1977 Ferguson 3734

Thorn 9600 Chassis

Model: Ferguson 3734


System:625 Line

Original List Price : £000.00 ( to be ascertained)

Valves: None

Transistors: Yes

Intergrated Circuits: Yes

CRT: A56 611X

General Info:

This set was acquired by a friend who when collecting a few sets for himself, thought of me when he saw this one. Subsequently given as a gift, many thanks Mikey.


Those that have followed my threads before over on Vrat 1.0  will know I’ve tackled many Thorn 3000 SMPSU, they used to intimidate me when I first started working on them, however time and tackling many, has left me with a high regard for them and an equal fondness to work on them. Syclops still scares me, so its probably a good idea now we’re here on Vrat 2.0 that I recap how syclops functions.


The syclops control circuit has to ensure that both the line output transistor and the chopper transistor are driven synchronously by the same drive waveform, hence the term Syclops. SYnchronous Converter and Line Output Stage.

The syclops control panel therefore has to provide a drive waveform which is suitable for both these stages. The requirements of the line output stage are that the drive must be removed at the end of the scan to initiate fly-back and that the drive must be restored before the efficiency diode stops conducting at time t2. (see below)

The only requirement of the SMPSU is that the on/off time of the drive can be varied in order to keep supplies constant despite variations in the mains input and/or changing load demands made on the supplies. Since the drive must be on between t2 and t3 for the line output stage, the only way that on/off time can be altered is to vary the point at which the on time occurs between t1 and t2.


The maximum on time is set by the syclops control circuit to limit the dissipation of the switch mode transistor to a safe level. It is possible to turn the drive on during this period without affecting the operation of the line output stage because the line output transistor is reverse biased by the negative potential at its collector due to conduction of the efficiency diode which produces the first half of the scan.

Another Cool Feature

The 9600 offers diagnostic sockets TSA (Scanning) and TSB (Power Supply). The 9600 can be operated with any suspect daughter board removed without causing damage to the set. Therefore making use of the diagnostic sockets TSA & TSB you can determine if the expected voltages are present along with removing the supply rail fuses, isolate the fault/trip condition.

The 9600 is a service engineers dream with amazing access to all the panels, which I will demonstrate in the next post. I have the suite of Thorn manufacturers service manuals to hand and I have to say they are the most comprehensive I’ve come across, I suppose with a complex chassis such as the 9600 its certainly a must have.

The Set 

The CRT is PIL A56 611X

Well that’s it for the opening of the thread except to say the sets fault condition is constant tripping. The next instalment will be me making use of the diagnostic ports to see if the source of the tripping can be narrowed down. Oh and I nearly forgot, on the side is a service sticker stating “frame collapse”. I’ve no idea if that is an old fault that was cleared, or why the set was laid up all those years ago.

Up Next……?

The chassis inspection, how it extends for ease of access and serviceability

Lets get intimate with a brief look under the bonnet

As I explained in the initial post the 9600 like all its predecessors, have the service engineer in mind. The 9600 chassis as a whole is withdrawn on two lower plastic rails, the chassis is released from its lock position by pushing in two tabs, lower left and right, finally unhooking the top plastic lock bar.

Now with the whole chassis extended back by about 8 inches it gives access to all the modules. The lower scanning module with E/W correction, left Switch Mode PSU – Line and Frame Oscillator Module – SYCLOPS Control Boards, the I.F -Video & Chroma board and the tuner/ audio module scan be extended and swung outwards.

Now we can now take a look at each one in turn

The Switched Mode Power Supply: With Syclops control, Line & Field Oscillator

The Scanning module & E/W correction module

I.F -Video & Chroma module

Touch Tuner – Audio Output

The Thorn A56 611X PIL Tube

As you can see quite a lot packed into this one. Its pretty neat that on the panels Thorn have supplied flow diagrams with voltages and expected waveforms, excellent for servicing reference. The daughter modules on the SMPSU can be removed and plugged in on the reverse side further aiding service access.

I suppose before I progress any further the next step should be to determine the state of the CRT. This will dictate if I should continue it might be a dud, it might be low but can be recovered or it luck is on my side, it might be a goodun.

Next…….instalment? CRT testing.


Excellent news, the A56 611X is a good one so the repair can go ahead.

And So It Begins……….

Power applied to the set, I hear sort of static rustle but dead! Removing the screening plate from the PSU gives me access to the PCB side and the TSB diagnostic pins.

