Receive Problem after Conversion Maxtrac 800 to 902

[RogDog]

A ham friend coverted a trunked Maxtrac and a conventional maxtrac to the 900 (33cm) band. The Trunked unit works fine. The Conventional has sensitivity of 45 uv. He brought it to me as I have experience with Maxtracs, although not with this conversion. I checked his work and it was all done properly. Eventually I found that the Q1 1st RF amp in the receiver was shorted. I replaced it with an NTE2402 and got .21 uv for 20dbq. Great! I put the unit back together and ran several tests both TX and RX. Then I found I was back to 45 uv sensitivity again. Q1 was shorted again. The NTE4202 was a semi-wild guess on my part, but since it developed such good sensitivity, I was suspicious that the failure was not the fault of the replacement transistor.

My supplier had no more 2402s in stock so I removed the Q1 M82D44 from a spare board and replaced the shorted NTE2402 on my friends board. I ran several careful tests with the exciter disconnected from the PA. All ok. I then ran several careful tests with the PA enabled and terminated in a 52 ohm load. All ok. I then fed the output into a mag 900 mhz antenna which exhibited about 30% reflected power. After a couple of trasnsmits, Q1 shorted again.

I ran the receiver input line from the PA into a service monitor and transmitted into 52 Ohms and the antenna and an open circuit. I saw no power down the receive leg. I also checked the switching diodes in the PA. They seemed OK.

Has anyone run into this while doing these mods? Like I said, the trunked unit works find, but it is a lower power unit than the conventional unit. 25 / 35 watts respectively. I showed about 18 / 26 respectively with a 50 watt 400-1000 MHZ bird meter. There appears to be a shottky dual diode on the output side of the first RF filter which should protect Q1 plus there is a 4.3 pf cap after the filter, between it an the base of Q1. Any ideas?

Thanks, RogDog

[Max-trac]

Are you converting a D45 800 radio up? And txing in the 902-903 range?
http://www.repeater-builder.com/maxtrac ... part2.html

Or converting a regular D37 to the ham band?

You might try putting a manual power control on it, the PA runs wide open and can put out way more than its ratings. It is not adjustable by rss when out of band.

[RogDog]

Max-trac,

The unit we are have trouble with is a D45 converted to the ham band.

Most of the posts I've read on this conversion seem to be using the converted units for receive only. Or, am I off base with that idea?

I was wrong about the diode I discribed. I thought it was a dual diode, but it is a single diode with the anode grounded.

I'm thinking you are right about the power output being the cause, but would think this would be a common occurance since no posts I've read mention very very poor receiver sensitivity after conversion.

RogDog

[Max-trac]

Moving a D45 to 902 transmit is not a common occurance, so there is probably not much experience. I have never done one, I have only read the repeater-builder info (see previous link). I have done LOTS of conversions to receive only.
If it is working fine and then gets blown after transmitting, I would get the transmitter tamed and tested into a dummy load running no more than 15w or so. It is working way out of its design range, and if things aren't tuned, the RF has to go somewhere!
The manual pot power control mod is easy, I would start there.

[WB6DGN]

Do you have access to a spectrum analyzer to look at that transmitter output? I wonder if you're getting spurious around the receive frequency.
Tom

[jackhackett]

hmm.. trying to remember if the receive cable plugs into the RF board on those radios, or is it soldered in? If plug in, you could disconnect it and connect to a service monitor to see how much transmit energy is coming out of it.

[RogDog]

Max-trac,

Thankyou very much for your input. My friend e-Mailed me after I sent him your ideas and told me that he had 'bumped up' the power in an attempt to get the unit transmitting on 927 MHz. It works fine from 902 - 910 and even 915, but drops off sharply above 915 MHz. I am unfamiliar with the ham repeaters on 900 Mhz. What is the standard Ham repeater frequency split on 900 MHz? I think he said he only needed 902.5 to 910 MHz on Tx.

At any rate I believe the problem on transmit is with the VCO not wanting to shift that high and NOT the PA or exciter.

Apparently my friend was able to adjust the power and he says he will reduce it to about 75%. Hopefully that will stop Q1 from frying. My thinking is that in conventional 800 MHz operation, the split is 45 MHz. That split along with the filtering that the two front-end filters provide, not much Tx RF can get to the receiver Q1. But, jumper out the filters and transmit on the receiver input frequency and bunches of RF has no reason other than the antenna switching diode to NOT get to Q1. You can get a nice little burn of an exposed antenna even at 2 or 3 watts. Q1 is not meant to see even a tenth of a watt, I wouldn't think. And, even if the HAM split is 10 or 15 MHZ, it really makes no difference since there is no effective filtering in this modified unit. I'd say our only hope is in the fact that the same can be said for the Trunked unit in which Q1 does not fry. There for, reducing TX power might work. I'll let you know.

One last question (for this post anyhow). What do most hams use to talk thru these repeaters? I ask that because you said there wasn't much call for transmitting on these converted units.

Thanks again,

Rog Dog

[RogDog]

Do you have access to a spectrum analyzer to look at that transmitter output? I wonder if you're getting spurious around the receive frequency.
Tom

Tom,

I question what difference spurs would make other than being undesireable for other reasons. Keep in mind, that for testing purposes, my friend made all channels simplex. ie. If receiving on 902, you also would transmit on 902 and ditto for 910 or any other channel. If the filters were in the picture, I would have some reason to agree with the spurious idea, but since they are jumpered out, I can't agree.

