Philips 671A

[Magic Eye]

Hi all
I have been working on a Philips 671A (170A chassis) and when I powered up the other day it seemed to pull in only a few stations, albeit some strong ones, and then after about 15 mins the sound went a bit distorted and weak. I also noticed that the EBL31 was too hot to touch.

I have replaced all the black Philips caps, plus C29, and restuffed C31 and C32. I am using Trader sheet 770.

I have taken some voltage readings as follows:
AZ31 cathode 250v
AZ31 anodes 252v
HT at C31 205v
The anode voltages seem high and I don't know what the HT is supposed to be, but 205v seems on the low side. I also noticed a small positive voltage on V4 (EBL31) top cap of 0.5v.

I am running on a short temporary aerial and it looks like the o/p transformer has been replaced a long time ago by an RS part specifically for these sets.

Any help much appreciated
thanks
Jon

[Magic Eye]

Decided to power up again last night on a much longer aerial, also provided an earth. This was worth doing as I heard many more stations.
However there is an annoying hum from the speaker and as I have replaced the reservoir and smoothing caps could this be a short in one of the valves, if so which is the most likely culprit?
The EBL31 is still running very hot, is this normal?

thanks
Jon

[Herald1360]

Eugh! What was the Philips designer on when that detector/output stage was conceived?

It does look like some positive voltage would be expected on the EBL31 grid- it returns to a tap on the cathode resistor.

I think to decide whether it's running too hot, you will have to figure out what its anode and screen currents are and compare with some datasheet values. Not so easy given that circuit- you may have to insert some 10ohm resistors strategically to do it.

At least have a look at the grid-cathode voltage with a high impedance meter and see if the bias voltage is about right.

[TrevorG3VLF]

You should measure the output valve (EBL31) current by measuring the voltage across the output transformer. The grid at 0.5V could be correct but any cathode bypass capacitor should be suspect.

Edit
Is C29 good and connected right?

[Boom]

Originally Posted by Herald1360

Eugh! What was the Philips designer on when that detector/output stage was conceived?

Probably a diluted drop of what their ancestors had when designing this 634A that I'm working on? Lovely Dutch gear indeed.

[Leon Crampin]

I've spent some time on the design of the 170A with a view to properly specifying the output transformer so that an accurate replacement could be made.

Philips were not fools, and in my view, the design is an excellent one other than being a little short of audio gain. Fully driving the output stage on a reasonable signal is easy to achieve if the set is properly aligned and the gain overall is higher than would be achieved with the same number of valves in a conventional set. The RF stage considerably enhances its performance on weak signals and on SW.

The cathode resistor for the EBL31 is split to allow for both the correct AGC voltage delay and the correct bias for the pentode. Use either or both resistors to determine the total EBL current by voltage measurement and Ohms' Law after checking the resistor values.

Measuring valve anode current using the resistance of the transformer primary is a bad idea as the copper winding has a considerable positive temperature coefficient of resistance. The resistance could be measured immediately prior to evaluation, but as there are linear resistors in the cathode circuit, there's no point.

AZ31s are not the most long-lived rectifiers and tend to fail premauturely with low emission.

The 170A and its clones was a good design - perhaps more elegant than might first appear.

Leon.

[TrevorG3VLF]

The output transformer is a large lump of metal and will not warm up much in a minute or two. Measuring the voltage across it will give the anode current directly which is what is required. I doubt if it will be more than 10% out.

The anode current will depend on many things, grid voltage, screen voltage, cathode resistors, cathode capacitor, heater/cathode leakage and valve characteristic.

[Leon Crampin]

The 170A transformer primary is wound with very thin wire - that's why so many fail. The bobbin will therefore heat much faster than the core and give an erroneous reading. Time would need to be allowed for the valve parameters to stabilise, so the most likely operating conditions would be a hot valve, hot bobbin and cold core.

Sometimes, a device which obeys Ohm's Law has real advantages. Other than transformer failure and weak AZ31 performance, the 170A output stage is not generally a problem.

Leon.

[Magic Eye]

Originally Posted by TrevorG3VLF

You should measure the output valve (EBL31) current by measuring the voltage across the output transformer. The grid at 0.5V could be correct but any cathode bypass capacitor should be suspect.

Edit
Is C29 good and connected right?

Thanks for all the replies.
Trevor- C29 is new and is connected correctly.

