Motorola PT400 question

[cahighwayman22]

I bought a PT400 VHF radio, and it receives ok, but the squelch doesn't work, and when I go to transmit, either the exciter board or something smokes, and it draws current as if their were a short. And help? Can't find the book either.

Trevor KI6KGN

[com501]

Are you using the stock alkaline battery pack or a nicad pack?

Both, as I recall, are positive ground in relation to the case....

Chances are, you may have a real interesting small boat anchor.

[WA6OXN]

Are you sure the battery is really charged up? Typically the squelch blows when the battery is low. Also, the lower the batt voltage the more current the TX might try to draw.

Just a thought...

[t95mwp]

Is that the radio you picked up last time at DeAnza?

[Bill_G]

Are we talking about the old lunchbox radio from the early 70's based on the Handy Talkie?

[WA6OXN]

HT200 boards all the way!

[cahighwayman22]

The battery is dead, and im using a power supply for the -15V, It does receive, but the squelch pot doesn't work, and when I go to transmit it looks like there is a short somewhere, maybe the transmitter. I really would like to fix it and use it for 2 meters.

[Bill_G]

The battery is dead, and im using a power supply for the -15V, It does receive, but the squelch pot doesn't work, and when I go to transmit it looks like there is a short somewhere, maybe the transmitter. I really would like to fix it and use it for 2 meters.

First, is it like the picture I posted above? If so, do you have the manual? Without a manual, I'd stop.

The squelch control problem could be as simple as the pot or the wiring. But, it could be dried caps. After all these years, they are probably all bad. The model used many germanium xters that will be difficult to find suitable replacements for. You do not want to learn how to thread a replacement discriminator coil in (if you can find one). Low output from the discriminator means low drive to the squelch circuit which means no squelch control. At least it sounds like your IF and limiting stages are functioning.

The power control boards were known to go down in a blaze of glory. They were one of the first dc coupled circuits used for power set. It can be a challenge to separate the power control board from the xmitter board to service the controlled stages. If this has the high power 15W PA, you will have trouble finding parts, and may have to roll it back to 5W from the xmit board.

[k2hz]

The squelch control problem could be as simple as the pot or the wiring. But, it could be dried caps. After all these years, they are probably all bad. The model used many germanium xters that will be difficult to find suitable replacements for. You do not want to learn how to thread a replacement discriminator coil in (if you can find one). Low output from the discriminator means low drive to the squelch circuit which means no squelch control. At least it sounds like your IF and limiting stages are functioning.

A common problem I saw with the HT200 series boards even when they were relatively new is that the IF or discriminator coils go open due to the very fine wire in the coils corroding. The symptom would be an open squelch and some degree of reduced sensitivity, depending on which coil is bad.

They are a real dog to retune even in the normal bandsplit and I am not sure if it will go down to 2M without modification. Given the age and condition of the unit and the cost of proper tx and rx crystals, I don't think it is a good candidate for a 2 M rig.

If you like the packset type radios, look for a PT500. They are based on the MT500 that is a newer and better radio.

[Bill_G]

A common problem I saw with the HT200 series boards even when they were relatively new is that the IF or discriminator coils go open due to the very fine wire in the coils corroding. The symptom would be an open squelch and some degree of reduced sensitivity, depending on which coil is bad.

They are a real dog to retune even in the normal bandsplit and I am not sure if it will go down to 2M without modification. Given the age and condition of the unit and the cost of proper tx and rx crystals, I don't think it is a good candidate for a 2 M rig.

If you like the packset type radios, look for a PT500. They are based on the MT500 that is a newer and better radio.

Yeah. That was happiness and joy to replace. Just like the PB1 pagers. You prayed someone painted the leads correctly and that you could scrape it and the enamel off without destroying the winding. At least they didn't overlap resistors and caps, or run them between the xters leads like the HT220. Lordy, someone stayed up nights to design that. Stairstepping the leads on the detector amp was always a good time.

The PT500 was a better radio, but by the time it came out, most people were switching to portables rather than packsets. I didn't get to see too many.

[WB6NVH]

Odd how different people have different experiences.

I serviced hundreds of HT-200 and PT-200/300 radios many years ago and never once found an open coil. I must have led a charmed life. What I ran across instead was many bad Permakay filters and the expected number of blown three-legged-fuses.

The high band HT-200 will tune into 2 Meters without issue so the PT series will as well, although I never took a 400 into 2 Meters so I don't know about the PA stage. Alignment is straightforward except that the receiver oscillator peak will require a millivoltmeter to read, contrary to what the manual says. Sensitivity will run 0.5 uv for 20 db.

This radio can be fixed. For the receiver, first I would look for dried out caps as the opinion above suggests, then do some voltage checks on the transistors around the squelch and audio section. For the short in the transmitter, probably a bad transistor or if you are lucky, just a pinched wire. I seem to recall there are some tantalum caps on the board too, and you may have a shorted one.

The pack sets are going to be very time consuming to learn and service from scratch. When you do them every day all day, it gets pretty easy, but let 30 years go by and you are starting from scratch again!

Applying reverse polarity accidentally will blow the audio output transistors in the receiver and several transistors in the transmitter PA if keyed in that condition. Since there is audio present, I doubt it was subjected to reverse polarity at any time.

Yes, crystals are expensive but they aren't any cheaper for the PT-500 either, and you need the late eeprom and a suitcase programmer to fix up a PX-300S, so they all have their disadvantages, in a way.