Batboard

I have an A7 Spectra head that isn't showing any signs of life. I've tried it on known good radios and known good interconnect boards. It looks like a brand new head or at least new front plastic and buttons since the protective plastic is still on the display. The caps don't look like they've leaked and nothing looks toasted on the board...are there any other common failure points on Spectra heads that I should be checking before I cannibalize an ugly head and stuff a working board into the new plastic?

MP 6.7 wrote:I have an A7 Spectra head that isn't showing any signs of life. I've tried it on known good radios and known good interconnect boards. It looks like a brand new head or at least new front plastic and buttons since the protective plastic is still on the display. The caps don't look like they've leaked and nothing looks toasted on the board...are there any other common failure points on Spectra heads that I should be checking before I cannibalize an ugly head and stuff a working board into the new plastic?

Unless you have the manual to match your head and have some good bench trouble shooting experience,
it isn't worth the time and effort to find the problem in either the high voltage section or the micro being
in a constant reset. Get another head off Ebay. As time goes on, these heads should be very cheap
and sold by the dozen. The narrow band push will saturate the market with these heads and the radios
that go with them over the next year or so. You can probably go by most radio shops and pull them out
of the dumpster right now. Maybe even ask them to set them aside for you to pick up.

Jim

There are some simple tests you can do with just a DC multimeter. But Like Jim says, there are at least four different version control head circuits.

See if 12 volts is at the FET switch and that the FET is turned on (later version boards). The output, 12 volts is the tab on the transistor. The pin closest to the board edge is the 12 volts into the FET. Someone may have moved a jumper, common.

We have even seen the mechanical on/off switches go bad on the earlier boards.

Followed by all the problems Jim mentioned.

There's a ceramic picofuse you can check too. Looking at the back of the head, it's located in the upper left corner just to the left and slightly above the big microprocessor.

Uh, That is the processor crystal and it will read open. There is no fuse in the control head.

Yeah, I kind of wondered about that since I tested it on a good head and it read open. I thought it was a fuse as well (sure looks like one) but since it was open I figured it was probably something else.

Anybody have an A7 with ugly plastic they're willing to part with?

You really should have a close board layout.

Common problems are the 10Mdf cap at the edge corrodes the thin foil trace supplying 12V,

If you have 12V at the FET switch, when the radio is turned on there must be over 20V at the gate. This is supplied by a square wave tripler and limited by a 36V zener near the FET. This is a multi section unit and I have seen partial shorts where the zener is now a lower voltage. If you remove it and the radio now turns on, it must be replaced.

This is a very high impeadence circuit and even a 10M input dvm will load the circuit so do not ever expect to read 36V at the gate.

A general comment - - physical leakage isn't the only way to tell if the electrolytics are bad. Best bet is an ESR meter. I have measured many which looked physically just fine but the ESR was already well above 100 Ohms.

The biggest problem I have seen is the failure of the fet that drives the display power supply.

Don't trust at-a-glance visual inspection for the two 10uF caps. I've seen several heads and radios where, to the unaided eye, the caps look perfectly all right -- until you detach them from the board, and find ugly corrosion underneath!

I almost lost my own UHF radio to that issue before I found out about the problem.

Happy tweaking.

kc7gr wrote:Don't trust at-a-glance visual inspection for the two 10uF caps. I've seen several heads and radios where, to the unaided eye, the caps look perfectly all right -- until you detach them from the board, and find ugly corrosion underneath!

I almost lost my own UHF radio to that issue before I found out about the problem.

Happy tweaking.

I know how that goes. I almost lost the command board in one of mine because of the same thing. They looked fine from above but once I pulled them off there was a good amount of damage and lifted traces. I was able to salvage it but barely.

I did change out the caps on the head, but they didn't bring it back to life. There was a little bit of leakage but no real damage; it all cleaned up nicely with isopropyl and a toothbrush.