I have carried out some measurements across R29 and R30 in the EBL31 cathode circuit.
R30 measured 170 ohms and dropped 6.3v so the current is 0.037A or 37mA
R29 measured 119 ohms and dropped 4.2v so the current is 0.035A or 35mA.

Does this look reasonable as Trader sheet 770 doesn't quote these values

thanks
Jon

[TrevorG3VLF]

My Wireless World data gives 36mA anode and 5mA screen so you are about right.

200V*37mA = 7.4W so it will get hot.

Philips data is the same, the maximum anode dissipation is 9W and screen 1.5W.

It is having an easy time !

[Magic Eye]

Hi
That's good news.
I still have the hum problem. Does anyone have any ideas what could be causing this?
thanks again
Jon

[TrevorG3VLF]

You could earth the grid of the EBL31 and hear if the hum stops. This would tell if the hum is originating before or in the output stage.

It has a strange sound output transformer circuit which I do not understand, there was an analysis of this circuit a few months ago.

[Leon Crampin]

If you earth the grid of the EBL31 on a 170A chassis you will completely upset the bias for the pentode, due to the AGC delay voltage being developed across both the bias resistor and an additional cathode resistor.

A quick prod around with a 'scope should nail this problem, but if this chassis produces low HT with hum and no other faults, I would suspect a low emission AZ31 (100 Hz hum) or an open HT winding on one half (50 Hz hum).

A low emission rectifier will produce high levels of ripple due to the increased time taken for the reservoir capacitor to recharge after each half-cycle where the rectifier is non conducting.

My 170A has an HT rail voltage of approximately 250V - and very little hum. Some of that residual hum is caused by magnetic coupling between the mains and output transformers.

Leon.

[Magic Eye]

Hmm, I'm getting 252v at each anode of the AZ31. 250v DC at the cathode and HT of 205v.
I pulled the AZ31 from my 170A and tried that. The HT only rose to 210v. So may be both rectifiers are tired (the "proper" 170A also needs restoration). I'm wondering if I should find a NOS AZ31....

thanks
Jon

[TrevorG3VLF]

Earthing the grid of the EBL31 will drop the standing current considerably but will not affect the gain greatly. It would only be done to check the hum location.

Scope - First catch you scope!
Surely, a longer capacitor charge time will give a lower ripple, at DC the ripple would be zero.

As I said, I do not understand the output transformer circuit but it appears to be giving hum compensation using R22. If the early part of the set is using too much current would this affect the balance?

I suggest measuring the voltage at each end of R22.

[Magic Eye]

OK I'll have to do this tomorrow now and will report back

thanks
Jon

[Herald1360]

Originally Posted by Leon Crampin

A low emission rectifier will produce high levels of ripple due to the increased time taken for the reservoir capacitor to recharge after each half-cycle where the rectifier is non conducting.

If a low emission rectifier behaves as a rectifier with a higher series resistance with the same current load (likely here since most of the current is the output valve which as a pentode is a reasonable constant current device), you get a lower dc (average voltage) but also lower p-p ripple voltage than for the normal rectifier. The p-p ripple as a percentage of the average dc does increase, though.

[Magic Eye]

Hi
I realised that I essentially measured the voltage across R22 earlier in the thread, but did it again anyway:
AZ31 cathode 250v
HT at C31 now 210v
The resistor itself, whilst looking a bit cooked, measured 1.5k. I am unsure what these readings tell us, apart from my HT still being on the low side. Leon said his was 250v.
I wondered if the low HT was due to a tired AZ31 (or something else dragging it down). It rises to over 300v on switch-on. Also wondered if the hum was due to a replacement o/p transformer having been fitted (long ago)?

regards
Jon

[TrevorG3VLF]

I asked for the voltage at each end of R22 since I was not sure what you meant about HT.

If my higher maths are correct, the current through R22 should be 24.7mA, you measure 27mA. That looks right.

The EBL31 anode should be 243V, a bit lower than AZ31 cathode, that looks right.
The EBL31 screen should be 215V, you measure 210V. That looks right.

So I come to the conclusion that there is nothing wrong. Why do you think the voltages are low?

If the output transformer has been replaced, does it have a tapped primary and a feedback winding?

[Magic Eye]

Hi
I thought the HT voltage was on the low side as Leon had posted earlier that the HT on his set was 250v. Certainly on the face of it all seems OK. The replacement o/p transformer is an RS part that must have been fitted many years ago. It has the correct windings and gives a list of the models it fits, including the 671a.
I will monitor the him after I have put it back together- I need to refinish the cabinet.
Thanks for everyone's help.
Jon