Help me troubleshoot Philips Valve radio 15RB505

[Hiten123]

Hello everyone,

This is Hiten From India. I have Philips Valve radio Name : Prestige Model No. 15RB505.00Z bought by my dad in 70s. It needs to be repaired. Here is information about it.

Brief history of problem and information And Direct link to Service Manual : Link

1) I am graphic designer by profession with very little knowledge about electronics. But I know basic rudimentary concepts. and I can take precautions of high voltages on valve radio

2) The radio was stored away for decades. 4/5 years a go I brought it out and it was playing. However I only played it for few mins. then stored away again. 2 years back I again played it with its pickup input playing 78 shellac record player (Ceramic cartridge). After few mins. with little burst sound the radio stopped working.

3) Now we are underlockdown and I have time to see it. I opened it and Resistor R15 gets very very hot in 3/4 seconds. and light plume of smoke was seen. So immediately stopped.

4) Testing I have done : Kindly check this link for detailed tests I have done. LINK

5) On above forum a gentleman helped me troublshoot some sections of the radio. Putting in only Rectifier valve EZ80 and Valve ECL82 and with pickup input radio is playing fine.

6) If I put ECH81 and EBF89 valve No sound from MW or SW bands and from the pickup input The sound is little less compared to with out putting in ECH81 and EBF89.

6) I have tested some capacitors with multimeter with capacitance testing. They are C34(Power supply), C35(Power supply), and some other capacitors details are in above link.

7) I interchanged each valves one by one (as I have spare of each) But problem remains.

Can anyone guide me further to troubleshoot this radio.




Thanks and regards

[Station X]

https://www.radiomuseum.org/r/philips_15rb505_00z.html

R15 is part of the smoothing arrangements and is in series with one of the speaker windings. HT current also flows through it. Does it still get excessively hot with just the rectifier and audio valves plugged in?

Before you go any further I suggest changing C36 the audio coupling capacitor. If this is electrically leaky it could cause extensive damage to the set.

[Radio Tech]

Hi Hinten

Without me looking at the circuit diagram I would suggest that you locate the cathode of the HT rectifier temporary lift this and check voltage, then work along to the main HT smoothing capacitor, this will be something like either a 16+16 or 32+32 uF I would suggest if this cap has been in there a long time then replace it, there are some nice axial types around from a company called Jellyfish Audio and finally take the output valve out and check voltages.

Best wishes

Ken

[petervk2mlg]

If R15 is getting very hot and smoking that means too much current is being passed.
If C34 is leaky, then it could be the culprit. Changing C36 has already been mentioned. This is very important. Also change C24, C20.
Turning the set to MW check that the local oscillator in the ECH81 is running. An easy way to check this is by bringing a working transistor radio on the MW band close to your Philips set. Tune the Philips across the MW band and if the local oscillator is running you should hear a beat or slight interference in the transistor radio as the oscillator in it interacts with the oscillator in the Philips. Another way of checking is to see if there is a small negative voltage on the grid of the oscillator in the ECH81 (pin 9). This is best done with an analog meter. You'll see the needle jump backwards if it's set to a 10volt DC range or similar. Positive probe to pin 9 and negative probe to chassis. (You can reverse the probes if you like and will see a positive movement of the needle in that case.)
I'd also be mindful that switches can be sources of trouble. See if you can clean the wave change switches and operate them a few times to make sure good contact is being made inside them. Checking voltages as marked on the circuit is a very good good idea too.

[Hiten123]

Station X,
Yes with only rectifier and amplifier valve plugged in the R15 gets very hot. Original value of R15 is 1.5K (10 w) according to schematic. My radio has 1 K (10W). It was told to me that even with R15 of 1K, It should not get extremely hot within 3/4 second. Will change C36 soon. We are under lockdown and shops are closed.
Radiotech
I am very noob So you will have to correct me. By lifting Cathode of Rectifier do you mean I should disconnect that pin wire and measure voltages by connecting common Probe of my multimeter to chassis and testing probe to the cathode ? OR measure Voltages as it is with all connected. Will be changing C34+C35 soon. They are specified in manual as 32uf+32Uf. I measured mine as 27uf and 37uf. Dont know if they are leaky or not.
petervk2mlg
1) Dont know if C34 is leaky. I think It measures 27uf (Original specified is 32uf)
2) C24 and C20 are measured and are within the range mentioned in the manual. Dont know if they are leaky.
3) I measured ECH81 (Pin 9) with Digital Meter. It shows -0.25V on 20V multimeter range. that is Negative voltages.
4) I have cleaned selectors and controls.

Gentlemen,
As I have very basic knowledge. I will collate testing/information done on another thread and post it here in single post. So it may be helpfull.

Regards.

[Station X]

I appreciate the difficulty in obtaining components during lock down.

Check the voltage on B3 control grid (pin 3) wrt to chassis. If it reads close to zero volts, C36 is OK for the moment, but should be replaced ASAP.

Also check the voltage on B3 cathode (pin 2) this will give us an idea of the current passing through B3 and whether it's excessive.

