Radiomobile 4200 help

[PaulR]

This car radio comes in two halves with most of the components in the "power pack" part. The owner said that it worked to an extent but the volume distorted at higher volumes. I thought "that", or as it has a p-p amp "those" caps and decided to replace all the paper caps in the main section.

There is now a very strange fault in that the volume control does nothing at all. The set comes on at medium volume but it will not go up or down. Shorting the centre tag has no effect at all and the earthy end of the track doesn't seem to be connected to earth. Clearly the af valve is getting a signal from anothe path.

I have double checked my wiring and cannot find anything wrong, but I assume I must have made a mistake somewhere. I also cannot understand how the circuit around the volume control is meant to work. There seems to be two S2 tone controls. I can see how the one adjacent to V5 works, but I cannot understand that near the volume control.

Can anyone please help with regard to the non-working volume control? Also if the workings of the circuit could be explained I might have more of a chance at fault finding for myself.

All references are to the Radiomobile circuit diagram which I obtained from this forum.

Many thanks

Paul

[Steve_P]

The centre of the volume control goes to pin 1 of the DH77. Of the two ends, one goes to a switch to C24. The other end goes to earth via C26, and DC via R16 and R19.

If the control is OK, then check the parts listed. It could be the valve, but that is unlikely.

Cheers,

Steve P.

[PaulR]

Thanks Steve

I cant fathom what the switch by C24 is doing. One pole seems to be connected to the screening of the lead to V4, two seem identical and the third puts C25 into the circuit. Also, it looks like one end of the volume pot is also connected to the screening of the lead to V4. As you say the whole lot is earthed but via C26. Is it C26 some sort of interference suppression?

The volume output is unaffected even if I short the pot wiper terminal to the lower end (ie connected to the mysterious switch) of the track.

[Steve_P]

Possibly. The switch is not directly connected to Pin 1 of the valve. That part of the diagram is not that well drawn. The earthing seems to go via the switch but that is all.

Meter the switch out and see if it's OK, change all the capacitors and look to see if the volume control pot is making full connection inside. Sometimes they come disconnected inside too. Also check points 6, 7 and 8 and make sure they are the right way round too.

Cheers,

Steve P.

[PaulR]

I have just had another look at it and the sound is unaffected if I completely disconnect the lead from pin 6 of the multi pin connector to pin 1 of the valve leaving nothing connected to the grid at all. The amp must be getting the signal in some other way, but I cannot see any mistakes I have made when I replaced the caps. Very odd!! The owner says that the volume control was working when he took it out of the car, but it wouldnot go particularly loud and was a bit distorted.

For reference, the front end and the controls are in the front section of the two cases whilst the rest from V3 onwards are in the back part which is hidden when it is in the car (a 1952 MkV1 Bentley). The two are connected by a multi pin connector.

[Steve_P]

Look for wiring faults and soldering faults. Try a new valve. This is a common fault on some domestic valve sets.

Cheers,

Steve P.

[PaulR]

Thanks Steve. It is nice to know that it isn't uncommon!

I will check everything again. The valve holder connections are a bit delicate and I have already had to "borrow" a spare from the rectifier so it is possible that another has broken.

The valve in question os a DH77 and I have swapped the two in the set about with no improvement.

Paul

[Steve_P]

This could have been done in the past. Try a known good one.

Cheers,

Steve P.

[DangerMan]

Hi Paul,
any chance of a snippet of the circuit, showing the relevant area?
Pete

[PaulR]

I am not sure how to show only part of the diagram (to keep within forum rules) dcan anyone help?

[geofy]

Originally Posted by PaulR

I am not sure how to show only part of the diagram (to keep within forum rules) dcan anyone help?

If it is a PDF format these are sometimes (annoyingly) copy protect. But Paintshop Pro is one program that can take copies of sections. Or use pencil and paper and draw it out.

If it is one of these radios with a long connecting lead between the two sections, is the lead damaged somewhere along its length. Or a lead left 'floating' between power pack and radio where the controls are.

[Station X]

Get the circuit on your PC's monitor screen and hit the "Print Screen" key. This will save the screen to your scratchpad. Open MSPaint, which comes as part of Windows and click "Edit" then "Paste". Save the document as a jpg.

Open the jpg using a photo editing application and crop it.

Or you can use MS Paint to crop the image. Use the selection tool ( a dotted square) to select the part of the circuit you want and copy it. You can then paste this into a new document.

[PaulR]

I am attaching part of the circuit diagram.

It has a long lead connecting the two together. I have gone over my wiring and everything seems correct. I will try resoldering later this evening.

The signal must be coming down the interconnecting lead somehow and I wondered whether I should try disconnecting each lead in turn to see which is actually carrying the signal - unless anyone can think of another approach.

Steve - I have a valve tester but no replacement valve at the moment. Were you thinking it had gone low on emission or some sort of internal short?

Thanks


Paul

[Herald1360]

Interesting circuit. The "bottom" of the volume control is grounded by C26 1uF and the grid of the audio amp triode in V4 is returned DC wise via the volume control and R16 to the junction of R18/R19 in the cathode circuit, so the grid bias is the voltage across R18 only. I think that the cathode may be higher DC wise than needed for bias to give a delay to the AGC voltage. I think that the switched tone correction only has two effective states, one with bass cut provided by C25 and the others flatter in response, otherwise it seems pointless?

The rest looks upmarket - push pull output no less!

[geofy]

Originally Posted by Herald1360

switched tone correction only has two effective states, one with bass cut provided by C25 and the others flatter in response, otherwise it seems pointless?

The rest looks upmarket - push pull output no less!

The bass cut is probably to make speech clearer, at higher volume levels in a car. I wonder if the screen has punctured the inner lead on the volume control and shorted it.

[PaulR]

Originally Posted by Herald1360

The rest looks upmarket - push pull output no less!

Well It does come from a Bentley!

I will check the volume control centre lead, but the fault is totally unaffected even when the lead to the control grid of V4 is completely disconnected from its connection to the socket on the inside of the "power" box. The signal must be getting into the amp some other way.

If it helps, some of the settings of the tone control make the volume more or less disappear.

[DangerMan]

Hi Paul,
the fact that the volume control doesn't work, and the volume is always less than maximum but greater than zero could be because the signal is not coming from the main detector or even via the audio pre-amp V4. The fact you can disconnect pin 1 with no effect seems to confirm this.
I would first check for a short circuit between pin 6 and 7 of V4, or more likely C29 connected accidentally to pin6. This would enable the AGC detector to provide an audio signal directly to the later stages of the audio amp.
The two switches are as described ... left hand part (on the diagram) rolls off the bass in one position by feeding it via a low value capacitor. The right hand part rolls off the treble in two positions; one is a mild cut with C30 and R22 in circuit, the other is more severe with position 11 directly grounded.
There is one position where neither HR nor LF roll off occurs.
TTFN
Pete

[PaulR]

I feel such a fool! I had connected C29 to pin 6 of V4. I even had to extend its lead to do so. Anyway, the volume control is working OK now. Very many thanks Pete.


Can I ask another question about the set? There are instructions about what speakers to connect to the different tappings on the o/p transformer. The owner wants to add another speaker of indeterminate impedance. I realise that the secondary of the o/p transformer should always have a load, but would it do any harm if the speakers were put in parallel so maybe halving the overall impedance?

Thanks


Paul

[Herald1360]

You won't harm a valve set with too low a speaker impedance; you may reduce the total power output available a bit, is all.

[PaulR]

Thanks Chris. That is what I thought.

Paul