Philips B3G75U

[Sideband]

Unfortunately I don't have the circuit to hand as I am at work and my on-line source doesn't have that circuit listed. I take it that V1 and V2 are in the FM tuner section? Are they UF80's? More than likely there is a decoupler somewhere that has been missed and now gone very leaky. R7 value of 2.2k suggests an HT feed. I'll dig the circuit out when I get home tonight unless of course someone else on here can help in the meantime.

SB

PS. One thing that might be worth checking. With all the work you did on the volume control, make sure it's not something silly like a component that has been moved and is now touching where it shouldn't.

[Wage Slave]

Thank you again SB that's really good of you. It is the FM tuner section and they are UF80s. I have the diagram but can't post all of it here for obvious reasons. PM perhaps?

[Station X]

If I'm looking at the right circuit, R7 is in the anode circuit of V2. Check C18, C19 and C60.

[Roy]

A quick look at the circuit in the Trader sheet suggests that C17 is the obvious suspect, however this is a 1000pF ceramic capacitor and therefore unlikely to be at fault.

The wiring inside the screened VHF tuner box is rather cramped, so it is possible that something is touching where it shouldn't. R7 & R6 are very close to the metalwork, and there is a paper insulator to prevent contact - this may be punctured. A resistance check from R7 to chassis might reveal something.

By the way, are you working from the Trader sheet 1342 or the Philips manual? The component numbers are not the same.

Good luck

Roy

[Wage Slave]

That sounds right. I am working from the Trader Service sheet at the moment. C60 is then 50uf and I replaced it last week. It seems fine on MW.

I'll check C18 and 19.

[Wage Slave]

I noticed the R6/7 juction and it was touching the paper insulator. There isn't a hole and I moved it a millimetre or so away. There isn't a short now and I don't think there was either.

[Station X]

I was looking at the Philips manual for the B3G97U. In this case C17 mentioned by Roy is C60.

[Station X]

Let's sing from the same hymn sheet, ie the Trader sheet. The caps to check are C15, C16 and C17.

[Wage Slave]

Right. Thank you all very much. I will get on with checking C15, C16 and C17 according to the Trader sheet.

[Wage Slave]

C17 seems fine. It shows OC on the 20M range of my DVM anyway.

[Sideband]

No point in checking caps for leakage on the resistance range of a DMM as the voltage across the cap will only be a fraction of a volt. The capacitor could well be breaking down at a higher voltage. You need to test capacitors at or near their rated voltage (use a megger). If this is not possible, then substitution is the best way forward.

SB

[Roy]

With care, it should be possible to unhook one end of C17 from the tag of L7, then re-test, monitoring the voltage. The tuner may become unstable with C17 disconnected, but it may help to narrow down the location of the fault.

It is incredibly cramped in that corner, so removal & replacement of these capacitors will not be easy.

Roy

[Wage Slave]

OK. I don't have a megger available to me so I'm not sure how to proceed. I would be happy to do so by substitution if necessary.

Roy, when you say I might lift C17 and monitor voltage what exactly did you mean? I can lift C17 reasonably easy. R6 an R7 are currently lifted.

[Station X]

C17 seems fine. It shows OC on the 20M range of my DVM anyway.

That may well be so, but a DMM only puts out a few volts. When subjected to full HT C17 may well break down. An analogy might be a sticking door. Push it gently and nothing will happen. Kick it and it will fly open.

Try disconnecting the capacitors and see what happens then. Currently is obviously flowing through R7 to chassis by some route.

[Station X]

If R7 is lifted then it will be impossible for it to get hot. Try a resistance measurement between the anode of V2 and chassis.

[Roy]

By "monitoring the voltage" I mean measuring the DC voltage at the junction of R7 & R6, with the receiver operating. The Trader sheet lists the expected voltages, in this case V2 anode should be 160V.

However, if you are not experienced in performing voltage checks on live equipment, don't do it.

With one end of C17 disconnected, but everything else in place, you could do a quick test to see if the resistor overheats again.

[Wage Slave]

Originally Posted by Station X

If R7 is lifted then it will be impossible for it to get hot. Try a resistance measurement between the anode of V2 and chassis.

Yes. I haven't powered it since the new resistor started smoking. I lifted it to get access to C17. I'll pop it back before any further tests.

Anode of V2 to chassis is OC.

[Wage Slave]

Originally Posted by Roy

By "monitoring the voltage" I mean measuring the DC voltage at the junction of R7 & R6, with the receiver operating. The Trader sheet lists the expected voltages, in this case V2 anode should be 160V.

However, if you are not experienced in performing voltage checks on live equipment, don't do it.

With one end of C17 disconnected, but everything else in place, you could do a quick test to see if the resistor overheats again.

Thanks, I see the logic now and why C17 is the chief suspect. I have done checks on live equipment when I worked on the rigs and am aware of the hazards peculiar to this set. I have run out of time at the moment so, unfortunately it will have to wait a bit.

There are too many kids, noises and sundry distractions here just at the moment. I might get a bit of time later tonight with luck.

In the meantime, many thanks again to you, and of course Station X, for your kindness. I'll be very disappointed if I can't get this thing working again.

[Wage Slave]

You people are astonishing in your ability to fault-find by very remote control. C17 is our troublemaker. Taken out of circuit, R7 no longer cooks and the set starts working as an FM radio again. Not perfectly but pretty well.

Voltage at the R6/R7 junction (ie anode V2) was 160V at startup which dropped fast to 125V or so and then gradually rose back up to 152V.

The parts shop is closed tomorrow for their one day a week off so I will have to go on Wednesday to see what they have. I take it that any ceramic capacitor rated .001uf will do as long as it can handle, say, 200V?

[Sideband]

Originally Posted by Wage Slave

C17 is our troublemaker. Taken out of circuit, R7 no longer cooks and the set starts working as an FM radio again. Not perfectly but pretty well.

Excellent! The poor operation is probably due to the fact that the stage is not decoupled at the moment and probably oscillating slightly.

There's a wealth of experience on here with many of us having been in the trade for years. A lot of us became quite adept at diagnosing problems over the phone when customers rang up with queries! Some of us worked in major service departments and gave advice when field engineers rang up with problems (I did). You soon get to know common circuit arrangements and associated problems.

Well done in finding the fault. You've learned a lot as well (including how to strip a volume control). You can also see why checking capacitors (in this case C17) on a DMM is pretty pointless unless you are looking for a dead short. All this will help with your next restoration job....!


SB

PS If the original component was a ceramic type, you should replace it with similar to ensure stability. 250V should give ample margin for operation. In practice, if you have trouble finding a ceramic with adequate voltage rating, you will probably be able to use a good quality polyester type. You can't do this in all circuit positions however.