[Hiten123]

Thanks Station X,

Since I have only 4/5 seconds to test. I have put a small fan at distance on R15 and ECL82.

I checked the voltages on B3 (ECL82). Multimeter set on 200V DC. Multimeter Common probe to chassis Test probe to test. Pin 3 = 0V DC and Pin 2 = 1.2 V DC.

[Hiten123]

Gentlemen,

please ignore the voltages I have mentioned in post #7. The valve rectifier needs to be warmed up to give proper supply.

I have decided to wait and replace C34+C35. and other parts; since there is chance that lockdown will be lifted within a week or so.

Will further update soon

Best Regards

[Station X]

1.2 VDC across the cathode resistor R14 indicates that the valve has at least started to conduct. As the cathode temperature rises the current through R14 will increase as will he negative bias on the control grid. Eventually a steady state will be reached and the cathode voltage should be around 12 VDC.

If R14 has gone low in value or C33 is electrically leaky, the current could be excessive. It would be worth checking the resistance of R14.

[petervk2mlg]

I think you could have a short somewhere after R15. With the set off measure the resistance from it to chassis.

[Hiten123]

thanks station X and petervk2mlg. I have mostly checked for shorts around R15. As said there is high likely hood that lockdonw in my town will be lifted in a week. So as R15 is getting hot within seconds I cant measure voltages etc. throughly. I am newbie and link given in post 1 people have guided me for some part of troubleshooting. Will check further in a weeks time or so.
I thank you all very much
take care and stay safe
Best regards.

[Radio Tech]

Hi Hiten

Yes I did mean disconnect the wire from the cathode of rectifier and work from there but as you have lockdown in your country and cannot get replacement parts yet does not help. But with Station x help and others helping you will get there solving the problem. Good luck, press on, don't give in.

Ken

[Hiten123]

sorry to say our lockdown is extended by 2 weeks. So Meanwhile I will be cleaning the old radio. Will update as soon as possible.
I appreciate help all of you.
warm regards

[vu2nan-nandu]

Hi Hiten,

You would do well to check all the resistors and electrolytic capacitors. At least one lead of each electrolytic would need to be disconnected before checking.

Then replace all bad resistors and electrolytic capacitors.

Next replace all the tubes with the spare tubes.

Further steps may be decided after this exercise.

Regards,

Nandu.

[Hiten123]

Thanks Nandu,
will do so onece lockdown is lifted.
Regards

[Hiten123]

Good News !!!
The radio is working.
I replaced R15 and C34 + C35. No Sound, Disapointed I tested each valve with another spare one. No result. Put back original valves. Decided to check Rectifier valve voltages. Rectifier Pin 3 had around 100V so Checked Transformer Secondary. It showed varying voltages. around 260V/280V. My multimeter is not good. I turned off everything. Checked new parts connections and started again. The radio started working. Once there were very low level faint fire cracker like sounds (three to four in no.) but radio kept working. So turned it off. Played in again after 3/4 hours. Now it is working.

Thanks everyone. Feels good that a product made 50 years ago (Philips) is still working.

1) Was the problem due to low value (1K instead of 1.5 K resistor R15) and old caps.
2) There is thermal fuse between windings of power transformer. Could it be intermitent ?

Thanks again everyone. WIll try to post pictures again after assembly.

[Chris55000]

Hi!

I've not found in my experience, thermal fuses in incorporated into mains transformers to be a source of intermittent defects, but breaks in the fine wire leads from the windings themselves where they're soldered to connecting tags, etc., is very possible, particularly where old varnish and/or old sleeving tends to hold the leadout wire from the windings in place!

In general, when you're considering intermittent defects and noisy operation in general of very old sets, the items to concentrate on are:–

1) Solder–joints, switch contacts, volume and tone controls, contact between valve holder socket contacts and to the valve base pins themselves;

2) Intermittent breaks in wiring, and defects of it's insulation, e.g., perished rubber;

3) Leadout wires from transformers, as mentioned a bit earlier;

4) Old paper and polystyrene capacitors, which can leak badly (electrically, not necessarily physically), intermittent open–circuits in capacitors;

5) High value resistors over about 10k in value, for increase in value or open circuits;

6) Effects of damp and corrosion in inductors and transformers;

7) Intermittent connections to electrodes in valves.

The above are given roughly in order of the most likely to the least likely.

Chris Williams

[crackle]

Originally Posted by Chris55000

Hi!



1) contact between valve holder socket contacts and to the valve pins themselves;

Number one I suspect. Give the valves a little wobble and see if there is any bad crackling.
Mike

[ekjdm14]

Originally Posted by crackle

Number one I suspect. Give the valves a little wobble and see if there is any bad crackling.
Mike

Seconded. This is especially true of Noval type valves with their smaller pins and less contact area than Octal. Quite a few times radios I've recommissioned have been silent until the valves are wiggled.

Glad to hear you've made some progress though, well done, it's always rewarding to hear the fruits of your labour.

[Hiten123]

Yes. Cleaning contacts and servicing is next.
I once again thank you all.
stay safe and take care.