Beomaster 4400 Type 2419: Continuing the Reassembly of the front panel switches

Continuing on from the previous post with the reassembly of the Beomaster 4400 front panel switches, I will also continue showing the differences in the switch panel components between early serial number units and later serial number units.

I did look up the B&O service manual addendum on the change with the switch panel.
According to that documentation the "early serial number" switch assemblies are for Beomaster 4400 serial numbers prior to SN 1524001.  Units with serial numbers 1524001 and on have the new switch assemblies.

That bears out on the Beomaster 4400 units I have seen.  The serial number for the Beomaster 4400 of this project is 1510043.

Here are some photos of the Beomaster 4400 front panel for both versions side by side.

From the front the difference is in the way the slider controls mount to the front panel.
The later serial number panel mounted the controls with screws from the front while the early serial number panel used metal spring clips. For that reason the early serial number front panels included a narrow mounting bracket for the slider controls to rest on.
The metal button panel frame that the switch assembly mounts to also changed due to the different style of switch actuators.

This picture shows the type of slider controls the Beomaster 4400 uses for the early serial number and later serial number units.

On two the circuit boards and wiring.
Both types of front panels have a small LED glued in place for the Stereo Indicator.
It is difficult to detach that LED assembly when disassembling the front panel so I started cutting the two wires that connect to the LED assembly.  When reassembling the panel I install a small 2-pin connector for the cut wires. That makes it a little easier in servicing the front panel.

The switch assembly is mounted to the front panel on a metal, button panel frame. 
It is secured with two screws.
The important thing here is to make sure all of the plastic switch actuators are pressed against their respective push buttons.

Now that the switch assembly is secured onto the front panel it is a good idea to double-check the mechanical operation of the switches again.

While I was working on the front panel assembly I took a little detour to replace the two 10,000uF electrolytic capacitors for the ±35 VDC output amplifier rails.  These are the two capacitors that are mounted on a bracket just to the side of PCB 5.

I also took a little time to install the two 100Ω trimmer resistors for the no-load current adjustment of the output amplifier. On those two trimmers I always go with multi-turn trimmers as they are easier to fine tune.

Here is the left channel trimmer.

Here is the right channel trimmer.

Returning to the reassembly of the front panel I decided to prepare the new incandescent lamps in the front panel wire harness.

Three of the five original lamps still worked but since I have to replace two I am going ahead and replacing all five with proper lamps from Martin's Beoparts store.

Here are the original lamps removed.

Here are the new lamps installed.

The next step of the front panel reassembly is to re-attach the circuit boards that mount to the switch assembly.  They are how the switches electrically connect to the Beomaster.

First is the PCB 6, 2-piece board.  
It has a normal, rigid PCB part and a flexible part to connect to the FM tuner preset dials.

The next PCB to attach to the switch assembly is PCB 9.

Here is a picture of the circuit boards reattached to the switch panel.

The front panel reassembly has gone smoothly so far.  
It is at the point where the wire harnesses get reattached.
In some cases that just means plugging in some connectors but unfortunately for me it also means quite a few loose wires that do not have a connector.  
That is the reason you have to take a lot of photos during the de-soldering of those wires when the front panel is being disassembled. 
I have those of course and my task for next time is to gather all of that information and carefully solder all of the individual, loose wires back in place.

Beomaster 4400 (2419): Into the Recap

These seventies era Beomaster receivers have a low profile and are quite a challenge to open up into a service position. I like the way the eighties era Beomaster units were designed with some consideration for the service technicians. Oh well, it is what it is. The Beomaster 4400 is an improvement over the Beomaster 4000. There are some board connectors.

Here is the Beomaster opened up and ready for recapping.

The PC5 board has the most recapping work and does not unplug completely for removal. I disconnected enough wires to it so I can get to where I need to with regards to replacing capacitors but I will tackle the PC5 board after I recap the smaller boards.

I started with the PC3 - Stereo Decoder and Indicator Circuit board. It is a good one to warm up on.

Here is the recapped board. For now I am leaving the trimmer resistors as they are on the FM boards. I will replace them as necessary during the Beomaster performance testing. 

Next are the PC1 and PC2 FM boards. There are just three capacitors to replace between them.

