28-07-2025, 08:22 AM
Hello all.
I'm writing to give a follow up to the repair of my newly acquired
Colour Bar Generator, a Unaohm EP 685A.
After obtaining four pages of the schematic, I figured that I had enough to have a chance of getting it going. My thanks go out to the
Golborne Vintage Radio Forum, being the source of the schematics.
I personally like the look of this piece of test equipment. it is not often that a piece of test equipment is useful, and looks good too.
Looking at the casings of many of the transistors, I guess that it was built around 1972. I guess a more accurate date of manufacture would have come from reading the date codes on the I.C.'s.
Once I had the schematics I opened up the and removed the two piece case to reveal the internals. Not a good look. The leads to the mains transformer, which were sleeved with spaghetti, had decomposed. It looks like the spaghetti had turned into some sort of acid which had caused the
in-contact extension leads, and enamel wire to corrode and decompose.
The M 205 fuse holder and 200mA fuse also were damaged by the fumes of the decomposing leads.
Round Two: What to Do?
It was far too early to give up and have the pattern generator as a static display only. I am one of crazy people who still repairs old televisions.
Anyway, it was not going to fix itself, so I took a lot of photos and removed the transformer. So I cut back the old leads and soldered on new leads and used heat-shrink tubing to cover the leads. Next I re-fitted the transformer, connected the leads, plugged in the mains lead, and flicked the lever on the Destruct Switch. Hmm. Dead. Time to break out my multimeter. Mains switch, 70 ohms in the on position. no wonder it did not destruct. I by-passed the switch for the time being. Now I had a pretty little red light on the front panel.
Round Three:Testing.
When in doubt, stop guessing and engage the brain. Nothing from the modulator. Alright I will somehow make room on my workbench for my ancient C.R.O. and see what is really going on. Bingo, I mean, success. I had some nice waveforms, although erratic, from the Video Out socket.
Spraying contact cleaner into the rear of the ten pattern select switches and working them in and out a number of times fixed up the patterns and also made them stable. While the contact cleaner was close to hand I cleaned the sound modulation switch and the mains on-off switch, by injecting contact cleaner through the opening around the switch lever. This was not successful when it came to cleaning the five switches of the R.F. attenuator. All were open and none responded to cleaning. The corrosive fumes from the spaghetti tubing had worked their way throughout the entire generator. Like a number of things, the switches in the attenuator could not be removed for replacement, so....I bypassed them.
Round Four: More repair work.
I promise, I will get around to cap replacement later, but for now there is still more to be done. EVERY switch contact was suspect. I removed the R.F. out band/frequency selector unit, and cleaned its contacts and band switches. Removing this unit required the use of a screwdriver with a 300mm shaft and the removal of the rear cover.
All the pots needed cleaning as well as the metal to metal contact to the pot's wiper connection.
So how does it work? Quite well. I particularly find the function “test 3” useful. When I get around to posting the photos, you will see that I made a mistake when labelling the R.F. modulator connections. Whenever you look at the modulator and see B5 (Band 5) just realise that ought to read B4.
This looks all nice now, but I am sure when I copy and paste this articleto the website, the formatting will go completely to hell.
Desmond.
I'm writing to give a follow up to the repair of my newly acquired
Colour Bar Generator, a Unaohm EP 685A.
After obtaining four pages of the schematic, I figured that I had enough to have a chance of getting it going. My thanks go out to the
Golborne Vintage Radio Forum, being the source of the schematics.
I personally like the look of this piece of test equipment. it is not often that a piece of test equipment is useful, and looks good too.
Looking at the casings of many of the transistors, I guess that it was built around 1972. I guess a more accurate date of manufacture would have come from reading the date codes on the I.C.'s.
Once I had the schematics I opened up the and removed the two piece case to reveal the internals. Not a good look. The leads to the mains transformer, which were sleeved with spaghetti, had decomposed. It looks like the spaghetti had turned into some sort of acid which had caused the
in-contact extension leads, and enamel wire to corrode and decompose.
The M 205 fuse holder and 200mA fuse also were damaged by the fumes of the decomposing leads.
Round Two: What to Do?
It was far too early to give up and have the pattern generator as a static display only. I am one of crazy people who still repairs old televisions.
Anyway, it was not going to fix itself, so I took a lot of photos and removed the transformer. So I cut back the old leads and soldered on new leads and used heat-shrink tubing to cover the leads. Next I re-fitted the transformer, connected the leads, plugged in the mains lead, and flicked the lever on the Destruct Switch. Hmm. Dead. Time to break out my multimeter. Mains switch, 70 ohms in the on position. no wonder it did not destruct. I by-passed the switch for the time being. Now I had a pretty little red light on the front panel.
Round Three:Testing.
When in doubt, stop guessing and engage the brain. Nothing from the modulator. Alright I will somehow make room on my workbench for my ancient C.R.O. and see what is really going on. Bingo, I mean, success. I had some nice waveforms, although erratic, from the Video Out socket.
Spraying contact cleaner into the rear of the ten pattern select switches and working them in and out a number of times fixed up the patterns and also made them stable. While the contact cleaner was close to hand I cleaned the sound modulation switch and the mains on-off switch, by injecting contact cleaner through the opening around the switch lever. This was not successful when it came to cleaning the five switches of the R.F. attenuator. All were open and none responded to cleaning. The corrosive fumes from the spaghetti tubing had worked their way throughout the entire generator. Like a number of things, the switches in the attenuator could not be removed for replacement, so....I bypassed them.
Round Four: More repair work.
I promise, I will get around to cap replacement later, but for now there is still more to be done. EVERY switch contact was suspect. I removed the R.F. out band/frequency selector unit, and cleaned its contacts and band switches. Removing this unit required the use of a screwdriver with a 300mm shaft and the removal of the rear cover.
All the pots needed cleaning as well as the metal to metal contact to the pot's wiper connection.
So how does it work? Quite well. I particularly find the function “test 3” useful. When I get around to posting the photos, you will see that I made a mistake when labelling the R.F. modulator connections. Whenever you look at the modulator and see B5 (Band 5) just realise that ought to read B4.
This looks all nice now, but I am sure when I copy and paste this articleto the website, the formatting will go completely to hell.
Desmond.
