Roberts R600 IF module problem.

[davidgem1406]

I have 2 Roberts R600 that I am refurbishing.
One unit was fully working with the LP1171 containing the original AF11x transistors.
The other unit firstly had an audio stage fault, soon dealt with, by replacing 1 transistor. It now had no FM or AM, this was put down to the AF11x transistors, 1 or more, having the tin whisker problem.

First I stripped out both units and removed the IF modules. I then rebuilt 1 unit minus the LP1171. Fly leads were added so that this chassis, fully working, could be used as a test bed for the LP1171 modules.

I first connected the fully working module and having removed the base cover took the DC voltage readings from the transistors.

(here I will cut out things that I did with the second module that produced zero results)

After taking the measurements I proceeded to replace the AF11x transistors one at a time starting with TR1. This I replaced with an AF127, checked that all was still working. I then did the same with TR2 followed by TR3 using AF106's. I now had a fully working module, I closed it up and checked one more time, fine.

Now the second LP1171 was wired in place of the first module having had all 3 AF11x's replaced by an AF127 and 2 x AF106's.
The results were that FM was working but appears to be at reduced gain, as compared to the first module, whilst AM was non-existent.
The DC voltage conditions were checked and found to be pretty much the same as the fully working module, on both AM and FM.

The only conclusion I can draw from this is that the AM oscillator coil has a fault? or has anybody else some other idea.

I was however thinking of changing C9 as shown on the diagram for the LP1164 module that is much the same as the LP1171.
The diagram etc for the LP1164 is attached thanks very much to the work done by another member, Ron Bryan, thank you Ron.

Also attached are all the strip out drawings I made for the R600, and a module pinout cross reference I made up.

Happy new year and regards to all
Dave.

[Station X]

I'm sure I remember replacing an AF115 transistor in an AM IF Mullard Module with an AF12X type and finding it wouldn't oscillate.

Further details escape me so I can't say whether the replacement was unsuitable or faulty.

[Radiocruncher]

Thanks for sharing the info you have obtained Dave. Very useful. I have done lots of these modules and have used mainly AF125's. I did have a batch in that were faulty though so wondering if you tested them before fitting. I'm not familiar with the AF106 or of it being used as a substitute. I have had electrolytics fail in these before so I change them all now after having to take a module back out of a set.

Good luck

Graham

[davidgem1406]

Hello Graham,
Thanks for the reply, I have been thinking about electrolytic's so will start changing them.

Tha AF106 will run to 260Mhz but I have no idea of the hfe.
I have used them in for TR2 and TR3 in these modules and they work fine. They don't seem to want to work as TR1.

I also have AF125's as well, the AF125 and AF127 are given to have the same characteristics and both will work as TR1

Dave.

[davidgem1406]

I have now done some further investigation into this problem.

The component numbers given refer to those on the circuit for the LP1164 module (attached to the original posting) the circuit of the LP1171 is the same in respect to this part of the circuit.

The module is working on FM only, AM is silent not even any noise.

I replaced C4 and C9 being the only 2 capacitors connected to the AM side, this had no effect whatever.
Di and D2 were checked and found to be in good order. Disconnecting R17 killed the FM indicating that AGC was working.

The AM oscillator coil is a grounded centre tapped primary and a single winding secondary and included in the LP1171 module.
Windings were measured with the following result: centre tap to one end 3 ohms, and to the other end zero ohms. The secondary was at 0.25 ohms. Measured with an avo 8 MK V on ohms x 1 range.

With all these results I can only conclude that the AM Oscillator coil is faulty.
Unless somebody has other suggestions/ideas.

Thanks
Dave.

[ronbryan]

The only diagram I have for the LP1171 is for the Heathkit Severn AM/FM radio. It has no component values shown. On that circuit, L201, the extreme left hand adjuster on the bottom row, looking at the top of the module, is the AM oscillator coil. This is shown as a double wound transformer, with the 'lower' ends of both windings connected to the module -7.5V rail. There is no centre tap, so it does not agree with your findings, unless the two pins with zero ohms beween them are actually joined.

On this Heathkit Severn circuit, the oscillator coil switching is hard to decipher at a glance and the module pinout is different to the one Roberts use so it will take a while to decipher how the first stage is supposed to oscillate on MW and LW.

Can you try a good AF125 in the mixer/oscillator position before you go further with the coil (or have you tried that already?).

Ron

[ronbryan]

Dave

I've had a look at the way the AM oscillator works on the LP1171 in the Severn and this is what I believe happens.

Oscillator coil L201 must include the isolated (and unlabelled) winding shown between pins 3 and 6 of the LP1171 module, so there should be three coupled windings connected to five pins.

On AM, the FM IF transformer is shorted out, the FM emitter bypass capacitor C202 is switched to be a coupling capacitor connected to oscillator coil L201 secondary.

L201 primary is tuned by the oscillator section of the variable capacitor via the selected padding capacitor.

R402 is the damping resistor required by the Mullard data sheet. Feedback coupled from the coil in the collector circuit to the coil in the emitter circuit causes the circuit to oscillate at the tuned frequency.

Have you compared the resistance readings with the oscillator coil for the other working unit and can you confirm that the three windings are configured as I describe?

Ron

[davidgem1406]

Hello Ron,
Thanks for the input, I also have that Heathkit circuit but have not looked at it.
I have tried AF125 and AF127 as TR1 everything works for FM but nothing on AM.
As for the AM oscillator coil both ends are used and definitely centre taped, unless it's a short circuit, 1 end has an input whilst the other goes to C1
If there are 3 windings and two have 1 end grounded then that could look like my centre tapped winding where the centre is grounded.

The completed and working module I have now rebuilt and fitted to a chassis so I can not check now. I did not make a comparison before doing this, sorry.

As the faulty module was worked, using a known good chassis, the only thing I can be sure of it that the module is at fault.
I had also assumed that as FM is working it had to be the AM coil, but now just maybe!

If I have another R600, but I don't think so off hand, I can strip that one down as it will need the transistors changed anyway.
I will have to get back on that one.