First is to check what voltage I have on pin 9, I find there is 580V. Next to check F511 which supplies 34V to the 24V regulator, which in turn supplies the field Osc an output stages, width and height compensation circuit, E-W raster correction and the IF decoder modules.

Here I find F511 was open,  I tried another just in case but it blew straight away.

According to the manual if F511 is blown check for a short circuit on the 34V line.  A resistance check of PL518/1  to chassis should be: +ve lead to pin PL518/1 and -ve to chassis – charges to 450R. Now -ve lead to PL518/1 and +ve to chassis – charges to 850R. My tests give 2.8M and OL respectively.

I removed W525 from circuit, tested and its failed, so that’s the cause of F1 blowing. Typical it decided to fail whilst I was trying to track the cause of F511 blowing. I do have a large stock of diodes so should have one to replace it.

after replacing the bridge rectifier. Well I didn’t proceed with another power on, something was nagging in the back of my mind, something was not sitting right. I had visions of powering on again only to fry yet another fairly expensive diode.

I’ve tested C530, C531, R511 and R522 all were OK.

I then removed and tested C515, no shorts, its slightly higher in capacitance and its ESR is fine, therefore I’d say its OK. Now to see how it reforms.

As I suspected  from the previous test voltage readings, nowt wrong with the chopper transistor, at least I have the opportunity to reapply some fresh heat compound. OK reassembly time.

Power applied, we have h.t and EHT – 22kV and 561V at TSB/9 so the choppers running. The set is staying up in that state.

However we’re back to where we were before the bridge rectifier expired, with a possible short on the 24V regulated rail as F511 has blown again. At least its staying up so that’s where the next investigations begin.

ohm’d TSA/2 to chassis, it should not read less the 150R, well its much lower at .51R certainly think we have a short.

Time to check the 24 regulator transistor VT812 and its driver VT813. Removed VT812 (regulator) & VT813 (driver) both are OK, so something else in the 24V regulated rail is shorted. Five to midnight, that’s it for tonight, back at the bench tomorrow.

Back into the fray, removed and checked C803, its OK. Then its the turn of the Zener W819, it uses and MVS240, not a type of zener package I’ve ever seen before. Anyway that tests fine with .7587V forward bias drop across it and the reverse bias is infinite as it should be…….. The hunt continues.

o be certain zener W819 wasn’t breaking down under load, I hooked it up to my variable power supply with a series 1KR resistor. Cranked it up from 15V- 27.5V, if its working under load it should stop at 24V if its not, then it will  track the PSU above 24V. Again its perfectly fine doing what its supposed to do, regulating to 24V despite receiving more.


PSU supplying 27.5V zener regulating at 24.1V 

So with the 24V regulator proven to be OK, I disconnected PL4 from the signals module, now the 24V rail stays up without blowing F511, so yes, likely one of the many blue tants.

With the 24V rail isolated from the signals board we now have activity on screen, namely a field collapse. So the sticker I found on the side of the set describing why this set was originally laid up is still true and that the fault was never addressed. Next two tasks, track down the cause of the short on the signals board, then find the cause of the field collapse.

With regards to the 24V short, I know many would say just replace all the tant’s, well I’m not going to do that, I want to diagnose the culprit. To that end I’m going to remove the signals module and try powering it from the bench supply, in theory this should enable me to track the culprit much quicker.

In case any engineers looking in are concerned about the field collapse burning a line in the phosphor, I can assure you the brightness and contrast are at a minimum. The photo makes the line look very bright when in fact its very dim and the set was only on for a few seconds to take shot. I anticipated a field collapse as there was a service sticker on the side of the set stating field collapse, I had a feeling it had not been repaired.

OK so leaving the field collapse for now I’ll concentrate on getting the supply rails stable and F511 not blowing. This is simple to do without the set as I can power the culprit module from the bench supply.

Feeding in 24V from the bench PSU to PL4-1 -ve, PL4-5 +ve the voltage collapses to 2V with a current draw of around 300mA. Watching and feeling components after a while I noticed the voltage jumped up to 16V with an associated drop to 200 mA draw. The voltage needle erratically dropping a couple of volts before rising back to 16V.

This is very symptomatic of a short partially clearing and a cap trying to charge, hmmmm now which one. I may just cheat and use the ESR meter for an in circuit check of those tant’s to see if it can locate it.

