Farnell TSV 70 MK2 Power Supply Repair (1975) - Page 1
Forum section: electronics / repair

author: killingtime
author_rank: Regular Contributor
timestamp: May 02, 2025, 01:28:40 am
content: Hi,I'm restoring a Farnell-TSV-70-MK2 350W power supply. Design is from 1975 and it uses discretes.I have the service manual with schematic but there's a zener diode I can't find any information on - HS MR56. I think it's a 5.1v zener but can't be sure. The one that's installed is measuring 6.5V under reverse bias @ >20mA. It's being used as a voltage regulator. 1/2W going from the size. The usual search engines have nothing on this part.At ~40+ years old the alu capacitors all test good and aren't leaking. Spotted two RIFA caps on the output, one of which is cracked. Any other suggestions welcome.Thanks.

author: themadhippy
author_rank: Super Contributor
timestamp: May 02, 2025, 01:37:31 am
content: QuoteUnless i had other clues id be leaning towards it being 5.6V

author: George Edmonds
author_rank: Frequent Contributor
timestamp: May 02, 2025, 03:13:29 am
content: Almost certainly a 5.6V Zener, as you measured.You do not say what is the fault with this power supply, it helps if you do as the Crystal balls are not working well at present

author: killingtime
author_rank: Regular Contributor
timestamp: May 02, 2025, 12:11:40 pm
content: Quote from: George Edmonds on May 02, 2025, 03:13:29 amI measured 6.5V which is a bit high.That particular zener is the reference voltage for regulated output, so I'll replace it. I was up until the wee hours today testing other components on different PCBs and came across another MR56 that measured ~5.5V under reverse bias so that confirms it. Attached @ 30mA. What confused me is the service manual states the output from this particular regulator circuit provides a 5.1V reference...hence the question.Quote from: George Edmonds on May 02, 2025, 03:13:29 amThe output defaults to maximum voltage at switch on and stays there. Way over spec.After much testing, I've found two low current transistors that feed the 2N3055 pass transistor bank which have gone short circuit, so they've been giving the base pin of that bank the full DC unregulated rail (ouch). Looks like a cascade failure. More testing awaits. I hope the whole pass bank hasn't failed as well. Those transistors are a pita to get to in the wind tunnel heat sink.Thanks.

author: factory
author_rank: Super Contributor
timestamp: May 02, 2025, 03:45:39 pm
content: It's possible the full part number of the Zener is MR56H or MR56-H, this is an AEI part, I haven’t found a datasheet but owner of this webpage has some old datasheets (not on the webpage);http://www.wylie.org.uk/technology/semics/BTHAEI/BTHAEI.htmThe 5.6V Zener used as the reference, would have been chosen for it's low temperature coefficient, which tends to be worse for higher or lower voltage parts, check the datasheets of modern parts to select one with a low TC.David

author: MathWizard
author_rank: Super Contributor
timestamp: May 02, 2025, 04:39:12 pm
content: Now an easy mod, that would be more stable than zeners, would be to add in an adjustable current sink, like a 3-terminal TO-92 TL431, or even a TO220 LM317. I made an old Farrnel PSU and it works pretty good, but yeah I used 1/4W zeners and some of them they burned out pretty easy. So I added some TL431's.

author: killingtime
author_rank: Regular Contributor
timestamp: May 02, 2025, 05:04:09 pm
content: Quote from: factory on May 02, 2025, 03:45:39 pmThanks, will do.I've done some more testing. Good news is that the NPN pass bank of 6 BJTs on the misbehaving side hasn't failed closed circuit. Mega Ohms between C-E. The bad news is that the HFE is quite a bit lower than the non-abused bank.Picture and schematic attached to save the guess work.A single 2N3055 NPN in emitter follower config (on its own pcb - VT17) drives the base of 6 2N3055 NPN in a wind tunnel heat sink, also in emitter follower config (VT18-23).This power supply is actually two independent supplies, placed in parallel or series depending on a mechanical front switch for 35v or 70V operation. So that's 14 2N3055s. 7 for each supply. This makes it easier to see if one side is faulty as you have a good side to compare with.After disconnecting all the wires to the driver pcb with the single 2N3055, and the 6 parallel BJTs (stud connection on the wind tunnel) I got the following hfe readings. Also had to lift a few resistors on the pcb to stop them messing with the hfe test.Good side hfe: Driver 19, Array 18Bad side hfe: Driver 9, Array 14.According to the spec sheet for a NPN 2N3055, I should be seeing at least 20 for the hfe, and that's at high current. Small signal gain should be 50+.Do you think I can get away with 14 on the pass bank NPNs? I'll change all the other transistors out for new.Hfe on a common collector BJT config (emitter follower) has no impact on voltage gain (~1) but it does alter the input impedance and current gain which will be a bit lower in the above case. So that just means the driver circuitry will have to work a bit harder to maintain regulation.Thanks.

author: fzabkar
author_rank: Super Contributor
timestamp: May 02, 2025, 06:00:56 pm
content: https://www.sourceresearch.com/StoreWork/CrossList.cfmMR56C-H crosses to NTE136A which is a 1W 5.6V 5% DO-41 Zener.