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 Return to top of page · Post #: 1 · Written at 10:20:09 AM on 1 June 2017.
Robbbert's avatar
 Location: Sydney, NSW
 Member since 18 September 2015
 Member #: 1801
 Postcount: 1113

The other day I found a Philips 112A in the pile. It had no valves and the speaker was missing. The transformer is mounted on the back of the speaker with rivets.

I have another radio that looks identical but has different valves, and they go in different places. The schematic I drew at the time was similar but not identical to the 112A. Luckily there was a barely readable sticker on the back which said what valves went where.

First thing was to deal with the speaker. I fished out one from my speaker-spares box. Then I drilled out the rivets. Now recently I had scrapped a test chassis which had never worked well, and from that I obtained a speaker transformer which happened to fit the speaker. The transformer was mounted with screws which of course I'd put in the wrong way around. The nut has to face the back of the radio, or else the bolt fouls the dial mechanism.

In the radio there was a new electrolytic already fitted, so I only had to replace a few wax and mud capacitors. The mud types are the notorious Techno brand which are truly dreadful.

A 1M resistor connected to the output valve's grid measured 1.5M so was replaced.

The speaker was then fitted to the radio, followed by the valves. (6X5GT, 6V6GT, EBF35, ECH33)

After switching on/off a few times to reform the caps it was time to test. A loud continual clunk-clunk noise could be heard, which could be turned down with the volume control. Listening to the local oscillator on a communications receiver gave a pure tone, which meant the IF valve (EBF35) was the problem. Touching it changed the noise, and it turned out that the red shield was disconnected. A careful look showed a wire sticking out from the base, and it measured continuity to pin 2. I tinned the end of this wire, then wrapped aluminium foil around the valve and made sure it connected to the wire. The foil was then taped into place. A new test revealed the problem was fixed, and stations could be heard.

The audio quality showed that a small alignment was needed, however this radio has no adjustments at all (despite the diagram saying otherwise). I guess you're supposed to unwind wire from the capacitors which is a cheap cop-out. Despite all of that the radio worked well enough. It was mounted into its case and put into the collection.


The dial glass is missing, just like on my other similar radio. Seems they are not easy to get.


 
 Return to top of page · Post #: 2 · Written at 11:55:10 AM on 1 June 2017.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3865

I have mentioned this this week. Motor boating in Philips sets, where feedback is taken from the OP transformer, is commonly due to the reversal of the primary or secondary and the wrong phase is fed back.

Also appearing in "Radio Waves": A while back I did submit to them a method (with pictures) of using conductive glue to replace the metalisation (totally) on Philips valves. The set that it (ECH35) is in (Breville 730) sat on a table running from 08:00 to around 16:00 at a display at Winton Raceway (Vic) without issue, this past weekend.

Marc


 
 Return to top of page · Post #: 3 · Written at 12:40:30 PM on 1 June 2017.
Robbbert's avatar
 Location: Sydney, NSW
 Member since 18 September 2015
 Member #: 1801
 Postcount: 1113

where feedback is taken from the OP transformer

This radio doesn't have such a connection.


 
 Return to top of page · Post #: 4 · Written at 8:34:51 PM on 1 June 2017.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3865

I did not look at the circuit, but several sets do that and Philips sets with it are really fussy about the phase. The other area that I have always found to be a problem in the realm of instability, is that around the second detector (detector / 1st audio).

Changing out outside foil caps, putting things in that area in a new place, often exacerbated by marginal shielding, can see that area totally destabilised. Having to add shielding & doing a bit of tidying up in that area is never unusual.

Naturally valves with defective shielding, 6U7 / 6D6 being one of the worst un-shielded, all need consideration, when things go a bit pear shaped.


 
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