Marconi 2955A bad LOPT?

[Torkrench]

I just picked up a 2955a and I'm attempting to repair. From what I can tell the functions seem to be working although I've only tested the audio generator so far and switched through different modes blind. The CRT isn't working and the fuse on the +12v line inside is bad. When I place an ammeter in place of the fuse I get about 4 amps current draw on the +12v line. If I lift a leg of D11, the draw drops to about .7 amps. With everything connected, I get about 10v on the +22v out. I've checked all of the electrolyrics on the board and they are all fine. I've checked most diodes and haven't found any shorts so far. I'm getting a 15khz square wave at the base of TR3.

From what I've gathered so far, I suspect the LOPT is dead. Does that sound right? Is there a better way to know for sure that it really is the LOPT? I'm hoping I can locate one of the kits to repair the unit as it seems to be otherwise perfectly fine.

Also, with 12v across pins 3 and 4 of the CRT (heater pins) should the CRT glow if it is getting no other signals?

[Torkrench]

I just realized that someone in the past has lifted one side of R18 and R17 of the AB3 board (character generator). That essentially broke the serial video output line to the CRT drive board.

I suspect that was done by someone while troubleshooting the issue with the CRT, but I'm wondering, could a lack of signal from that line cause the excessive +12vdc current to the CRT drive board and low voltage on the +24vdc output? I'm still leaning towards a bad LOPT.

[Biggles]

It's a known weak spot on early 2955s but there was a modification brought out by MI which involved a different LOPT and associated component changes. I am assuming that the later ones were better but I couldn't be sure. The other thing to be aware of is the fact that if the master oscillator stops working, the whole display goes off as I think the line oscillator is driven by it. It's a long time since I have looked at the circuit so I am going by memory, although I have repaired a few in my time. OM345 modules are a known trouble spot, as are the attenuators. I believe there is a 2955 or at least an MI group somewhere out there on the web who may be able to give you more help. I also once had an electrolytic capacitor on the input of one of the power rails go short and this fried the inductor in series with it. I think it may have been on the demodulator board but it was a while ago.
Alan.

[Torkrench]

Yeah, I had the coil get hot when I first powered it up. Should have had the unit on a variac because it hadn't been powered on in a little while. I was running it on a current limited 12V external supply so I think that helped keep the coil from burning up. From looking at the schematic, it appears that was most likely caused by a shorted electrolytic. Seems like the cap opened after a few seconds so it didn't burn the coil completely, just got it a bit discolored. When I get the display working I'll replace it along with pretty much all of the electrolytics.

This has the old LOPT and I ordered a replacement from EB5AGV in the UK. hopefully that will work for me. From using the unit blind, the audio frequency generator works correctly, so I'm hopeful that other components are good. Relays switch as I change though modes, but the RF generator relies on +24v that is provided by the LOPT, so I can't test that until the +24v side is working correctly.

Do you think that if the master oscillator was not working that the LOPT wouldn't work? I'm getting no high voltage and there is that high +12v current draw by the LOPT.

[Biggles]

Without digging the manuals out I am not sure what the master oscillator drives but I seem to remember that a failed oscillator stops the machine lighting up altogether. I suppose you could try driving it from an external 10MHz? source via the input on the back panel. I have service manuals for the 2955, but not the 2955A. I have only fixed one 2955A and I managed that with my manuals for the 2955 but I can't be sure how different they are circuit wise.
Alan.

[Torkrench]

Finally got my LOPT kit installed and the CRT is good now. It's failing all self tests. I did find and replace a shorted electrolytic capacitor on the AA3 board. From what I've tested so far, the rest of the power rails seem to be fine. What I'm getting is:

1.0 - failed code 11
1.1 - failed code 13
1.2 - failed code 15
1.3 - failed code 17
1.4 - failed code 19
2.0 - failed code 21
2.1 - failed code 23
2.2 - failed code 25
2.3 - failed code 27
3.0 - failed code 34
3.1 - failed code 36
3.2 - failed code 37
3.3 - failed code 3A

It looks like all of the functions work fine in the TX test mode. Audio frequency generator works fine. I can get it to generate RF, but if the unit is set at 1MHz, a 22MHz signal is generated. 2MHz generates around 30MHz signal. In the service manual under fault diagnostics - oscillator switching, I'm not getting any of the measurements they say I should getting.

P.S. I did some more testing and found that it appears to be some kind of offset. The unit at 1MHz has about a 22MHz offset. at 500MHz there is a 50MHz offset. At 800MHz there is an 80MHz offset. By offset I mean the frequency output from the 2955 is higher than the setting on the screen.

As far as modulation settings go, everything works like it should. Power output is consistent all the way through from 1MHz to 800MHz.

Any ideas?

[Torkrench]

Ok, last update for the night. In the section of the service manual for checking the loop amplifier. It states IC107 pin 6 should be 3vdc at 165MHz and 17vdc at 259MHz. I'm getting a higher reading at both frequencies. If I remove PLD and power R3 directly, I can get the unit to be right on frequency if I set the voltage just right. I suspect someone has gotten in and turned something to affect the voltage at pin 6 of IC107. I just can't find any information anywhere on setting the voltage of pin 6.

[Dickie]

Removing the PLD link opens the synthesizer PLL. The voltage on pin 6 of IC107 (the loop amplifier) will be whatever is needed to make the loop lock at the programmed frequency.
If you are seeing 165MHz at 3V approx. and 259MHz at 17V approx. then the 165-259MHz VCO is likely to be working OK. The voltages don't have to be accurate, I would guess +/- 1V is OK. The 3-17V range is near the limits of the output range of IC107.

Ideally you would follow a logical fault finding process to get this going, but a strong possibility is failure of the divider amplifier IC1 (OM350). If you search for the various threads on the OM350 on the forum there are ways of quickly checking its functionality and also possible repair strategies.

[Torkrench]

Thanks for the reply Dickie. I was going over the unit one more time last night and found a resistor that I had lifted one of the legs of when I was locating a shorted capacitor. R139, which connects part of the +9vdc rail on the AA3 board to several components. As soon as I soldered it back in place everything was good again.

In the end the only issues with this unit were the bad LOPT and one shorted electrolytic. There is an LED out on the front panel for one of the RF input/outputs and it's showing some signal in TX test mode with nothing connected, so I need to troubleshoot that, but there are no covers on the RF deck and the LOPT board is still hanging outside of the radio. Once I put those back on I'll see if there is still a signal showing up.

[mole42uk]

One typical fault condition is where the connectors between the various boards on the underside go high resistance. I don't remember exactly, but there are small links between each board that rely on gold contacts to make the connection. If these aren't good, some self-tests fail.

Also, if the 10MHz clock hasn't been running, it is possible for the EPROMs to become corrupt.

[Biggles]

The LED out on the RF input select must be a common problem. On the same machine with the shorted supply electrolytic I repaired, the LED on the low power input failed to light. I thought that it may be a dry joint but soon discovered it was a faulty LED. I hooked up a good LED across the PCB pads and it lit. The faulty LED is difficult to get at so I didn't replace it. The ports were working ok so it doesn't really matter. When the high power LED is off, then the low power input is active. Simple.
Alan.