Not that we need any further evidence but ohm’d PL4-5 to PL4-1 it should be around 800R. As can be seen its a dead short at .56R ( middle reading on the meter is the lead delta reading).

I’ve had the current limit off for a while and nothing ( i.c’s, trans, caps etc) is hot to the touch, I don’t think freezer spray will help at this stage but I can give it a go.

Some progress I found C171 6.8 uF/35V a decoupler from pin 5 of ic4 is one of the culprits.

I now have the voltage back and holding at 16V but still with over 200mA draw. Its a step in the right direction but we still have another one in there causing a short. When I now ohm pin PL4/1- PL4/5 instead of  having .51R I now have 530R, this is a lot closer to the expected 800R.

So to recap where we are

  • There is a short on the IF/decoder module causing F511 on the PSU to blow.
  • Ohm’ing PL4/1 – PL4/5 on the IF/decoder revealed .51R it should be around 800R
  • Powering the 24V rail into the IF/decoder via a bench PSU, we had 300mA current draw with the voltage collapsed
  • C171 was discovered to be contributing to the fault. Removing it allows the voltage to climb to 16V with 200mA current draw
  • Checked and removed the possible candidates C196, C140, C146 and IC4, they are all innocent, the fault is still present.

This morning I put the IF/decoder module back on the bench PSU feeding in 24V with current limit enabled. The voltage sits at 16V with around 200mA. I then removed current limit, the voltage jumps up to 24V with the current draw increasing to 300mA, surely something must now get hot!

Finger checked, I found IC3 SN76227DN was getting very hot, as is IC1. IC3 is in the luminance channel, contains a DC controlled amplifier which is controlled by the colour control R150. IC1 is the IF amp. I removed IC3 and IC1 the fault still remained so its not the chips. Found C173 6.8 uF that decouples the base of VT104 was leaky, removed and now ohms between PL4/1 – PL4/5 is up to 780R.  Back on the PSU with current limit on, we only have 16V with 200mA draw, remove current limit voltage goes up to 24V with 300mA draw.

As PL4/1 PL4/5 should be around 800+R there’s still one final faulty component to trace, I’m beginning to suspect the 6.8’s. A collegue suggested checking C203 so expediting it up the check-list reveals that its slightly high ESR, risen in value to 1.17uF which is negligible and no leakage whatsoever.  Due to the ESR I replaced it, curiously this has now moved the ohm reading between PL4/1 PL4/5 to 795R. But still there’s something else dragging it down. On the PSU with current limiter on its now reaching 18V with 185 mA current draw.  Taking limit off and the voltage rises to 24V with a current draw of 210 mA.

I’ll keep on my path of checking, must be close now.

4 thoughts on “1977 Ferguson 3734

  1. That’s the set I’ve been looking for! This one was on Fools and horses when Granddad was alive in the show, also this was built for Television for Schools and Colleges and University’s. The only difference was that this set was in a very large cabinet with doors and hood. Inside was painted in black while outside it was a dark brown wood colour.

    The set had two neon lights in the front, Red for power, Orange for Monitor mode. It had VHF / UHF aerial sockets, I think it had base band video in and out, same type as CB radio aerial in. The Audio was 5 pin in and out.

    I used to connect my Radio Aid for TV sound direct to my hearing aids, mind you I was the only kid was allowed to plug this radio aid in or out, those were the days.

  2. The 9600 version that was built for educational establishments was actually the 9653 chassis which had been modified to provide baseband video and audio out and also contained a mains isolating transformer to allow this to be done. The chassis was housed in an oversized cabinet, with doors and a shade which was all mounted on a Unicol wheeled stand. The Unicol stand also contained a shelf for a VCR to rest on. There was definitely a 26″ “schools” model using the 9653 chassis and I think I may have seen reference to a 22″ inch version too.

    The school sets were provided via Radio Rentals Contracts and servicing was often provided by the DER leg of the Thorn rental outlets. I certainly serviced a fair few 9653, and the earlier 3700 (based on the 3500 chassis) sets in schools in the North East during my time with DER.

  3. When these were working the quality of the picture was very good
    the rental versions had tint and tone controls
    but they were a bit complex for a thorn esp the syclops control

  4. The Ferguson 9K6 model you have looks just like the 9800 was the 9800 the later set?. Before the TX range I remember it had a diode split transformer and an inertia start for the power supply


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