RF Power Amp repair - Page 1
Forum section: electronics / repair

author: veedub565
author_rank: Frequent Contributor
timestamp: November 13, 2020, 05:42:17 pm
content: Hi, just looking for a few pointers on repairing this RF Power amp. It's a 200W Microset SR200 (circuit diagram attached)Everything seems to work as it should, all the LED's light up when they should, the relays click over when they should..... it just doesn't output anything.In normal operation the signal path is routed straight through from input to output. Put 10W in get 10W(ish) out. Switch the RX amp in and yep that works fine. Switch the Power amp on, put 10W in and get 1W out.Nothing obviously blown up, checked all the tants for shorts, checked all the semiconductors for shorts, checked the fuses. I get 13.8v at "IN_VA"I noticed I only get about 0.7v at "IN_VB" and "IN_VB" measures 4.5 ohm to ground. Not sure that is correct or not. Base Emitter on the output transistors also measures 4.5 ohm to ground.I'm leaning towards the Output transistors being at faultnot a cheap to just swap out and try it though. Need to be sure if it is them or not.

author: bob91343
author_rank: Super Contributor
timestamp: November 13, 2020, 07:21:39 pm
content: Verify that the relay contacts are doing their job. If the input doesn't reach the amplifier, or the output doesn't reach the antenna, that would cause it.

author: veedub565
author_rank: Frequent Contributor
timestamp: November 13, 2020, 07:47:38 pm
content: Thanks Bob, I was just heading down that route.It seems with Power and RX OFF, then the default position of RL1/1A and RL2/2A is a straight through line to the Ant. With Power "ON" when I key up I can hear relay clicking noises. This should be RL1 & 1A, I'm wondering if it's possible 1A is switching but 1 isn't.With the Power "ON" and the rig keyed up, output to a dummy load. Using a high impedence probe I'm only measuring mW at C17, maybe 10mW on the base of the output transistors. That's with about 10W on the input.If RL1 isn't switching then the RF in could be bypassing the amplifier stage altogether. Winding up at RL1A whichhasswitched to the output of the amp stage. Just a theory,

author: jdragoset
author_rank: Frequent Contributor
timestamp: November 13, 2020, 09:56:30 pm
content: Seems low VB-IN low (0.7 volts) is suspiciousCheck DC voltage drop across Power Amp Switch, when closed.Also VB-Out to VB-IN DC current if possible.

author: veedub565
author_rank: Frequent Contributor
timestamp: November 14, 2020, 03:57:13 pm
content: Ok I may have made an error last night with my measurements.With "POWER" off I measure very little RF at C17, a few mW. With "POWER" on I get at least 2W at C17 (my RF probes overloads at 2W unfortunately) This would seem to suggest that RL1 at least is switching and diverting RF input to the input of the amplifier stage.What is perhaps a bit odd, is that the closer to the base of the output transistors I get, the less power there is. >2W at C17, 1.8W at C18, 1.3W at C39, 0.3W at the base of the output transistor. 1.3W on the collector of the output transistor, and approx 2W coming out.Couple of thoughts..1. Maybe with "POWER" on, the rig I'm using is seeing high VSWR on the input and reducing the power going in. I need to check this.2. RL1 may be switching, but RL1A may not be and so the output transistors would be trying to drive an open circuit. And they won't thank you for that.The 0.7v at VB-IN seems low, but "ON_AIR" light is coming on which would suggest it is correct.

author: veedub565
author_rank: Frequent Contributor
timestamp: November 14, 2020, 05:18:43 pm
content: Checked the VSWR with an insertion unit placed between the output of the rig and the input of the amplifier.With "POWER" offVSWR = 1.15:1With "POWER" onVSWR = 3.94:1And just for verification purposes, with the rig going straight into the dummy load ( amp not connected)VSWR = 1.10

author: veedub565
author_rank: Frequent Contributor
timestamp: November 14, 2020, 06:44:57 pm
content: Quote from: jdragoset on November 13, 2020, 09:56:30 pmI must have made a mistake on this too. Perhaps a dodgy connection. Just rechecked this and it's 12.8V on "OUT-VB" and on "IN-VB"So confirmed RL1 is switching, confirmed 12.8v on "IN-VB", confirmed 13.8v on ""IN-VA", confirmed no s/c tants, the rest of the system seems to be working ok... there's not a lot else left, the inductors look ok, the resistors look ok. The apparent 4.5 ohm from base to emitter, I tracked that to R31 & R36 which are 4.2 ohm resistors to gnd.Looking like the output transistors, they are a PITA to remove, and not sure how I'd test them anyways.