...and here are those boards after the recap.

Now for the PC4 - Preamplifier board.

On this board I also changed out the six 50KΩ trimmer resistors as I recapped the board.

Tomorrow I will tackle the large, PC5 board and the two reservoir capacitors.

Beogram 4002: Working the Main Board

On this project I am starting with the electrical restorations. The main circuit board is first. As usual I will replace electrolytic and tantalum capacitors that are 2.2uF and less with WIMA MKS capacitors. If there is room I sometimes replace 4.7uF capacitors as well. In this case the fit is too tight to install 4.7uF WIMA capacitors so I will stick with electrolytic replacements on those.

Here is the main board again as I received it.

Here is the speed select relay removed along with the speed trimmer resistors.

I am replacing the 45 RPM and 33 RPM speed control trim pots with sealed, multi-turn trimmers. For the speed selection relay I am using Beolover's custom RPM relay component. It is a sealed SMD relay mounted on a special adapter board so it is a direct replacement for the original relay (A Siemens relay in this case).

A key component on this board is the 1C10 capacitor. Beolover's exhaustive testing of the rebuilt DC platter motors has determined that the value of the 1C10 capacitor has a measurable bearing on the stability of the platter motor speed. In his testing of the DC platter motor from this Beogram unit he determined that a 10uF capacitor provides the most stable speed circuit. 

Here is the completed main board with its new parts.

Beomaster 6000 (2702) restoration: recapping output PCB + trimmers

After the complete dismantling of the Beomaster 6000 quad it is time to start the restoration work. I decided to tackle the output PCB first (not the darlingtons yet). This PCB houses the 4 power output driver networks and the bank of push buttons for different actions like: front and rear speakers, loudness, hi/low filters, mono, stereo, SQ decoder, ambio sound and AFC / silent tuning. This bord is long and small and sits in a metal subframe. It is not very well fixed in place and therefore, due to warping of the board, the risk of a short circuit between the print side of the board and the metal chassis is high. NEVER attempt to power up the board while is sits in the metal frame without properly isolating the board from the chassis. I usually put a piece of carton in between (that stays in there permanently). But since the whole unit is now disassembled, a more permanent solution will need to be found (see my next blog about this).

Replacing electrolyte capacitors and tantalum's in a vintage electronic unit is always a good idea. They start drying out, leak or loose there value. As you will see on the picture below, one of the ROE caps had a serious crack and was leaking. Very common. I always replace electrolyte capacitors with quality 105C ones that have a longer lifespan. The tantalum's are also replaced with quality electrolytes. I like to use MKS's, but they did not fit in the narrow space on this board. I have seen exploded tantalum's in the past (in a Beocord 5000) and several ones that where short circuit. This failure mode, specifically on older types, is called Field Crystallization and is the major reason for degradation and catastrophic failures of solid tantalum capacitors. It causes a dielectric breakdown and happens very fast (milliseconds).

The recapped output PCB

As you may notice, I also changed the trimmers for the bias current setting (quiescent current). The originals are small 1-turn types that are a bit difficult to reach and not easy to set at the correct value. Moreover, the risk of making a short circuit when setting them with a non isolated screwdriver is high (some readers will probably smile now...!). I replaced them with encapsulated 22 turn trimmers. I prefer to use the long type since they give a bit more stability and makes it easier to reach the small turning knob. Off course, one needs to redirect the wires to make them fit into the 3 existing holes from the old trimmers. It's a good idea to make sure the turning direction of the new ones versus the old ones is the same (fully anti-clockwise for lowest current setting).

And here's a view of how the trimmers are rewired. Make sure the wires are not touching each other. Use some isolating tape or heat shrink tubes.

Earlier on I noted already that one of the front right output stage darlington transistors was fried. It was time now to check if the driver section was still OK. And it was not. The emitter resistor of 0,12 ohm was "open" and the BC312 was also fried. I have spare original ones available so replaced them as well. 

This is a picture of all the caps, trimmers, resistor and transistor that where replaced on this board. Old en new together !

And here are the 4 tantalum's that where replaced with electrolytes.

Next I will tackle the subframe with the 8 darlington transistor and there cooling plates.