We do, however, thankyou for considering this problem with us.

Rog Dog

[RogDog]

hmm.. trying to remember if the receive cable plugs into the RF board on those radios, or is it soldered in? If plug in, you could disconnect it and connect to a service monitor to see how much transmit energy is coming out of it.

Jack,

Read the third line of my original post. The RF line does unplug, which makes it fairly simple to test the RF feedback down the receive path. I did so and saw no appreciable RF. Would have thought I'd see something. However, the transistor never blew under any circumstances while the receive cable was unplugged from the RF board jack. So I'm quite confident that it must be some RF coming down that line. Reducing the RF Transmit power will hopefully take care of this. I will let you all know. Thankyou very much for your input.

Rog Dog

[Tom in D.C.]

To answer one of your questions, the ham split is 25 mHz. Transmit on
902 and receive on 927. Simplex operation (except when testing, setting
up, etc.) is of course on 927. Also, even though you're apparently trying
to get an 800 mHz radio UP to work on 900, when we modify actual
900 radios to work the "new" hamband we have to modify the RSS's
.MDF file in most cases. None of the Motorola factory radios are designed/
set up to work between 902 and 927. Ham repeaters use a variety of
converted units, usually Spectras, but also GTXs. Operation on 900 is
about the same as on the other FM bands. In my area we have a few
900 repeaters but there is still not much activity.

900 mHz GTX radios are relatively cheap these days, and you'd only have
to mod the .MDF file to get it on the hamband, in case you get really
discouraged with your present situation.

[jackhackett]

Jack,
Read the third line of my original post.

In my defense, I was drunk at the time.
What am I saying.. I'm drunk all the time.

Strange problem.. good luck with it.

[WB6DGN]

Keep in mind, that for testing purposes, my friend made all channels simplex.

You're absolutely right, I missed that point. I also missed the part about bypassing the front end filters, I was thinking it had 915 filters in it. Please feel free to use "WhiteOut" on my post.
Tom

[RogDog]

To answer one of your questions, the ham split is 25 mHz. Transmit on
902 and receive on 927. Simplex operation (except when testing, setting
up, etc.) is of course on 927. Also, even though you're apparently trying
to get an 800 mHz radio UP to work on 900, when we modify actual
900 radios to work the "new" hamband we have to modify the RSS's
.MDF file in most cases. None of the Motorola factory radios are designed/
set up to work between 902 and 927. Ham repeaters use a variety of
converted units, usually Spectras, but also GTXs. Operation on 900 is
about the same as on the other FM bands. In my area we have a few
900 repeaters but there is still not much activity.

900 mHz GTX radios are relatively cheap these days, and you'd only have
to mod the .MDF file to get it on the hamband, in case you get really
discouraged with your present situation.

Tom in DC,

Thanks for the info Tom. I don't know, but strongly suspect my friend did
the .MDF file stuff. I'll send him a copy of your post and see if it helps any.

I'm not familiar at all with the GTX units. What is the general Motorola
model number for these, (assuming it is a Motorola unit)???

Thanks,
Rog Dog

[Tom in D.C.]

GTX is a Motorola 800 and 900 mHz line of mobiles and HTs.
Model numbers beginning with:
M11U... are 800 mHz mobiles.
H11U... are 800 mHz portables.
M11W...are 900 mHz mobiles.
H11W...are 900 mHz portables.

Do an eBay search for GTX and you'll find a ton of them listed; well,
maybe not a ton but quite a few. The disadvantage of the GTX vis-a-vis
the Spectra is usually not as many channels and much lower max power in
the mobiles. The portables are the same frame, battery, etc. as the GP300, but
they have a limited type of display that really only tells you what channel
you're on. Also, look up the GTX on http://www.batlabs.com under Model Specific
Information for more info.

[RogDog]

GTX is a Motorola 800 and 900 mHz line of mobiles and HTs.
Model numbers beginning with:
M11U... are 800 mHz mobiles.
H11U... are 800 mHz portables.
M11W...are 900 mHz mobiles.
H11W...are 900 mHz portables.

Do an eBay search for GTX and you'll find a ton of them listed; well,
maybe not a ton but quite a few. The disadvantage of the GTX vis-a-vis
the Spectra is usually not as many channels and much lower max power in
the mobiles. The portables are the same frame, battery, etc. as the GP300, but
they have a limited type of display that really only tells you what channel
you're on. Also, look up the GTX on http://www.batlabs.com under Model Specific
Information for more info.

Thanks again Tom,

My friend assured me that he did not do the .mdf thing. Thanks fo the GTX info. I've not had the pleasure of working on those.

RogDog

(Hey, if Rog is Rodge, then Dog should be Dodge. Right??? )

[RogDog]

Update to Receive problem after mods.

I had my friend reduce the output power (via software) by about 30%. This seemed to do the 'trick' as I transmitted to timeout on 900, 905, 910, 915 & 920 on each of his three mag mount antennas, one of which was the one with about 25% reflected, and did not blow the Q1 RF transistor on any of them. My thinking is that the combination of a bad CR2 line protection diode and too much reflected power with effectively no RF filtering, caused Q1 to stress out and short. If I'm wrong about that it will be the first time ever!

I have an unrelated question, but will start a new post for it soon.

Thanks all,

Rog Dog