author: TheMG
author_rank: Frequent Contributor
timestamp: November 15, 2020, 06:27:41 pm
content: Quote from: veedub565 on November 14, 2020, 03:57:13 pmThat's normal. The impedance of an RF transistor is much lower than 50 ohms. The input network of the amplifier aims to not only divide the input power between the multiple transistors, but also transforms down to a lower impedance.Similarly, on the output of the transistors the impedance is very low, and there is a matching network to bring the impedance up to 50 ohms.What this means is, without knowing the exact input and output impedance of the transistors in question, the readings from your RF probe don't mean much. You can use it for comparative measurements, but that's about it.Did you measure the DC voltage at the base of each transistor (with PTT applied and no RF input)? It should be around 0.7V. Also I don't know if you have the specs on what the bias current is supposed to be for this amplifier, but similarly you should measure the collector current (again, with PTT on but no RF).Taken out of circuit, BJT RF power transistors can be tested pretty much like any other transistor. But since you've got a linear amplifier a basic check is to look for a sensible bias voltage at the bases, and current draw at the collectors, which if you get both would indicate that the transistors are at least working to some extent.Do you have more documentation than just the schematic? There isn't even component part numbers indicated on there...

author: veedub565
author_rank: Frequent Contributor
timestamp: November 16, 2020, 10:33:38 am
content: Here's the datasheet for the RF Transistor and the user manual. That's all the information I've been able to find on this.The circuit diagram isn't the most helpful having no part numbers or component values. Makes it tricky matching it up with the parts on the actual board, which has no component references printed on it.I've been a bit hesitant at probing around with a DVM while applying any RF to it, I will use the PTT as this seems a safer bet all round. I did try connecting the PTT, it seemed a bit odd as soon as I connected the ground wire the unit clicked into "ON-AIR" state. I was expecting to connect the ground wire, and then it would go to "ON-AIR" state whenever the positive lead was attached. I may have got this wrong thinking PTT went back to the PSU. So I just need to connect the two PTT pins together.I really want to be able to say for sure if it is or isn't the power transistors.I will take some more measurements and report back

author: veedub565
author_rank: Frequent Contributor
timestamp: November 16, 2020, 05:53:47 pm
content: OK confirmed 0.7v on the base of the transistors with PTT on and no RF going in. Collector voltage is 13.8vRegards measuring collector current am I going from collector to gnd, or putting the meter in series somewhere?The whole thing is only drawing 863mA now in total, with no RF going in and PTT on. I did notice that R32 got hot, its 7W 39ohm, one of the 4 resistors between the two transistor bases (two are 39ohm two are 0.1ohm)

author: TheMG
author_rank: Frequent Contributor
timestamp: November 16, 2020, 09:00:16 pm
content: Normal for R32 to get quite warm. It's dropping about 13V so dissipating about 4.3W.R35 (the other 39ohm resistor) should be getting similarly warm.These resistors provide the bias current to the bases of the two transistors.Collector current measured in series.

author: veedub565
author_rank: Frequent Contributor
timestamp: November 16, 2020, 09:06:11 pm
content: Thanks, I'll lift one leg of L1 and get the meter in there.BTW I ruled out RL1A being a possible cause. Using a DVM on ohms I confirmed It is switching the output of the amplifier stage to the antenna.

author: veedub565
author_rank: Frequent Contributor
timestamp: November 17, 2020, 04:05:10 pm
content: Collector current reads 0.03mA (meter in series between L1 and collector)So the RF transistors have 0.7v on the base, 13.8v on the collector, and a collector current of 0.03mA. Thats with PTT, no RF going in

author: jdragoset
author_rank: Frequent Contributor
timestamp: November 17, 2020, 06:05:32 pm
content: Based on 0.7 volt B to E drop and base circuit resistance you should have a base bias current 0f 0.3A.Hfe minimum of 10 should give a 3 amp collector current.

author: veedub565
author_rank: Frequent Contributor
timestamp: November 17, 2020, 07:00:38 pm
content: Definitely nothing like 3A at the collector

author: TheMG
author_rank: Frequent Contributor
timestamp: November 17, 2020, 08:57:52 pm
content: Since you have bias current and collector supply voltage but virtually no collector current at all, combined with the fact that the amplifier is behaving like an attenuator, I'd say it's a fair assumption at this point that the transistors are toast.Once you remove them from circuit you can triple-check by measuring with a multimeter. To desolder these, heat one lead to melt the solder and use a small screwdriver or a pick to lift the lead away from the board. If some solder remains, use wick underneath the lead until there is no more solder bridging between the lead and PCB pad. Repeat for all 4 leads.When purchasing replacement transistors, it is advisable to purchase a matched pair to ensure optimum linearity.

