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Old 14th Oct 2018, 12:01 pm   #1
Christoffrad
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Join Date: Jun 2018
Location: Cambridge, Cambridgeshire, UK.
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Default Philips 634A sensitivity

I recently bought a Philips 634A. Complete and in good condition but missing V4. (TDD4) I have replaced the missing TDD4 + HT electrolytics and C39 all other capacitors test OK on the bridge and resistors on multimeter. but sometimes sensitivity is (or drifts) low, but is restored by touching V1 grid (to create a crackle) or even momentarily causing a glitch on the mains. Also the sensitivity switch has little or no effect even though the bias can be seen to change slightly. Voltages look generally OK. Any ideas?

BTW I originally thought that this radio required a SD4 rather than the TDD4. I even bought a SD4 prior to collecting the set in readiness. And in so doing I've made a couple of interesting discoveries.... The Mullard SD4 that I bought in The Netherlands was manufactured with the European B6 rather than the 7 pin base even though it has all the normal markings of a SD4 so it was effectively a E444. The socket for V3 in this set has fixing screws rather than the rivets used on the other bases and accepts the TDD4. I had assumed that this was an post production mod (as described on the Trader Sheet) but then I noticed that the top cap (suited for the TDD4) carried the same red sealing paint that was applied elsewhere around the set. So it appears that The UK Philips factory made changes for the TDD4 prior to shipping. The circuitry however is otherwise unchanged.

Chris
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Old 14th Oct 2018, 12:42 pm   #2
cathoderay57
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Default Re: Philips 634A sensitivity

Hi Chris, I have never played with this particular set but I have restored many of a similar design and vintage. I have encountered these symptoms 2 or 3 times. Most common fault was a dry joint on the flexible wires to the speaker speech coil at the point where they are often riveted to a solder tag on the speaker frame. The dry joint between the rivet and the tag is easily bridged with a short length of wire. The crackle effect seems to provide enough voltage at the speaker terminals to overcome the dry joint temporarily. Failing that I would look elsewhere for dry joints, notably dirty or corroded valve pins and sockets, loose valve sockets, dirty wavechange switch contacts or a bad soldered joint somewhere. Occasionally oxidation of the internal wires inside the valve pins or top cap can cause problems such as this. Good luck, intermittent faults are always a challenge. Cheers, Jerry Oh and if there is an extension speaker socket with a make-and-break contact set these often give similar trouble
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Old 14th Oct 2018, 12:50 pm   #3
Christoffrad
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Default Re: Philips 634A sensitivity

Thank you, I'll look out for that. The fact that no mechanical movement was needed to restore performance made me assume it to be an electronic issue but I see your reasoning too.
Chris
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Old 14th Oct 2018, 1:26 pm   #4
PJL
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Default Re: Philips 634A sensitivity

Your symptoms are typical of a problem around the aerial coil, perhaps the waveband switch.

The capacitor blocks in these philips sets normally need rebuilding. A capacitor bridge will not check for leakage or ESR.
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Old 14th Oct 2018, 1:38 pm   #5
Panrock
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Default Re: Philips 634A sensitivity

Chris, I received your PM. I've replied to you separately but append my ideas here too.


> Are you familiar with the electronics of it? I have replaced the
missing TDD4 + HT electrolytics and C39 all other capacitors test OK on
the bridge and resistors on multimeter. but sometimes sensitivity is (or
drifts) low but is restored by touching V1 grid or even momentarily
causing a glitch on the mains.


Flick finger against L7/L8/L9 can. Check end caps of R1 and R27 are
making good contact. Swap over V1 and V2 and see if the fault moves to V2.

> Also the sensitivity switch has no effect even though the bias can be
seen to change slightly. Voltages look generally OK.


Check R30. Check ohms across closed S6. Check R19 and R22.
Are the RF valves the correct original types and tested as good? The
vari-mu characteristics must be correct for best results.

Steve
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