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 Return to top of page · Post #: 1 · Written at 9:00:44 PM on 28 May 2017.
STC830's Gravatar
 Location: NSW
 Member since 10 June 2010
 Member #: 681
 Postcount: 842

30 years ago repaired a Kriesler Beehive as a love job (new speaker and resistor instead of filter choke). The chap I did this for is in his eighties now and is de-cluttering, so he decide to give it back to me with a nice pale green Philips MM1 mantle.

The Philips was said to be working but very faintly, so decide to have a look inside. Straight away I saw that there were four high voltage filter caps, the two chassis mounted metal originals, both leaking, and two axials. Trouble was that the axials had been patched-in in parallel with the chassis mounted caps, actually soldered onto their terminals!

I guess it must have been ten to five on a Friday afternoon.


 
 Return to top of page · Post #: 2 · Written at 9:39:18 PM on 28 May 2017.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 5340

I guess it must have been ten to five on a Friday afternoon.

I think more likely a DIY by someone without the requisite knowledge.


 
 Return to top of page · Post #: 3 · Written at 8:15:45 AM on 29 May 2017.
STC830's Gravatar
 Location: NSW
 Member since 10 June 2010
 Member #: 681
 Postcount: 842

When I see the chap next will ask him if he knows who did the repair.


 
 Return to top of page · Post #: 4 · Written at 9:40:31 AM on 29 May 2017.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3855

Depending on the structural integrity of the cap: I have been known to modify a tag strip & solder the earth tag, to it. That solves the issue of wires and isolating the cap. Often some slackers leave them floating in mid air.

The reality is that the old cap must be isolated. and wires left safe.


 
 Return to top of page · Post #: 5 · Written at 5:42:21 PM on 29 May 2017.
STC830's Gravatar
 Location: NSW
 Member since 10 June 2010
 Member #: 681
 Postcount: 842

I intend to use a tag strip attached to the chassis somewhere and leave the original chassis mounted caps in place. Hopefully leakage through these is the reason for the poor performance, though there are plenty of papers caps as well to be replaced.

While the general case aesthetic condition is good with just some slight "burns" from the plasticiser in the cord, the chrome plated die cast dial escutcheon is badly corroded on the upper surfaces. Will see if I can come up with a replacement before I restore its electronics.


 
 Return to top of page · Post #: 6 · Written at 9:06:59 PM on 29 May 2017.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3855

With a Philips, they often have an electrolytic across a back bias resistor (CT to Chassis): That is a trap for young players. The negative goes to the centre tap and the positive to chassis. Quite often that gets messed up & will destroy the bias & cap.

All Wax paper & Electrolytic caps should be replaced & do not forget to check the resistors as you go.

Poor performance can be deceiving. It is never unusual to find every adjustment that moves adjusted in a futile attempt to make it run, when things go wrong. I had one recently where even the IF transformers had been messed with top & bottom. That set apparently picked up only two stations before it died.

If you replace parts in the RF section, it needs to be re-calibrated. starting with the IF transformers & a modulated signal that's strength is not enough to cut in the AGC (if fitted). Then you sort out the oscillator,, dial & antenna circuit. There was a big wow factor for the owner of this particular set, with it running on the bench and being able to pick up a multitude of stations as it traversed the band.

Methodology & Happy campers are good value.


 
 Return to top of page · Post #: 7 · Written at 10:03:29 PM on 29 May 2017.
STC830's Gravatar
 Location: NSW
 Member since 10 June 2010
 Member #: 681
 Postcount: 842

Thanks for the tips Marcc.

Have found the schematic in July '95 Radio Waves, which gives RF alignment instructions and a voltage table.

The back bias does come from the CT. CT goes to chassis through a 47ohm resistor. The bias from this resistor goes through a 3.3Mohm resistor, then through a pi filter consisting of a 2.2Mohm resistor and two 0.047μF caps, presumably to reduce hum. So no confusing electrolytic.

A feature that might be of interest to those with asbestos sheeting fitted as a heat shield, is that the shield in this radio appears to be a piece of a lino floor tile, complete with pattern. I understand that in those days these tiles were asbestos reinforced so would provide a shield but not as effective as asbestos sheeting. Relevant to heat shielding is that the output (6M5) valves has the socket inset into the chassis to lower its top to the same height as the other valves.

Some of the adjustment screws have wax applied to them and do not appear to have been interfered with. So hopefully I will escape previous ignorant adjustments.


 
 Return to top of page · Post #: 8 · Written at 10:29:00 PM on 29 May 2017.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3855

I would as it has been messed with, consider IFT adjustment, unless there is no equipment to do it. The backbias is only developed across the 47 Ohm resistor CT to chassis (chassis positive) and is normally that of the OP tube grid.

Sometimes there is a 0.1mfd, or so, cap across it others 25μF, some nothing. The bias is dependent on the valves working properly, as the sum total of all cathode current passes through it.

What model is it?


 
 Return to top of page · Post #: 9 · Written at 9:33:20 AM on 30 May 2017.
STC830's Gravatar
 Location: NSW
 Member since 10 June 2010
 Member #: 681
 Postcount: 842

The model is MM1.

It has some nice design features including, as well as the recessed output valve already mentioned, spring loaded attachment screws for the grill moulding to the chassis. This means that as the case to chassis screws are tightened up at the back, the strain is taken up by the spring loading arrangement.

Regarding my comments about the back bias arrangements, I have misread the circuit - the arrangement is for grid bias as the AGC diode is in the circuit. There are no cathode bias arrangements, except for the output valve where it is is provided by a 220ohm resistor, no bypass cap. So it looks as if the CT bias arrangement saves a couple of bypass electrolytics.

If the alignment is way out will apply a by-ear alignment procedure described by EA many years ago, and then try instruments also described in the EA article.

Half expected feedback arrangements but can't see any.


 
 Return to top of page · Post #: 10 · Written at 9:58:09 AM on 30 May 2017.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3855

The 132L I have, salvaged from a recycler was a bit of a disaster in the audio feedback area: It was disconnected. It comes off of the OPT secondary. Along with several other misdemeanours, it was clearly dumb hacker work.

It is clear that the OPT failed, so the goose put a new one in, primary or secondary, wired in reverse: Clever. That of course with most Philips sets like that causes it to Motorboat (Oscillate) due to the reversed phase. So being incapable of understanding why it all went awry, it was disconnected.

To add insult to injury, in drilling out whatever held the transformer to the speaker, the bit continued through the cone; Which stuffed it. It ended up distorted & poling. 6" speakers of course are less common, but I managed to get a NOS one for an early Magna door and do a bit of OP transformer re-locating to get it a mounting point.

Do beware of the Philips mains transformer, I have seen several with windings escaping from the side of the wrap. That does not mean that they have failed. But it does mean that some Mar is needed and care. Do not use bare fingers, that will cause corrosion.

Marc


 
 Return to top of page · Post #: 11 · Written at 5:17:57 PM on 30 May 2017.
STC830's Gravatar
 Location: NSW
 Member since 10 June 2010
 Member #: 681
 Postcount: 842

The mains transformer looks good, only a few bits of paper come adrift but no windings visible. It has of a burnt smell about it,so I think it has been overloaded at some stage. But seeing as strong local stations do come through I think it has survived.

There are no signs of other work apart from the new electrolytics, which job is neatly done if misguided.

Getting back to the schematic, an odd feature is that the HT for the output valve comes from the first HT filter electrolytic, instead of the second where all of the others get their HT.


 
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