Picture is worth thousand words - allegedly Confucius. The photos still pixelate however, one of the Ecaps in the other photo shows 25V.

The President's noted are back biased, and 25V is a common cathode bias cap. Philips and very few others use them across the back bias where used. So we either have a cathode bypass cap or perhaps a cap across back bias. However, the latter is normally 40V & positive to chassis. So something achieved.

At this point the two unreadable, whitish caps could be around 16μF and the filters and if its original perhaps two cathode bypass caps: Not committed to that one yet, bur Astor did it.

Marc

Thanks Mark, I'll email some higher res pics to you shortly, looking at the chassis compared to the M55 circuit there does spear to be some difference - so still at a bit of a loss as to what it is and should look like
Scott

And what a difference clear photos make. That has been seriously got at. The two caps in the RH top corner are far from original the dark one is English looks like an off the bench one but the voltage rating is right (now is it polyester & not paper?) and the silver one is oil filled. They are as bad as paper & more toxic, it has to go.

The electrolytics are fascinating and it will need to be worked out as to what they are on. We have five: Two from one era, three from another: Bizarre? How many are in parallel & if it was properly repaired, what the hell are the two red ones doing, still in situ? They should have been replaced? 4μF is an unusual value in a valve radio. That may be useful.

This makes ID just a whole lot more confusing to the young players and old.

Its a case of back to square one & find the transformer HV Centre tap. It is either direct to chassis, or via a resistor. 125 Ohms if it is what I thought it was originally, but now have doubts about.

How many valve sockets has this got? despite a two gang tuner there seems to be something obscure next to the gang & hardly visible in the top of chassis photo, behind a can.

Marc

A couple of thoughts. There is a list of radio models floating about. I have a copy. Tedious but a superb document for looking for sets with a certain valve combination.

Second: 4μF In a more modern set. HMV were one that in some models had a cap of around that value with a parallel Non polarised RF bypass cap across it, on the screen of the Pentagrid. Might we have similar or is it C1 of the filter keeping the volts down?

Marc

Hi Marc,
I'm really appreciate your input on all this. I'm out of town till Wednesday but will get back to you with some answers to your questions then. It's really got me stumped.
Perhaps I could call you??
Cheers
Scott

I would note that I was fixing these things before I left school. I really am long past the point of bothering to panic if there is no diagram. With some you are dreaming if you think its going to happen. STC Chassis #59 and an AWA one off being examples.

So one moves on & replaces what is there, not worrying if its obviously an original part. The alternative is to draw a circuit by reverse engineering it.

That's for me, merely a case of initially getting the AutoCad & dropping in the requisite pre-drawn Valves (bases), transformers, etc in the place they are on the chassis & then playing join the dots. Putting different coloured marker dots on the valve elements pins / sockets helps. E.g Plate, Red; Screen, Blue; Cathode, Black and so on.

If you fix these things, too hard & panic are not qualities to be found useful in fixing the customers problem.

Marc

That makes sense. So I guess then if possible, re wire it to put a 6M5 back as the audio OP tube would be the go. As I'm fairly new to all this, your guidance would be appreciated

I am not a fan of modifying until it it is established as to what you have.

I will send Brad a drawing of a typical 6M5 output. If this is back biased Rk and its cap will be missing and the pin grounded. Unused pins may have other stuff on them (tags of convenience). Look at it and what you have; It will either match or it won't. That is view socket wiring side.

That drawing should give an idea on how to go about reverse engineering it.

Marc

Photo uploaded to Post 23.

Thanks Marc,

Ok this is what's currently the wiring for this tube (still don't know what the tube is)

1. Has 5 connections
- .4μF electro + side with - side to gnd
- resistor to pin 1 of the next valve 6BD7
- resistor which is terminated on the OP transformer
- 8μF electro + side with neg side to gnd
- resistor to coil and directly connected to 2nd coil

2. Has 2 connections
- Goes to tone control
- .02 μF cap with other side connected to pin 1 of 6BD7

3. Goes to gnd
4. Heater (gnd)
5. Heater
6. No connection
7. OP transformer and unknown cap to gnd
8. No connection
9. No connection


Hope that makes some sense....

Scott

Looking at the speaker connection, 2 pins go to the OP transformer as per above and the other 2 directly to the speaker itself.
It really doesn't make any sense to me at all.
Hopefully you may have some idea on it all.
Cheers

I'd still say it is a 6M5.

That's fine and I'm happy to put a 6M5 in, but I'm guessing we would have to modify the current setup to make it work.

It looks like our slacker has paralleled the 4μF & 8μF and it was originally a 16μF cap. I thought it had too many. Cathode pin 6M5 grounded says its back biased, like the Presidents mentioned

We modify naught: We try to get it back to spec. That wiring is consistent with a 6M5 so we leave it as it is and just replace the caps & check its resistors. Resistor terminated on the transformer is odd?

There should be one of around 470K from pin 2 to the HV centre tap rather than chassis. Make sure that is what you are looking at? Finding that CT is important. That point on the wiring plate(?) should be perhaps 170 Ohms from it to chassis.

If its missing it won't run.

The paper caps, filter caps & other maintenance is going to be done before power is again applied.

Note 6CK6 comment if you cannot get a 6M5 which may actually be what is in it currently? Join pin 6 to pin 3.

Marc

Hi again,
Does anyone have the schematic for the President Tasma M51? We have restored the audio output side which is now working fine with a 6M5 installed. But the radio side is not working at all.