author: veedub565
author_rank: Frequent Contributor
timestamp: November 17, 2020, 09:20:52 pm
content: I suspected the power transistors but its great to be able to nail it down with a bit more certainty. So next step I'll remove them and as suggested triple check. I have access to an Atlas component tester (not mine) so presumably that would work connected to base, collector and one of the 4 emitter pins.I was planning on using the Hakko hot air gun to remove it. Maybe use the underboard heater too. And I was going to use capton tape or small heatshields to protect surrounding components.Sourcing a replacement pair could be tricky. There's loads on a certain Internet auction site, but ranging from £10 to £60 each it seems a minefield of fakes and factory rejects.

author: TheMG
author_rank: Frequent Contributor
timestamp: November 18, 2020, 12:57:11 am
content: Actually if you have a hot air station that would be a good way to remove it too, especially if you can remove the board from the heatsink first.RF Parts is a good source, they sell them as matched pairs too:https://www.rfparts.com/2sc2782-mp.htmlOf course, you do pay a premium, but at least you know what you're getting. Yes, it's expensive, but RF power transistors in general aren't cheap and these are discontinued so remaining stock of them is decreasing over time.

author: veedub565
author_rank: Frequent Contributor
timestamp: November 20, 2020, 03:06:28 pm
content: Wow that was a PITA to remove, even with the underboard heater and a hakko it took some time and patience. Board will need cleaning up and the big blobs of solder removed before trying to refit the new one with a soldering iron. Or would you hot air the new one on too?Only pulled the one off so far, tested with a meter and it reads open circuit base-emitter base-collector emmitter-collector. So confirmed its dead, blown open.Unfortunatlu RFparts won't export these parts to the UK. A reliable source in the UK has quoted £67 each! and can buy two from the same batch. Not cheap are they.

author: veedub565
author_rank: Frequent Contributor
timestamp: November 20, 2020, 03:47:18 pm
content: Second one is out now too. Interestingly it reads slightly different to the first oneBase-collector 200ohmBase-emitter 3.7MohmCollector emitter 3.8Mohm**edit**Top tip, if you've been pointing a hot air gun, set to 415 degC, at a pair of tweezers for 5min.... don't touch the end of the tweezers. Surprise surprise they are hot 🔥 😖

author: veedub565
author_rank: Frequent Contributor
timestamp: December 08, 2020, 09:48:47 am
content: Ok so I have replaced the power transistors, starting off gently I tested with 5W input and I get around 20-25W output. So it does appear to be amplifying now.Unfortunatly I have noticed that the input VSWR is going up very high and clearly not correct. With the Amplifier section and RX preamp section disconnected, I still see high VSWR on the input. Removing C1 improves things greatly. Possibly this is now the original fault.Any idea's ?

author: dl6lr
author_rank: Frequent Contributor
timestamp: December 08, 2020, 10:36:17 am
content: Quote from: veedub565 on December 08, 2020, 09:48:47 amThis is somehow irritating as you stated "With "POWER" off VSWR = 1.15:1". In POWER off, the RF PTT circuit is still involved but it does not trip the relais.Removing C1 will render the RF PTT functionality inoperable. Beware: if you put RF to the amp with the Rx Amp switched on, you will toast the Rx Amp if PTT is not actively driven!Check C1, C2, D1, D2, R1, CP1. CP1 is effective only in SSB. This is a voltage doubler, test with Rx Amp off Tx Amp off, a dummy to the output and PTT disconnected. With minimal RF to the input read the DC voltage across C2 with a high impedance meter. It should be roughly the double peak value of the RF. So if you feed 10mW, C1 should have between 0.7 and 1.4V (depending on R2, R3 and TR2, TR4).Bernd

author: veedub565
author_rank: Frequent Contributor
timestamp: December 08, 2020, 11:56:40 am
content: Default position for the relays with everything off is straight through. The RX circuit is disconnected unless RX is switched on. Amplifier section is disconnected unless power on is switched on. In this straight through configuration the relays do not operate even when PTT is activated or RF applied.I will try your suggestions, C1 is now back in circuit.One thing I noticed that was the SWR increased as more power was applied. perhaps there is a problem with RF being fed back form the output.

author: veedub565
author_rank: Frequent Contributor
timestamp: December 08, 2020, 09:13:40 pm
content: C1 and C2 check out ok. D1 and D2 both measure 1.6Mohm forward and reverse, which seems a bit odd.