower. Both R6 and R7 are within tolerance however.

Since the coil resistances read ok and the switches operate correctly I'm struggling to see what could be wrong in the input circuit. The only other component is C1 which shunts the MW and LW coils to chassis in operation. This reads o/c to DC on the multimeter, and shorting across it in operation simply cuts off the signal. It doesn't look like this is the problem then. Perhaps it would be worth removing the protective can from the r.f. coil assembly to have a look inside?

[ms660]

You might be in a poor reception area, the only real way to compare receiver sensitivity between wave bands is with a known good signal generator.

Lawrence.

[Voxophone]

But if the strength of the signal in my area was the problem, surely bypassing the input tuned circuit would not increase the volume?

I don't have a problem with MW on any of my other sets apart from the Marconi.

[ms660]

It was just a suggestion, if your happy with the signal strength at the antenna then checking the alignment might be worth doing after all the usual suspects have been dealt with such as switchery, capacitors, shorts, opens, valve etc.

If checking the alignment then remember that the IF must be in alignment first in order for everything else to follow.

Lawrence.

[PJL]

It doesn't necessarily mean the problem is with the first tuned circuit as the alignment may be wrong.

At this point I would say follow the RF alignment procedure but you can't due to the irreversible trimmers.

Do you have a signal generator? If so you could try to see if you get two images indicating an alignment problem.

Another option would be to replace C5 (trader 50pF) with a small trimmer and try to see if you can get a better peak.

Failing that you will need to take the covers off the RF transformers.

[Voxophone]

The trimmer C35 in the MW aerial circuit is the hex nut type so I should be able to adjust this one at least with the proper tool. The wire wrap trimmer in the MW circuit is in the oscillator section if I remember rightly.

I've tried moving the improvised wire aerial to various different points through the first tuned circuit and the results seem to confirm that the loss of volume takes place between the primary and secondary of the r.f. transformer, with the switches being ok.

Since I can't turn the adjustment at the moment, I tried touching two different fixed ceramic caps in parallel with the r.f. secondary. A 10 pF cap increased the volume somewhat, while a 47 pF one descreased it. I suppose this would suggest a peak somewhere between the two values. The Philips hex nut trimmer for MW is described as 30 pF in the Trader sheet.

[PJL]

Are you able to tell if the station is appearing at the right point on the dial?

I thought they were all wire type as in another Philips I have. If its like a beehive trimmers, a wooden tooth pick will do for a trimmer. Just scrape the red wax off carefully with a small screwdriver and loosen off the trimmers.

[Voxophone]

Most of the trimmers (apart from C43 and C44 in Trader) are the type pictured above in post 21 which I think is the 'beehive' type you describe. The problem I have at the moment is that I don't really have anything suitable to turn the nuts with.

Regarding station positions, I think they are more or less okay; however I can't be absolutely certain as I am yet to re-mount the dial glass in the cabinet. I'm in the process of replacing the felt surround and removing the foil paper covering from the floor - which is proving to be easier said than done...

[PJL]

Once the wax is removed they move quite freely. In fact so freely they can be difficult to set accurately.

[crackle]

Why remove the foil paper covering the floor of the cabinet? It could be forming part of the screening for the radio.
It may even in some instances have a minor effect on the alignment of the tuning.
Mike

[Voxophone]

The original foil paper is in very poor condition as one of the old electrolytics had leaked and dumped a load of black sludge all over it, besides eating a hole in the top of the can! I intend to replace it with new. Does anyone know the best stuff to use?

Regarding the trimmers, I managed to have a go at moving them using the housing of a Bic pen. Moving the MW trimmer C35 a little either way reduces the volume very slightly. This suggests that its original position was correct. I don’t really want to disturb these any more than necessary since the set is working quite well, so based on this result I will probably leave the trimmers and perhaps install a modern trimmer in parallel with the original.

Alternatively I might put in a wire jumper so that the r.f. tuned circuit is shorted out when switches to MW – effectively connecting the aerial direct to C2 and the V1 grid. The cans covering the r.f. transformers look like they would be quite difficult to remove. They are secured by bent lugs on the steel chassis and I am not altogether sure that removing these would not dislodge the entire coil assembly.

[GeoffK]

If C35 is already set correctly and responds to movement, trying to open the can will not help and could damage the coil, and the clamps will never go back down right, don't think a parallel trimmer will help, you could try opening the aerial shunt C1 by sliding a thin paper between S4 contacts to isolate it, not sure what effect this will have. It is aluminium foil in the bottom of the case, if only part has been covered in sludge, just cover that part and keep the rest, why did the front grill end up in such a mess.

[Voxophone]

I've already started removing the foil, both good and bad. To be honest there's so much capacitor slime all over it that it would be better to start from new. The affected area is about five or six inches square and the slime is a quarter inch thick in places!

The cloth was like that when I bought the set. Fortunately the speaker is ok.

[PJL]

You need to free up all 3 trimmers and follow the alignment procedure to fix this. It's quite possibly a fault but the only way of diagnosing it will be to see if the coils peak properly.

[Edward Huggins]

Use thick quality kitchen foil. It will mould nicely to the cabinet where needed. Edward