Diode replaced, problem solved. Lets hope it stays that way.

Thank you everyone!

Let me try to revive an old thread...

My Beogram is a type 5527 and i've just downloaded the 'new' service manual that Peter kindly uploaded.

Where to start..... Problem is that the arm will lower at the 12" position even with no LP on the platter.

I traced bad wiring to the photocell in the arm - that was replaced. All of the small tantalum beads were replaced just for peace of mind (all the correct polarity)

Problem continues... I got the 'scope out to see what was going on. The photocell is giving an output.

Connected the scope up to compare to the diagrams under 'Detector arm circuit' listed on page 7.4 in the service manual.

Measuring at the base of TR3, the waveform just appears as noise. Stimulating the photocell with the platter doesn't appear to do anything.

Next, onto measuring the collector. I get approx 3v instead of the specified 4 when in a steady state. On stimulation, the waveform looks similar to the diagram (with more noise), but the waveform stays between 2.5 and 3.5v. If I shine a very bright light onto the platter, it the wave will reach 0v at the trough, and go slightly past 6v at peak. The arm still lowered though even with a bright light there.

I didn't have chance to measure the base of TR4 yet... I had to put all of the equipment away before I had the chance.

The resistors in that part of the circuit all measure within spec, and the supply +21v looks nice and clean.

I did change TR3 to a BC549 (similar spec to original), which yielded neared to 4v at the collector, but that was about all.

I'll try to have another look at it tomorrow, but if anyone can suggest anything in the meantime, I would appreciate it.

Ben,

I take it the sensor arm lamp is shining OK on the record and is of the correct wattage ?
All aluminum surfaces installed ?
Given those;
- Check that the lamp is sitting correctly, with its filament
right above the focus lens and as close to it as possible.
- Check the grounding connection to the carriage.
- The lamp can have degarded. Its light output degrades with age.

The sensor cell gives only a few hundreds of a volt, not easy to measure without
disturbing the signal.
Instead, I recommend you measure at the base of 1TR4 where the signal amplitude
(reflections from the platter ribs) should be about 1 Volt offset to apprx 20V DC.

Martin

Hi Martin,

The arm lamp is correct, but is old, so maybe degraded.

When measuring the photocell output before the decoupling capacitor (2.2uf), I can see where the ribs on the platter are, but after the capacitor, it all just becomes too noisy.

I didn't have the aluminium surfaced installed when I was measuring with the oscilloscope, but they have been installed previously, and the arm still lowered.

I'll get all of the gear back out tomorrow and have a look at TR4 - Will take a picture of the display on my 'scope and upload it.

Thanks,

Ben

Ok, so I tried to measure the base of 1TR4, I would get only 21V steady. 1TR4 tested to be short circuit between base and emitter. This was replaced and here is the image as viewed at the base of the new 1TR4... looks pretty close to the one in the workshop manual.

Peak to peak, its approx 1V with the bright light shining on the platter, without its approx 800mV.

The deck still was lowering the arm though...

I soldered a wire to the base of 1TR6, so I could reassemble the deck and observe the waveform, and hey presto, the arm continued over the platter! I placed an LP on the deck, and the arm would lower.

There must have been a dry joint on transistor TR6 that was fixed when soldering on my flying lead.

I still need to sort out the lamp in the detector arm though... I can't always have a desk lamp pointed at the deck! What is the specification of this? In the manual, it just says 'Lamp 6V'.

Nice troubleshooting! Generally, I resolder EVERY joint on all the PCB's on any vintage machine I get in for restoration - time consuming, but well worth it!

Nice scope! Definitely among those considered for my Wishlist.

The vast majority of vintage B&O lamps are 25mA or 30mA - you can't go wrong if you choose a rating like that.

Menahem

Thanks for that... it seems as if 6V 30ma are available from my sources, so i'll try that.

I think I may have some ultrabright white LED's that I could try first though, then I don't have to worry about failure. Anybody upgraded with LED's before?

Yeah, the scope is quite a nice one... my late 70's Gould recently expired, so I treated myself to this LeCroy. I could either have a new no name Chinese 20MHz analogue scope, or this used high end digital scope for less money.

In this post, I dealt with the issue of changing to LED's, of which I am a fan, where possible.

http://forum.beoworld.org/forums/t/12646.aspx

That particular lamp on the sensor arm, I did not change, because it is relatively easy to change (do not need to dismantle the whole turntable) after a burn-out, which is relatively rare, and more importantly, the angle of the bulb is important to get the right concentration of light for the reflecting surface, so an LED, which is highly directional, may not be suitable.

But we're always willing to hear differently from you, if you succeed. I'd try a 3mm LED, instead of the regular 5mm units. Check the voltages - you will probably need a dropping resistor.

Menahem

Not only is the position of the lamps filament with regards to the lens system and the amount of light
output important. The type of light also needs to be right since the sensor is neither a photocell,
LDR nor photo-transistor but rather a voltaic cell.
A LED will not output enough energy to activate the system (at least they did not about five years ago when I
tried it) and LED's do not produce a continuos output but are switched on a high frequency.
As mentioned, I also tried the LED conversion but didn't manage to reach a working solution so I stick with lamps.
I have suitable lamps in stock if you cannot get one locally.

Martin

Right again, Martin :-)

Well it turns out that an LED will work!

Installed a 5mm ultra bright led yesterday, and the electronics side of the deck all work great.

The 5mm LED was very tight to fit, I had to shave down the flange at the end, so there was a flat on both sides of the LED, but it works great.

Anyway, another problem:

When playing a disc that is warped slightly, the arm doesn't follow the disc up and down - the cantelever of the stylus is! Luckily, i'm not using a decent cartridge or disc so far.

There is a thin brass coloured shaft, which it appears that the arm balances upon. I tried lubricating this with a sewing machine type oil, but i'm not sure which part of the shaft to lubricate - the end opposite the tracking force adjustment, or directly under the arm? Could the spring to do with the force adjustment have something to do with the problem?

Finally, what do people use instead of tool 3624009 for making sure the arms are tracking correctly?