This rig was bought as a “spares or repair” rig, and on top of that the rig had a history of having been over on the “dark side”, ie used for illegal QRO 27MHz CB work.
It is however fitted with the FM board (obviously given its history) and that was a “plus”.
My other ‘101ZD has the AM board (if you don’t know, you can’t have both in a 101ZD, it’s either / or!
The description of the fault was that the rig was working fine until one morning it was switched on, and there was a loud buzz from the mains transformer, together with that “hot” smell which many of us recognise as something overheating which shouldn’t be.
The radio was switched off by the previous owner and tried again briefly a couple of times and was declared “u/s” and put on EBay.
This is how it came to me.
Repair Strategy
Leaving the mains connection well out of the way of temptation I took the covers off for an appraisal of what I’d bought, and produced a small list of defects.
This included the missing removable screens from the PA compartment.
This would have little if any effect on performance, but it puts a nice barrier between someone with the top cover off, and the 900V HT line which sits on the top of the PA choke, and anode connections to the output valves! More on this later.
Another defect noted was an incorrect mains fuse had been fitted (wrong size, type and rating) in the mains supply (worrying …).
Now to look at the main fault, which seemed to point squarely at the good old Rectifier A board.
I did the “external dc supply” trick by applying 17V dc to C06 the main low voltage supply reservoir, and the rig fired up on receive, taking 600mA or so from this supply and 700mA or so when switched to transmit.
So far so good.
A quick poke around the circuit with an ohm meter revealed that one arm of the HT rectifier diodes (the one supplying the positive end of the HT) was short circuit and I realised having thought about it that this was more than likely the main fault.
Another Rectifier A board to unsolder and remove all those wires!
This time I decided to tackle the disconnection of the board from the opposite side from where done it on the previous occasion – ie from the HT connection side (see photos).
This also enabled me to swing the board out vertically so that the underside of the board could be inspected, and the faulty components removed.
As it happens the reverse of the board was in very good condition, no burns or marks, just many years’ accumulation of dust which could be cleaned away very easily.
All four rectifiers were removed, along with their respective 470k ohm voltage equalising resistors and replacement parts ordered.
It turned out that two of the resistors were so high in value that they were virtually open circuit, and the other two were about 50% high.
I ordered beefier diodes and when they arrived I realised they were physically too big to fit exactly where the originals had been, so I decided to rebuild the rectifier assembly “ugly” style on the top taking care not to let high voltage parts get too close to things they shouldn’t be close to.
I did however spend a little time pondering on the abuse this part of the circuit gets under operational conditions:-
At switch on there is a bank of capacitors with zero volts across them connected to one end of the rectifier arm, and 900V dc at the other end.
So we have a big voltage for them to cope with together with the inrush or charging current for the capacitors.
That’s quite a hard life, and the reason why many HT circuits have “soft start” or “step start” arrangements to try to keep things reasonable.
Plus if in a previous life the rig has been thrashed running lots of FM power and where the CB operator may not have had too much clue about tuning and efficiency and other considerations which Radio Amateurs always think about (!) then there would have been a requirement for these little diodes to keep pumping up the reservoir manfully trying to keep up with the current drain from it.
That is a hard life, and no wonder the rectifiers eventually gave up the ghost!
By "beefing up" the rectifier with higher-specification replacement diodes I admit that there is a danger of causing something else to fail should the root cause have been incorrectly identified, or if another fault develops. The weakest link in the chain will break, and I have effectively replaced a weak link with a stronger one, not knowing where the next weakest link will be.
However, the balance of probabilities is that I should be OK.
A short note about carbon composition resistors.
It has been well known in the world of electronics that high-value carbon composition resistors have a nasty habit of increasing in value ("going high") as they age, particularly if they are exposed to heat or thermal cycling.
This probably happened in this radio, and may well have contributed to the failure of the rectifiers.
I replaced the failed ones in this radio with high-voltage thick film resistors (Farnell Part Number 129-2566) – not the cheapest option, but I thought it would be a good idea.
The rectifier diodes were replaced with type 1N5408 which are rated at 3A 1kV.After replacing the above components and taking some precautions such as leaving out one of the board mounting bolts which was looking uncomfortably close to one of my “ugly” rectifier arms, the board was re-wired and the radio powered up using a Variac (borrowed from work!).
This allowed me to apply mains volts slowly and progressively until finally full mains and HT was achieved. I now had a working rig, sort of!
(to be continued ...)